902.5 Traffic Control Signal Features (MUTCD Chapter 4D)
- 1 902.5.1 General (MUTCD Section 4D.01)
- 2 902.5.2 Traffic Signal Operation
- 3 902.5.3 Signal Phasing
- 4 902.5.4 Control Features for Non-coordinated Signals
- 5 902.5.5 Coordination
- 5.1 902.5.5.1 How Coordination Works
- 5.2 902.5.5.2 Planning a Coordinated System
- 5.2.1 902.5.5.2.1 Determine System Limits
- 5.2.2 902.5.5.2.2 Determine Timing Plans to Use
- 5.2.3 902.5.5.2.3 Determine Cycle Lengths for Timing Plans
- 5.2.4 902.5.5.2.4 Determine Phase Times and Sequence for Each Intersection
- 5.2.5 902.5.5.2.5 Determine Offsets and Transition in Each Timing Plan
- 5.2.6 902.5.5.2.6 Adaptive Control
- 5.3 902.5.5.3 How to Interconnect for Coordination
- 5.3.1 902.5.5.3.1 Determine Type of Interconnect
- 5.3.2 902.5.5.3.2 Types of Interconnect
- 5.4 902.5.5.4 Communication between Controllers
- 5.5 902.5.5.5 Diamond Interchanges
- 6 902.5.6 Controller Assembly Components
- 7 902.5.7 Detectors
- 8 902.5.8 Responsibility for Operation and Maintenance (MUTCD Section 4D.02)
- 8.1 902.5.8.1 Agreements
- 8.2 902.5.8.2 Signal Operations
- 8.3 902.5.8.3 Signal Maintenance
- 8.3.1 902.5.8.3.1 Preventive Maintenance
- 8.3.2 902.5.8.3.2 Conflict Monitor and Malfunction Monitor Testing
- 8.3.3 902.5.8.3.3 Emergency Maintenance
- 8.3.4 902.5.8.3.4 Maintenance Limits
- 8.3.5 902.5.8.3.5 Equipment Replacement and Repair
- 8.4 902.5.8.4 Record Keeping
- 9 902.5.9 Provisions for Pedestrians (MUTCD Section 4D.03)
- 10 902.5.10 Meaning of Vehicular Signal Indications (MUTCD Section 4D.04)
- 11 902.5.11 Application of Steady Signal Indications (MUTCD Section 4D.05)
- 12 902.5.12 Signal Indications – Design, Illumination, Color, and Shape (MUTCD Section 4D.06)
- 13 902.5.13 Size of Vehicular Indications (MUTCD Section 4D.07)
- 14 902.5.14 Positions of Signal Indications Within a Signal Face – General (MUTCD Section 4D.08)
- 15 902.5.15 Positions of Signal Indications Within a Vertical Signal Face (MUTCD Section 4D.09)
- 16 902.5.16 Positions of Signal Indications Within a Horizontal Signal Face (MUTCD Section 4D.10)
- 17 902.5.17 Number of Signal Faces on an Approach (MUTCD Section 4D.11)
- 18 902.5.18 Visibility, Aiming, and Shielding of Signal Faces (MUTCD Section 4D.12)
- 19 902.5.19 Lateral Positioning of Signal Faces (MUTCD Section 4D.13)
- 20 902.5.20 Longitudinal Positioning of Signal Faces (MUTCD Section 4D.14)
- 21 902.5.21 Mounting Height of Signal Faces (MUTCD Section 4D.15)
- 22 902.5.22 Lateral Offset (Clearance) of Signal Faces (MUTCD Section 4D.16)
- 23 902.5.23 Signal Indications for Left-Turn Movements – General (MUTCD Section 4D.17)
- 24 902.5.24 Signal Indications for Permissive Only Mode Left-Turn Movements (MUTCD Section 4D.18)
- 25 902.5.25 Signal Indications for Protected Only Mode Left-Turn Movements (MUTCD Section 4D.19)
- 26 902.5.26 Signal Indications for Protected/Permissive Mode Left-Turn Movements (MUTCD Section 4D.20)
- 27 902.5.27 Miscellaneous Applications for Left Turns
- 28 902.5.28 Left Turn Lanes
- 29 902.5.29 Guidelines for Use of the Flashing Yellow Left Arrow (FYA)
- 29.1 902.5.29.1 Installation Strategy and Guidelines
- 29.2 902.5.29.2 Equipment for Reconstructed Signals
- 29.3 902.5.29.3 Equipment for New Construction
- 29.4 902.5.29.4 Signal Hardware Purchases (Internal Retrofit)
- 29.5 902.5.29.5 Regional Cooperation
- 30 902.5.30 Signal Indications for Right-Turn Movements – General (MUTCD Section 4D.21)
- 31 902.5.31 Signal Indications for Permissive Only Mode Right-Turn Movements (MUTCD Section 4D.22)
- 32 902.5.32 Signal Indications for Protected Only Mode Right-Turn Movements (MUTCD Section 4D.23)
- 33 902.5.33 Signal Indications for Protected/Permissive Mode Right-Turn Movements (MUTCD Section 4D.24)
- 34 902.5.34 Overlaps and Right Turn Phasing
- 35 902.5.35 Signal Indications for Approaches with Shared Left-Turn/Right-Turn Lanes and No Through Movement (MUTCD Section 4D.25)
- 36 902.5.36 Signal Timing
- 37 902.5.37 Detector Settings
- 37.1 902.5.37.1 Stop Bar Detectors
- 37.2 902.5.37.2 Advance Detectors and Gap Reduction
- 37.3 902.5.37.3 Detection on High-Speed Approaches
- 37.4 902.5.37.4 Delay and Extend Detector Settings
- 37.5 902.5.37.5 Locking and Non-Locking Detector Setting
- 37.6 902.5.37.6 Recalled Phases
- 37.7 902.5.37.7 Detector Call Switching
- 37.8 902.5.37.8 Phase Re-Service
- 38 902.5.38 Preemption and Priority Control of Traffic Control Signals (MUTCD Section 4D.27)
- 38.1 902.5.38.1 Turn Restrictions During Preemption (MUTCD Section 8B.08)
- 38.2 902.5.38.2 Traffic Control Signals at or Near Highway-Rail Grade Crossings (MUTCD Section 8C.09)
- 38.3 902.5.38.3 Traffic Control Signals at or Near Highway-LRT Grade Crossings (MUTCD Section 8C.10)
- 38.4 902.5.38.4 Use of Traffic Control Signals for Control of LRT Vehicles at Grade Crossings (MUTCD Section 8C.11)
- 39 902.5.39 Flashing Operation of Traffic Control Signals – General (MUTCD Section 4D.28)
- 40 902.5.40 Flashing Operation – Transition Into Flashing Mode (MUTCD Section 4D.29)
- 41 902.5.41 Flashing Operation – Signal Indications During Flashing Mode (MUTCD Section 4D.30)
- 42 902.5.42 Flashing Operation – Transition Out of Flashing Mode (MUTCD Section 4D.31)
- 43 902.5.43 Power Outages at Signalized Intersections
- 43.1 902.5.43.1 Temporary Stop Signs at Signalized Intersections
- 43.2 902.5.43.2 Start up from Dark at Signalized Intersections
- 43.3 902.5.43.3 Battery Backup Systems at Signalized Intersections
- 44 902.5.44 Temporary and Portable Traffic Control Signals (MUTCD Section 4D.32)
- 45 902.5.45 Lateral Offset of Signal Supports and Cabinets (MUTCD Section 4D.33)
- 46 902.5.46 Use of Signs at Signalized Locations (MUTCD 4D.34)
- 47 902.5.47 Use of Pavement Markings at Signalized Locations (MUTCD Section 4D.35)
902.5.1 General (MUTCD Section 4D.01)
Support. The features of traffic control signals of interest to road users are the location, design and meaning of the signal indications. Uniformity in the design features that affect the traffic to be controlled, as set forth in this article, is especially important for the safety and efficiency of operations.
Standard. When a traffic control signal is not in operation, such as before it is placed in service, during seasonal shutdowns, or when it is not desirable to operate the traffic control signal, the signal faces shall be covered, turned, or taken down to clearly indicate that the traffic control signal is not in operation.
Support. Seasonal shutdown is a condition in which a permanent traffic signal is turned off or otherwise made non-operational during a particular season when its operation is not justified. This might be applied in a community where tourist traffic during most of the year justifies the permanent signalization, but a seasonal shutdown of the signal during an annual period of lower tourist traffic would reduce delays; or where a major traffic generator, such as a large factory, justifies the permanent signalization, but the large factory is shut down for an annual factory vacation for a few weeks in the summer.
Standard. A traffic control signal shall control traffic only at the intersection or midblock location where the signal faces are placed.
Midblock crosswalks shall not be signalized if they are located within 300 ft. from the nearest traffic control signal, unless the proposed traffic control signal will not restrict the progressive movement of traffic.
Guidance. A midblock crosswalk location should not be controlled by a traffic control signal if the crosswalk is located within 100 ft. from side streets or driveways that are controlled by STOP signs or YIELD signs.
Engineering judgment should be used to determine the proper phasing and timing for a traffic control signal. Since traffic flows and patterns change, phasing and timing should be reevaluated regularly and updated if needed.
Traffic control signals within 1/2 mile of one another along a major route or in a network of intersecting major routes should be coordinated, preferably with interconnected controller units. Where traffic control signals that are within 1/2 mile of one another along a major route have a jurisdictional boundary or a boundary between different signal systems between them, coordination across the boundary should be considered.
Support. Signal coordination need not be maintained between control sections that operate on different cycle lengths.
For coordination with grade crossing signals, see EPG 902.5.38 and MUTCD Chapter 8.
902.5.2 Traffic Signal Operation
Support. Traffic signals can operate independently of any other traffic control signal ("isolated" operation) or their operation can be related to other traffic control signals ("coordinated" operation) forming a traffic control signal system.
Traffic control signals can be operated in pretimed, full-actuated, or semi-actuated control.
902.5.2.1 Pre-timed Control
Support. Pre-timed controllers direct traffic to stop or permit it to proceed according to a predetermined fixed cycle length and a division of the fixed cycle time between the various approaches to the intersection regardless of the actual vehicle demand. The sequence in which the signal indications are shown, and the time-relation of the signal to other signals are also pre-selected. Any or all of these features can be changed to accommodate specific needs.
In pre-timed operation, signal sequence is controlled by signal plans, which define the order of the signal intervals that are displayed. The amount of time given to each interval in a signal plan is determined by a timing plan. The time of the day at which specific timing and/or signal plan programs begin or end can be predetermined locally or remotely.
Pre-timed control can work well at intersections with tight spacing (i.e. diamond interchanges or central business district). However, traffic actuated controllers are preferred at most intersections: actuated controllers can run pre-timed but pre-timed controllers cannot run actuated.
902.5.2.2 Fully-actuated Control
Support. A fully actuated controller uses detection for all movements to determine the display and duration of vehicle and/or pedestrian movements at an intersection. The controller is able to skip those movements where no demand is present.
Fully actuated controllers have the flexibility of operating fully actuated, semi-actuated or pre-timed. The type of operation can also be changed by time of day.
Fully actuated controllers are available in two primary types, the NEMA, National Electrical Manufacturers Association, or the Type 170/2070 (Caltrans TEES, Caltrans Transportation Electrical Equipment Specifications). Both are keyboard entry and software driven machines. The NEMA controllers make up the bulk of the actuated controllers used in the state.
NEMA TS1 standards (refer to EPG 902.5.6.1 NEMA TS1) define the basic operating parameters of the controller as well as the inputs and outputs of the unit. This has led to the interchangeability of NEMA controllers between manufacturers. Most of the manufacturers have enhanced the operating software, adding many features that can be unique to that make and model. However, to be certified as a NEMA controller the basic operating functions are identical.
The NEMA TS2 standard (refer to EPG 902.5.6.1 NEMA TS2) expands the features of the TS1 standard, providing a higher level of standardization and interchangeability of equipment. TS2 also defines many of the features that have evolved in the current NEMA controller technology. Some TS2 functionality can be part of a TS1 cabinet.
The Type 170/2070 (refer to EPG 902.5.6.1 Type 170/2070) controller defines the hardware of the controller and allows the user to choose the software to run the controller. This has led to a great and almost total interchangeability of the controller and cabinet hardware but has left the user to evaluate and standardize on the software to run the intersection. Most of the features of the software are similar to the NEMA parameters. Although some hybrid versions of the 2070 have been created that can be used in NEMA cabinets, the true 170/2070 style controllers are designed for mounting in a 19” rack and meet Caltrans TEES (Caltrans Transportation Electrical Equipment Specifications).
All fully actuated controllers are able to respond to the traffic at the intersection. Minimum green times for called phases as well as extensions, when there continues to be traffic present, are programmable. There are also maximum green times, when a phase must be terminated to serve other calls, as well as yellow change and red clearance intervals that are programmable. There are other features available on a per phase basis such as pedestrian movements, added initial, maximum initial, minimum gap, time to reduce, time before reduction, minimum and maximum recalls. These features allow the fully actuated controller to serve the traffic in the most efficient manner.
The phases in an actuated controller can be assigned or grouped in many ways to provide unique operations that best serve the intersection's needs. The most common is the dual ring eight-phase configuration. This arrangement allows for separate through and left turn phases for up to four approaches. The opposing left turns are typically on concurrently, either leading or lagging. Through the software, the phases can be repositioned to provide a lead-lag left operation if so desired. Examples of typically used configurations are found in Signal Phasing and Layout Examples.
Efficient actuated operation is dependent on the type and placement of the detectors. Poor detector placement can have a serious impact on the delay and capacity of an intersection. Different types of detection are discussed in EPG 902.5.7 and EPG 902.15.3. Typically, stop bar presence detection is used. A common presence detector is a 6 ft. x 30 ft. area. Mainline through detection can be supplemented with advanced detectors located a distance from the stop bar based on the speed of approaching traffic.
Full actuation and advance vehicle detection on high-speed approaches are typically used to reduce the frequency of vehicles in the “dilemma zone”. The dilemma zone is the area where the onset of the yellow change interval creates difficulty for the driver to decide whether to stop or proceed.
902.5.2.3 Semi-actuated Control
Support. Semi-actuated intersection control refers to intersections where a fully actuated controller is used but one or more phases are not actuated. Typically, the mainline through phases are not actuated and the side streets and the mainline lefts are actuated. Timing for the non-actuated phases can be accomplished by using recalls. The non-actuated phases will remain in green until there is a call from one of the other movements and the minimum green timer is expired. The actuated phases work the same as in fully actuated control. To avoid or minimize unnecessary delay at isolated intersections fully actuated control is preferable to semi-actuated control.
902.5.2.4 Comparison of Pre-timed and Actuated Control
Support. Each principal type of traffic signal control, pre-timed and actuated, posses certain advantages not afforded by the other. The following discussion is intended to bring out basic differences in the different types of control, as to their operating characteristics and suitability for various traffic requirements. It is to be remembered that each type of control is capable of being modified in various ways for improved efficiency and flexibility.
With basic pre-timed control, a consistent and regularly repeated sequence of signal indication is given to traffic. Pre-timed control is particularly adaptable to intersections where it is desired to coordinate signal operation with closely spaced intersections.
Actuated control differs basically from pre-timed control in that signal indications are not of fixed length, but are determined by and conform within certain limits to the changing traffic flow or to the background cycle, if coordinated. The length of the cycle and sequence of phases might or might not remain the same from cycle to cycle since phases will not be serviced unless there are detections from waiting vehicles or pedestrians.
902.5.2.5 Advantages of Pre-timed Control
Support. Advantages of pre-timed control include the following:
- Consistent starting time and duration of intervals can facilitate more precise coordination with adjacent traffic signals.
- Pre-timed control can permit the operation of two or more very closely spaced intersections to operate at maximum efficiency.
- Pre-timed control is not dependent on vehicle detectors. Thus, the maintenance needs can be reduced.
- Pre-timed control can be more acceptable than actuated control in areas where large and fairly consistent pedestrian volumes are present, and where confusion might occur with the operation of pedestrian push buttons.
902.5.2.6 Advantages of Actuated Control
Support. Advantages of actuated control include the following:
- Can provide maximum efficiency at intersections where fluctuations in traffic cannot be anticipated or programmed with pre-timed control.
- Can provide maximum efficiency at complex intersections where one or more movements are sporadic or subject to variation in volume.
- Can provide the advantages of continuous signal control even in periods of light traffic without causing unnecessary delay to traffic on the major street.
- Can be used at locations where traffic signal control is warranted for only brief periods during the day.
902.5.3 Signal Phasing
Support. The phasing of a signal determines the order that movements are serviced. A study of traffic movements at the intersection is made to determine permitted and controlled movements. From this, the number and sequence of traffic phases is determined, which in turn determines the interval or color sequence and types of signal indications to be used. In general, the most efficient operation is obtained with the fewest possible phases; however, each signal installation is designed to provide safe and efficient control of conflicting traffic movements.
Guidance. The following articles provide guidelines for selecting phasing. Several examples are also shown in Signal Phasing and Layout Examples.
The typical phase arrangement at most intersections is with eight phases grouped into two sets of movements, or "rings". NEMA designates the assignments as follows:
A general form for actuated controller sequencing of an intersection is available. However, districts might have their own forms that are specific to the controller or software that is used at the intersection.
Phase assignment should be kept uniform in accordance with this ring structure whenever possible. Mainline left turns should be assigned to phases 1 and 5, with mainline through movements assigned to phases 2 and 6. Side street left turns should be assigned to phases 3 and 7, with through movements assigned to phases 4 and 8. Other phase numbering schemes can be used, but consistency should be maintained throughout the district.
As shown above, mainline leading left indications are displayed first. Phases on opposite sides of the "barrier" cannot operate together (such as phases 2 and 4). Phases on the same side of the barrier in one ring can run concurrently with phases in the other ring (e.g. phase 3 can be on at the same time as phase 8). Only one phase per ring can be on at any given time. (For example, if phase 2 is on, then no other phases in ring 1, such as phases 1, 3, or 4 can be on at the same time.)
902.5.4 Control Features for Non-coordinated Signals
Support. Control features for non-coordinated signals include:
- 1. Isolated Operation. Actuated operation can effectively minimize traffic delays at locations where coordination is not a consideration.
- 2. Traffic Density Timing. The traffic density timing feature provides for the initial green interval and/or the allowable traffic gap that ends the green interval, to be automatically adjusted according to traffic flow variations.
- The added initial setting increases the minimum green interval according to the number of vehicle actuations received during the preceding red interval (some controllers use both actuations received during the yellow and red intervals). This setting is intended for use where the shortest practical minimum green interval is not adequate for the traffic that can be stored between the advance detectors and the stop bar. The initial part of the green interval (minimum initial), remains short when few cars have arrived during the red interval, but it increases (added initial) a fixed amount per actuation during the red interval up to the maximum initial. At intersections with advance detection only (no stop bar detection), vehicles need more time to enter the intersection before they lose the right of way.
- Gap reduction which allows the passage time to decrease as the speed of the flow increases is another setting commonly used at locations with advance detection.
- Features of density control are discussed in EPG 902.5.37.2.
- 3. Recall Operation. Each phase of a controller is equipped with a recall feature. With recall disabled, the phase responds only to its detectors. With recall enabled, the controller can place a call to service that phase without vehicles being detected. This is accomplished by the controller placing a single call back to the phase when the clearance interval is initiated.
- Recall comes in several variations and has different uses. Minimum recall is normally used on the mainline through phases. Minimum recall guarantees the timing of the minimum green for the phase selected with additional green available through actuations. Using this feature on the mainline through is advantageous since during periods of light activity the controller will rest with the main street through green indications on. Maximum recall guarantees the timing of the maximum green interval for the phase selected. This feature is normally used when the detection for a phase has been disabled due to failure or removal. In this instance, the phase will always be serviced for the maximum green interval programmed that is to be adjusted accordingly.
- Many actuated controllers also have a feature called “soft recall” that allows the selected phase to be serviced only if there are no other conflicting calls. This feature can be used on the main street during periods of light traffic. The advantage over minimum recall is the controller can skip servicing the main street, if no real calls exist, and move on to the next called phase. This can provide a quicker response during the periods of light traffic.
- For more information about Recall Phases refer to EPG 902.5.37.6.
Support. Coordination can provide nearly uninterrupted travel, resulting in one of the greatest benefits to motorists. Coordination provides many benefits other than less delay: the reduction of stops, crashes, fuel consumption, emissions, and driver frustration makes coordination one of the best values for the dollars spent.
902.5.5.1 How Coordination Works
Support. The key to coordinating signals is a common cycle length among all signals within the limits of the system. However, using a fraction of the cycle length, such as a half-cycle, can be used in some cases in order to reduce side-street delay while still maintaining mainline coordination. The start of a certain phase at each intersection, called the coordinated phase (usually the mainline through), is synchronized to a system reference point. This system reference provides a signal to each controller once every cycle. The cycle length is the total time it takes for a signal to serve all phases. The amount of time it takes for the coordinated phase at an intersection to start after the system reference is called the offset. Different offsets along a series of coordinated signals will provide different starting times of the coordinated phases. By setting offsets based on the speed of traffic in a certain direction, stops can be greatly minimized by starting the mainline green at that point where traffic from a previous signal just reaches the next signal.
Another important factor in coordination is what cycle length to run. Because volumes of traffic fluctuate throughout the day, different cycle lengths and offsets can be run to more efficiently handle the changing demands of volume in the system. Higher cycle lengths are generally used in the morning and evening peak times. Peak lunch times can also necessitate an increased cycle length. Weekend traffic can also justify cycle times that differ from weekday timing. Off-peak times use shorter cycle lengths. In a coordinated system, the time of day that each cycle length begins and ends must be the same for every controller for progression to work.
902.5.5.2 Planning a Coordinated System
902.5.5.2.1 Determine System Limits
Identification of how many signals to coordinate is the first step in building a signal system. Generally, knowledge of the arterial flow is a good start. Arterial streets where you can expect a group of vehicles to travel from one signal to another without a significant loss of vehicles turning off to another arterial or development can give reasonable limits.
The upper limit on spacing for coordinated signals has been assumed to be 1/2 mile. However, if the flow of vehicles can be maintained at a greater distance, then there is likely no reason to disregard additional signals in the system. Computer software can be used to establish the limits of a coordinated system. The "coupling index (CI)" formula can also be used if computer software is not available:
- I = V / L
- I = Coupling Index
- V = 2-way volume on the link in vehicles per hour
- L = Length of link in ft.
The link is bounded by two signalized intersections. The units of the CI are meaningless.
For planning purposes, a CI equal to or greater than 0.3 during any hour indicates the possibility of including the signals within the system. For analysis of existing systems, a CI equal to or greater than 0.5 indicates the signals is to be coordinated during the hour analyzed, if they can be operated on equal cycle lengths. The CI formula provides a very simplistic method of determining the system limits.
The final factor in determining coordination limits is engineering judgment. Factors to consider for including a signal within a coordinated system are whether the intersection is over-saturated, already part of a system for the intersecting arterial or can be serviced with the system's cycle length.
All newly constructed signals should be reviewed for coordination and implemented if needed. Non-coordinated signals should be reviewed periodically to determine if coordination is to be provided.
902.5.5.2.2 Determine Timing Plans to Use
Guidance. The purpose of different timing plans is to match the traffic conditions in order to provide the best cycle lengths and split times. In most systems, the basic timing plans used during a week are:
- 1. Off-Peak
- 2. AM Peak
- 3. PM Peak
- 4. Late-Night Operation
Off-Peak operation generally uses lower cycle lengths than AM and PM peaks. Late-night operation can be free control or a short cycle length to minimize delays for the side street when signal coordination control is desired.
In addition, the traffic conditions along a system might not be adequately handled by only one off-peak plan. Mid-day and weekend patterns can greatly differ from off-peak patterns and could require separate plans. Even AM and PM peaks might require more than one plan if volumes and distribution greatly vary during those times.
Ideally, a 24-hour count of each intersection is to be obtained for at least a five-day period that includes a weekend but not a major holiday. This provides typical weekday and weekend traffic conditions. If every intersection cannot be counted, the major intersections are to be counted in this manner. From these counts, the times when traffic characteristics change in amount and/or directional distribution are to be charted.
In order to change from one timing plan to another, a transition period occurs which disrupts the coordination flow. Therefore, the start and end times of each plan should occur well before and after the worst hour during which the timing plan runs. If the transition occurs during times of peak flow, then any benefits provided by the additional plan can be lost by the disruption in coordination. If the transition cannot be made without disrupting the coordinated flow, then one plan should be used for a longer period in order to keep traffic moving. The timing plan chosen might not provide ideal conditions throughout the entire time it is in effect but should be set up to handle the worst hour of traffic during that time.
In addition to regular time-of-week timing plans, yearly conditions might require other timing plans. Arterials servicing seasonal tourist locations, large shopping centers and large schools will likely require the capability of timing plans that satisfy those seasonal conditions. For the best timing results, counts should be done that will take these conditions into account.
902.5.5.2.3 Determine Cycle Lengths for Timing Plans
Guidance. In most coordinated systems, the cycle length requirements differ from intersection to intersection for a timing plan. In order to run a coordinated system, a common cycle length will have to be chosen. Typically, computer software is used to determine appropriate cycle lengths. However, if computer software is unavailable the following procedures will provide a rough, but effective, cycle length to apply to an arterial.
902.5.5.2.3.1 Determine Minimum Cycle Lengths
Standard. The minimum cycle length in each system shall be determined by, the critical intersection, the intersection with the highest volume to capacity ratio.
Guidance. The critical intersection will likely have side street volumes that demand a significant amount of green time. In order to achieve any benefits of coordination, the proper amount of green time will need to be allotted to both the through traffic and the side street traffic. Two methods for determining minimum cycle length are:
(A) Critical Lane Volumes
|Sum of Critical Lane Volumes||Number of Phases|
|* Intersection might be over capacity. Maximum recommended cycle is 120 seconds.|
The table above presents a way of determining minimum cycle length based on the sum of critical lane volumes and number of phases at the intersection.
(B) Highway Capacity Manual (HCM)
Appendix II for the "Signalized Intersections" chapter in the HCM presents a more detailed method of determining minimum cycle length. It is recommended to use the "Pre-timed Signals" procedure in the "Allocation of Green Time" section regardless of the type of control used, since a free-operating, fully-actuated signal will have to run nearly pre-timed operation in a coordinated system.
902.5.5.2.3.2 Final Determination of Cycle Length
Guidance. The determination of the minimum cycle length at the critical intersection should provide a starting point as to a practical cycle length for the system, but it should not be interpreted as an absolute value. Slight variations up or down might be required to best meet the demands of each intersection. Most arterial optimization software will require the user to input a lower and upper range for proper performance. If optimization software is not being used, the cycle length that provides the best service for the critical intersection should be used as the system's cycle length for that timing plan.
902.5.5.2.4 Determine Phase Times and Sequence for Each Intersection
Guidance. Computer software determines phase times and sequence when determining the cycle length. When calculating by hand, green times and sequences for each intersection should be determined after a cycle length has been determined. Since the purpose of coordination is to favor the progression of the coordinated phase, every effort is to be made to maximize the amount of green time and provide the best sequence to that phase.
902.5.5.2.4.1 Phase Times
Guidance. In order to run a pre-determined cycle length, actuated phases must be terminated, or forced off, after a certain amount of time regardless of the presence of vehicles. Actuated phases can still gap-out or be skipped if no calls are present but cannot extend past its force-off time for that timing plan. Non-actuated phases must extend to their entire force-off time. This force-off time per phase usually includes the green time and change period times.
If optimization software is being used, the program will generally give a force-off distribution for each intersection in the system. For non-computer planning, there are several methods to determine phase times. As with the cycle length determination, the HCM Appendix II in the "Signalized Intersections" chapter provides a basic procedure to allocate green time.
902.5.5.2.4.2 Phase Sequence
Guidance. When more than two phases are used, the sequence in which the indications are displayed must be determined for each timing plan. The usual choice is when to display a protected left turn in relation to the mainline green: at the start (lead) or the end (lag). The use of lead-lag for protected left turns on the mainline can greatly affect the progression. (See EPG 902.5.27.1 Leading and Lagging Left-Turns.)
If optimization software is being used, the program will generally give a mainline sequence at each intersection that maximizes the green band. Side street sequencing is usually left up to the user. There might be some timing plans where the side street sequence becomes a factor in coordination. A heavy left turn onto the mainline might call for a sequence that puts the side street left turn indications at a point in the cycle that allows for the group to clear a downstream intersection.
902.5.5.2.5 Determine Offsets and Transition in Each Timing Plan
Guidance. Once the cycle length, phase timings, and sequence have been determined, it is necessary to determine when to begin the coordinated phase in relation to the master cycle offset reference. The master cycle offset reference is a defined point during a 24-hour period to run the background cycle. In most cases, midnight is used as the master cycle reference point. The offset for each intersection will, if properly set, provide for progressed flow in the desired direction.
Option. Different offsets may be used for each timing plan at each intersection.
Guidance. The most common way to represent the flow of traffic on an arterial is the use of a time-space diagram. This type of graphical representation uses an x-y axis plot of red and green time, phase sequences, cycle lengths, and intersection spacing to display how well a platoon of vehicles moves from one end of the arterial to the other. The x-axis is a scale of the intersections by feet, and the y-axis represents the time scale in seconds (the x- and y-axis might be switched depending on the program used to create it.). Multiple cycles are laid out on the y-axis, with solid lines representing mainline red time. A green band represents the flow of vehicles whose slope represents the speed of the platoon along the artery. An example is shown below.
It will be assumed the user has a computer with a time-space diagram program available. Hand-drawn time-space diagrams are cumbersome, difficult to lay out, and nearly impossible to modify when changes are needed on the street. All further discussion on time-space diagrams will be based on the computer applications.
902.5.5.2.5.1 Offset Determination
Guidance. Determination of the offset using computer software can be accomplished by the following:
- (1) Enter basic information into the software program
- a. Distances between intersections, in feet. Measured from the center of each intersection.
- b. Phase times and sequence for each intersection.
- c. Speed of platoon between each intersection. For some arterials, it might be possible to maintain the same speed from end to end, but real conditions usually make this impractical to assume. Conditions such as closely spaced intersections, steep grades, and areas of heavy traffic will likely degrade free-flow speeds. Initially, the 85th percentile speeds, if known, can be used.
- d. Local controller's offset reference point. This is the point in the local cycle that the offset is referenced to and is the starting point of the local cycle (cycle zero). This point is typically at the beginning or end of the coordinated movements, usually the mainline throughs. Movements which are to be favored under coordination are designated as coordinated phases and the offset reference will be related to these movements. Many controllers offer options for the location of the offset reference.
- One or both of the coordinated phases typically start at local cycle point zero. If only one coordinated phase begins at local cycle zero with the other coordinated phase starting later in the cycle, this is referred to as "start of first coordinated green". If a second coordinated phase begins at local cycle zero after the start of the first coordinated phase, this setup is called "start of last coordinated green". If both coordinated phases begin together at cycle zero, either reference can be used. Additional options for offset reference are available in many controllers, see the manual of your specific controller for more options.
- e. Initial offset time, if available. If previous analysis used arterial optimization software to arrive at the cycle length and phase times, it likely provided an offset based on the desired direction of progression. This provides a good starting point but will likely need to be adjusted to match field conditions. If hand-calculation methods were used, the initial offset can be entered as zero and adjusted within the time-space diagram program.
- (2) Fine-Tune the Green Band
- After the basic information is entered, the user can open the time-space diagram to view the initial conditions entered. If all the information was accurately entered, this view will show what the user can expect to see on the street under the given conditions. The display will show, beginning at either end, the start and end of the green time projected in the direction of travel by straight lines. The slope of the lines is a function of the travel speed of the platoon. The area between these two lines is referred to as the green band. The bottom line of the green band represents the first car leaving the first intersection, with the top line the last car to clear before the indications turn red for mainline.
- Ideally, the goal is to keep this green band unbroken from one end of the arterial to the other for the direction the user wants to favor during that timing plan. However, in some cases, it might be more efficient to break the green band if a larger green band can be obtained following the break. For instance, a 30-second green band would probably be more efficient than a 10-second green band through the system. It is not uncommon that obtaining a continuous green band is impossible through the entire system particularly in large systems where two-direction progression is desired or where signal spacing is not optimal.
- During AM or PM peaks, it is common to favor one direction of flow. Off-peak plans usually require progression in both directions. In order to adjust green bands through the green time at an intersection, the user can usually move the intersection's phase time display up or down in relation to the y-axis directly on the screen and instantly see the effect on the green bands. This is graphically changing the offset time of the intersection. Once the user has adjusted all the intersections to show the best green bands, the final offset values is to be recorded for programming into the on-street controller. If all values have been entered correctly into the time-space diagram program, and then into the controllers, the user should see similar conditions on the street as shown on the computer screen.
902.5.5.2.5.2 Timing Plan Transition Determination
Support. When a controller changes timing plans, it needs a way to change to different phase times, offsets and possibly phase sequences as smoothly as possible in order to minimize the effect on progression. The two major transition methods are discussed here, since they are available on most every brand of controller, and either can be selected to best suit conditions.
- (1) Dwell Method. For this method, the controller will stop its cycle countdown and dwell at the local offset point in the cycle for either a predetermined amount of time, or until the master cycle zero point is reached, whichever comes first. Most controllers will dwell in the green time of the coordinated phases.
- The major advantage of this method is the transition can be completed in one cycle length if no set dwell time is programmed. Disadvantages to this method become more apparent as cycle lengths increase. Indefinite dwell times can cause up to a one-cycle delay before resuming normal operation. This becomes critical if the side street demand is high, or if the offset dwell point is on the side street and mainline has to stop more frequently during transition. If side street demand is low the use of the dwell method with a high or indefinite dwell time will allow for a quick transition with minimal impact.
- (2) Shortway Method (also commonly known as smooth transition). For this method, the cycle length is varied, it can be either higher or lower than its standard value until the proper offset is achieved. The amount of variance is usually no more or less than 20% of the desired cycle length. The major advantage of this method is it allows all other phases to be served with at least minimum green time, which is critical at intersections with many phases. The drawback to this method is that it might take several cycles to achieve the proper offset.
902.5.5.2.6 Adaptive Control
Support. Most traffic control systems today are based on time-of-day schedules where the traffic signal settings (cycle length, green times, offsets) are set by time-of-day based on historical data on traffic demand (e.g., am peak hour turning movement counts). However, some signal systems with two-way communication or central communication systems can escape the need to predict traffic flow and better account for variations such as weather, incidents, or major traffic generator events.
Traffic responsive systems rely on user-defined timing plans consisting of cycle length, split times, and offsets, but instead of a scheduled time for enacting plans, a traffic responsive system will select a plan based on observed volumes and occupancies. There is no guarantee that a traffic responsive system will have a plan for the observed conditions, therefore plans must be developed to handle a needed situation in advance.
Since the response to the variation in volume and/or occupancy is a change in timing plans for these types of systems, care will be needed to ensure that timing plans enacted by the system are in operation for a significant minimum duration to prevent frequent timing plan transitions.
Traffic adaptive systems discard the need for timing plans based on cycle lengths, splits, and offsets. Instead, these systems generally respond to changes in traffic on a system-wide basis quite rapidly on a cycle-by-cycle basis for each intersection. Other than for system failure backup purposes, storage of traditional timing plans is not required – a traffic adaptive system continually computes the traffic control plan.
A truly traffic adaptive system will adjust the settings at traffic signals based on real-time data on traffic conditions, and can best respond to unexpected or unplanned events, such as incidents, special events, weather, etc., since they adapt the timings based on observed traffic data. Similarly, adaptive systems will improve performance over time-of-day plans when the traffic patterns have a high degree of variability. Also, adaptive systems will reduce the adverse effects of offset transition, preemption, and transit priority. However, extensive traffic detector instrumentation is required, and intersection controller equipment for adaptive systems is often more complex than for the other control categories.
|Adaptive Traffic Signals in Lee’s Summit|
|See also: Research Publications|
Support. Installation of adaptive traffic signal systems are recommended for further consideration for corridors where traffic demand changes quickly or in an unpredictable manner, where traditional timing plans are unable to accommodate coordination in two directions of travel, or where travel times are at least 50 percent higher than free flow travel times after signal timing plans have been optimized.
Traffic Adaptive “Light”
A hybrid of both the responsive and adaptive systems, adaptive “light” systems retain the need for traditional timing plans and fixed schedules for timing plan implementation but can change the split at each phase of the traffic signal cycle based on traffic measurements upstream of the intersection and demand on minor movements. Small changes in cycle time and offset are made during time periods ranging from each cycle to a few minutes. Benefits include the ability to adjust timing plans without the requirement to manually generate new plans – developed plans can be left in operation for a longer time and not require re-optimization. Another benefit is that the need for additional detection is far less for a “light” system than a fully adaptive system.
For additional information refer to FHWA’s Traffic Control Systems Handbook.
902.5.5.3 How to Interconnect for Coordination
Guidance. The method of coordination explained above is the same for every controller in a system. How that data is communicated between controllers and what information needs to be accessed remotely should be determined when desiring signal interconnectivity.
902.5.5.3.1 Determine Type of Interconnect
Guidance. The most basic interconnection system only requires an output from a master to the locals with a signal that controls:
- 1. Offset Break
- 2. Dial 2 On
- 3. Dial 3 On
- 4. Flash/Free Operation On
The basic setup offers no communication back from the locals to the master, and can be adequately handled with seven-conductor hardwire. If the need for coordination on the arterial is critical, and the flexibility of remote monitoring of all intersections and multiple timing plans are needed, a closed-loop system with fiber optic communication should be used.
If there is doubt as to how well coordination will work, or funding for a permanent type of interconnect is not available, then time base coordination (TBC) utilizing internal clocks of the controllers should be used. This allows for a very good demonstration of how coordination will affect the arterial. Also, TBC can be used on intersections outside the existing system limits to determine if interconnection needs to be extended.
Not every controller cabinet in use is set up for interconnection. If the signal has been operating outside of a coordinated system, an interconnect panel or device might be necessary to complete a connection to the controller. The final connection is through the controller’s Ethernet port. If the controller lacks this port, another type of controller that can accept Ethernet connectivity will be needed.
902.5.5.3.2 Types of Interconnect
902.5.5.3.2.1 Time Base Coordination (TBC)
Support. If coordination is between signals is desired but interconnection is not feasible, time base coordination can be used. The most important component in a TBC system is a highly accurate clock at each controller. Controllers have clocks built in and can respond to internal offset breaks and timing program changes. The advantage of TBC is its lower initial cost. Disadvantages are that the clocks drifting can cause significant disruptions in coordination and any program change needs to be made to each individual controller. The only way to maintain clock synchronization, with no additional equipment, is physical presence to re-synchronize the controllers on a regular basis.
The time clock in each controller acts as the synchronizer. All controllers in the system are set to the proper time within one second of each other. It is imperative that clocks be off no more than one second of any other clock in the system in order to ensure accurate coordination.
Because there is no master controller in a TBC system, the locals must act as their own masters and be responsible for producing an accurate offset break. Even if the clocks are synchronized in a system, the master cycle offset reference point must be the same for proper coordination.
The following diagram shows how critical common offset reference can be to a TBC system:
- Example: A TBC system with two controllers running a 70 second cycle starting at 6 a.m.
- Controller 1 - Sync Reference: Midnight
At 6 a.m., this controller will look back to midnight and calculate when to time an offset break by starting the first 70-second cycle at midnight.
Controller 2 - Sync Reference: Start of Timing Plan
At 6 a.m., this controller will begin to time offset breaks every 70 seconds.
Due to the use of two different references, Controller 1's offset break is 30 seconds after Controller 2's. Therefore, if different offset references are used the coordination between the signals will be lost.
Because a TBC system does not have a master controller to give each local a signal as to what timing plan to run, each local is responsible for changing timing plans. The timing plans can be designated by different numbers (i.e. Cycle 3/Split 1 or Timing Plan 06) if the cycle length is similar, but for consistency, it is advisable to keep the same designation in each controller for each timing plan.
The start of each timing plan is typically at the same time for each controller, especially if the offset reference is the start of the timing plan. If all controllers are operating at a midnight reference for offset breaks, then timing plans which begin at slightly staggered times could be used with a slight delay in the onset of proper coordination. If the offset reference is at the start of the timing plan, then slightly staggered start times will give different times that the offset breaks start, and ruin coordination.
902.5.5.3.2.2 Ethernet-Over-Copper Interconnect
Support. For basic Ethernet-over-copper coordination, the controllers do not need to be the same model. However, the controllers’ internal command language must be the same. Failure to respond accurately to the desired timing plan can ruin any coordination effort.
One controller is to be designated as the master controller and send the proper coordination signals to the locals. CAT 6 cable is run into each cabinet and connected either directly to the controller, or through an ethernet switch in order to synchronize time clocks, switch coordination plans, and more.
Small systems isolated from central communication can use peer-to-peer communication to synchronize time clocks, switch coordination plans and more. The interconnect method can be CAT 6 or fiber optic cable (commonly run underground in conduit and pull boxes), by radio link between signals, or any combination of these methods.
902.5.5.3.2.3 Fiber Optic Cable Interconnect
Support. Fiber optic cable is the preferred method for interconnecting both short and long runs between signals. The fiber optic cable is run into each cabinet and connected to an internal controller modem or an Ethernet switch, which translates the optical information into data the controller can recognize.
Fiber optic interconnect has several advantages over copper wire interconnect cables. Fiber does not transmit electrical energy from lightning which helps prevent lightning damage to control equipment. The signal demands on the fiber optic capacity is small enough that the cable run can be used in the future for other uses such as real-time video surveillance and connection to ITS systems.
The disadvantages of fiber are the need for higher technical expertise to install and maintain the cable and expensive special equipment.
902.5.5.3.2.4 Wireless Interconnect
Support. Wireless interconnect is an option when it is not practical, physically or financially, to run conduit and fiber optic cable between the controllers. Types of wireless interconnect may include cellular, which connects back to the central system, or radio, which connects point to point.
902.5.5.3.2.5 Mixed Interconnection
Support. Not every system needs to have the same type of interconnection between all controllers. In some locations where hardwire interconnect can be installed between controllers and other links have physical barriers which prevent conduit, wireless interconnect might be a viable option to complete the system. Any mix of hardwire, fiber and wireless can be used as long as the end protocol is the same.
902.5.5.4 Communication between Controllers
Support. Regardless of the type of communication between controllers, several basic items will be received by all intersections to operate a system properly. More advanced systems can be remotely accessed and monitored from a central computer system.
902.5.5.4.1 Master and Local Controllers
Support. In all systems, one controller or central computer system is responsible for keeping an accurate clock running, generating an offset reference and storing timing plan start and stop times. A controller in this capacity is referred to as the master controller of the system, and it can also act as the controller for a specific intersection. It transmits this information to the other controllers in the system which are called local controllers. The locals can have similar information stored in them as the master (e.g. synchronized time clock, timing plan start and stop times), but use this information only as a backup in case of failure in communication with the master. The local controllers are responsible for having each timing plan the master might call for loaded into their memory. The timing plan in the local controllers must include phase times for each interval that add up to the proper cycle length called for from the master, and the offset value unique to that intersection. Offsets and what timing plans to run are transmitted from the master and are recognized by the locals. Every local must interpret the master signal as the same command. Failure by one local to recognize this signal will disrupt coordination in the system.
902.5.5.4.2 Closed Loop
Support. Closed loop systems allow for remote access to the local controllers through the master controller in order to monitor, change or upload information at each intersection. Any information that can be programmed or stored at a local controller can be remotely accessed in a closed loop system. Remote access can be through a computer connected to the master or front panel access to the locals at the master controller.
902.5.5.4.3 Central Control Systems
Support. In centralized systems, a central computer makes control decisions and directs the actions of individual controllers. Each intersection requires only a standard controller and communication link to the central computer.
Central systems have the following characteristics:
- They depend on reliable communications networks. Since real-time control commands are transmitted from the central computer to the local intersections, any interruption in the communications network forces the local controllers to operate without that real-time control and revert to its backup plan via time-based control. If an interruption in communication occurs while in coordination a transition is required from central control to local control. During this transition, signal coordination is usually lost for a short period of time. For this reason, communications networks for centralized systems usually include some form of reliable communications, such as fiber optics.
- They depend on reliable central computers. Without the central computers, centrally controlled systems cannot happen. When the central computer is down the system has the same problems as when the communications network is down, except that the problem affects all intersections, not just the few that are on that communications branch. Staff dedicated to healthy computer system operations is a must for reliable central control.
- They are expensive. Most of the cost in a central system is providing the communications networks – easily a much higher investment than the central system’s software.
- They provide excellent surveillance response time. The system’s communications network is reliable enough to allow mandatory real-time control communications. In most situations, this requirement ensures once-per-second return of surveillance information such as status of phases and detectors, and controller alarms requiring maintenance attention.
- They allow centralized control algorithms. This is the one area where centrally controlled systems have a distinct advantage over traditional “interconnect” systems – the ability to define a signal coordination plan by need instead of physical connection to each intersection. As long as a controller has some sort of communication link back to the central computer, the intersections so designated can run in coordination. Central control allows these system limits to vary by both time of day and on manual needs such as incident response for detour routes.
Many traffic adaptive systems require a central computer to calculate the optimization algorithm for the entire network. Only a centrally controlled system can provide this capability.
902.5.5.5 Diamond Interchanges
Support. Due to the close spacing of both ends of a standard or compressed diamond interchange it is extremely important to offer coordination between both ends of the interchange. This can be accomplished with pre-timed or actuated control.
This discussion primarily relates to standard diamond interchanges, however similar considerations can also be made for half diamond and other diamond interchange variations. If a signalized roadway (i.e. outer roadway) is very close to a ramp intersection, a different configuration might be required.
Option. Coordination between both ends may be accomplished by running a pre-timed operation during the critical peak times and actuated operation during the off-peak times. This option is to be evaluated carefully, as coordination during actuated operation may not be optimal.
Guidance. The merits of each setup should be evaluated for the best operation at each location. The following are recommended criteria for selecting the best setup. A cost comparison might also be helpful in deciding which setup to use.
Actuated Control with One Controller:
- Overall interchange operates below capacity.
- No more than two mainline or ramp left turn movements require critical coordination.
- There is sufficient left turn storage between the ramps.
Actuated Control with Two Controllers:
- Overall interchange operates below capacity during off-peak.
- None of the movements require critical coordination during off peak.
- There is sufficient spacing and left turn storage between ramps for non-coordinated operation during off-peak.
Pre-timed Control with Two Controllers:
- Overall interchange operates near or at capacity.
- Most or all of the mainline and ramp left turn movements require critical coordination.
- There is not sufficient left turn storage between the ramps.
Diamond Interchange Examples provides examples of phasing configurations for diamond interchanges.
902.5.6 Controller Assembly Components
902.5.6.1 Controller Unit
Support. The controller unit (CU) is a solid-state traffic actuated unit. The CU interfaces with a number of low voltage (logic level) input and output functions to control signal lamps, receive inputs from detectors, operate in coordinated systems, etc. Additional information on types of control is found in EPG 902.5.2 Traffic Signal Operation.
The NEMA TS1 is a common cabinet configuration for solid state controllers. The back panel for a TS1 cabinet configuration has terminals that are used to interface with the other devices in the cabinet, call certain features of the controller, as well as to display the indications on the street. Harnesses are provided to route the wiring from the controller to the rear of the back panel. By using jumpers on the front of the back panel, the inputs and outputs of the controller can be assigned. The physical makeup of the back panel is described in Sec 902.
The NEMA TS2 standard replaces much of the discrete cabinet wiring with high speed serial communications interfaces. In addition, the communications allow the CU, malfunction management unit (MMU), backpanel, and detector rack to exchange information on a regular basis, performing redundant checks on each other.
The TS2 Type 1 standard uses EIA-485 serial communications interfaces and Synchronous Data Link (SDLC) communication protocol to link the major cabinet components. The serial data is converted to analog inputs and outputs in the back panel and detector rack by a bus interface unit (BIU).
The back panel for a TS2 cabinet configuration is also used for the termination of controller inputs and outputs. Load switch drivers and other functions of the controller have terminals on the back panel that are used to interface with the other devices in the cabinet and to display the indications on the street. The back panel is linked to the CU through one or more BIUs. Load switch assignments and other back panel functions are configured through the controller software. Discrete wiring is still provided between the back panel and the MMU to monitor load switch outputs.
Cabinets for Type 170/2070 Controllers use a 19 in. rack assembly to secure equipment and follow Caltrans standards (California Department of Transportation). The controller unit and cabinet assemblies are attached to the racks. Cabinet assemblies consist of the power supply assembly, power distribution assembly (PDA), input file, and output file. The power supply and PDA provide power, circuit protection and surge suppression for cabinet equipment. The PDA houses the flasher and auto/flash switch. The input file houses card rack detectors, isolators and other input devices. The output file houses load switches, flash transfer relays and the monitor. Other auxiliary equipment can be rack mounted or mounted by other means. Terminations for wiring are made on the back of associated cabinet assemblies.
902.5.6.2 Conflict Monitor Unit / Malfunction Management Unit
Support. All solid-state controllers have a conflict monitor unit (CMU) or a malfunction management unit (MMU) to supervise the operation of the traffic signals. The primary purpose of this unit is to guarantee that conflicting signal indications are not displayed on the street at the same time. If such a conflict is detected, the unit will automatically put the intersection into a flashing condition. The intersection will remain in flash until the monitor unit is reset and the problem that caused the failure is corrected.
These monitors can also monitor the absence of signal indications on the street. The absence of a load on the output side of the load switch when that output is turned on will cause the monitor to put the intersection into a flashing condition. This occurs when all the bulbs of the same color on a particular phase are burned out or when a wiring failure causes loss of power to the indications.
Standard. For phases with only one signal head (i.e. a left turn phase with a single turn lane), load resistors shall be adequate for the output so that a single indication outage will not cause the intersection to go to flash.
Support. Each conflict monitor has a program card that is unique for that intersection. On the program card, jumpers are installed to tell the unit which movements, or channels, are considered compatible. Those positions not having jumpers are considered as conflicts and will trip the monitor.
The monitors also check the controller. If power is lost to the controller or if the internal 24-volt DC voltage of the controller is lost, the monitor will trip, and the intersection will go into flash. Some of the newer conflict monitors available exceed the minimum specifications set out by NEMA.
In NEMA TS2 cabinets, communications between the MMU and the CU allow the ability to monitor for fault conditions between the major cabinet components. Certain fault conditions will cause the intersection to go into flash. Some examples of these faults are the loss of serial communications, incompatibility between MMU program card and CU phase sequences and discrepancies between load switch outputs and CU phase outputs. In no case is a solid-state controller operated without a monitor unit.
902.5.6.3 Load Switches
Support. The operating voltages of the solid-state controller are 24 volts DC. This voltage must be converted to 120-volt AC in order to drive the signal indications. The load switch is a solid-state device that converts the 24-volt DC output from the controller to the 120-volt AC needed by the indications. Each load switch can handle three circuits. Normally, one switch is assigned for each phase and it handles the green, yellow and red outputs. A separate load switch is used to control pedestrian indications, if they are present at the intersection. Strategies used for flashing yellow arrow indications vary.
902.5.6.4 Auxiliary Interfaces
Support. Other auxiliary interfaces might also be needed in the controller cabinet. Examples of these include hardwire interconnect interfaces, closed loop system interfaces and preempt interfaces. These typically consist of a panel or unit that brings external inputs, outputs and communications into the CU.
902.5.6.5 Detector Interface
Support. The detector interface provides connections between the CU and the detection devices. In solid-state pre-timed and NEMA TS1 controllers, the connections are made through the back panel. In NEMA TS2 controllers, the detector inputs and outputs are linked to the CU through a BIU. In Type 170/2070 cabinets, the input file serves as the detector interface.
Support. The basic goal of a detector is to provide a valid input to the controller unit of the need to provide service. There are many types of detectors currently in use and more are consistently being developed. Detector types include, but are not limited to, pedestrian push buttons, inductive loops, video detection, and radar detection.
There are two primary types of detection: pulse (or passage) and presence. In pulse detection the detector provides a short instantaneous call to the controller that demand is present and then the call is dropped. Presence detection registers that demand is present and will retain the call so long as there is demand.
A resource for information on additional detector types, alternate loop designs, and many other aspects of detectors is FHWA’s Traffic Detector Handbook (also available through ITE).
902.5.7.1 Induction Loop Detectors
Support. Induction loop detectors consist of wire that is placed in the pavement that senses the passage or presence of metal objects (i.e. vehicles). The detectors’ inductance is based on the number of turns of wire in the saw cuts in the pavement, and the current flowing through them. The passage or presence of a metal mass changes the inductance of the loop. The detector amplifier then measures this change in inductance and when the set thresholds are exceeded, detection is registered.
902.5.7.1.1 Loop Configuration
Support. The most common arrangement of the loop detector is the quadrapole. This layout, a rectangle with an additional cut down the middle, provides the greatest sensitivity of detecting small vehicles, motorcycles, and bicycles while reducing the occurrence of cross talk between loops in adjacent lanes and false calls from adjacent lanes. The typical quadrapole loop is 6 ft. wide by 30 ft. long located at the stop bar in each lane. Quadrapole detectors shorter than 30 ft. are sometimes used when field conditions don’t allow for full size loops.
Another widely used loop configuration is the 6 ft. x 6 ft. square loop. This loop, which is centered in the lane, is typically used for detection in advance of the signal. This layout is more susceptible to cross talk and false detections but with proper adjustments of the amplifier, good performance can be achieved. This loop configuration is also typically used for vehicle counting. A variation of this loop is a diamond shaped loop. By turning the square loop 45 degrees, the more sensitive corners can be centered on the lane.
There are additional loop layouts, such as the skewed loop, round loops and several other variations available. See Comparison of Induction Loop Detector Designs and Standard Plan 902.50 for more information.
902.5.7.1.2 Induction Loop Detector Amplifiers
Support. Induction loop detector amplifiers are installed in the controller cabinet and are available in shelf-mount and rack-mount configurations. The shelf-mount units are self-contained and are connected to the controller backpanel through a wiring harness. Rack-mount units are installed in a card rack with separate power supplies. Type 170/2070 and NEMA TS2 controllers use only rack-mount detectors. The TS2 detectors have additional diagnostics that are linked to the CU through the serial communications.
Support. Probes are point detectors that are installed in the pavement. There are two types: micro-loops and wireless probes. They operate on a similar principal to a conventional loop detector. Micro-loops have a continuous lead in to the controller cabinet. Wireless probes sometimes require repeaters.
Probes are typically used at locations where the pavement is not able to support the cutting of a loop or right of way is limited. The principal drawback is a smaller detection zone, but through the use of several probes in an array, probes can closely simulate a long detector.
902.5.7.3 Microwave (Radar)
Support. Consisting of an emitter/sensor mounted either above or adjacent to the pavement, microwave detectors measure the Doppler shift in the microwave frequency and detect the passage of a vehicle. Simple microwave units are designed to place a call if an approaching vehicle is sensed for one lane or the entire approach. More advanced microwave detectors can define multiple zones of detection with one unit and can measure speeds. Microwave detectors are also directional; they can distinguish if a vehicle is approaching or leaving the detector.
A big advantage of microwave detectors is that they do not need to be installed in the pavement. This can allow for greater flexibility in installation as well as avoiding the problems associated with being in the pavement.
902.5.7.4 Video Detection
Support. Video detection consists of a video camera mounted above or adjacent to the pavement and a unit that processes the video signal to generate vehicle calls and other information. The processing unit uses software to draw zones of detection on the video output.
Video detection typically has a higher initial cost but offers the advantages of being completely out-of-pavement and allowing considerable flexibility in detector placement and configuration. Video detection can require one or several cameras to be effective and requires a rigid mounting location for the cameras. Higher mounting locations will provide more effective detection. One of the most common disadvantages of video detection is the potential for poor performance during inclement weather.
902.5.7.5 Closed Loop System Detectors
Support. The objective of system detectors is to gather data that the system master uses to make decisions on timing plan and offset patterns (see EPG 902.5.5 Coordination for more information). The data from these detectors can also be used as a monitoring tool for the system. The primary difference between system detectors and standard detectors is that system detectors do not have direct control over signal phase times. The master can use data from the system detectors to make system wide decisions based on parameters set by the user.
902.5.8 Responsibility for Operation and Maintenance (MUTCD Section 4D.02)
Guidance. Prior to installing any traffic control signal, the responsibility for the maintenance of the signal and all the appurtenances, hardware, software, and the timing plan(s) should be clearly established. The responsible agency should provide for the maintenance of the traffic control signal and all the appurtenances in a competent manner.
To this end the agency should:
- A. Keep every controller assembly in effective operation in accordance with its predetermined timing schedule. This includes checking the operation of the controller assembly frequently enough to verify that it is operating in accordance with the predetermined timing schedule, establishing a policy to maintain a record of all timing changes and ensuring that only authorized persons are permitted to make timing changes;
- B. Clean the optical system of the signal sections and replace the light sources as frequently as experience proves necessary;
- C. Clean and service equipment and other appurtenances as frequently as experience proves necessary;
- D. Provide for alternate operation of the traffic control signal during a period of failure, using flashing mode or manual control, manual traffic direction by proper authorities as might be required by traffic volumes or congestion, or by erecting other traffic control devices;
- E. Have properly skilled maintenance personnel available without undue delay for all signal malfunctions and signal indication failures;
- F. Maintain an inventory of spare equipment in order to minimize the interruption of traffic control signal operation as a result of equipment failure;
- G. Provide for the availability of properly skilled maintenance personnel for the repair of all components;
- H. Maintain the appearance of the signal displays and equipment; and
- I. Follow appropriate procedures to assure optimum operation and maintenance for all traffic signal control systems and equipment.
Support. The municipal and/or county agreement for a project located within a municipality or county contains the general requirements for cooperation between the state and the city or county for the efficient operations of traffic signals proposed by the state. An additional agreement is executed to cover the operation and maintenance of state-installed and owned traffic control signals and devices, highway lighting, signing and pavement marking (see EPG 153 Agreements and Contracts). Detailed information concerning the sequence for preparing and executing a project agreement is available in EPG 235.2.3 Project Agreements. All agreements are reviewed by the Chief Counsel's Office upon receipt of the necessary information from the district. It is important the agreements be completed as early in the development of plans for construction projects as possible. Copies of all agreements are submitted to Design for submission to FHWA as part of the Plans, Specifications and Estimate (P. S. & E.) documents.
Standard. It shall be the responsibility for each district to establish record keeping, operational and maintenance programs utilizing these guidelines.
902.5.8.2 Signal Operations
Guidance. All signals should be operated in the most efficient manner possible. If signals are not operated at optimum efficiency, their presence can be a detriment rather than an asset to efficient traffic control. The responsibility of general signal operations rests with district personnel as assigned.
902.5.8.2.1 Signal Timing
Support. The objective of signal timing is to assign right of way to traffic in such a manner as to minimize delays and reduce the probability of crash producing conflicts.
Standard. District engineering personnel shall be responsible for all signal phasing, computing all signal timing and maintaining a signal phasing and timing record of each signalized intersection.
Guidance. When timing signals, the following principles should be followed.
- 1. Keep the number of phases to a minimum. As the number of phases increase, the effective green time within the cycle decreases.
- 2. Use the shortest possible cycle length. The shortest cycle length that will clear traffic will produce the lowest average intersection delay.
3. The distribution of green times within a cycle should be based on the traffic volumes for each approach. Computer software programs are available to assist the engineer in developing the timing.
- 4. Refer to EPG 902.5.36 Signal Timing and EPG 902.6 Pedestrian Control Features to ensure that the proper times for minimum green, maximum green, red clearance interval, yellow change interval, WALK, flashing DON'T WALK, etc., are used.
902.5.8.2.2 Field Observation
Support. Any theoretical method of determining signal timing, no matter how precise, is only an approximation of real-world conditions. There is no substitute for field surveillance and adjustment to provide the most efficient signal timing at an intersection.
Guidance. Traffic signals should be observed each year to ensure proper operation and verify effective traffic flow. Four observation periods, a.m. peak, noon peak, p.m. peak and off peak, should be completed for each signal. However, observation periods should be determined based on the traffic volumes and the timing plan of the intersection. District staff should document observed signal data on an observation sheet, and the observation date should be recorded in the Transportation Management System (TMS) database.
Personnel completing observations should be knowledgeable on signal characteristics as well as department policies and guidelines. When completing observations employees must be alert to both increases and decreases in vehicle volumes and be able to determine if timing adjustments are required. They should either be prepared to adjust the timing accordingly, or to notify appropriate personnel to make the necessary adjustment. A Traffic Signal Observation Worksheet is typically used during observations to ensure all necessary aspects are checked.
A Traffic Signal Observation Worksheet should include:
- Green/Yellow/Red Timing – Verify the timing is adequate for the traffic.
- Pedestrian Timing – Verify the timing is adequate for pedestrians.
- Timing Sheet / Controller Consistency – Verify the timing sheet matches the controller program.
- Vehicle Detectors – Verify that the vehicle detection is functioning properly.
- Pedestrian Detection – Verify that the pedestrian push buttons are functioning properly.
- Signal Heads – Verify that all signal heads are properly aligned, all indications (including pedestrian) are in working condition, and that all visors and backplates are present and unbroken and retroreflective backplates are used when necessary.
- Time Clock / Date – Verify that the correct time and date is set in the controller.
- Coordination – Verify that the coordination is adequate and is processing traffic through the system as well as possible. It is recommended to drive the system in order to determine if coordination is adequate.
- Signing / Striping – Verify that the signing and striping are appropriate and visible.
902.5.8.3 Signal Maintenance
Support. Traffic signal maintenance activities can be divided into three major areas: preventive maintenance, emergency repairs, and follow-up work. Signal maintenance personnel are responsible for these tasks.
Guidance. If a traffic signal is dark for signal maintenance, consideration should be given to notifying the police agency whose jurisdiction the signal is located in. If the signal can be flashed instead of dark, a request for police assistance might not be necessary unless the time of day, traffic volumes and congestion dictate a need.
If it is necessary to turn the signal off or a malfunction occurs and traffic cannot be handled by a flashing operation or stop signs cannot be installed immediately, the police should be notified that their assistance is requested until the signal can be repaired or stop signs can be installed.
902.5.8.3.1 Preventive Maintenance
Support. Preventive maintenance (PM) for traffic signals , includes the systematic inspection, cleaning, testing, adjustment and completion of non-emergency repairs needed to ensure it will function as efficiently and reliably as intended throughout its expected life cycle.
A warranted and well-maintained signalized intersection is one of the best services that can be offered to the public by the department. When efficiently operated it represents a direct savings through reductions in delay, fuel consumption, and greenhouse gas emissions while providing safe intersection traffic control. In the long run, a well-executed preventive maintenance program will reduce the number of emergency maintenance calls and will help ensure reliable and efficient traffic control.
Standard. A schedule of inspections shall be prepared for the purpose of preventive maintenance in accordance with the recommended guidelines set forth herein and administrative supervision shall be maintained to see that maintenance is performed as scheduled. MoDOT’s standard for performing these inspections is to evaluate each traffic signal within a two-year time span.
Guidance. Several factors should be considered when preparing the preventive maintenance schedule to ensure the effort is focused in the most useful locations. Each traffic signal will differ as to the maintenance attention required at different times in its expected life cycle. Preventive maintenance efforts in addition to that described in the Department’s standard should be focused on traffic signals identified through review and consideration of the following.
- Review of the frequency of call reports
- A statistical report should be prepared at least once each year from the MoDOT Customer Service Center call report database. Those signals producing the highest 20% of call reports should be reviewed to determine if extra maintenance efforts could improve reliability. Call reports that proved to be signal timing related rather than maintenance related should be removed from this analysis and forwarded to engineering staff for review.
- Review of recent modifications, additions, or contract work
- Signals that have received recent modifications and newly constructed signals can certainly benefit from a thorough, post-project inspection. Inspection by trained signal maintenance personnel can help ensure that any potential maintenance issues are discovered early and resolved before problems develop. Any traffic signal that has undergone modifications or construction since the last inspection cycle should be considered for an additional preventive maintenance review.
- Review of previous inspection reports
- Traffic signals that might have structures or features nearing the end of their life cycle might need more frequent inspections to help determine and prioritize replacement needs. The type of signal structure, vulnerability to accident damage and exposure to extreme weather might also be cause for more frequent inspections. Signals needing this additional focus can be identified through review of any concerns noted on previous preventive maintenance inspection reports and should be considered for inclusion in the preventive maintenance inspection schedule.
Reports in checklist format are available to aid the preventive maintenance process for cabinet and control equipment and for signal supports, signal heads and pull boxes. More detailed preventive maintenance guidance can often be found in the manufacturer’s equipment manuals for the many varied components of signal structures and control.
A preventive maintenance program should include the following:
- LED Signal Indications
- Scheduled replacement of LED indications should be accomplished every 10 years. Inspection of the indications between changes should include visual verification that each indication is operating and is oriented properly within the signal head.
- Signal Heads
- All signal heads should be plumb and in proper alignment to be visible from the appropriate lane. Signal heads capable of programmable viewing angles should be checked for visibility and non-visibility from the appropriate lanes. The points of signal head attachment, especially on span-wire installations and post top mounting should be checked to see that all components are sound and secure. If top mounted signal heads are loose and cannot be tightened, consideration should be given to installing a longer pedestal post and side mounting the head. Check that all setscrews are in place and tight.
Overhead signals and associated hardware should be checked for proper clearance over the roadway. Signal section doors should be observed to see that they are tightly secured to the signal section housing. The condition of the signal heads should be evaluated to determine if replacement might be necessary. Backplates and signal visors should be checked for proper attachment and condition and replaced if necessary.
- Signal Supports
- Signal support posts should be plumb or raked as planned. Steel posts and mast arms should be inspected at seams and joints for signs of stress or fatigue. Particular attention should be directed to the base plate and mast arm plate joints and associated welds. Any indication of stress or cracking should be marked and called to attention for further evaluation. Bolts and nuts should be checked for tightness and rust. Where possible anchor bolts should be visually examined for signs of rust or deterioration. Steps should be taken to replace any missing handhole or post top covers. Wooden support poles should be checked for bending and general condition.
- Concrete signal post bases should be checked for settling or shifting, cracking, severe spalling or similar weather-related deterioration. The gap between the post foundation and the post base plate should be filled with galvanized wire screen. On older installations, grout might have been used to fill this gap. If this is the case, consider removing the grout and installing wire mesh if the grout is broken or appears to be retaining moisture.
- Guy cables, support cables, tether cables and anchors should be checked for tautness and for signs of strain. The cable's physical condition should be checked to see that there are no broken strands. Cable attachment clamps should be checked for tightness. Anchors should be checked for movement and general condition. Anchors and guy cables can loosen up over time as they are subjected to wind loads, changing soil conditions and even vehicle accidents. This loosening can affect clearance of the signal over the roadway. Clearance between the roadway and the bottom of signal heads is to be maintained between 16 and 19 feet.
- To minimize signal head movement, cable spans should not have excessive sag. Tether cables should be maintained in a straight line across the bottom of the signal heads. If excessive sag is found, the cable should be tightened. Minimizing movement does not only enhance signal head visibility but also reduces stress on the signal heads, cables and associated hardware.
- Vehicle and Pedestrian Detection
- Each detector should be observed to see if it is operating properly. Equipment is available for troubleshooting detectors. It is highly desirable to make and record the impedance and resistance of each loop when installed. Such information can be invaluable in trouble shooting induction loop detection systems. Each push button should be pressed to confirm that it actuates the pedestrian phase. Check pedestrian signs for condition and proper alignment.
- Pull Boxes
- All pull boxes should be checked for cracking of the concrete apron, and condition of the lid. Signs of settling could indicate sidewall failure. Consideration should be given to adding a drain if the pull box is holding water. Detector loop splices and the grounding system should also be checked for tightness and condition.
- The outside surface of the cabinet should be inspected for overall physical condition. Brush and vegetation should be under control in the area around the cabinet. Doors should fit tightly when closed and the gasket should seal properly. Locks and hinges should move freely and be lubricated if necessary. The inside of the cabinet should be clean, and all assemblies should be free of dust, pests or signs of moisture intrusion. Filters should be in good condition and replaced or cleaned to ensure proper ventilation. Ventilation fans should be checked for operation and the appropriate thermostat setting should be verified. Anchor bolts on the cabinet should be checked for tightness to the base. The conditions of the duct seal and cabinet seal as well as the ground rod clamp and wire should also be evaluated. Cabinet documentation including wiring diagrams, timing sheets, work records and intersection layout or plan sheets should be present. Police door lock, flash switch and manual control should be checked for proper operation.
- All terminals should be checked for tightness. Relays, plug-in modules, and connectors should also be checked for proper fit and operation. A voltage measurement should be made and recorded at the main voltage input terminal block. This voltage measurement should agree with the voltage measurement at the service disconnect and should be within the tolerances of the signal control equipment. Voltages at the field terminals should also be checked. Lightning protection devices should be examined to see that they are in condition to activate if a surge should occur. If no lightning protection is present at the service line input, it should be added.
- Flashers should be checked to ensure they operate with a flash rate of 50 to 60 times per minute.
- Verify that the cabinet timing sheet matches the programming currently in the controller. Check for the proper operation of programmed functions.
- Preemption equipment should be regularly checked for proper operation. Preemption equipment is typically installed at critical intersections (near railroad tracks or emergency vehicle routes) so malfunctioning preemption equipment should be repaired or replaced as quickly as possible.
- MoDOT’s preventive maintenance measures should extend only to the equipment owned and maintained by MoDOT. When preemption equipment is not owned, operated, and maintained by MoDOT, testing of preemption system operation should be coordinated with the other involved agency and can be scheduled separately from the preventive maintenance inspection.
- Utility Service
- The utility service should be examined to ensure that the electric meter enclosure, conduit and boxes are in sound condition and securely mounted. Surge protection should be present at the service. Circuit breakers should be sized appropriately and labeled as to their function. All electrical connections must be tight and the service should be properly grounded. Supply voltage should be measured at the service disconnect or main breaker and low or high supply voltages should be reported to the utility company for correction if tolerances are exceeded with which the signal control equipment is designed to work. If the service is an overhead drop, ensure that the drop and connection point is clear of tree limbs.
- Battery Backup Power Supplies
- Enclosures or cabinets for battery backup systems should be inspected using many of the same inspection points considered for traffic signal control cabinets. Inspect the cabinet for physical damage. Locks, hinges, doors, ventilation and moisture seal should all be evaluated. Battery condition and age should be considered and noted on the inspection form. Connection points should be examined to ensure they are securely fastened and free of corrosion. Control unit self-tests and power transfer tests can be very useful in validating the system. Careful consideration of the potential effect on signal operation is essential before scheduling and performing such testing.
902.5.8.3.2 Conflict Monitor and Malfunction Monitor Testing
Guidance. It is important to test monitors periodically to ensure their reliability. Monitors should be tested according to the following guidelines: Annual tests should be conducted and recorded as part of the annual preventive maintenance procedure. The documentation of these procedures should be stored with other permanent records. It is recommended that field tests be performed during low traffic volume periods in order to minimize disruption to traffic.
902.5.8.3.2.1 Conflict Monitor Unit Test Procedure
Guidance. The conflict monitor should be tested annually with a computerized conflict monitor tester. This is done by removing the intersection's monitor and running a complete test with the conflict monitor tester unit. If the test is to be completed in the field, a spare monitor should be installed temporarily while the test is being performed. Monitors can also be shop-tested by rotating pre-tested monitors to the field. Documentation of the tested monitor should include the following:
- 1. Date
- 2. Name of Technician
- 3. Location – includes intersection name, city and/or county
- 4. Serial number of conflict monitor
- 5. Comments regarding fail or pass conditions
The monitoring unit’s permissive programming card should be inspected for physical condition. The jumpers or diodes that establish monitor channel concurrency should also be evaluated and verified as correct for the intersection in question.
Failed monitors should either be repaired so that they pass the monitor test or replaced with a monitor that passes the test.
To ensure the reliability of the computerized monitor tester, a calibration of the unit should be done annually. The units need to be returned to the manufacturer in order for this calibration to be done properly. The districts should establish a yearly program of having their conflict monitor testers returned to the manufacturer and the recalibration performed.
902.5.8.3.2.2 Cabinet Test Procedures
Guidance. Two tests should be performed to check the cabinet wiring and operation as related to conflict monitoring functions. These tests should be performed as part of initial cabinet setup and should be repeated at the time of any cabinet wiring modifications or additions. These tests are not required during annual preventive maintenance monitor unit testing.
- (A) Conflict Test
- An actual conflict condition is induced by means of a jumper wire. This is most commonly done by placing one end of the jumper wire on a green terminal output and the other end on a conflicting green terminal output. Once power is applied to either green terminal, the conflict monitor should trip causing signal control to be transferred to flashing operation. A push button with leads and alligator clips can be helpful for performing this test.
- (B) Harness Test
- For solid-state pre-timed, NEMA TS1 and Type 170/2070 controllers, the monitor harness or harnesses should be tested for continuity and to ensure that all wiring to the monitor is intact and that the correct load switch circuit goes to the correct channel in the conflict monitor. The results of the test are recorded on the Conflict Monitor Continuity Checklist.
- Conflict Monitor Continuity Checklist
- This test should be performed during initial cabinet set-up and any time changes are made that affect the monitor wiring or operation (i.e. phasing changes). This test can also be performed at other times as needed. The harness test is not required for NEMA TS2 controllers since the cabinet diagnostics continuously check cabinet conditions.
- Conflict Monitor Harness Tester presents the procedure for using a harness tester. The harness tester also shows connector terminations for 6 and 12-channel conflict monitors. More detailed information on conflict monitors is found in the NEMA Standards Publication No. TS-1, Traffic Control Systems. A similar device can also be developed for Type 170/2070 controllers. This tester can be helpful as a troubleshooting tool as it can be used to isolate some cabinet wiring problems.
902.5.8.3.3 Emergency Maintenance
Guidance. Occasionally traffic signals, flashers and lights malfunction or are damaged from vehicle crashes, acts of nature and/or other unexplained phenomenon. Some of these occurrences will constitute an emergency requiring an immediate response, while others might indicate a lower priority response. Sound judgment should be used when evaluating the priority of signal malfunctions or damage.
Generally, flashers have lower priority than traffic signals. Refer to the Incident Response Plan and Incident Response Priorities for detailed information regarding emergency response for traffic signals, flashers and lights.
902.5.8.3.4 Maintenance Limits
Guidance. No major repair should be attempted while the controller is in service. Where possible, the availability of a spare unit for each type of assembly minimizes interruption of signal operation when that unit must be removed for servicing or repair.
Field servicing of solid-state equipment should be limited to the exchange of modules and assemblies known to be good for those thought to be defective. Due to the sensitivity of some parts, it is important that they be stored in the shipping containers until used and then handled with minimum hand contact.
902.5.8.3.5 Equipment Replacement and Repair
Guidance. Sound judgment should be used when making decisions about replacing or repairing malfunctioning signal equipment. When possible, field repairs should be limited to operations that result in minimal down time to signal operation. Shop repairs are preferable where spare equipment can be installed to maintain signal operation.
When deciding to repair or replace a piece of equipment, consider these factors:
- - Relative condition of the piece of equipment.
- - Remaining usable life of the piece of equipment.
- - Cost of repairs in personnel time or repair shop costs vs. cost of new equipment.
- - Availability of replacement parts.
- - Is the piece of equipment functionally obsolete?
The evaluation of these factors might determine that it is more cost effective to replace a piece of equipment than to repair it.
Option. Repair work may be performed by district personnel or may be sent to a repair shop. This decision is based on the available expertise of personnel, time demands on the shop and the nature of the malfunction.
902.5.8.3.5.1 Crash Damage to Controllers
Standard. Occasionally a crash will result in the destruction of a signal controller. Whenever this occurs, MoDOT tries to collect for the damages incurred. The following depreciation schedule shall be applied when determining the present worth of an existing controller that has been damaged.
|Age, In Years||Worth, % of Original Value|
|16 or More||2|
902.5.8.3.5.2 Crash Damage to Signal Heads and Supports
Standard. Whenever damages occur to signal heads and their supports as the result of a crash, collection for damages shall also be attempted. Straight line depreciation shall be used with a 15-year life used for all signal heads and all temporary span wire supports. A 25-year life shall be used for all permanent supports.
902.5.8.4 Record Keeping
Standard. Each district shall keep and maintain documentation on each signalized intersection. Each district will be required to maintain (1) Signal Maintenance Files and (2) a Module and Assembly Repair File. The Signal Maintenance File shall be kept on a per intersection basis, whereas the Module and Assembly Repair File shall be kept on a module and assembly type basis. Refer to Figure 902.5.8.4.1 for guidelines regarding record keeping for signal-related information.
Standard. The information being recorded shall be accurate and legible. Completing documentation for each signalized intersection requires more time of those involved in signal operation and maintenance, but the time required to do so is considered minimal and over a period of time will more than justify the time spent.
It is important to retain records for the appropriate amount of time before information is discarded. For more specific details on document retention, review MoDOT’s Retention Schedule.
In general, design and construction-related information such as design plans and contracts are retained permanently. For signals constructed on contracts administered by Construction, Construction typically retains contract records. Design typically retains plans for projects that are let under the right of way and construction program. For signals constructed by permit, department forces, or a combination of department forces and contract work, district traffic is responsible for retaining design and construction documentation.
All other records pertaining to operation and maintenance of traffic signals shall be retained a minimum of seven years. Each district is responsible for retaining these records. The following reports and records should be maintained. Some example forms for various records are included, but districts can choose to modify them as needed in order to meet their specific needs.
- 1. Signal, Flasher, and Lighting Inventories
- Traffic Signal, Flasher, and Lighting Inventories
These inventories contain each signalized intersection, flasher installation, and lighting installation on MoDOT right of way. The inventories are part of the Transportation Management System (TMS). District personnel shall maintain the TMS signal, flasher, and lighting inventories on an ongoing basis. Guidance is available in the Traffic Signal, Flasher, and Lighting Training Manual.
- 2. Intersection Plans
- Intersection plans are provided for each signalized intersection, although plans for older installations can be difficult to find. At a minimum, the documentation shall be in the form of signal plan sheets from the final plans of the project or drawings. The plans shall show the lane widths, all signal indications and their location, detector placement, and approximate geometrics with regard to skew. Intersection plans shall be available in the district office, in the signal shop’s signal maintenance file, and can be scanned and stored in the signal inventory in TMS.
- 3. Signal Phasing and Timing Record
- Signal phasing and timing is determined, computed, and stored by district office personnel. Old signal phasing and timing shall be archived. This documentation shall be accessible electronically or at the controller cabinet, at the signal shop, and at the district office or TMC. It can also be stored within the signal inventory in TMS.
- 4. Traffic Signal Service History
- A historical record of the service history shall be kept in the controller cabinet. Entries shall be completed by anyone observing the operation of the equipment or performing maintenance. The date of the observation or call to the controller shall be indicated with a very brief note of explanation and initialed by the person entering the information. This form shall be filed in the signal shop whenever the sheet becomes full. See Figure 902.5.8.4.2 for an example form.
- 5. Preventive Maintenance (PM) Checklists
- There are checklists available for preventive maintenance: PM Checklist for Signal Supports, Heads, Pull Boxes and PM Checklist for Cabinet and Control Equipment. Each list contains a number of items relating to the intersection operation that shall be examined on a periodic basis in accordance with the recommended guidelines set forth herein. Each item on the list shall be examined during the PM check. A check opposite of each individual item indicates that item has been examined and found in proper operating and physical condition. On completion, the PM checklist shall be returned to the signal shop for review and subsequently placed in the signal maintenance file for that particular intersection.
- 6. Emergency Signal Maintenance Work Records
- This record is essentially an emergency work record that can be used to record any maintenance activity performed at an intersection that is not considered preventive maintenance. The record shall be maintained as either a paper copy or a computer database
- 7. Bench Repair Label
- The bench repair sticker shall be attached to any component, module, or assembly when it is removed from an intersection and taken to the signal shop for repair. When the unit is repaired, the label is completed, indicating what was done or replaced during the repair. When the unit is ready to be returned to service, the label is then placed in the module and assembly repair file for that particular unit.
- 8. Conflict Monitor Information
- Conflict monitor test results, cabinet test results, conflict monitor card programming and any other important information shall be kept in the signal shop. Forms for documenting conflict monitor programming include:
- - Figure 902.5.8.4.5 (Conflict Monitor Program Card)
- - Figure 902.5.8.4.6 (NEMA TS2 Conflict Monitor Program Card) and
- 9. Cabinet Drawings
- Cabinet drawings and other critical paperwork shall be kept in the controller cabinet, in the signal shop, and can also be stored in the signal inventory in TMS.
Guidance. District personnel should store all intersection photographs within the TMS signal inventory in TMS. Photographs should be updated as needed, but the older photos should always be retained. Intersection photographs that can be beneficial include, but are not limited to, photographs of the signal cabinets, components within the signal cabinet, and the power supply.
902.5.9 Provisions for Pedestrians (MUTCD Section 4D.03)
Support. EPG 902.6 Pedestrian Control Features contains additional information regarding pedestrian signals and EPG 902.7 Pedestrian Hybrid Beacons contains additional information regarding pedestrian hybrid beacons. EPG 642.4 Impact of the Project Category on ADA contains information regarding what types of signal projects must be accompanied by ADA-related improvements.
Standard. The design and operation of traffic control signals shall take into consideration the needs of pedestrians as well as vehicular traffic.
If engineering judgment indicates the need for provisions for a given pedestrian movement, signal faces conveniently visible to pedestrians shall be provided by pedestrian signal heads (see EPG 902.6 Pedestrian Control Features) or a vehicular signal face(s) for a concurrent vehicular movement.
Accessible pedestrian signals (see EPG 902.6.9 Accessible Pedestrian Signals and Detectors – General) that provide information in non-visual formats (such as audible tones, speech messages, and/or vibrating surfaces) shall be provided at all new installations of pedestrian signal accommodations and at other locations as described in EPG 642.4 Impact of the Project Category on ADA.
Where pedestrian movements regularly occur, pedestrians shall be provided with sufficient time to cross the roadway by adjusting the traffic control signal operation and timing to provide sufficient crossing time every cycle or by providing pedestrian detectors.
Guidance. If it is necessary or desirable to prohibit certain pedestrian movements at a traffic control signal location and it is not practical to provide a barrier or other physical feature to physically prevent the pedestrian movements, No Pedestrian Crossing (R9-3) signs (See MUTCD Section 2B.51 or EPG 903.5.22) should be used.
Standard (MUTCD Section 9D.02 Signal Operations for Bicycles). At installations where visibility-limited signal faces are used, signal faces shall be adjusted so bicyclists for whom the indications are intended can see the signal indications. If the visibility-limited signal faces cannot be aimed to serve the bicyclist, then separate signal faces shall be provided for the bicyclist.
On bikeways, signal timing and actuation shall be reviewed and adjusted to consider the needs of bicyclists.
902.5.10 Meaning of Vehicular Signal Indications (MUTCD Section 4D.04)
Support. No matter how complex the traffic signal installation or how elementary or sophisticated the control equipment, the sole link from the control mechanism to the driver is the signal indication. Unless the correct information is conveyed to and understood by the driver, the total installation might not function as desired.
It is important that the proper selection of indications and signing be made so a high degree of consistency can be maintained with regard to standardizing the meaning of the indications, thereby conveying to the motorist the type of operation to be encountered at the intersection. Intersection design, signal locations, indications and sequences are standardized wherever possible for better driver education, response and obedience, particularly in a series of adjacent signal installations.
The “Uniform Vehicle Code” (see EPG 900.1.11) is the primary source for the standards for the meaning of vehicular signal indications to both vehicle operators and pedestrians as provided in this Section, and the standards for the meaning of separate pedestrian signal head indications as provided in EPG 902.6.2.
The physical area that is defined as being “within the intersection” is dependent upon the conditions that are described in the definition of intersection in EPG 900.1.13.
Standard. The following meanings shall be given to highway traffic signal indications for vehicles and pedestrians:
- A. Steady green signal indications shall have the following meanings:
- 1. Vehicular traffic facing a CIRCULAR GREEN signal indication is permitted to proceed straight through or turn right or left except as such movement is modified by lane-use signs, turn prohibition signs, lane markings, roadway design, separate turn signal indications, or other traffic control devices.
- Such vehicular traffic, including vehicles turning right or left, shall yield the right-of-way to:
- (a) Pedestrians lawfully within an associated crosswalk, and
- (b) Other vehicles lawfully within the intersection.
- In addition, vehicular traffic turning left shall yield the right-of-way to other vehicles approaching from the opposite direction so closely as to constitute an immediate hazard during the time when such turning vehicle is moving across or within the intersection.
- 2. Vehicular traffic facing a GREEN ARROW signal indication, displayed alone or in combination with another signal indication, is permitted to cautiously enter the intersection only to make the movement indicated by such arrow, or such other movement as is permitted by other signal indications displayed at the same time. A U-turn movement from the left turn lane shall only be allowed during the protected only mode for left turns and lane-use signs R10-30c and the U-TURN ON GREEN ARROW R10-30b are installed.
- Such vehicular traffic, including vehicles turning right or left or making a U-turn movement, shall yield the right of way to:
- (a) Pedestrians lawfully within an associated crosswalk, and
- (b) Other vehicles lawfully within the intersection.
- 3. Pedestrians facing a CIRCULAR GREEN signal indication, unless otherwise directed by a pedestrian signal indication or other traffic control device, are permitted to proceed across the roadway within any marked or unmarked associated crosswalk. The pedestrian shall yield the right of way to vehicles lawfully within the intersection or so close as to create an immediate hazard at the time that the green signal indication is first displayed.
- 4. Pedestrians facing a GREEN ARROW signal indication, unless otherwise directed by a pedestrian signal indication or other traffic control device, shall not cross the roadway.
- B. Steady yellow signal indications shall have the following meanings:
- 1. Vehicular traffic facing a steady CIRCULAR YELLOW signal indication is thereby warned that the related green movement is being terminated or that a steady red signal indication will be displayed immediately thereafter when vehicular traffic shall not enter the intersection. The rules set forth concerning vehicular operation under the movement(s) being terminated shall continue to apply while the steady CIRCULAR YELLOW signal indication is displayed.
- 2. Vehicular traffic facing a steady YELLOW ARROW signal indication is thereby warned that the related GREEN ARROW movement or the related flashing arrow movement is being terminated. The rules set forth concerning vehicular operation under the movement(s) being terminated shall continue to apply while the steady YELLOW ARROW signal indication is displayed.
- 3. Pedestrians facing a steady CIRCULAR YELLOW or YELLOW ARROW signal indication, unless otherwise directed by a pedestrian signal indication or other traffic control device shall not start to cross the roadway.
- C. Steady red signal indications shall have the following meanings:
- 1. Vehicular traffic facing a steady CIRCULAR RED signal indication, unless entering the intersection to make another movement permitted by another signal indication, shall stop at a clearly marked stop line; but if there is no stop line, traffic shall stop before entering the crosswalk on the near side of the intersection; or if there is no crosswalk, then before entering the intersection; and shall remain stopped until a signal indication to proceed is displayed, or as provided below.
- Except when a traffic control device is in place prohibiting a turn on red, vehicular traffic facing a steady CIRCULAR RED signal indication is permitted to enter the intersection to turn right, or to turn left from a one-way street into a one-way street, after stopping. The right to proceed with the turn shall be subject to the rules applicable after making a stop at a STOP sign.
- 2. Vehicular traffic facing a steady RED ARROW signal indication shall not enter the intersection to make the movement indicated by the arrow and, unless entering the intersection to make another movement permitted by another signal indication, shall stop at a clearly marked stop line; but if there is no stop line, before entering the crosswalk on the near side of the intersection; or if there is no crosswalk, then before entering the intersection; and shall remain stopped until a signal indication permitting the movement indicated by such RED ARROW is displayed.
- 3. Unless otherwise directed by a pedestrian signal indication or other traffic control device, pedestrians facing a steady CIRCULAR RED or steady RED ARROW signal indication shall not enter the roadway.
- D. A flashing green signal indication has no meaning and shall not be used.
- E. Flashing yellow signal indications shall have the following meanings:
- 1. Vehicular traffic, on an approach to an intersection, facing a flashing CIRCULAR YELLOW signal indication is permitted to cautiously enter the intersection to proceed straight through or turn right or left except as such movement is modified by lane use signs, turn prohibition signs, lane markings, roadway design, separate turn signal indications, or other traffic control devices.
- Such vehicular traffic, including vehicles turning right or left, shall yield the right of way to:
- (a) Pedestrians lawfully within an associated crosswalk, and
- (b) Other vehicles lawfully within the intersection.
- In addition, vehicular traffic turning left shall yield the right of way to other vehicles approaching from the opposite direction so closely as to constitute an immediate hazard during the time when such turning vehicle is moving across or within the intersection.
- 2. Vehicular traffic, on an approach to an intersection, facing a flashing YELLOW ARROW signal indication, displayed alone or in combination with another signal indication, is permitted to cautiously enter the intersection only to make the movement indicated by such arrow, or other such movement as is permitted by other signal indications displayed at the same time.
- Such vehicular traffic, including vehicles turning right or left, shall yield the right of way to:
- (a) Pedestrians lawfully within an associated crosswalk, and
- (b) Other vehicles lawfully within the intersection.
- In addition, vehicular traffic turning left shall yield the right of way to other vehicles approaching from the opposite direction so closely as to constitute an immediate hazard during the time when such turning vehicle is moving across or within the intersection.
- 3. Pedestrians facing any flashing yellow signal indication at an intersection, unless otherwise directed by a pedestrian signal indication or other traffic control device, are permitted to proceed across the roadway within any marked or unmarked associated crosswalk. Pedestrians shall yield the right of way to vehicles lawfully within the intersection at the time that the flashing yellow signal indication is first displayed.
- 4. When a flashing CIRCULAR YELLOW signal indication(s) is displayed as a beacon (see EPG 902.12) to supplement another traffic control device, road users are notified that there is a need to pay extra attention to the message contained thereon or that the regulatory or warning requirements of the other traffic control device, which might not be applicable at all times, are currently applicable.
- F. Flashing red signal indications shall have the following meanings:
- 1. Vehicular traffic, on an approach to an intersection, facing a flashing CIRCULAR RED signal indication shall stop at a clearly marked stop line; but if there is no stop line, before entering the crosswalk on the near side of the intersection; or if there is no crosswalk, at the point nearest the intersecting roadway where the driver has a view of approaching traffic on the intersecting roadway before entering the intersection. The right to proceed shall be subject to the rules applicable after making a stop at a STOP sign.
- 2. Vehicular traffic, on an approach to an intersection, facing a flashing RED ARROW signal indication if intending to turn in the direction indicated by the arrow shall stop at a clearly marked stop line; but if there is no stop line, before entering the crosswalk on the near side of the intersection; or if there is no crosswalk, at the point nearest the intersecting roadway where the driver has a view of approaching traffic on the intersecting roadway before entering the intersection. The right to proceed with the turn shall be limited to the direction indicated by the arrow and shall be subject to the rules applicable after making a stop at a STOP sign.
- 3. Pedestrians facing any flashing red signal indication at an intersection, unless otherwise directed by a pedestrian signal indication or other traffic control device, are permitted to proceed across the roadway within any marked or unmarked associated crosswalk. Pedestrians shall yield the right of way to vehicles lawfully within the intersection at the time that the flashing red signal indication is first displayed.
- 4. When a flashing CIRCULAR RED signal indication(s) is displayed as a beacon (see EPG 902.12 Flashing Beacons) to supplement another traffic control device, road users are notified that there is a need to pay extra attention to the message contained thereon or that the regulatory requirements of the other traffic control device, which might not be applicable at all times, are currently applicable. Use of this signal indication shall be limited to supplementing STOP (R1-1), DO NOT ENTER (R5-1), or WRONG WAY (R5-1a) signs, and to applications where compliance with the supplemented traffic control device requires a stop at a designated point.
902.5.11 Application of Steady Signal Indications (MUTCD Section 4D.05)
Standard. When a traffic control signal is being operated in a steady (stop-and-go) mode, at least one indication in each signal face shall be displayed at any given time.
A signal face(s) that controls a particular vehicular movement during any interval of a cycle shall control that same movement during all intervals of the cycle.
Steady signal indications shall be applied as follows:
- A. A steady CIRCULAR RED signal indication:
- 1. Shall be displayed when it is intended to prohibit traffic, except pedestrians directed by a pedestrian signal head, from entering the intersection or other controlled area. Turning after stopping is permitted as stated in Item C.1 in EPG 902.5.10.
- 2. Shall be displayed with the appropriate GREEN ARROW signal indications when it is intended to permit traffic to make a specified turn or turns, and to prohibit traffic from proceeding straight ahead through the intersection or other controlled area, except in protected only mode operation (see EPG 902.5.25 and EPG 902.5.32), or in protected/permissive mode operation with separate turn signal faces (see EPG 902.5.26 and EPG 902.5.33).
- B. A steady CIRCULAR YELLOW signal indication:
- 1. Shall be displayed following a CIRCULAR GREEN signal indication in the same signal face.
- 2. Shall not be displayed in conjunction with the change from the CIRCULAR RED signal indication to the CIRCULAR GREEN signal indication.
- 3. Shall be followed by a CIRCULAR RED signal indication except that, when entering preemption operation, the return to the previous CIRCULAR GREEN signal indication shall be permitted following a steady CIRCULAR YELLOW signal indication (see EPG 902.5.38).
- 4. Shall not be displayed to an approach from which drivers are turning left permissively unless one of the following conditions exists:
- (a) A steady CIRCULAR YELLOW signal indication is also simultaneously being displayed to the opposing approach;
- (b) An engineering study has determined that, because of unique intersection conditions, the condition described in Item (a) cannot reasonably be implemented without causing significant operational or safety problems and that the volume of impacted left-turning traffic is relatively low, and those left-turning are advised that a steady CIRCULAR YELLOW signal indication is not simultaneously being displayed to the opposing traffic if this operation occurs continuously by the installation near the left-most signal head of a W25-1 sign (see EPG 903.6.39) with the legend ONCOMING TRAFFIC HAS EXTENDED GREEN; or
- (c) Drivers are advised of the operation if it occurs only occasionally, such as during a preemption sequence, by the installation near the left-most signal head of a W25-2 sign (see EPG 903.6.39) with the legend ONCOMING TRAFFIC MAY HAVE EXTENDED GREEN.
- C. A steady CIRCULAR GREEN signal indication shall be displayed only when it is intended to permit traffic to proceed in any direction that is lawful and practical.
- D. A steady RED ARROW signal indication shall be displayed when it is intended to prohibit traffic, except pedestrians directed by a pedestrian signal head, from entering the intersection or other controlled area to make the indicated turn. Turning on a steady RED ARROW signal indication shall not be permitted.
- E. A steady YELLOW ARROW signal indication:
- 1. Shall be displayed in the same direction as a GREEN ARROW signal indication following a GREEN ARROW signal indication in the same signal face, unless the GREEN ARROW signal indication and a CIRCULAR GREEN signal indication terminate simultaneously in the same signal face.
- 2. Shall be displayed in the same direction as a flashing YELLOW ARROW signal indication following a flashing YELLOW ARROW signal indication in the same signal face, when the flashing arrow indication is displayed as part of a steady mode operation, if the signal face will subsequently display a steady red signal indication.
- 3. Shall not be displayed in conjunction with the change from a steady RED ARROW or flashing YELLOW ARROW signal indication to a GREEN ARROW signal indication, except when entering preemption operation as provided in Item 6(a).
- 4. Shall not be displayed when any conflicting vehicular movement has a green or yellow signal indication or any conflicting pedestrian movement has a WALKING PERSON (symbolizing WALK) or flashing UPRAISED HAND (symbolizing DONT WALK) signal indication, except that a steady left-turn YELLOW ARROW signal indication used to terminate a flashing left-turn YELLOW ARROW signal indication in a signal face controlling a permissive left-turn movement as described in EPG 902.5.24 and EPG 902.5.26 shall be permitted to be displayed when a CIRCULAR YELLOW signal indication is displayed for the opposing through movement. Vehicles departing in the same direction shall not be considered in conflict if, for each turn lane with moving traffic, there is a separate departing lane, and pavement markings or raised channelization clearly indicate which departure lane to use.
- 5. Shall not be displayed to terminate a flashing arrow signal indication on an approach from which drivers are turning left permissively unless one of the following conditions exists:
- (b) An engineering study has determined that, because of unique intersection conditions, the condition described in Item (a) cannot reasonably be implemented without causing significant operational or safety problems and that the volume of impacted left-turning traffic is relatively low, and those left-turning drivers are advised that a steady CIRCULAR YELLOW signal indication is not simultaneously being displayed to the opposing traffic if this operation occurs continuously by the installation near the left-most signal head of a W25-1 sign (see EPG 903.6.39) with the legend ONCOMING TRAFFIC HAS EXTENDED GREEN; or
- 6. Shall be terminated by a RED ARROW signal indication for the same direction or a CIRCULAR RED signal indication except:
- (a) When entering preemption operation, the display of a GREEN ARROW signal indication or a flashing arrow signal indication shall be permitted following a steady YELLOW ARROW signal indication.
- (b) When the movement controlled by the arrow is to continue on a permissive mode basis during an immediately following CIRCULAR GREEN or flashing YELLOW ARROW signal indication.
- F. A steady GREEN ARROW signal indication:
- 1. Shall be displayed only to allow vehicular movements, in the direction indicated, (or to allow a U-turn to the left when in the left turn lane during protected only mode and if lane use signs R10-30c and U-TURN ON GREEN ARROW R10-30b are used), that are not in conflict with other vehicles moving on a green or yellow signal indication and are not in conflict with pedestrians crossing in compliance with a WALKING PERSON (symbolizing WALK) or flashing UPRAISED HAND (symbolizing DONT WALK) signal indication. Vehicles departing in the same direction shall not be considered in conflict if, for each turn lane with moving traffic, there is a separate departing lane, and pavement markings or raised channelization clearly indicate which departure lane to use.
- 2. Shall be displayed on a signal face that controls a left-turn movement when said movement is not in conflict with other vehicles moving on a green or yellow signal indication (when U-turns to the left are allowed during a protected only mode, a right-turn GREEN ARROW signal indication for conflicting right turners shall not be simultaneously displayed) and is not in conflict with pedestrians crossing in compliance with a WALKING PERSON (symbolizing WALK) or flashing UPRAISED HAND (symbolizing DONT WALK) signal indication. Vehicles departing in the same direction shall not be considered in conflict if, for each turn lane with moving traffic, there is a separate departing lane, and pavement markings or raised channelization clearly indicate which departure lane to use.
- 3. Shall not be required on the stem of a T-intersection or for turns from a one-way street.
Option. An indication for a right turn vehicle is normally not required if there is a channelizing island that creates a "free" right turn. A yield sign would normally control a “free” right turn. In cases where right turn motorists cannot readily judge conflicting movements, it may be necessary to place channelized right turns under signal control. If not otherwise prohibited, steady red (only for left turns), yellow, and green turn arrow signal indications may be used instead of steady circular red, yellow, and green signal indications in a signal face on an approach where all traffic is required to turn or where the straight-through movement is not physically possible.
Support. [902.5.35 Signal Indications for Approaches with Shared Left-Turn/Right-Turn Lanes and No Through Movement (MUTCD Section 4D.25) EPG 902.5.35] contains information regarding the signalization of approaches that have a shared left-turn/right-turn lane and no through movement.
Standard. If supplemental signal faces are used, the following limitations shall apply:
- A. Left-turn arrows shall not be used in near-right signal faces.
- B. Right-turn arrows shall not be used in far-left signal faces. A far-side median-mounted signal face shall be considered a far-left signal for this application.
The following combinations of signal indications shall not be simultaneously displayed on any one signal face:
- A. CIRCULAR RED with CIRCULAR YELLOW;
- B. CIRCULAR GREEN with CIRCULAR RED.
Additionally, the above combinations shall not be simultaneously displayed on an approach as a result of the combination of displays from multiple signal faces unless the display is created by a signal face(s) devoted exclusively to the control of a right-turning movement and:
- A. The signal face(s) controlling the right-turning movement is visibility-limited from the adjacent through movement or positioned to minimize potential confusion to approaching road users, or
- B. A RIGHT TURN SIGNAL (R10-10R) sign (see EPG 902.5.30 through 902.5.33) is mounted adjacent to the signal face(s) controlling the right-turning movement.
The following combinations of signal indications shall not be simultaneously displayed on any one signal face or as a result of the combination of displays from multiple signal faces on an approach:
- A. CIRCULAR GREEN with CIRCULAR YELLOW;
- B. GREEN ARROW with YELLOW ARROW pointing in the same direction;
- C. RED ARROW with YELLOW ARROW pointing in the same direction; or
- D. GREEN ARROW with RED ARROW pointing in the same direction.
Guidance. No movement that creates an unexpected crossing of pathways of moving vehicles or pedestrians should be allowed during any green or yellow interval, except when all three of the following conditions are met:
- A. The movement involves only slight conflict, and
- B. Serious traffic delays are substantially reduced by permitting the conflicting movement, and
- C. Drivers and pedestrians subjected to the unexpected conflict are effectively warned thereof by a sign.
902.5.12 Signal Indications – Design, Illumination, Color, and Shape (MUTCD Section 4D.06)
Standard. Each signal indication, except those used for pedestrian signal heads and lane-use control signals, shall be circular or arrow.
Letters or numbers (including those associated with countdown displays) shall not be displayed as part of a vehicular signal indication.
Strobes shall not be used within or adjacent to any signal indication.
Except for the flashing signal indications and the pre-emption confirmation lights that are expressly allowed by the provisions of this article, flashing displays shall not be used within or adjacent to any signal indications.
Each circular signal indication shall emit a single color: red, yellow or green.
Each arrow signal indication shall emit a single color: red, yellow or green.
The arrow, which shall show only one direction, shall be the only illuminated part of an arrow signal indication.
Arrows shall be pointed:
- A. Horizontally in the direction of the turn to indicate a turn at approximately or greater than a right angle, or
- B. Upward with a slope at an angle approximately equal to that of the turn if the angle of the turn is substantially less than a right angle, or
Except as provided in the Guidance below, the requirements of the publication entitled “Vehicle Traffic Control Signal Heads” (see EPG 900.1.11) that pertain to the aspects of the signal head design that affect the display of the signal indications shall be met.
Guidance. The intensity and distribution of light from each illuminated signal lens should comply with the publications entitled “Vehicle Traffic Control Signal Heads” and “Traffic Signal Lamps” (see EPG 900.1.11).
Standard. References to signal lenses in this section shall not be used to limit signal optical units to incandescent lamps within optical assemblies that include lenses.
All signal indications shall be Light Emitting Diode (LED).
Support. Research has resulted in signal optical units that are not lenses, such as, but not limited to, light emitting diode (LED) traffic signal modules. Some units are practical for all signal indications, and some are practical for specific types such as visibility-limited signal indications.
Guidance. If a signal indication is so bright that it causes excessive glare during nighttime conditions, some form of automatic dimming should be used to reduce the brilliance of the signal indication.
902.5.13 Size of Vehicular Indications (MUTCD Section 4D.07)
Standard. There shall be two nominal diameter sizes for vehicular signal indications: 8 in. and 12 inches.
Except for emergency-vehicle traffic signals, 12 in. signal indications shall be used for all signal sections in all signal faces.
Eight-inch circular signal indications are used in signal faces only for the flashing yellow signal indication on the major street in an emergency-vehicle traffic control signal (see EPG 902.8.2).
902.5.14 Positions of Signal Indications Within a Signal Face – General (MUTCD Section 4D.08)
Support. Standardization of the number and arrangements of signal sections in vehicular traffic control signal faces enables road users who are color vision deficient to identify the illuminated color by its position relative to other signal sections.
Standard. Unless otherwise provided for a particular application, each signal face at a signalized location shall have three, four, or five signal sections.
A single-section signal face shall be permitted at a traffic control signal if it consists of a continuously-displayed GREEN ARROW signal indication that is being used to indicate a continuous movement and positioned vertically upward to indicate a straight-through movement.
The signal sections in a signal face shall be arranged in a vertical or horizontal straight line.
If a signal section that displays a CIRCULAR YELLOW signal indication is used, it shall be located between the signal section that displays the red signal indication and all other signal sections.
Support. Fig. 902.5.14 illustrates some of the typical arrangements of signal sections in signal faces that do not control separate turning movements. Figures 902.5.24 through 902.5.26 illustrate the typical arrangements of signal sections in left-turn signal faces. Fig. 902.5.34 illustrates the typical arrangements of signal sections in right-turn signal faces.
902.5.15 Positions of Signal Indications Within a Vertical Signal Face (MUTCD Section 4D.09)
Standard. In each vertically-arranged signal face, all signal sections that display red signal indications shall be located above all signal sections that display yellow and green signal indications.
In vertically-arranged signal faces, each signal section that displays a YELLOW ARROW signal indication shall be located above the signal section that displays the GREEN ARROW signal indication to which it applies.
The relative positions of signal sections in a vertically-arranged signal face, from top to bottom, shall be as follows:
- CIRCULAR RED
- Steady left-turn RED ARROW
- CIRCULAR YELLOW
- CIRCULAR GREEN
- Steady left-turn YELLOW ARROW
- Flashing left-turn YELLOW ARROW
- Left-turn GREEN ARROW
- Steady right-turn YELLOW ARROW
- Right-turn GREEN ARROW
Option. In a vertically-arranged signal face, signal sections that display signal indications of the same color may be arranged horizontally adjacent to each other at right angles to the basic straight-line arrangement to form a clustered signal face, such as those used for protected-permissive right turns. (See Fig. 902.5.34).
Standard. Such clusters shall be limited to two or three different signal sections that display signal indications of the same color.
The signal section that displays a flashing yellow arrow signal indication during steady mode operation:
- A. Shall not be placed in the same vertical position as the signal section that displays a steady yellow arrow signal indication, and
- B. Shall be placed below the signal section that displays a steady yellow arrow signal indication.
902.5.16 Positions of Signal Indications Within a Horizontal Signal Face (MUTCD Section 4D.10)
Other than in emergency vehicle signals, MoDOT does not typically utilize horizontal signal faces unless engineering judgment deems necessary because of poor visibility or low clearance.
Standard. In each horizontally-arranged signal face, all signal sections that display red signal indications shall be located to the left of all signal sections that display yellow and green signal indications.
In horizontally-arranged signal faces, each signal section that displays a YELLOW ARROW signal indication shall be located to the left of the signal section that displays the GREEN ARROW signal indication to which it applies.
The relative positions of signal sections in a horizontally-arranged signal face, from left to right, shall be as follows:
- CIRCULAR RED
- Steady left-turn RED ARROW
- CIRCULAR YELLOW
- Steady left-turn YELLOW ARROW
- Flashing left-turn YELLOW ARROW
- Left-turn GREEN ARROW
- CIRCULAR GREEN
- Steady right-turn YELLOW ARROW
- Right-turn GREEN ARROW
The signal section that displays a flashing yellow arrow signal indication during steady mode operation:
- A. Shall not be placed in the same horizontal position as the signal section that displays a steady yellow arrow signal indication, and
- B. Shall be placed to the right of the signal section that displays a steady yellow arrow signal indication.
902.5.17 Number of Signal Faces on an Approach (MUTCD Section 4D.11)
Standard. The signal faces for each approach to an intersection or a midblock location shall be provided as follows:
- A. If a signalized through movement exists on an approach, a minimum of two primary signal faces shall be provided for the through movement. If a signalized through movement does not exist on an approach, a minimum of two primary signal faces shall be provided for the signalized turning movement that is considered to be the major movement from the approach (also see EPG 902.5.35).
- B. Each signalized approach lane shall have one signal face centered over it.
- C. Primary signal indications shall be located on the far side of the intersection.
- D. All signal faces shall have backplates . For approaches with operating speeds equal to or greater than 55 miles per hour, backplates shall have a retroreflective border.
- E. See EPG 902.5.23 through 902.5.26 for left-turn signal faces.
- F. See EPG 902.5.30 through 902.5.33 for right-turn signal faces.
Support. The use of backplates enhances the contrast between the traffic signal indications and their surroundings for both day and night conditions, which is also helpful to older drivers.
Guidance. Retroreflective backplate borders should be installed at signalized intersections where enhanced signal visibility and conspicuity is desired in daytime and/or nighttime conditions. Such signalized intersections include, but are not limited to:
- signalized intersections with operating speeds greater than 45 mph
- signalized intersections with busy or confusing backgrounds
- signalized intersections with automated enforcement
- isolated signalized intersections
- signalized intersections that are the first signalized intersection upon entering a city or in an unexpected area.
When retroreflective borders are installed, not all backplates on that approach or at that signalized intersection are required to have a retroreflective border, engineering judgment is required. For example, a retroreflective border for a left turn signal head or for a right turn signal head is not required when the through signal faces have retroreflective borders and retroreflective borders are not required on the sidestreet when the mainline approaches have retroreflective borders.
In addition to the primary signal faces, one or more supplemental pole-mounted or overhead signal faces should be considered to provide added visibility for approaching traffic that is traveling behind large vehicles.
See Fig. 902.5.17 and Table 902.5.17 for signal face information.
Table 902.5.17 Recommended Minimum Number of Primary Signal Faces for Through Traffic on Approaches
|Number of Through Lanes on Approach||Total Number of Primary Through Signal Faces for Approach*||Minimum Number of Overhead-Mounted Primary Through Signal Faces for Approach|
|4 or more||4 or more||4 or more|
|NOTE: * A minimum of two through signal faces is always required (See EPG 902.5.17). These recommended numbers of through signal faces may be exceeded. Also, see cone of vision requirements otherwise indicated in EPG 902.5.19.|
Option. Where a movement (or a certain lane or lanes) at the intersection never conflicts with any other signalized vehicular or pedestrian movement, a continuously-displayed single-section GREEN ARROW signal indication may be used to inform road users that the movement is free-flow and does not need to stop.
Support. In some circumstances where the through movement never conflicts with any other signalized vehicular or pedestrian movement at the intersection, such as at T-intersections with appropriate geometrics and/or pavement markings and signing, an engineering study might determine that the through movement (or certain lanes of the through movement) can be free-flow and not signalized.
Guidance. If two or more left-turn lanes are provided for a separately controlled protected only mode left-turn movement, or if a left-turn movement represents the major movement from an approach, two or more primary left-turn signal faces should be provided.
If two or more right-turn lanes are provided for a separately controlled right-turn movement, or if a right-turn movement represents the major movement from an approach, two or more primary right-turn signal faces should be provided.
Support. Locating primary signal faces overhead on the far side of the intersection has been shown to provide safer operation by reducing intersection entries late in the yellow interval and by reducing red signal violations, as compared to post-mounting signal faces at the roadside or locating signal faces overhead within the intersection on a diagonally-oriented mast arm or span wire.
902.5.18 Visibility, Aiming, and Shielding of Signal Faces (MUTCD Section 4D.12)
Standard. The primary consideration in signal face placement, aiming, and adjustment shall be to optimize the visibility of signal indications to approaching traffic.
Road users approaching a signalized intersection or other signalized area, such as a midblock crosswalk, shall be given a clear and unmistakable indication of their right-of-way assignment.
The geometry of each intersection to be signalized, including vertical grades, horizontal curves, and obstructions as well as the lateral and vertical angles of sight toward a signal face, as determined by typical driver-eye position, shall be considered in determining the vertical, longitudinal, and lateral position of the signal face.
Guidance. The two primary signal faces required as a minimum for each approach should be continuously visible to traffic approaching the traffic control signal, from a point at least the minimum sight distance provided in Table 903.6.28 in advance of and measured to the stop line. This range of continuous visibility should be provided unless precluded by a physical obstruction or unless another signalized location is within this range.
There should be legal authority to prohibit the display of any unauthorized sign, signal, marking, or device that interferes with the effectiveness of any official traffic control device (see Section 11-205 of the “Uniform Vehicle Code”).
At signalized midblock crosswalks, at least one of the signal faces should be over the traveled way for each approach.
Standard. If approaching traffic does not have a continuous view of at least two signal faces for at least the minimum sight distance shown in Table 903.6.28, a signal ahead (W3-3) sign (see EPG 903.6.28) shall be installed to warn approaching traffic of the traffic control signal.
Option. If a sign is installed to warn approaching road users of the traffic control signal, the sign may be supplemented by a Warning Beacon (see EPG 902.12.3).
A Warning Beacon used in this manner may be interconnected with the traffic signal controller assembly in such a manner as to flash yellow during the period when road users passing this beacon at the legal speed for the roadway might encounter a red signal indication (or a queue resulting from the display of the red signal indication) upon arrival at the signalized location.
If the sight distance to the signal faces for an approach is limited by horizontal or vertical alignment, supplemental signal faces aimed at a point on the approach at which the signal indications first become visible may be used.
Guidance. Supplemental signal faces should be used if engineering judgment has shown that they are needed to achieve intersection visibility both in advance and immediately before the signalized location.
If supplemental signal faces are used, they should be located to provide optimum visibility for the movement to be controlled.
Standard. In cases where irregular street design necessitates placing signal faces for different street approaches with a comparatively small angle between their respective signal indications, each signal indication shall, to the extent practical, be visibility-limited by signal visors, optically limited heads, or other means so that an approaching road user’s view of the signal indication(s) controlling movements on other approaches is minimized.
Signal visors shall be used on signal faces to aid in directing the signal indication specifically to approaching traffic, as well as to reduce “sun phantom,” which can result when external light enters the lens.
Option. Special signal faces, such as visibility-limited signal faces, may be used such that the road user does not see signal indications intended for other approaches before seeing the signal indications for their own approach, if simultaneous viewing of both signal indications could cause the road user to be misdirected.
Limiting the visibility of the indication is to be carefully done according to the manufacturer's recommendations. A single section of an optically limited head can be combined with conventional sections to create the needed visibility control. Due to the increased cost of optically limited heads, they may only be used at locations where needed.
Standard. Optically limited heads shall not be installed on a span wire since the control of the indication will be lost because of the movement of the span wire mounted head.
Standard. The inside of signal visors (hoods) and the front surface of backplates shall have a dull black finish to minimize light reflection and to increase contrast between the signal indication and its background.
902.5.19 Lateral Positioning of Signal Faces (MUTCD Section 4D.13)
Standard. At least one and preferably both of the minimum of two primary signal faces required for the through movement (or the major turning movement if there is no through movement) on the approach shall be located between two lines intersecting with the center of the approach at a point 10 ft. behind the stop line, one making an angle of approximately 20 degrees to the right of the center of the approach extended, and the other making an angle of approximately 20 degrees to the left of the center of the approach extended. The signal face that satisfies this requirement shall simultaneously satisfy the longitudinal placement requirement described in EPG 902.5.20 (see Fig. 902.5.20).
If both of the minimum of two primary signal faces required for the through movement (or the major turning movement if there is no through movement) on the approach are post-mounted, they shall both be on the far side of the intersection, one on the right and one on the left of the approach lane(s).
The required signal faces for through traffic on an approach shall be located not less than 8 ft. apart measured horizontally perpendicular to the approach between the centers of the signal faces.
If more than one separate turn signal face is provided for a turning movement and if one or both of the separate turn signal faces are located over the roadway, the signal faces shall be located not less than 8 ft. apart measured horizontally perpendicular to the approach between the centers of the signal faces.
Guidance. If a signal face controls a specific lane or lanes of an approach, its position should make it readily visible to road users making that movement.
Standard. If an exclusive left-turn or right-turn lane is present on an approach, the primary separate turn signal face shall be offset from the centerline of the lane by 2 ft. to prevent the signals from blocking the view of the opposing approach.
Option. The primary separate turn signal face may be positioned such to not be any further to the right than the extension of the right-hand edge of the exclusive turn lane or any further to the left than the extension of the left-hand edge of the exclusive turn lane.
Standard. Supplemental turn signal faces mounted over the roadway shall not be subject to the positioning requirements in the previous paragraph.
Standard. For new or reconstructed signal installations, on an approach with an exclusive turn lane(s) for a left-turn (or U-turn to the left) movement and with opposing vehicular traffic, signal faces that display a CIRCULAR GREEN signal indication shall not be post-mounted on the far-side median or mounted overhead above the exclusive turn lane(s) or the extension of the lane(s).
If supplemental post-mounted signal faces are used, the following limitations shall apply:
- A. Left-turn arrows to the left shall not be used in near-right signal faces.
- B. Right-turn arrows to the right shall not be used in far-left signal faces. A far-side median-mounted signal face shall be considered a far-left signal for this application.
902.5.20 Longitudinal Positioning of Signal Faces (MUTCD Section 4D.14)
Standard. Except where the width of an intersecting roadway or other conditions make it physically impractical, the signal faces for each approach to an intersection or a midblock location shall be provided as follows:
- A. A signal face installed to satisfy the requirements for primary left-turn signal faces (see EPG 902.5.23 through 902.5.26) and primary right-turn signal faces (see EPG 902.5.30 through 902.5.33), and at least one and preferably both of the minimum of two primary signal faces required for the through movement (or the major turning movement if there is no through movement) on the approach shall be located:
- 1. No less than 40 ft. beyond the stop line,
- 2. No more than 180 ft. beyond the stop line unless a supplemental near-side signal face is provided, and
- 3. As near as practical to the line of the driver’s normal view, if mounted over the roadway.
- The primary signal face that satisfies this requirement shall simultaneously satisfy the lateral placement requirement described in EPG 902.5.19 (see Fig. 902.5.20).
- B. Where the nearest signal face is located between 150 ft. and 180 ft. beyond the stop line, engineering judgment of the conditions, including the worst-case visibility conditions, shall be used to determine if the provision of a supplemental near-side signal face would be beneficial.
Guidance. Supplemental near-side signal faces should be located as near as practical to the stop line.
902.5.21 Mounting Height of Signal Faces (MUTCD Section 4D.15)
Standard. The top of the signal housing of a vehicular signal face located over any portion of a highway that can be used by motor vehicles shall not be more than 25.6 ft. above the pavement.
The bottom of the signal housing and any related attachments to a vehicular signal face located over any portion of a highway that can be used by motor vehicles shall be at least 16 ft. above the pavement.
The bottom of the signal housing (including brackets) of a vehicular signal face that is vertically arranged and not located over a roadway shall be a minimum of 9 ft. and a maximum of 19 ft. above the sidewalk or, if there is no sidewalk, above the pavement grade at the center of the roadway.
Option. Where engineering judgment supports the installation of a supplemental signal indication, a mounting height greater than 19 ft. above the sidewalkmay be used.
902.5.22 Lateral Offset (Clearance) of Signal Faces (MUTCD Section 4D.16)
Standard. Signal faces mounted at the side of a roadway with curbs at less than 15 ft. from the bottom of the housing and any related attachments shall have a horizontal offset of not less than 2 ft. from the face of a vertical curb, or if there is no curb, not less than 2 ft. from the edge of a shoulder. Examples of these offsets can be seen in the below images.
902.5.23 Signal Indications for Left-Turn Movements – General (MUTCD Section 4D.17)
Support. Left-turning traffic is controlled by one of four modes as follows:
- A. Permissive Only Mode—turns made on a CIRCULAR GREEN signal indication or a flashing left-turn YELLOW ARROW signal indication after yielding to pedestrians, if any, and/or opposing traffic, if any.
- B. Protected Only Mode—turns made only when a left-turn GREEN ARROW signal indication is displayed.
- C. Protected/Permissive Mode—both modes can occur on an approach during the same cycle.
- D. Variable Left-Turn Mode—the operating mode changes among the protected only mode and/or the protected/permissive mode and/or the permissive only mode during different periods of the day or as traffic conditions change.
Standard. U-turns to the left shall only be permitted at protected only mode left turns.
Option. In areas having a high percentage of older drivers, special consideration may be given to the use of protected only mode left-turn phasing, when appropriate.
Standard. During a permissive left-turn movement, the signal faces for through traffic on the opposing approach shall simultaneously display green or steady yellow signal indications. If pedestrians crossing the lane or lanes used by the permissive left-turn movement to depart the intersection are controlled by pedestrian signal heads, the signal indications displayed by those pedestrian signal heads shall not be limited to any particular display during the permissive left-turn movement.
During a protected left-turn movement, the signal faces for through traffic on the opposing approach shall simultaneously display steady CIRCULAR RED signal indications. If pedestrians crossing the lane or lanes used by the protected left-turn movement to depart the intersection are controlled by pedestrian signal heads, the pedestrian signal heads shall display a steady UPRAISED HAND (symbolizing DONT WALK) signal indication during the protected left-turn movement.
A protected only mode left-turn movement that does not begin and terminate at the same time as the adjacent through movement shall not be provided on an approach unless an exclusive left-turn lane exists.
A yellow change interval for the left-turn movement shall not be displayed when the status of the left-turn operation is changing from permissive to protected within any given signal sequence.
If the operating mode changes among the protected only mode and/or the protected/permissive mode and/or the permissive only mode during different periods of the day or as traffic conditions change, the requirements in EPG 902.5.24 through 902.5.26 that are appropriate to that mode of operation shall be met, subject to the following:
- A. The CIRCULAR GREEN and CIRCULAR YELLOW signal indications shall not be displayed when operating in the protected only mode.
- B. The left-turn GREEN ARROW and steady left-turn YELLOW ARROW signal indications shall not be displayed when operating in the permissive only mode.
Option. Additional static signs or changeable message signs may be used to meet the requirements for the variable left-turn mode or to inform drivers that left-turn green arrows will not be available during certain times of the day.
Support. EPG 902.5.23 through 902.5.26 describe the use of separate signal faces for controlling left-turn movements.
Separate left-turn signal face controls only the left-turn movement and cannot serve as one of the two required primary signal faces for the adjacent movement (usually the through movement) because it displays signal indications that are applicable only to the left-turn movement. If a separate left-turn signal face is mounted overhead at the intersection, it is positioned over the extension of the left-turn lane. In a separate left-turn signal face, a flashing left-turn YELLOW ARROW signal indication is used to control permissive left-turning movements.
EPG 902.5.19 contains provisions regarding the lateral positioning of signal faces that control left-turn movements.
It is not necessary that the same mode of left-turn operation or same type of left-turn signal face be used on every approach to a signalized location. Selecting different modes and types of left-turn signal faces for the various approaches to the same signalized location is acceptable.
Option. A signal face that is shared by left-turning and right-turning traffic may be provided for a shared left-turn/right-turn lane on an approach that has no through traffic (see EPG 902.5.35).
Support. Guidelines are available to aid in determining the proper left turn phasing for signalized intersections.
|Left Turn Phasing Warrants|
Left turn indications at signalized intersections are designed so they are neither overly restrictive nor inconsistent from the driver's point of view. The Left Turn Phasing Warrants are available in an interactive spreadsheet for safety warrants and capacity warrants to determine the amount of protection to be given to a left turn movement. These warrants are based upon accepted safety and capacity values for signalized intersections.
When factors such as sight distance, speed of opposing vehicles, etc. make permissive turns undesirable, the permissive left turn option is removed. Safety warrants are checked first; if an approach requires protected-only phasing for safety reasons, it is unnecessary to check the capacity warrants.
Once safety considerations are satisfied, Capacity Warrants will need to be analyzed. Capacity Warrants are divided into three parts: Permissive-Only left turns, Protected/Permissive left turns, and Protected-Only left turns. This criterion is used when designing or upgrading a signal installation.
In order to provide the proper phasing at an intersection, it will be necessary to check Capacity Warrants for several hours for each approach. For example, if only the peak hour is checked, the phasing will most likely be too restrictive for the rest of the day. It is recommended that the peak periods plus a sample of off-peak hours be checked before choosing the phasing.
When traffic volumes at an intersection are approaching the thresholds listed in the capacity warrants variable left turn phasing can be used by time of day. Variable left turn phasing allows for the selection of either protected only, protected/permissive, or permissive only left turn phasing. This can be used to provide appropriate phasing for varying volumes throughout the day. The protected left turn phase can be omitted by time of day and flashing yellow arrow operation allows for removal of the permissive left turn in addition to removal of the protected left turn phase. It can be used only on approaches with “positive” signal lane control, in that each approach lane has its own signal indication. Refer to EPG 902.5.29 for more information on flashing yellow arrow indications. Otherwise, the most appropriate left turn phasing is chosen based on the results of the Capacity Warrants.
When the flashing yellow arrow indication is used to provide variable phasing, each hour during a typical day is evaluated to determine proper phasing throughout the day. The Variable Left Turn Worksheet can help evaluate each hour during the day. During initial installation the flashing yellow arrow indication can allow the selection of more restrictive phasing initially and then change to a less restrictive mode if appropriate.
Note that some overlap may occur when analyzing the volumes at each approach (i.e., the data for one hour might satisfy parts of the criteria for both permissive-only and protected/permissive left turns). Therefore, it will be necessary to check at least two of the three parts of the criteria. When an overlap does occur, previous experience and/or evaluation studies at the location is to indicate whether the situation is better served by the more or less restrictive phasing that is determined using the criteria.
The left turn phasing guidelines, below, give safety and capacity considerations for selecting left turn phasing. The interactive spreadsheet allows for the user to directly enter criteria and see suggested thresholds based on these formulas. These guidelines are used when reviewing design plans and when modifying the phasing of an existing installation.
Guidance. Guidelines for Variable Left-Turn Phasing
This is a guide for the selection of variable left turn phasing hour-by-hour. Guidelines based on safety and capacity are provided.
Definition of Terms
The following terms are used in these guidelines :
- VLT = The left turn volume per hour per approach.
- (VLT)pp = The number of vehicles attempting to make permissive left turns during the permissive part of a protected/permissive left turn per hour per approach.
- VO = The opposing volume per hour per approach per lane (excluding free right turn volume and volume serviced by a separate right turn phase).
- cp = The cycle length (in seconds) when those volumes occur using permissive-only phasing1.
- cpp = The cycle length (in seconds) when those volumes occur using protected/permissive phasing1.
- gp = The green time (in seconds) common to both VLT and VO during that cycle using permissive-only phasing1.
- gpp = The green time (in seconds) common to both (VLTLT)pp and VO during that cycle using protected/permissive phasing1.
- TP = The time allocated to the protected left turn movement using protected/permissive phasing1.
- 1 These green times are used in the calculations regardless of the existing phasing. For phasing configurations not currently used it will be necessary to develop realistic timing for that phasing configuration. A signal timing computer program can be helpful in developing this timing.
Adjustment of Left Turn Volumes
This evaluation considers the number of vehicles attempting to make permissive left turns during the permissive part of a protected/permissive left turn. Therefore, the effects of protected left turns should be eliminated. This can be handled using the following method:
- (VLT)pp = VLT - VP,
where the variable VP is the number of left turn vehicles served by the protected left turn indication. If this formula yields a negative number, use 0 for (VLT)pp. Assuming that vehicles enter the intersection at a rate of 2 seconds/vehicle, the volume using the protected movement in a one-hour period is:
- VP =
- Safety Criteria
- Protected Only Left Turns
- Note: Protected-Only left turns should be provided full-time when any one of the following criteria are satisfied:
- A. Number of Opposing Through Lanes ≥ 3
- B. Sight Distance:
- < 125 ft. for 20 mph
- < 150 ft. for 25 mph
- < 200 ft. for 30 mph
- < 250 ft. for 35 mph
- < 325 ft. for 40 mph
- < 400 ft. for 45 mph
- < 475 ft. for 50 mph
- < 550 ft. for 55 mph
- C. Number of Correctable Accidents By Upgrading to Protected Only Phasing > 5 over 12 months
- Note: The 5 correctable accidents should involve the SAME Left Turn approach. Only those approaches satisfying that criteria should be upgraded.
- D. Number of Observed Traffic Conflicts > 48 Conflicts / 11 Hour Day
- Note: Conflicts occur when motorists on the OPPOSITE APPROACH must respond to the actions of motorists making the subject left-turn movement. Therefore, conflicts should be measured by observing the intersection from the opposite approach. Only those approaches satisfying the criteria should be upgraded.
- E. Speed (prevailing)
- ≥ 50 mph AND ≥ 2 opposing through lanes
- = 45 mph AND a study indicates that the number of gaps is insufficient to turn safely
- F. ≥ 2 left turn lanes. However, if there are two left turn lanes and one opposing through lane with low speed and low volume protected/permissive might be considered using engineering judgment.
- G. Unusual intersection geometrics that make permissive left turns difficult.
- Protected/Permissive Left Turns
- Note: Protected/Permissive left turns should be provided when the following criteria is satisfied.
- A. Number of Observed Traffic Conflicts > 29 Conflicts / 11 Hour Day
- Note: The number of conflicts are those occurring on the OPPOSITE APPROACH that are caused by the subject left-turn movement. Only those approaches satisfying the criteria should be upgraded.
- Capacity Criteria
- Permissive-Only Left Turns
- Note: Permissive-Only left turns are an option when one of the criteria in (A.) is satisfied in conjunction with (B.).
- A. VLT < 100 Vehicles per Hour
- VLT < 2 Vehicles per Cycle1
- VO < 100 Vehicles per Hour
- 1 This criteria is only valid if observations at the intersection show that drivers tend to make left turns during the clearance interval on a regular basis. These field checks should be made during the hour(s) in which either the highest left turn volume or the highest opposing volume occurs.
- B. VLT + VO < 600 x (gp/cp)
- Protected/Permissive Left Turns
- Note: Protected/Permissive left turns should be provided when one of the criteria in (A.) is satisfied in conjunction with one of the criteria in (B.).
- A. VLT > 100 Vehicles per Hour AND VO > 100 Vehicles per Hour
- VLT > 2 Vehicles per Cycle1 AND VO > 100 Vehicles per Hour
- VLT + VO > 600 x (gp/cp)
- B. (VLT)pp + VO < 1200 x (gpp/cpp)
- (VLT)pp x VO < 50,000
- B. (VLT)pp + VO < 1200 x (gpp/cpp)
Protected-Only Left Turns
- Note: Protected-Only left turns should be provided when any one of the following criteria are satisfied.
- A. (VLT)pp + VO > 1200 x (gpp/cpp) for 3 or more hours if considering permanent phasing change
- B. (VLT)pp x VO> 50,000 for 3 or more hours if considering permanent phasing change
- Protected left turn movements should be provided with an adequate turn bay or a separate turning lane, depending upon the volumes using the intersection and the existing intersection geometry. Protected-Only left turns should not be used with shared lanes unless split phase operation is used.
902.5.24 Signal Indications for Permissive Only Mode Left-Turn Movements (MUTCD Section 4D.18)
Standard. For a separate left-turn signal face being operated in a permissive only left-turns mode, a CIRCULAR GREEN signal indication shall not be used in that face.
If a separate left-turn signal face is being operated in a permissive only left-turn mode and a flashing left-turn YELLOW ARROW signal indication is provided, it shall meet the following requirements (see Fig. 902.5.24):
- A. It shall be capable of displaying the following signal indications: steady left-turn RED ARROW, steady left-turn YELLOW ARROW, and flashing left-turn YELLOW ARROW. Only one of the three indications shall be displayed at any given time.
- B. During the permissive left-turn movement, a flashing left-turn YELLOW ARROW signal indication shall be displayed.
- C. A steady left-turn YELLOW ARROW signal indication shall be displayed following the flashing left-turn YELLOW ARROW signal indication.
- D. It shall be permitted to display a flashing left-turn YELLOW ARROW signal indication for a permissive left-turn movement while the signal faces for the adjacent through movement display steady CIRCULAR RED signal indications and the opposing left-turn signal faces display left-turn GREEN ARROW signal indications for a protected left-turn movement.
- E. During steady mode (stop-and-go) operation, the signal section that displays the steady left-turn YELLOW ARROW signal indication during change intervals shall not be used to display the flashing left-turn YELLOW ARROW signal indication for permissive left turns.
- F. During flashing mode operation (see EPG 902.5.41), the display of a flashing left-turn YELLOW ARROW signal indication shall be only from the signal section that displays a steady left-turn YELLOW ARROW signal indication during steady mode (stop-and-go) operation.
- G. If the permissive only mode is not the only left-turn mode used for the approach, the signal face shall be the same separate left-turn signal face with a flashing YELLOW ARROW signal indication that is used for the protected/permissive mode (see EPG 902.5.26) except that the left-turn GREEN ARROW signal indication shall not be displayed when operating in the permissive only mode.
Standard. U-turns to the left shall not be allowed during permissive-only mode.
902.5.25 Signal Indications for Protected Only Mode Left-Turn Movements (MUTCD Section 4D.19)
Standard. A separate left-turn signal face is provided for a protected only mode left turn. It shall meet the following requirements (see Figure 4D-10):
- A. It shall be capable of displaying, the following signal indications: steady left-turn RED ARROW, steady left-turn YELLOW ARROW, and left-turn GREEN ARROW. Only one of the three indications shall be displayed at any given time. A signal instruction sign shall not be required with this set of signal indications.
- B. During the protected left-turn movement, a left-turn GREEN ARROW signal indication shall be displayed.
- C. A steady left-turn YELLOW ARROW signal indication shall be displayed following the left-turn GREEN ARROW signal indication.
- D. If the protected only mode is not the only left-turn mode used for the approach, the signal face shall be the same separate left-turn signal face that is used for the protected/permissive mode (see EPG 902.5.18 and 902.5.26) except that the flashing left-turn YELLOW ARROW signal indication shall not be displayed when operating in the protected only mode.
Standard. U-turns to the left shall only be allowed during the left GREEN ARROW of protected only mode if lane use sign R10-30c and U-TURN ON GREEN ARROW R10-30b signs are installed.
902.5.26 Signal Indications for Protected/Permissive Mode Left-Turn Movements (MUTCD Section 4D.20)
Standard. For a separate left-turn signal face being operated in a protected/permissive left-turn mode, a CIRCULAR GREEN signal indication shall not be used in that face.
If a separate left-turn signal face is being operated in a protected/permissive left-turn mode and a flashing left-turn YELLOW ARROW signal indication is provided, it shall meet the following requirements (see Fig. 902.5.26):
- A. It shall be capable of displaying the following signal indications: steady left-turn RED ARROW, steady left-turn YELLOW ARROW, flashing left-turn YELLOW ARROW, and left-turn GREEN ARROW. Only one of the four indications shall be displayed at any given time.
- B. During the protected left-turn movement, a left-turn GREEN ARROW signal indication shall be displayed.
- C. A steady left-turn YELLOW ARROW signal indication shall be displayed following the left-turn GREEN ARROW signal indication.
- D. During the permissive left-turn movement, a flashing left-turn YELLOW ARROW signal indication shall be displayed.
- E. A steady left-turn YELLOW ARROW signal indication shall be displayed following the flashing left-turn YELLOW ARROW signal indication if the permissive left-turn movement is being terminated and the separate left-turn signal face will subsequently display a steady left-turn RED ARROW indication.
- F. It shall be permitted to display a flashing left-turn YELLOW ARROW signal indication for a permissive left-turn movement while the signal faces for the adjacent through movement display steady CIRCULAR RED signal indications and the opposing left-turn signal faces display left-turn GREEN ARROW signal indications for a protected left-turn movement.
- G. When a permissive left-turn movement is changing to a protected left-turn movement, a left-turn GREEN ARROW signal indication shall be displayed immediately upon the termination of the flashing left-turn YELLOW ARROW signal indication. A steady left-turn YELLOW ARROW signal indication shall not be displayed between the display of the flashing left-turn YELLOW ARROW signal indication and the display of the steady left-turn GREEN ARROW signal indication.
- H. During steady mode (stop-and-go) operation, the signal section that displays the steady left-turn YELLOW ARROW signal indication during change intervals shall not be used to display the flashing left-turn YELLOW ARROW signal indication for permissive left turns.
- I. During flashing mode operation (see EPG 902.5.41), the display of a flashing left-turn YELLOW ARROW signal indication shall be only from the signal section that displays a steady left-turn YELLOW ARROW signal indication during steady mode (stop-and-go) operation.
- J. A signal instruction sign shall be required with this set of signal indications. The sign shall be a “LEFT TURN YIELD ON FLASHING ARROW” (R10-27a).
Standard. U-turns to the left shall not be allowed during this mode.
902.5.27 Miscellaneous Applications for Left Turns
902.5.27.1 Leading and Lagging Left-Turns
Support. A leading left turn is a left turn that precedes or is accompanied by the first through movement in a direction. A lagging left turn is a left turn that follows the last through movement or is on at the end of the green time for a through movement.
At locations where a left turn lane is needed but cannot be provided, some relief is achieved by the use of a leading or lagging green period for the direction of traffic with the heavy left turn.
Leading and lagging left turn phasing is typically used to improve coordination on mainline routes where modifying the left turn phasing will provide a significant improvement in coordination. Lead-lag protected-permissive phasing is not normally used on uncoordinated approaches. Lead-lag phasing can be helpful where a short left-turn bay exceeds its capacity. The lagging left can prevent the turn bay overflow from blocking through traffic.
902.5.27.2 Split Phasing
Support. Split phasing is servicing a street one approach at a time. Because split phasing is a very inefficient use of green time, other alternatives, including geometric improvements are often preferred. Split phasing greatly impedes coordination if used on a main line and decreases the efficiency of the whole intersection by increasing the amount of time needed to serve both approaches separately. This phasing is used when the intersection geometry (i.e. offset intersection) doesn’t allow the operation of concurrent phases. For intersections with existing split phasing, use care when modifying phasing to include permissive left turns.
902.5.27.3 Alternate Sequences
Support. When needed, the normal NEMA ring sequence can be altered to fit operating conditions, with restrictions as detailed in EPG 902.5.3. The most common application is to provide lead-lag left turns. Assume this phasing assignment for the following intersection:
In order to allow the northbound phase 1 to become a lagging left turn, the sequence of phase 1 and phase 2 must be programmed to switch. Under this alternate sequence, the ring structure would be:
This ring sequence starts with southbound left and through, and ends on the left side of the barrier with northbound left and through. Sequencing on the right side of the barrier is unchanged. More than one alternate sequence can be programmed.
902.5.28 Left Turn Lanes
Guidance. It is nearly always desirable to have left turn lanes at any intersection, whether or not signals are present, if there is a need for that turning movement. The warrants outlined in NCHRP 457 – “Engineering Study Guide for Evaluating Intersection Improvements”, will be used as a basis for determining the need for left turn lanes.
Dual Left Turns
Guidance. It is generally agreed that two left turn lanes is to be considered when the left-turn volume exceeds 300 VPH. Where dual left turn lanes are provided, the second left turn lane has been found to operate at 80% or less of the efficiency of the first left turn lane. A capacity analysis of the intersection is to be made when considering dual left turns. If all the movements operate at a satisfactory level of service, than the dual left turn may not be needed. The installation of dual lefts typically requires expensive geometric and signal modifications.
Geometric considerations must also be carefully reviewed at a potential dual left turn. Ideally, dual lefts are to be two exclusive turn lanes. Using a shared lane for the second left turn lane is to be carefully analyzed before using. A shared through and left lane reduces the capacity for the through movement and usually requires split phasing. Turning radii from each of the left turn lanes is to be designed to accommodate side-by-side turning of the largest vehicles typically using that approach. See Markings for Dual Left Turn Lanes for dual left turn striping. Advance signing for the dual left is recommended.
Where the flow of the dual left turn lanes will carry traffic downstream must be taken into account. There is to be at least two through lanes of traffic of sufficient length to handle the completion of the dual turning lanes. Any lane reduction after a dual left turn is to take place far enough away from the intersection so as to provide adequate merging distances. Turning one lane of the dual left into another downstream turning lane is to be clearly signed in advance of the dual left turn.
902.5.29 Guidelines for Use of the Flashing Yellow Left Arrow (FYA)
|Flashing Yellow Arrow animation|
|Left Turn Phasing Warrants|
|Flashing Yellow Arrow brochure|
|NCHRP Study 493|
|NCHRP Study 123|
Support. This subarticle is a starting point for installations and other steps needed to successfully implement the flashing yellow arrow (FYA) for left turns at signalized intersections. The flashing yellow arrow has replaced the circular green ball for the permissive movement for left turns.
Standard. The turning movement for FYA operation must have a separate signal head. Shared indications are not allowed.
Support. Verify the installation of a stacked 4-section head is possible at a location with an existing 5- or 3-section head. If the turn head has a circular green being shared as the 2nd through indication, then a dedicated signal head will be needed for the 2nd through. Check field wiring to insure there are enough dedicated conductors to drive the new signal heads. More conductors will be needed for a FYA head than a 5-section head, since each indication must be driven from the cabinet and not jumpered off the adjacent through.
Standard. A LEFT TURN YIELD ON FLASHING ARROW (R10-27a) sign shall be mounted adjacent to the head.
When a new signal is being built or where an existing signal is being reconstructed, the FYA shall be used for the permissive left turn movement. All new cabinets shall be configured for FYA operation on all left turns.
Support. Verify the sign can be installed. Any existing left turn sign will need to be removed.
902.5.29.1 Installation Strategy and Guidelines
Guidance. Carefully considering where to first install the FYA can help with public acceptance. The Left Turn Phasing Warrants (safety and capacity) worksheet should be help determine what type of left turn signal phasing should be used.
The FYA will likely require a 16-position backpanel at locations with any current or anticipated signalized pedestrian control and/or dedicated overlaps, where pedestrian channels are put in the last 4 channels. However, the FYA components, if not immediately used, should be deactivated for future use.
Support. Run “Left Turn Phasing warrants” worksheet for safety and capacity warrants. Volume. Collect volume data for each approach where the FYA will be installed. This includes at a minimum, the proposed left turn movement and opposing through volumes during times where phasing may possibly change.
For each FYA approach, enter the relevant data into the “Left Turn Phasing” worksheet to determine 24 hour-by-hour what phasing option can be used. Follow recommended phasing choice for each hour.
If on an approach the number of “protected-only” hours in a day are numerous, then consideration to full-time “protected-only” phasing should be given.
902.5.29.1.1 Removing Protected-Only Turn Phasing: When Not To
Standard. Protected-only left turn phasing shall not be removed if opposing sight distance is inadequate for permissive left turns, operating speed is too great, or there are too many opposing through lanes. The Left Turn Safety Warrants still apply.
Guidance. If the approach passes Left Turn Safety Warrants, exercise engineering judgment before going from protected-only to FYA protected-permissive. Consider whether the protected-only left turn was installed for other safety reasons (crash prevention when under less restrictive phasing), before operating a possible FYA installation protected-permissive.
902.5.29.1.2 Installation – Best Candidates
Guidance. When considering locations for initial installation look for:
- a) Isolated signals OR
- b) Adjacent signals within a coordinated signal corridor within common political boundaries (minimum two signals) AND
- c) One of the following conditions for each of the signals:
- Mainline left turn approach(es) that currently operate protected/permissive.
- Mainline permissive-only left turn approach(es) having the capability to run part-time protected/permissive or protected-only.
Upgrading sidestreet approaches as well at these chosen signals is preferred.
Additional installations could also include:
- Adjacent signals to first installation and possibly across another political boundary
- Another corridor in another political entity well away from other upgraded locations
- Nearby coordinated corridor with heavy protected-permissive left turn phasing
902.5.29.2 Equipment for Reconstructed Signals
- Cabinet. Examine the existing cabinet to determine how many load switch bays are unused and how many pedestrian load switches are in operation. Each FYA approach will require either one open load switch OR one in-use pedestrian load switch (to utilize the unused yellow channel) in addition to the current load switch for a protected permissive turn.
- Cabinet Modification. Contact the cabinet vendor to provide red-lined cabinet prints that will show the needed changes. Once installation is completed, acquire 4 clean prints that reflect the final modifications.
- Controller. Controller must be capable of FYA operation. Some controllers are too old to be upgraded to FYA operation. If utilizing an existing controller, check with controller’s vendor for capability.
- Conflict Monitor. FYA operation requires a special conflict monitor. Check with signal equipment vendors for applicable models. Consider purchasing backup conflict monitors so if the installed model fails, an applicable conflict monitor is immediately available.
902.5.29.3 Equipment for New Construction
- Cabinet. Develop plans with the ultimate number of FYA approaches accounted for. Install the cabinet with an appropriate number of load switches configured for full implementation of both left turn and right turn FYA. This may require a 16-position backpanel for locations with pedestrians and overlap movements.
- Controller and Conflict Monitor. Regardless if Sec 1092 requires FYA logic, including a footnote on the D-37C sheet to reinforce FYA logic is necessary for new controllers and conflict monitors.
- Signal Heads. Make sure FYA indications are noted on the intersection plan sheet and quantity included on D-37A sheet for those approaches where FYA will be installed on the project.
- Other Plan Items. On D37-D sheet, include notation on which indications are to be wired to FYA outputs. On D38-A sheet, make sure the FYA indications are included in the Phasing Sequence chart.
902.5.29.4 Signal Hardware Purchases (Internal Retrofit)
Support. If the decision has been made to upgrade an existing signal by MoDOT forces, then plenty of lead time must be given to have all of the needed equipment on hand and ready to install. The district might elect to utilize a private contractor to do all or some of the retrofit. Purchasing of the equipment and/or contractor services will be done through General Services with detailed information given on the following items:
|St. Louis District contract submittal example|
- Quantity of items
- Traffic Control Plan
- Cabinet print numbers
- Specify Commission furnished vs. contractor furnished items clearly (if any)
- Completion date
- Specific days and times of installation allowed due to local traffic conditions
- Disposition of removed equipment
For a district’s first installation, it is advisable to acquire the upgraded controller and monitor a few weeks in advance of the installation date so in-depth bench testing can be done by all interested parties.
902.5.29.5 Regional Cooperation
Support. Installation of the FYA at another agency’s signals might not be possible. One of the critical requirements of this option is that each lane on the approach have its own signal head. The left turn signal head cannot be sharing its circular indications with the adjacent through lanes. If an agency does not use this type of control, they cannot use the FYA. If the adjacent agency’s signals are using positive lane control, then cooperation in the form of simultaneous FYA installations can be explored.
902.5.30 Signal Indications for Right-Turn Movements – General (MUTCD Section 4D.21)
Support. Right-turning traffic is controlled by one of four modes as follows:
- A. Permissive Only Mode—turns made on a CIRCULAR GREEN signal indication, after yielding to pedestrians, if any.
- B. Protected Only Mode—turns made only when a right-turn GREEN ARROW signal indication is displayed.
- C. Protected/Permissive Mode—both modes occur on an approach during the same cycle.
- D. Variable Right-Turn Mode—the operating mode changes among the protected only mode and/or the protected/permissive mode and/or the permissive only mode during different periods of the day or as traffic conditions change.
Right turns on a CIRCULAR RED signal indication are permitted, after stopping unless a traffic control device is in place prohibiting a turn on red (see Item C.1 in EPG 902.5.10).
Standard. If pedestrians crossing the lane or lanes used by the permissive right-turn movement to depart the intersection are controlled by pedestrian signal heads, the signal indications displayed by those pedestrian signal heads shall not be limited to any particular display during the permissive right-turn movement.
During a protected right-turn movement, the signal faces for left-turn traffic, if any, on the opposing approach shall not simultaneously display a steady left-turn GREEN ARROW or steady left-turn YELLOW ARROW signal indication,. If pedestrians crossing the lane or lanes used by the protected right-turn movement to depart the intersection are controlled by pedestrian signal heads, the pedestrian signal heads shall display a steady UPRAISED HAND (symbolizing DONT WALK) signal indication during the protected right-turn movement.
During the phase where a steady CIRCULAR RED signal indication is displayed for a right-turn movement and conflicting u-turns are allowed during the steady left-turn GREEN ARROW signal indication, in protected only mode, a RIGHT ON RED MUST YIELD TO U-TURN (R10-30) sign shall be installed for the right-turn. (If the conflicting right turn is controlled by a yield sign, a RIGHT TURN MUST YIELD TO U-TURN (R10-30a) sign shall be installed).
When U-turns from a protected only mode for left turns are allowed, a right turn GREEN ARROW signal indication for a conflicting right turn movement shall not be simultaneously displayed.
A protected only mode right-turn movement that does not begin and terminate at the same time as the adjacent through movement shall not be provided on an approach unless an exclusive right-turn lane exists.
A yellow change interval for the right-turn movement shall not be displayed when the status of the right-turn operation is changing from permissive to protected within any given signal sequence.
If the operating mode changes among the protected only mode and/or the protected/permissive mode and/or the permissive only mode during different periods of the day or as traffic conditions change, the requirements in EPG 902.5.31 through EPG 902.5.33 that are appropriate to that mode of operation shall be met, subject to the following:
- A. The CIRCULAR GREEN and CIRCULAR YELLOW signal indications shall not be displayed when operating in the protected only mode.
- B. The right-turn GREEN ARROW and right-turn YELLOW ARROW signal indications shall not be displayed when operating in the permissive only mode.
Option. Additional static signs or changeable message signs may be used to meet the requirements for the variable right-turn mode or to inform drivers that right-turn green arrows will not be available during certain times of the day.
Support. EPG 902.5.30 through EPG 902.5.33 describe the use of separate signal faces for controlling right-turn movements.
Separate right-turn signal face controls only the right-turn movement and cannot serve as one of the two required primary signal faces for the adjacent movement (usually the through movement) because it displays signal indications that are applicable only to the right-turn movement. If a separate right-turn signal face is mounted overhead at the intersection, it is positioned over the extension of the right-turn lane.
EPG 902.5.19 contains provisions regarding the lateral positioning of signal faces that control right-turn movements.
It is not necessary that the same mode of right-turn operation or same type of right-turn signal face be used on every approach to a signalized location. Selecting different modes and types of right-turn signal faces for the various approaches to the same signalized location is acceptable.
Option. A signal face that is shared by left-turning and right-turning traffic may be provided for a shared left-turn/right-turn lane on an approach that has no through traffic (see EPG 902.5.35).
902.5.31 Signal Indications for Permissive Only Mode Right-Turn Movements (MUTCD Section 4D.22)
Standard. If a separate right-turn signal face is being operated in a permissive only right-turn mode and a steady CIRCULAR GREEN signal indication is provided, it shall meet the following requirements (see Fig. 902.5.31):
- A. It shall be capable of displaying a steady CIRCULAR RED, steady CIRCULAR YELLOW, and a steady CIRCULAR GREEN. Only one of the three indications shall be displayed at any giventime.
- If the CIRCULAR RED signal indication is sometimes displayed when the signal faces for the adjacent through lane(s) are not displaying a CIRCULAR RED signal indication, a RIGHT TURN SIGNAL (R10-10R) sign (see EPG 903.5.30) shall be used unless the CIRCULAR RED signal indication in the separate right-turn signal face is shielded, hooded, positioned, or designed such that it is not readily visible to drivers in the through lane(s).
- B. During the permissive right-turn movement, a steady CIRCULAR GREEN signal indication shall be displayed.
- C. A steady CIRCULAR YELLOW signal indication shall be displayed following the steady CIRCULAR GREEN signal indication.
- D. When the separate right-turn signal face is providing a message to stop and remain stopped, a steady CIRCULAR RED signal indication shall be displayed with a NO TURN ON RED (R10-11a) sign, if it is intended that right turns on red not be permitted.
- E. If the permissive only mode is not the only right-turn mode used for the approach, the signal face shall be the same separate right-turn signal face with a steady CIRCULAR GREEN signal indication that is used for the protected/permissive mode (see EPG 902.5.33) except that the right-turn GREEN ARROW signal indication shall not be displayed when operating in the permissive only mode.
902.5.32 Signal Indications for Protected Only Mode Right-Turn Movements (MUTCD Section 4D.23)
Standard. If a separate right-turn signal face is provided for a protected only mode right turn, it shall meet the following requirements (see Fig. 902.5.31):
- A. It shall be capable of displaying a steady CIRCULAR RED, steady right-turn YELLOW ARROW, and right-turn GREEN ARROW. Only one of three indications shall be displayed at any given time. If the CIRCULAR RED signal indication is sometimes displayed when the signal faces for the adjacent through lane(s) are not displaying a CIRCULAR RED signal indication, a RIGHT TURN SIGNAL (R10-10R) sign (see EPG 903.5.30) shall be used unless the CIRCULAR RED signal indication is shielded, hooded, positioned, or designed such that it is not readily visible to drivers in the through lane(s).
- B. During the protected right-turn movement, a right-turn GREEN ARROW signal indication shall be displayed.
- C. A steady right-turn YELLOW ARROW signal indication shall be displayed following the right-turn GREEN ARROW signal indication.
- D. When the separate right-turn signal face is providing a message to stop and remain stopped, a steady CIRCULAR RED signal indication shall be displayed with a NO TURN ON RED (R10-11a) sign, if it is intended that right turns on red not be permitted.
- E. If the protected only mode is not the only right-turn mode used for the approach, the signal face shall be the same separate right-turn signal face that is used for the protected/permissive mode (see EPG 902.5.33) except that a steady CIRCULAR GREEN signal indication shall not be displayed when operating in the protected only mode.
902.5.33 Signal Indications for Protected/Permissive Mode Right-Turn Movements (MUTCD Section 4D.24)
Standard. If a separate right-turn signal face is being operated in a protected/permissive right-turn mode and a steady CIRCULAR GREEN signal indication is provided, it shall meet the following requirements (see Figure 902.5.31).
- A. It shall be capable of displaying a steady CIRCULAR RED, steady CIRCULAR YELLOW, steady CIRCULAR GREEN, steady right-turn YELLOW ARROW, and steady right-turn GREEN ARROW. Only one of the three circular indications shall be displayed at any given time. Only one of the two arrow indications shall be displayed at any given time. If the CIRCULAR RED signal indication is sometimes displayed when the signal faces for the adjacent through lane(s) are not displaying a CIRCULAR RED signal indication, a RIGHT TURN SIGNAL (R10-10R) sign (see EPG 903.5.30) shall be used unless the CIRCULAR RED signal indication in the separate right-turn signal face is shielded, hooded, positioned, or designed such that it is not readily visible to drivers in the through lane(s).
- B. During the protected right-turn movement, a right-turn GREEN ARROW signal indication shall be displayed.
- C. A steady right-turn YELLOW ARROW signal indication shall be displayed following the right-turn GREEN ARROW signal indication.
- D. During the permissive right-turn movement, a steady CIRCULAR GREEN signal indication shall be displayed.
- E. A steady CIRCULAR YELLOW signal indication shall be displayed following the steady CIRCULAR GREEN signal indication if the permissive right-turn movement is being terminated and the separate right-turn signal face will subsequently display a steady red indication.
- F. When a permissive right-turn movement is changing to a protected right-turn movement, a right-turn GREEN ARROW signal indication shall be displayed immediately upon the termination of the steady CIRCULAR GREEN signal indication. A steady CIRCULAR YELLOW signal indication shall not be displayed between the display of the steady CIRCULAR GREEN signal indication and the display of the steady right-turn GREEN ARROW signal indication.
- G. When the separate right-turn signal face is providing a message to stop and remain stopped, a steady CIRCULAR RED signal indication shall be displayed with a NO TURN ON RED (R10-11a) sign, if it is intended that right turns on red not be permitted.
902.5.34 Overlaps and Right Turn Phasing
Support. An "overlap" provides a green or flashing yellow arrow indication for a traffic movement during the green intervals of two or more phases. The overlap green indication can be displayed during the change period between two or more phases if these phases are consecutive. Overlaps are also used to facilitate the operation of flashing yellow arrows for permissive turn movements. An overlap can be integrated into an actuated controller to supplement the flow of traffic. A simple application of an overlap is shown below:
For this case, a through movement is labeled "OLA" (OverLap A). OLA indication can be green while either phase 1 or phase 2 is green. While phase 1 is timing out the change period in transition to phase 2, the OLA indication remains green since OLA follows concurrently. OLA indication is red when phase 3 is green or yellow.
Overlaps need not be used when normal NEMA dual ring structure can be utilized. In the previous example, the overlap is to be assigned to phase 6. A more practical application is for signalizing a right turn movement as shown below:
In this case, the right turn phase is labeled "OLA" (Overlap A). OLA will display a green right while either phase 1 or phase 7 is green and will display a ball green when phase 2 and phase 6 are green. It receives no additional time, since its time comes from phases 1 and 7.
Support. A lane that is shared by left-turn and right-turn movements is sometimes provided on an approach that has no through movement, such as the stem of a T-intersection or where the opposite approach is a one-way roadway in the opposing direction.
Standard. When a shared left-turn/right-turn lane exists on a signalized approach, the left-turn and right-turn movements shall start and terminate simultaneously, and the red signal indication used in each of the signal faces on the approach shall be a CIRCULAR RED.
Support. This requirement for the use of CIRCULAR RED signal indications in signal faces for approaches having a shared lane for left-turn and right-turn movements is a specific exception to other provisions in this article that would otherwise require the use of RED ARROW signal indications.
Standard. The signal faces provided for an approach with a shared left-turn/right-turn lane and no through movement shall be one of the following:
- A. Two or more signal faces, each capable of displaying CIRCULAR RED, CIRCULAR YELLOW, and CIRCULAR GREEN signal indications, shall be provided for the approach. This display shall be permissible regardless of number of exclusive left-turn and/or right-turn lanes that exist on the approach in addition to the shared left-turn/right-turn lane and regardless of whether or not there are pedestrian or opposing vehicular movements that conflict with the left-turn or right-turn movements. However, if there is an opposing approach and the signal phasing protects the left-turn movement on the approach with the shared left-turn/right-turn lane from conflicts with the opposing vehicular movements and any signalized pedestrian movements, a left-turn GREEN ARROW signal indication shall also be included in the left-most signal face and shall be displayed simultaneously with the CIRCULAR GREEN signal indication.
- B. If the approach has one or more exclusive turn lanes in addition to the shared left-turn/right-turn lane and there is no conflict with a signalized vehicular or pedestrian movement, and GREEN ARROW signal indications are used in place of CIRCULAR GREEN signal indications on the approach, the signal faces for the approach shall be:
- 1. A signal face(s) capable of displaying CIRCULAR RED, YELLOW ARROW, and GREEN ARROW signal indications for the exclusive turn lane(s), with the arrows pointing in the direction of the turn, and
- 2. A shared left-turn/right-turn signal face capable of displaying CIRCULAR RED, left-turn YELLOW ARROW, left-turn GREEN ARROW, right-turn YELLOW ARROW, and right-turn GREEN ARROW signal indications, in an arrangement of signal sections that complies with the provisions of EPG 902.5.15 and EPG 902.5.16.
Support. Figs. 902.5.35.1, 902.5.35.2 and 902.5.35.3 illustrate application of these Standards on approaches that have only a shared left-turn/right-turn lane, and on approaches that have one or more exclusive turn lanes in addition to the shared left-turn/right-turn lane.
902.5.36 Signal Timing
Support. Once the proper phasing has been determined for an intersection, the proper timings for the signal indications must be developed in order to function efficiently.
902.5.36.1 Green Interval
902.5.36.1.1 Minimum Green
Guidance. The minimum green is the shortest green time of a phase. Green times that are too short can lead to frequent and needless stops.
The following minimum green times are generally recommended:
- Mainline Through Movement: 10 seconds
- Side street Through Movement: 7 seconds
- Protected Left Turn: 7 seconds
Option. In some cases, the minimums may can be set higher. For heavily traveled mainline throughs lacking back detection, a higher minimum may be desired to reduce the chances of the controller quickly cycling off the mainline green to other phases.
Standard. Minimum green shall not be set below five seconds for green indications.
902.5.36.1.2 Maximum Green
Guidance. Maximum green times are set on an actuated controller as low as possible, but high enough to adequately handle most of the vehicle demands. Maximum greens set too low result in less flexibility in the phase's timings based on detector activity, since there is very little time between the minimum and maximum for the fluctuation in traffic. Maximum greens set too high can result in unnecessary delays during periods of detector failures and increase the delay for other approaches.
The following maximum green times are recommended:
- Mainline Through Movement: 40 to 70 seconds
- Left Turn Movements: 15 to 50 seconds
- Side street Through Movements: 20 to 50 seconds
Observation is the final factor in deciding the proper setting for maximum green. In low volume and/or low speed situations, lower maximums might be advantageous. Some approaches might need more than the usual times, at different times of the day.
Support. Controllers allow for different maximum settings to be enacted through the time clock. This is useful if heavy demand on a certain phase can be accurately predicted and set to a time of day. This is also useful with semi-actuated control where the mainline timing is controlled by a maximum recall since mainline demand typically changes by time of day. Controller clocks should be set by a central signal system if utilized; however, if there is not a central signal system it is recommended to set the controller clock using www.time.gov to remain consistent.
Guidance. Although the concept of a cycle length is usually reserved for pre-timed control and coordinated actuated control, it can be applied to isolated, actuated control. The temptation to set all maximums at an isolated intersection extremely high should be avoided. Maximum settings too high result in longer delay for other approaches and defeat the flexibility of actuated control by creating needless backups. See EPG 902.5.5 Coordination for more discussion on cycle lengths for all types of control.
902.5.36.2 Yellow Change and Red Clearance Intervals (MUTCD Section 4D.26)
Standard. A steady yellow signal indication shall be displayed following every CIRCULAR GREEN or GREEN ARROW signal indication and following every flashing YELLOW ARROW signal indication displayed as a part of a steady mode operation. This requirement shall not apply when a CIRCULAR GREEN or a flashing YELLOW ARROW signal indication is followed immediately by a GREEN ARROW signal indication.
The exclusive function of the yellow change interval shall be to warn traffic of an impending change in the right of way assignment.
Support. The yellow change interval and the red clearance interval (all indications displaying red) are required to prepare the intersection for the transfer of right of way. These intervals permit vehicles that are either within the intersection or so close to it that they cannot comfortably stop to clear the intersection, and to permit those vehicles that can come to a comfortable stop to do so.
|Change Interval Timing Worksheet|
Standard. The total time for the yellow change interval and the red clearance interval is the change period. All change periods shall be updated using the criteria below.
Support. To determine the appropriate yellow change interval and red clearance interval, ITE has developed a Kinematic Model – Formula shown below. The duration of change and clearance intervals, as well as the appropriateness of red clearance intervals, is a topic with no clear consensus. The following formula is developed based on a kinematic model of stopping behavior to determine the duration of the yellow and red indications and is in common use throughout the country.
- CP = nondilemma change period (yellow plus all red), seconds
- t = perception-reaction time, recommended as 1.0 second
- V = approach speed (recommended as 85th or 15th percentile speed), ft/sec
- g = percent grade (positive for upgrade, negative for downgrade), expressed as a decimal
- a = deceleration rate, recommended as 10 ft/sec2
- W = width of intersection from stop line to end of far side crosswalk (if present), ft.
- L = length of vehicle, recommended as 20 ft.
Support. A spot-speed study on an approach to an intersection will produce a range (or distribution) of speeds. The 85th percentile speed is used to determine the yellow change and red clearance interval. A spot-speed study is performed during free flow conditions (typically during off-peak periods) to obtain the highest 85th percentile speed to be used in the equation. It is important, however, to also consider slower traffic going through the intersection at the 15th percentile speed. Low speeds and wide intersections or large left turn radii are a combination that can require a longer change period (yellow plus all-red). For this reason, it might be necessary to calculate the equation using both the 85th and 15th percentile speeds. Engineering judgment is used to determine whether the 85th or 15th percentile speed be used.
Using this equation for approaches with steep downgrades yields such long intervals that they appear unreasonable to drivers as well as the engineer. The remedy is not to ignore the physics of the situation when an unusually long phase change period results from a steep grade or from high approach speeds. The remedy might come from other devices such as warning signs, advanced detection or other countermeasures.
Districts can use the Change Interval Timing Worksheet to calculate the yellow change and red clearance intervals. Keep a printout of the spreadsheet with signal correspondence for future reference.
The sum of the first two terms in the formula is the yellow change interval and the red clearance interval is the last term. The purpose of the yellow change interval is to warn approaching traffic of the imminent change in the assignment of right of way. The first two terms include a reaction time, a deceleration element and approach speed, which are all necessary to determine a yellow change interval that will either allow the driver to come to a stop or proceed through the intersection. The red clearance interval is used to provide additional time following the yellow change interval before conflicting traffic is released. The last term in the formula includes the intersection width, length of vehicle and approach speed, which are all necessary in determining the intersection clearing time.
Standard. The yellow change interval shall not be less than three seconds.
Guidance. The yellow change interval should not exceed six seconds. The red clearance interval should not exceed six seconds.
Option. Based on engineering judgment, adjustments to the yellow change interval and red clearance interval may be made provided for each interval:
- 1) Yellow and all-red are not less than the calculated values using t = 1 second and a = 10 ft/sec2 AND
- 2) Yellow does not go below the appropriate minimum of 3 seconds AND
- 3) Yellow and all-red do not go above the appropriate maximum of 6 seconds OR
- 4) Special conditions exist (e.g. Single Point Urban Interchange, Continuous Flow Interchange, Dual Lefts, etc.).
Standard. If the conditions are such that the district recommends using values that result in a yellow change interval less than what is calculated by the formula (using the values of t = 1 second and a = 10 ft/sec2) and/or a red clearance interval less than what was calculated by using the formula, then a high level of discussion/collaboration with Central Office Traffic Division shall be made.
Support. EPG 902.5.11 contains provisions regarding the display of steady CIRCULAR YELLOW signal indications to approaches from which drivers are allowed to make permissive left turns.
Standard. The durations of yellow change intervals and red clearance intervals shall be consistent with the determined values within the technical capabilities of the controller unit. The duration of a yellow change interval shall not vary on a cycle-by-cycle basis within the same signal timing plan.
The duration of a red clearance interval shall not be decreased or omitted on a cycle-by-cycle basis within the same signal timing plan.
Standard. Except for warning beacons mounted on advance warning signs on the approach to a signalized location (see EPG 903.6.28), signal displays that are intended to provide a “pre-yellow warning” interval, such as flashing green signal indications, vehicular countdown displays, or other similar displays, shall not be used at a signalized location.
Support. The use of signal displays (other than warning beacons mounted on advance warning signs) that convey a “pre-yellow warning” have been found by research to increase the frequency of crashes.
902.5.37 Detector Settings
Support. With actuated control and properly timed detectors, the green time can be distributed to the needed movement and taken away when demand is gone. In order to keep this rotation of phases moving along without dwelling on a movement with little demand, the settings must be programmed to match the type, size and location of the detectors.
902.5.37.1 Stop Bar Detectors
Support. The most common placement of detection senses the vehicle at the stop bar. This location is generally used for minor approaches, mainline turning lanes, and low-speed mainline through lanes. When this location is used, the type of detection is set for "presence". Presence detection allows for a call to be placed to the controller whenever a vehicle is in the zone of detection, and the call is not removed until the vehicle leaves the detected zone.
The setting to control the time to elapse without a vehicle call before changing phases is the "passage" or "gap" time. This timer will reset if a vehicle is detected before the max green has timed out and extend the green indication. If the timer reaches zero before another call is placed, the controller "gaps out" and serves the next phase.
Settings for the passage time vary on how much of a gap to allow between successive vehicles before the following vehicle loses the right of way. Generally, the larger the detection zone the shorter the passage time.
902.5.37.2 Advance Detectors and Gap Reduction
Support. Advance detectors are used for through movements on the major roadway to create a detection zone in advance of the stop bar, especially if the 85th percentile speeds on the major roadway are 45 mph or more. These detectors are typically placed a distance of five seconds behind the stop bar at the 85th percentile speed. Because of the long spacing from the stop bar, sole use of the standard passage time would result in extended green times with large gaps. Gap reduction control provides a means of decreasing the passage time as the speed of the flow increases.
The settings for gap reduction control are (terms may vary from brand to brand, consult controller manual for terminology):
- 1. Passage Time. When used in gap reduction control, passage time becomes the maximum gap time allowed. Its purpose is to provide sufficient time so a vehicle moving at the prevailing speed can travel from the detector to the stop bar.
- 2. Minimum Gap. This is the smallest gap time allowed, and is the lowest time allowed after gap reduction has occurred.
- 3. Time Before Reduction. This setting is the time to postpone the start of gap reduction from passage time to minimum gap time. This time is usually set up to allow any queue to accelerate up to a free flow speed.
- 4. Time To Reduce. After "Time Before Reduction" has reached zero, this timer begins. This is the time allowed for gap reduction to go from its passage time to the minimum gap time. Reduction is linear.
- 5. Added Initial. This setting allows the controller to increase the minimum green time to account for the vehicles stored between the detector and the stop bar at the beginning of the green interval. Without this setting, the minimum green would have to be set high enough to insure all vehicles could clear the intersection. This time is set based on the number of seconds added for each detection while the phase is not green, usually 0.5 to one second added to the programmed minimum per actuation.
The benefit of these settings is best realized when stop bar detection is not used on the approach or is de-activated during the approach green interval.
902.5.37.3 Detection on High-Speed Approaches
Support. With signals installed on high-speed roadways (85th percentile speed of 45 mph or more), a single advance detector may not be able to be placed in the proper location to keep vehicles from the "dilemma zone" conflict. The "dilemma zone" is the area approaching the intersection where drivers first see a yellow indication but is too far away to proceed through the intersection, and too close to stop comfortably. If an advance detector is placed too close to the intersection, it may not detect fast vehicles in time to control them with gap timing. If placed too far back with high gap times, the mainline will be needlessly favored with long green times.
One option using in-ground detection is the placement of two pulse detectors per lane per approach spaced far enough apart to take a high-speed vehicle through the intersection. One detector is placed eight seconds in advance of the stop bar and the second is placed five seconds in advance of the stop bar (based on an operating speed at the 85th percentile). Minimum gap is set to three seconds using gap reduction control.
This allows for a vehicle approaching the intersection at the 85th percentile speed to hit the first detector and extend their call for at least three seconds once gap reduction is finished. At the 85th percentile speed, they will hit the next detector five seconds away from the stop bar, and extend their call another three seconds. After this second extension, the vehicle is two seconds away from the stop bar. This is close enough to allow it to clear during the yellow and red intervals. Any vehicles moving faster than the 85th percentile speed will hit the five second detector before the gap times out, and vehicles traveling slower will gap-out before reaching the five second detector.
Advanced detection can also be helpful to the motorist by enabling the signal to terminate a conflicting phase (when possible an appropriate) while the motorist is still approaching the intersection; thus providing the motorist a “quicker” green light upon arriving at the intersection.
Table 902.5.37.3 is a reference for detector placement.
Table 902.5.37.3 Vehicular Distance Traveled
|Speed||Time, in seconds|
|Distance Traveled in Feet|
902.5.37.4 Delay and Extend Detector Settings
- (A) Delay Settings. When a vehicle travels into the detection zone, the detector amplifier immediately receives the call. In some cases, the call might not be needed immediately. A common situation is a dedicated right turn lane and stopping the opposing direction is usually not needed. A delay is programmed to keep the call from registering in the controller until a certain amount of time has passed. This time might be programmed in some detector processors, or in the controller. After the programmed time has passed, the call is recognized by the controller.
- In NEMA controllers, care is taken as to where the delay time is programmed. If the delay time is set up in the detector processor, then every call going through that detection zone will be delayed. This will cause quick gap-outs if the delay time is near the gap time and no other normal detection is set up for that movement. If delay time is programmed in the controller, then the delay time is for all detectors in a movement, but delay is usually turned off when that movement is green. This will not allow for an immediate call in a lane where detection delay is needed when facing a red indication.
- In 170 controllers, delay time can be programmed independently for each detector input.
- (B) Extend Settings. In other cases, the detector call might need to be longer than the time the vehicle is within the detection zone. An extension time can be programmed into either the detection processor or controller to hold the call for a certain period of time. Once the vehicle leaves the detection zone, the extend timer begins to countdown, and holds the call in until reaching zero. Common applications are where the detection zone is in advance of the stop bar, and the call is needed until the vehicle passes the stop bar. Advance detectors on pulse setting for dilemma zone prevention are another application. Extension time allows the call to stay on while the vehicle clears the intersection.
- As with delay settings, care is taken as to what movements need extension timing.
902.5.37.5 Locking and Non-Locking Detector Setting
Support. When a signal is red for an actuated movement with no recall option, the vehicle detection is registered in the controller whenever a vehicle enters the detection zone. When the vehicle is allowed to leave the intersection before getting a green indication, usually on a right turn on red, it might not be necessary to call that movement if all vehicles have left the detection zone. The detector input for that movement can be set to "non-locking" in order to keep the call from stopping opposing directions. The movement will be served with green if a vehicle remains in the detection zone while set to non-locking. If the movement is set for "locking", then a call remains for that movement until it is served with a green indication, regardless of the presence of vehicles after the initial call.
Commonly, non-locking is used for dedicated right turn lanes, and protected-permissive left turn lanes. Locking is usually for through lanes and protected left turn lanes. Other situations, such as odd detection zone locations, might require a different locking technique. Locking detector setting is also recommended for instances where it has been observed a frequent and recurring problem of motorists “overshooting” the stop bar and leaving the detection zone; thus failing to get served by that phase.
902.5.37.6 Recalled Phases
Support. The use of the recall feature for an approach can increase the potential for drivers to receive a green signal before they reach the intersection, thus minimizing the number of stops. There are three main recall options:
- 1. Min(imum) Recall. This will place a continuous request for service for minimum green on the selected phases. A recalled phase will stay green if no other conflicting calls have been received.
- 2. Max(imum) Recall. This will place a continuous request for service of maximum green on the selected phases. Once the maximum time has expired, the next phase will be served if there is a detected call. It is commonly used for movements without detectors in semi-actuated approaches.
- 3. Soft Recall. This places a continuous request on the selected phase only in the absence of any conflicting calls. Soft recall is only used at fully actuated signals. This is similar to minimum recall, except that a phase with soft recall may be skipped in the absence of actual demand. Soft Recall is typically used when you want the signal to “default” back to a movement in the absence of any traffic.
Guidance. Recommended uses of minimum and soft recall are as follows:
- Min(imum) Recall. Is typically beneficial at fully actuated intersections with an obvious mainline that will need to be served every cycle can be placed under minimum recall. Where detector reliability is an issue, minimum recall is also beneficial
- Soft Recall. Is typically beneficial where volumes are more balanced or less predictable and where the signal has protected left turn phases. Soft recall allows for a more efficient use of the ring structure than minimum recall does.
Using an 8-phase intersection with standard ring structure, and phases 2 and 6 assigned are to the mainline through directions.
There are no vehicles in any detection zone when a vehicle enters the detection zone for phase 4 (sidestreet through).
The signal cycles to serve the phase 4 with minimum green.
Just as the minimum green starts, another vehicle enters the detection zone for phase 3 (sidestreet left), and no vehicle calls on any mainline phases.
- Minimum Recall: After serving phase 4, the signal will cycle to phases 2 and 6 and show at least the minimum green time (or more if any other actuations on that phase), and then serve phase 3.
- Soft Recall: After serving phase 4, the signal will recognize phases 2 and 6 with no vehicle calls and not serve them and go right to phase 3.
902.5.37.7 Detector Call Switching
Support. Controllers allow for detector calls to be transferred to phases other than the ones assigned. This feature is very useful on approaches with protected-permissive left turn phasing and detection on a left turn lane and the opposing through lanes.
Example: Take a standard 4-way intersection with phase 1 as the northbound left turn, phase 6 the northbound through, phase 2 southbound through, and standard ring structure:
Phase 1 is a protected-permissive left turn. After phase 1 gaps or maxes out, phase 2 goes green along with phase 6 until they both max or gap-out. Without detector switching, a vehicle waiting to make a yielding northbound left turn would not be detected and would be susceptible to gap-outs caused by phase 6 detectors, even though time is left in the max timer for a yielding left turn. When detector switching is programmed for phase 1 calls to be transferred to phase 6, the vehicle waiting on the phase 1 detector is then placing a call on phase 6 once phase 1 goes yellow and continues to call phase 6 until leaving the detector or reaching phase 6's max time. Once phase 6 goes yellow, the phase 1 detection returns to phase 1 and allows the protected left turn to come up next cycle.
Advantages to this setting are the reduction of quick changes of phases late at night with sporadic traffic, and the reduction of yielding left-turn conflicts.
902.5.37.8 Phase Re-Service
Support. Phase re-service, or conditional re-service, allows for the standard phase sequence to reverse and display green for non-conflicting directions that have already been served. The most common application is re-serving protected left turns when the opposing throughs gap-out.
Example: Take a standard actuated 4-way intersection with phase 1 a northbound left turn, phase 6 the northbound through, phase 2 southbound through, and standard ring structure:
In this case, the phase 1 indication is a protected-only left turn. After phase 1 is served, phase 2 begins its green time along with phase 6. Re-service of the odd-numbered phase is allowed under these conditions:
- 1. The even phase in the same ring (phase 2 in this example) has gapped out and is resting in green.
- 2. There is a call across the ring barrier to another phase (a side street call for this example).
- 3. The even phase in the opposite ring is still extending and there is enough time left in its max timer. This time must be equal to or greater than the re-serviced phase's minimum green time plus opposing through phase yellow and all-red time.
During the re-service period, gap control is timed by phase 6 detectors and not phase 1 detectors. Therefore, if the northbound through gaps out with demand still present on the northbound left turn, both directions will terminate together.
Advantages to this setting are in reducing the delay for re-serviced left turns. Again, care is to be taken when using this setting at a coordinated intersection. Re-service will not be possible when used with coordinated phases, and available time to re-service side street phases will be almost non-existent. This setting works best at isolated intersections, or when coordinated signals are not running coordination (Free Operation).
902.5.38 Preemption and Priority Control of Traffic Control Signals (MUTCD Section 4D.27)
Option. Traffic control signals may be designed and operated to respond to certain classes of approaching vehicles by altering the normal signal timing and phasing plan(s) during the approach and passage of those vehicles. The alternative plan(s) may be as simple as extending a currently displayed green interval or as complex as replacing the entire set of signal phases and timing.
Support. Preemption control (see definition in MUTCD Section 1A.13) is typically given to trains, boats, emergency vehicles, and light rail transit.
Examples of preemption control include the following:
- A. The prompt displaying of green signal indications at signalized locations ahead of fire vehicles, law enforcement vehicles, ambulances, and other official emergency vehicles;
- B. A special sequence of signal phases and timing to expedite and/or provide additional clearance time for vehicles to clear the tracks prior to the arrival of rail traffic; and
- C. A special sequence of signal phases to display a steady red indication to prohibit turning movements toward the tracks during the approach or passage of rail traffic.
Priority control (see definition in MUTCD Section 1A.13) is typically given to certain non-emergency vehicles such as light-rail transit vehicles operating in a mixed-use alignment and buses.
Examples of priority control include the following:
- A. The displaying of early or extended green signal indications at an intersection to assist public transit vehicles in remaining on schedule, and
- B. Special phasing to assist public transit vehicles in entering the travel stream ahead of the platoon of traffic.
Some types or classes of vehicles supersede others when a traffic control signal responds to more than one type or class. In general, a vehicle that is more difficult to control supersedes a vehicle that is easier to control.
Option. Preemption or priority control of traffic control signals may also be a means of assigning priority right of way to specified classes of vehicles at certain non-intersection locations such as on approaches to one-lane bridges and tunnels, movable bridges, highway maintenance and construction activities, metered freeway entrance ramps and transit operations.
Standard. During the transition into preemption control:
- A. The yellow change interval, and any red clearance interval that follows, shall not be shortened or omitted.
- B. The shortening or omission of any pedestrian walk interval and/or pedestrian change interval shall be permitted.
- C. The return to the previous green signal indication shall be permitted following a steady yellow signal indication in the same signal face, omitting the red clearance interval, if any.
During preemption control and during the transition out of preemption control:
- A. The shortening or omission of any yellow change interval, and of any red clearance interval that follows, shall not be permitted.
- B. A signal indication sequence from a steady yellow signal indication to a green signal indication shall not be permitted.
During priority control and during the transition into or out of priority control:
- A. The shortening or omission of any yellow change interval, and of any red clearance interval that follows, shall not be permitted.
- B. The shortening of any pedestrian walk interval below that time described in EPG 902.6.6 shall not be permitted.
- C. The omission of a pedestrian walk interval and its associated change interval shall not be permitted unless the associated vehicular phase is also omitted or the pedestrian phase is exclusive.
- D. The shortening or omission of any pedestrian change interval shall not be permitted.
- E. A signal indication sequence from a steady yellow signal indication to a green signal indication shall not be permitted.
Guidance. For emergency or railroad preemption, different controllers will call these special movements with different names. Care is to be taken to thoroughly review a controller manual to make sure the programmer has positively identified which setting will accomplish the best preemption routine. Thorough testing, both on the shop bench and at the intersection, is a must before leaving the controller to operate the intersection. Testing is to also be done as part of the yearly preventive maintenance schedule.
Support. The following 8-phase intersection with a railroad crossing is used as an example:
Upon preemption input, the signal will terminate any movement other than southbound through and left (phases 1 & 6). This can be done with a special minimum green that is less than the normal programmed times. The yellow change interval, and any red clearance interval that follows, shall not be shortened or omitted.
If or after these movements have terminated with a red indication, the controller will time out the southbound through and left (phases 1 & 6) for a pre-determined time in order to clear out vehicles which may be left on the tracks. The amount of time given to this movement is critical, and must be sufficient for the slowest of vehicles to get away from the tracks.
After the southbound direction has cleared, and while the preemption input is running, it is allowable for the westbound through and left (phases 3 & 8), eastbound through (phase 4), and northbound protected left (phase 5) to cycle. The protected eastbound left movement cannot be brought up (phase 7). If phase 7 is a protected/permissive left turn, a red indication must be displayed for the left turn movement. This is best accomplished with a FYA display. This special cycle allows for the side street to move along since their movements are not in conflict with the tracks.
After the preemption input has dropped off, northbound and southbound through directions (phases 2 & 6) can be favored with a green indication for a pre-determined time before resuming normal signal operation.
The following 8-phase intersection with a nearby emergency vehicle entrance is used as an example:
Upon preemption input, the emergency driveway signal goes from flashing yellow to solid yellow to solid red. The indications for the driveway will then go ball green. At the controller north of the driveway, the controller begins its preemption routine. The first item to determine is whether to delay the start of the routine. If the emergency vehicle driveway is too far away from the signalized intersection, the routine may expire before the emergency vehicles have cleared the intersection. This delay time must be determined in order to achieve proper operation.
When the preemption routine begins, the signal will terminate any movement other than northbound through and left. This may be done with a special minimum green that is less than the normal programmed times. The yellow change interval, and any red clearance interval that follows, shall not be shortened or omitted.
After other movements have terminated with a red indication, the controller will dwell in northbound left and through green for a time that allows for northbound vehicles between the emergency driveway and the signal to clear out before the arrival of emergency vehicles. It will remain in green for these directions to allow for emergency vehicles to clear the intersection.
After the northbound dwell has timed out, the heavy directions that were stopped is to be brought up with a green indication for a pre-determined time before resuming normal signal operation.
Guidance. Except for traffic control signals interconnected with light rail transit systems, traffic control signals with railroad preemption or coordinated with flashing-light signal systems should be provided with a back-up power supply.
When a traffic control signal that is returning to a steady mode from a dark mode (typically upon restoration from a power failure) receives a preemption or priority request, care should be exercised to minimize the possibility of vehicles or pedestrians being misdirected into a conflict with the vehicle making the request.
Option. During the change from a dark mode to a steady mode under a preemption or priority request, the display of signal indications that could misdirect road users may be prevented by one or more of the following methods:
- A. Having the traffic control signal remain in the dark mode,
- B. Having the traffic control signal remain in the flashing mode,
- C. Altering the flashing mode,
- D. Executing the normal start-up routine before responding, or
- E. Responding directly to initial or dwell period.
Guidance. If a traffic control signal is installed near or within a grade crossing or if a grade crossing with active traffic control devices is within or near a signalized highway intersection, MUTCD Chapter 8C should be consulted.
Traffic control signals operating under preemption control or under priority control should be operated in a manner designed to keep traffic moving.
Traffic control signals that are designed to respond under preemption or priority control to more than one type or class of vehicle should be designed to respond in the relative order of importance or difficulty in stopping the type or class of vehicle. The order of priority should be: train, boat, heavy vehicle (fire vehicle, emergency medical service), light vehicle (law enforcement), light rail transit, rubber-tired transit.
Option. A distinctive indication may be provided at the intersection to show that an emergency vehicle has been given control of the traffic control signal (see Section 11-106 of the Uniform Vehicle Code). In order to assist in the understanding of the control of the traffic signal, a common distinctive indication may be used where drivers from different agencies travel through the same intersection when responding to emergencies.
If engineering judgment indicates that light rail transit signal indications would reduce road user confusion that might otherwise occur if standard traffic signal indications were used to control these movements, light rail transit signal indications complying with MUTCD Section 8C.11 and as illustrated in MUTCD Figure 8C-3 may be used for preemption or priority control of the following exclusive movements at signalized intersections:
- A. Public transit buses in “queue jumper” lanes, and
- B. Bus rapid transit in semi-exclusive or mixed-use alignments.
902.5.38.1 Turn Restrictions During Preemption (MUTCD Section 8B.08)
See EPG 903.20.8 Turn Restrictions During Preemption for information.
902.5.38.2 Traffic Control Signals at or Near Highway-Rail Grade Crossings (MUTCD Section 8C.09)
Standard. The appropriate provisions of EPG 902 relating to traffic control signal design, installation and operation shall be applicable where traffic control signals are used to control road users instead of flashing-light signals at highway-rail grade crossings.
Traffic control signals shall not be used instead of flashing-light signals to control road users at a mainline highway-rail grade crossing.
Guidance. If a highway-rail grade crossing is equipped with a flashing-light signal system and is located within 200 feet of an intersection or midblock location controlled by a traffic control signal, the traffic control signal should be provided with preemption in accordance with EPG 902.5.38 Preemption and Priority Control of Traffic Control Signals.
Coordination with the flashing-light signal system, queue detection, or other alternatives should be considered for traffic control signals located farther than 200 ft. from the highway-rail grade crossing. Factors to be considered should include traffic volumes, highway vehicle mix, highway vehicle and train approach speeds, frequency of trains and queue lengths.
The highway agency or authority with jurisdiction and the regulatory agency with statutory authority, if applicable, should jointly determine the preemption operation and the timing of traffic control signals interconnected with highway-rail grade crossings adjacent to signalized highway intersections.
Standard. Information regarding the type of preemption and any related timing parameters shall be provided to the railroad company so that they can design the appropriate train detection circuitry.
If preemption is provided, the normal sequence of traffic control signal indications shall be preempted upon the approach of trains to avoid entrapment of highway vehicles on the highway-rail grade crossing.
This preemption feature shall have an electrical circuit of the closed-circuit principle, or a supervised communication circuit between the control circuits of the highway-rail grade crossing warning system and the traffic control signal controller. The traffic control signal controller preemptor shall be activated via the supervised communication circuit or the electrical circuit that is normally energized by the control circuits of the highway-rail grade crossing warning system. The approach of a train to a highway-rail grade crossing shall de-energize the electrical circuit or activate the supervised communication circuit, which in turn shall activate the traffic control signal controller preemptor. This shall establish and maintain the preemption condition during the time the highway-rail grade crossing warning system is activated, except that when crossing gates exist, the preemption condition shall be maintained until the crossing gates are energized to start their upward movement. When multiple or successive preemptions occur, train activation shall receive first priority.
Guidance. If a highway-rail grade crossing is located within 50 ft. (or within 75 ft. for a highway that is regularly used by multi-unit highway vehicles) of an intersection controlled by a traffic control signal, the use of pre-signals to control traffic approaching the grade crossing should be considered.
Standard. If used, the pre-signals shall display a steady red signal indication during the track clearance portion of a signal preemption sequence to prohibit additional highway vehicles from crossing the railroad track.
Guidance. Consideration should be given to using visibility-limited signal faces (see definition in EPG 900.1.13) at the intersection for the downstream signal faces that control the approach that is equipped with pre-signals.
Option. The pre-signal phase sequencing may be timed with an offset from the downstream signalized intersection such that the railroad track area and the area between the railroad track and the downstream signalized intersection is generally kept clear of stopped highway vehicles.
Standard. If a pre-signal is installed at an interconnected highway-rail grade crossing near a signalized intersection, a STOP HERE ON RED (R10-6) sign shall be installed near the pre-signal or at the stop line if used. If there is a nearby signalized intersection with insufficient clear storage distance for a design vehicle, or the highway-rail grade crossing does not have gates, a No Turn on Red (R10-11, R10-11a, or R10-11b) sign (see MUTCD Section 2B.53) shall be installed for the approach that crosses the railroad track, if applicable.
Option. At locations where a highway-rail grade crossing is located more than 50 ft. (or more than 75 ft. for a highway regularly used by multi-unit highway vehicles) from an intersection controlled by a traffic control signal, a pre-signal may be used if an engineering study determines a need.
If highway traffic signals must be located within close proximity to the flashing-light signal system, the highway traffic signals may be mounted on the same overhead structure as the flashing-light signals.
Support. EPG 902.3.11 Warrant 9, Intersection Near a Grade Crossing describes the Intersection Near a Grade Crossing signal warrant that is intended for use at a location where the proximity to the intersection of a grade crossing on an intersection approach controlled by a STOP or YIELD sign is the principal reason to consider installing a traffic control signal.
EPG 902.5.38 Preemption and Priority Control of Traffic Control Signals describes additional considerations regarding preemption of traffic control signals at or near highway-rail grade crossings.
902.5.38.3 Traffic Control Signals at or Near Highway-LRT Grade Crossings (MUTCD Section 8C.10)
Support. There are two types of traffic control signals for controlling vehicular and LRT movements at interfaces of the two modes. The first is the standard traffic control signal described in EPG 902, which is the focus of this Section. The other type of signal is referred to as an LRT signal and is discussed in EPG 902.5.38.4 Use of Traffic Control Signals for Control of LRT Vehicles at Grade Crossings.
Standard. The provisions of EPG 902 Signals and EPG 902.5.38.2 relating to traffic control signal design, installation and operation, including interconnection with nearby automatic gates or flashing-light signals, shall be applicable as appropriate where traffic control signals are used at highway-LRT grade crossings.
If traffic control signals are in operation at a crossing that is used by pedestrians, bicyclists, and/or other non-motorized road users, an audible device such as a bell shall also be provided and shall be operated in conjunction with the traffic control signals.
Guidance. When a highway-LRT grade crossing equipped with a flashing-light signal system is located within 200 ft. of an intersection or midblock location controlled by a traffic control signal, the traffic control signal should be provided with preemption in accordance with EPG 902.5.38.
Coordination with the flashing-light signal system should be considered for traffic control signals located more than 200 ft. from the crossing. Factors to be considered should include traffic volumes, highway vehicle mix, highway vehicle and LRT approach speeds, frequency of LRT traffic, and queue lengths.
If the highway traffic signal has emergency-vehicle preemption capability, it should be coordinated with LRT operation.
Where LRT operates in a wide median, highway vehicles crossing the tracks and being controlled by both near and far side traffic signal faces should receive a protected left-turn green phase from the far side signal face to clear highway vehicles from the crossing when LRT equipment is approaching the crossing.
Option. Green indications may be provided during LRT phases for highway vehicle, pedestrian and bicycle movements that do not conflict with LRT movements.
Traffic control signals may be installed in addition to four-quadrant gate systems and automatic gates at a highway-LRT crossing if the crossing occurs within a highway-highway intersection and if the traffic control signals meet the warrants described in EPG 902.3 Traffic Control Signal Needs Studies.
At a location other than an intersection, when LRT speeds are less than 25 mph, traffic control signals alone may be used to control road users at highway-LRT grade crossings only when justified by an engineering study.
Typical circumstances may include:
- A. Geometric conditions preclude the installation of highway-LRT grade crossing warning devices.
- B. LRT vehicles share the same roadway with road users.
- C. Traffic control signals already exist.
Support. EPG 902.5.38 contains information regarding traffic control signals at or near highway-LRT grade crossings that are not equipped with highway-LRT grade crossing warning devices.
EPG 902.3.11 Warrant 9, Intersection Near a Grade Crossing describes the Intersection Near a Grade Crossing signal warrant that is intended for use at a location where the proximity to the intersection of a grade crossing on an intersection approach controlled by a STOP or YIELD sign is the principal reason to consider installing a traffic control signal.
Guidance. When a highway-LRT grade crossing exists within a signalized intersection, consideration should be given to providing separate turn signal faces (see definition in EPG 900.1.13) for the movements crossing the tracks.
Standard. Separate turn signal faces that are provided for turn movements toward the crossing shall display a steady red indication during the approach and/or passage of LRT traffic.
Guidance. When a signalized intersection that is located within 200 ft. of a highway-LRT grade crossing is preempted, all existing turning movements toward the highway-LRT grade crossing should be prohibited.
Support. EPG 902.5.38.1 contains information regarding the prohibition of turning movements toward the crossing during preemption.
EPG 902 Signals contains information regarding signal phasing and timing requirements.
902.5.38.4 Use of Traffic Control Signals for Control of LRT Vehicles at Grade Crossings (MUTCD Section 8C.11)
Guidance. LRT movements in semi-exclusive alignments at non-gated grade crossings that are equipped with traffic control signals should be controlled by special LRT signal indications. See MUTCD Section 8C.11 for more information regarding these LRT signal indications.
Support. EPG 902.5.38 contains information about the use of LRT signal indications shown in MUTCD Figure 8C-3 for the control of exclusive bus movements at “queue jumper lanes” and for the control of exclusive bus rapid transit movements on semi-exclusive or mixed-use alignments.
Option. Standard traffic control signals may be used instead of LRT traffic control signals to control the movement of LRT vehicles, see EPG 902.5.38.3.
Standard. If a separate set of standard traffic control signal indications (red, yellow, and green circular and arrow indications) is used to control LRT movements, the indications shall be positioned so they are not visible to motorists, pedestrians, and bicyclists, see EPG 902.5.18 Visibility, Aiming, and Shielding of Signal Faces.
If the LRT crossing control is separate from the intersection control, the two shall be interconnected. The LRT signal phase shall not be terminated until after the LRT vehicle has cleared the crossing.
902.5.39 Flashing Operation of Traffic Control Signals – General (MUTCD Section 4D.28)
Standard. The light source of a flashing signal indication shall be flashed continuously at a rate of not less than 50 or more than 60 times per minute.
The displayed period of each flash shall be a minimum of 1/2 and a maximum of 2/3 of the total flash cycle.
Flashing signal indications shall comply with the requirements of other Sections of this Manual regarding visibility-limiting or positioning of conflicting signal indications, except that flashing yellow signal indications for through traffic shall not be required to be visibility-limited or positioned to minimize visual conflict for road users in separately controlled turn lanes.
Each traffic control signal shall be provided with an independent flasher mechanism that operates in compliance with this article.
The flashing operation shall not be terminated by removal or turn off of the controller unit or of the conflict monitor (malfunction management unit) or both.
A manual switch, a conflict monitor (malfunction management unit) circuit, and, if appropriate, automatic means shall be provided to initiate the flashing mode.
Support. There are two types flashing operation: all-direction malfunction flash and all-direction programmed flash.
All-direction malfunction flash is the flash operation that occurs when the signalized intersection is operating inappropriately and is initiated by the conflict monitor. All-direction programmed flash is the flash operation that is programmed into the controller to flash the signal during specific times every day.
Standard. All-direction malfunction flash is permitted to be made at any time.
Actuated intersections shall not have all-direction programmed flash, they shall be operational at all times. For pretimed intersections, every effort shall be made to upgrade the signalized intersection from pretimed to actuated operation.
Guidance. Several factors should be used with good engineering judgment when making the decision to implement all-direction programmed flashing operation. Among these is crash history and sight distance. If correctable crashes were a significant warrant for installation, then all-direction flashing operation should not be acceptable at any time.
Standard. If sight distance is below minimum sight distances shown in Table 903.6.28 Requirements for Advance Traffic Control Sign Placements, then no flashing operation shall be considered.
Guidance. If an engineering study supports the use of all-direction programmed flash at a fully actuated intersection, then consideration can be given to flashing isolated fully actuated signals yellow/red where high-speed approaches are involved. It has been common to leave a fully actuated signal operating free at all times. However, if sight distance is adequate and mainline volumes are light, then yellow/red all-direction flash can reduce the chances for crashes since mainline traffic will not have to stop for a single vehicle on an opposing approach.
Fully actuated signals within a coordinated system should be analyzed for possible free operation when other signals within the system are on all-direction flash. It can be more beneficial to all-direction flash the fully actuated signal yellow/red with the other signals in order to be consistent with the rest of the system.
Support. An in-house MoDOT safety study identified that crashes at locations with late night/early morning flash tend to occur before 2 a.m. and in FHWA’s publication Signalized Intersections: Informational Guide (FHWA-HRT-04-091), it was found that that an estimated 78% reduction in right-angle collisions and an estimated 32% reduction in all collisions can be expected with the removal from late night/early morning flash mode.
Support. EPG 902.6 Pedestrian Control Features and EPG 902.6.9 Accessible Pedestrian Signals and Detectors – General contain information regarding the operation of pedestrian signal heads and accessible pedestrian signal detector pushbutton locator tones, respectively, during flashing operation.
902.5.40 Flashing Operation – Transition Into Flashing Mode (MUTCD Section 4D.29)
Standard. The transition from steady (stop-and-go) mode to flashing mode, if initiated by a conflict monitor (malfunction management unit – “MMU”) (malfunction flash) or by a manual switch, shall be permitted to be made at any time it is in fail safe operation.
Programmed changes (programmed flash) from steady (stop-and-go) mode to flashing mode shall be made under either of the following circumstances:
- A. At the end of the common major-street red interval (such as just prior to the start of the green in both directions on the major street), or
- B. Directly from a CIRCULAR GREEN signal indication to a flashing CIRCULAR YELLOW signal indication, or from a GREEN ARROW signal indication to a flashing YELLOW ARROW signal indication, or from a flashing YELLOW ARROW signal indication (see EPG 902.5.23 to EPG 902.5.33) to a flashing YELLOW ARROW signal indication in a different signal section.
During programmed changes into flashing mode, no green signal indication or flashing yellow signal indication shall be terminated and immediately followed by a steady red or flashing red signal indication without first displaying the steady yellow signal indication.
902.5.41 Flashing Operation – Signal Indications During Flashing Mode (MUTCD Section 4D.30)
Guidance. When a traffic control signal is operated in the flashing mode, a flashing yellow signal indication should be used for the major street and a flashing red signal indication should be used for the other approaches unless flashing red signal indications are used on all approaches.
Yellow/red flash is generally used for intersections where traffic volumes during flashing operation greatly favor one direction. Yellow indications are flashed for the major street and red flashed for the minor street.
Red/red all-direction flashing operation is used for intersections where volumes during flashing operation on all approaches are approximately equal. Complex intersections (5-leg, large widths, multiple dual left turn approaches) will likely require red/red flash, regardless of volumes.
When a system of interconnected signals is flashed, care is to be taken to be consistent in flash color for the major street so as not to violate driver expectancy. A red/red flashing signal in the middle of a system of yellow/red flashing signals could lead to violations of the flashing red along the major street due to the expectancy of yellow flashing indications.
Fail-safe operation for a malfunctioning signal leads to all-direction flashing operation when tripped by a conflict monitor. Cabinet wiring determines all-direction flashing operation when a malfunction has occurred, this generally means malfunction flash will be red/red since it is wired the same as the start up from dark flash (see EPG 902.5.43 Power Outages at Signalized Intersections).
Standard. When a traffic control signal is operated in the flashing mode, all of the green signal indications at the signalized location shall be dark (non-illuminated) and shall not be displayed in either a steady or flashing manner, except for single-section GREEN ARROW signal indications as provided elsewhere in this subarticle.
Flashing yellow signal indications shall be used on more than one approach to a signalized location only if those approaches do not conflict with each other.
When a traffic control signal is operated in the flashing mode, one and only one signal indication in every signal face at the signalized location shall be flashed.
Standard. No steady indications, other than a single-section signal face consisting of a continuously-displayed GREEN ARROW signal indication that is used alone to indicate a continuous movement in the steady (stop-and-go) mode, shall be displayed at the signalized location during the flashing mode. A single-section GREEN ARROW signal indication shall remain continuously-displayed when the traffic control signal is operated in the flashing mode.
If a signal face includes both circular and arrow signal indications of the color that is to be flashed, only the circular signal indication shall be flashed. A common application of this is a five-section head with a yellow arrow and circular yellow. The circular yellow shall be flashed, and the yellow arrow remain dark.
All signal faces that are flashed on an approach shall flash the same color, either yellow or red, except that separate turn signal faces (see EPG 902.5.23 and EPG 902.5.30) shall be permitted to flash a CIRCULAR RED or RED ARROW signal indication when the adjacent through movement signal indications are flashed yellow.
The appropriate RED ARROW or YELLOW ARROW signal indication shall be flashed when a signal face consists entirely of arrow indications. A signal face that consists entirely of arrow indications and that provides a protected only turn movement during the steady (stop-and-go) mode or that provides a flashing YELLOW ARROW signal indication for a permissive turn movement during the steady (stop-and-go) mode shall be permitted to flash the YELLOW ARROW signal indication during the flashing mode if the adjacent through movement signal indications are flashed yellow and if it is intended that a permissive turn movement not requiring a full stop by each turning vehicle be provided during the flashing mode.
Support. For dual or protected-only left turns programmed to flash, the indications should flash yellow arrow when the purpose of the phasing was high volume. When the phasing is in place for safety reasons, the indications for the left should flash red.
Whether the signal flashes by cabinet wiring or controller programming, care is to be taken to not flash all indications at the same time. This causes a severe load fluctuation that reduces the life of the electrical equipment and can lead to malfunctions when the signal is in operation.
902.5.42 Flashing Operation – Transition Out of Flashing Mode (MUTCD Section 4D.31)
Standard. All changes from flashing mode to steady (stop-and-go) mode shall be made under one of the following procedures:
- A. Yellow-red flashing mode: Changes from flashing mode to steady (stop-and-go) mode shall be made at the beginning of the major-street green interval (when a green signal indication is displayed to through traffic in both directions on the major street), or if there is no common major-street green interval, at the beginning of the green interval for the major traffic movement on the major street.
- B. Red-red flashing mode: Changes from flashing mode to steady (stop-and-go) mode shall be made by changing the flashing red indications to steady red indications followed by appropriate green indications to begin the steady mode cycle. These green indications shall be the beginning of the major-street green interval (when a green signal indication is displayed to through traffic in both directions on the major street) or if there is no common major-street green interval, at the beginning of the green interval for the major traffic movement on the major street.
Standard. The steady red clearance interval provided during the change from red-red flashing mode to steady (stop-and-go) mode should have a duration of 6 seconds.
Guidance. When changing from the yellow-red flashing mode to steady (stop-and-go) mode, if there is no common major-street green interval, the provision of a steady red clearance interval for the other approaches before changing from a flashing yellow or a flashing red signal indication to a green signal indication on the major approach should be considered.
Standard. During programmed changes out of flashing mode, no flashing yellow signal indication shall be terminated and immediately followed by a steady red or flashing red signal indication without first displaying the steady yellow signal indication.
Option. Because special midblock signals that rest in flashing circular yellow in the position normally occupied by the green signal indication do not have a green signal indication in the signal face, these signals may go directly from flashing circular yellow (in the position normally occupied by the green signal indication) to steady yellow without going first to a green signal indication.
902.5.43 Power Outages at Signalized Intersections
Standard. Each district shall develop a power outage plan for signalized intersections that includes information about the installation, use, and recovery of Temporary Stop Signs (TSS) and the installation of battery backup systems. This subarticle provides information for these items.
902.5.43.1 Temporary Stop Signs at Signalized Intersections
Support. Temporary Stop Signs (TSS) refer to stop signs that meet the MUTCD stop sign design requirements for regulatory signs and are temporarily installed at signalized intersections where the traffic signals cannot function due to damage and/or power outage. These temporary placements include but are not limited to roll-up stop signs, temporary mounts on the signal vertical upright, or stop signs mounted on other crash worthy devices.
Standard. Such signs shall remain at the intersection until power at the non-functioning signalized intersection has been restored (see EPG 902.5.43.1.4 Recovery).
902.5.43.1.1 Conditions For Use
Guidance. TSS should be erected at locations where a signalized intersection is non-functioning. A non-functioning signalized intersection is defined as an intersection that is equipped with a traffic signal that is damaged and/or without power which cannot display proper indications to control traffic.
After verifying that the signal is non-functioning, Districts should contact the appropriate utility company to notify them of the power outage, if applicable, and to determine if power will be restored in a reasonable amount of time (at the District’s discretion). In which case, TSS are not required. If used, the TSS should be deployed as soon as practical depending on location of the signalized intersection and the stored TSS. Districts should also request police assistance for traffic control if they are not already present at the site or aware of the power outage. Outside of normal business hours, it might be necessary for the electrician or maintenance personnel to directly contact the highway patrol or local police and the power company. When a signalized intersection is non-functioning, then TSS should be installed when one of the following conditions is met:
- When the traffic signal is both damaged and without power, or
- When the traffic signal is without power and restoration of power using an alternate power source is not possible.
Standard. When TSS are utilized at a signalized intersection that is non-functioning, the District shall decide whether the power shall be disconnected or whether the signal should be switched to flash to avoid conflicts when power is restored. If switched to flash, the flash shall be red-red since TSS will be installed on all approaches, if used, at a signalized intersection without power (dark signals are to be treated like a 4-way stop according to the Missouri Driver’s Guide). The TSS shall not be displayed at the same time as any signal indication is displayed other than a flashing red.
A request shall be made of the nearest maintenance building, emergency responder, or external emergency responder (whomever stores the TSS) to bring stop signs to the intersection. Personnel or emergency responders instructed in signal operation shall disconnect the power or switch the signal to flash operation (external emergency responders will do this in the signal cabinet police door) before placing the TSS. Without this change in operation, the traffic signal could return to steady (stop-and-go) mode within seconds after the signal is repaired or power is restored, which would cause conflicts between the signal and the TSS (conflicting green or yellow indications with a stop sign for the same approach). The signal shall be visible to traffic on all approaches and all these approaches will flash upon restoration of power (see EPG 902.5.43.2 for more information regarding Startup from Dark).
Guidance. When law enforcement is present at a non-functioning signalized intersection to direct traffic, then the TSS that have been placed should be covered or removed to avoid conflicts (the law enforcements authority supersedes the TSS).
Option. If it has been determined that the power outage will last for an extended amount of time (at the district’s discretion) the signal heads may be covered to reduce the confusion of approaching motorists.
Guidance. If signal heads are covered, the appropriate enforcement agency should be advised and asked to occasionally monitor the intersection. Also, the power company should be advised and asked to notify proper personnel when the power is restored.
902.5.43.1.2 Location and Placement
Standard. The signalized intersection locations for installation of TSS shall meet the conditions of use in EPG 902.5.43.1.1 and shall be at the discretion of the district. Each District shall develop a list of critical signalized intersections to establish a priority for TSS installation. The TSS prioritized installation list developed by each district shall be in each district’s Power Outage Plan.
Guidance. The installation of TSS should begin at the identified critical intersections and should be prioritized as follows (as applicable to each district):
- 1. Signals with railroad preemption
- 2. Signals with a speed limit greater than 50 mph
- 3. Signals with a high accident rate
- 4. Intersections difficult to flag or require multiple flaggers (non-routine roadway configurations/geometry, SPUIs, multi-lane approaches, etc.)
- 5. Signals with high volumes (freeway type off-ramps, major roadways, etc.)
- 6. Signals with frequent power outages
- 7. Signals located at schools.
As battery backup systems are installed (see EPG 902.5.43.3 Battery Backup Systems at Signalized Intersections) at signalized intersections, districts should re-evaluate their list of prioritized intersections for the installation of TSS.
Standard. On all roadways, TSS shall be placed in a location where they are visible to all lanes. On two-way roadways, stop signs shall be erected on the right-hand side of all approaches. On divided highways, stop signs shall be erected on both the right and, if possible, on the left-hand side of all approaches.
Guidance. If the power outage is widespread, additional personnel should be requested to help with the placement of the signs.
902.5.43.1.3 Storage and Distribution
Guidance. Each District should store enough TSS to be deployed at the critical signalized intersections identified in the District Power Outage Plan.
Standard. TSS shall be distributed by the district to the district’s maintenance personnel or emergency responders or external emergency responders on an as-needed basis. It shall be the responsibility of the district to develop a means of distribution.
Standard. TSS shall remain at the intersection until power at the non-functioning signalized intersection has been restored. Power will remain disconnected or the signal will flash until TSS are removed. Immediately following TSS removal, personnel or emergency responders instructed in signal operation shall restore signal operation in accordance with the procedures set forth in EPG 902.5.43.2 Steady (stop-and-go) Mode for transition to steady (stop-and-go) mode.
The recovery of the TSS shall be accomplished by using the district’s maintenance personnel or emergency responders or external emergency responders by either of the following:
- Complete removal from each intersection.
- Stockpiling outside of the intersection to avoid conflicts with the signalized intersection (stockpiled signs shall not be faced towards the traveling public and stored not to damage sheeting) and stored in a location to not become a roadside hazard.
A detailed recovery plan for each intersection with TSS shall be developed in the district’s Power Outage Plan.
902.5.43.2 Start up from Dark at Signalized Intersections
Standard. When a signalized intersection has been damaged and/or is without power the district shall have either disconnected the power or switched the signal to flash to avoid conflicts when power is restored. If switched to flash, the flash shall be red-red since TSS will be installed on all approaches, if used, at a signalized intersection without power (dark signals are to be treated like a 4-way stop according to the Missouri Drive’s Guide). If TSS are in place, the power shall remain disconnected or the signal shall operate in flash mode until TSS are removed and personnel or emergency responders instructed in signal operation restore signal operation.
Steady (stop-and-go) Mode
Standard. When power is reconnected or when the signal is switched from flash to steady (stop-and-go) mode, the controllers shall be programmed for startup from flash. The signal shall flash red-red for 7 seconds and then change to steady red clearance for 6 seconds followed by beginning of major-street green interval or if there is no common major-street green interval, at the beginning of the green interval for the major traffic movement on the major street.
902.5.43.3 Battery Backup Systems at Signalized Intersections
Standard. Each district shall develop a list of critical signalized intersections to establish a priority for the installation of Battery Backup Systems (BBS) as part of the district’s Power Outage Plan.
Guidance. The installation of BBS should begin at the identified critical intersections and should be prioritized as follows (as applicable to each district):
- 1. Signals with railroad preemption
- 2. Signals with a speed limit greater than 50 mph
- 3. Signals with a high accident rate
- 4. Intersections difficult to flag or require multiple flaggers (non-routine roadway configurations/geometry, SPUIs, multi-lane approaches, etc.)
- 5. Signals with high volumes (freeway type off-ramps, major roadways, etc.)
- 6. Signals with frequent power outages
- 7. Signals located at schools.
Each district’s prioritized installation list for BBS should be based on their traffic conditions and needs. The prioritized TSS installation list will need to be reevaluated as BBS are installed.
Standard. BBS shall be capable of operating at a minimum of 2 hours in steady (stop-and-go) mode and a minimum of 2 hours in flash operation.
Guidance. Any signalized intersection with BBS should have a generator socket for extended operation.
902.5.44 Temporary and Portable Traffic Control Signals (MUTCD Section 4D.32)
Support. A temporary traffic control signal is generally installed using methods that minimize the costs of installation, relocation, and/or removal. Typical temporary traffic control signals are for specific purposes, such as for one-lane, two-way facilities in temporary traffic control zones (see EPG 902.9), for a haul-road intersection, or for access to a site that will have a permanent access point developed at another location in the near future.
Standard. Advance signing shall be used when employing a temporary traffic control signal. A temporary traffic control signal shall:
- A. Meet the physical display and operational requirements of a conventional traffic control signal.
- B. Be removed when no longer needed.
- C. Be placed in the flashing mode when not being used if it will be operated in the steady mode within 5 working days; otherwise, it shall be removed.
- D. Be placed in the flashing mode during periods when it is not desirable to operate the signal, or the signal heads shall be covered, turned, or taken down to indicate that the signal is not in operation. Flash times outlined in EPG 902.5.39 are not applicable to temporary traffic signals.
Guidance. A temporary traffic control signal should be used only if engineering judgment indicates that installing the signal will improve the overall safety and/or operation of the location.
The use of temporary traffic control signals by a work crew on a regular basis in their work area should be subject to the approval of the jurisdiction having authority over the roadway.
A temporary traffic control signal should not operate longer than 30 days unless associated with a longer-term temporary traffic control zone project.
For use of temporary traffic control signals in temporary traffic control zones, reference should be made to MUTCD Section 6F.84.
902.5.45 Lateral Offset of Signal Supports and Cabinets (MUTCD Section 4D.33)
Guidance. The following items should be considered when placing signal supports and cabinets:
- A. Reference should be made to the American Association of State Highway and Transportation Officials (AASHTO) Roadside Design Guide (see MUTCD Section 1A.11) and to the Americans with Disabilities Act Accessibility Guidelines for Buildings and Facilities (ADAAG).
- B. Signal supports should be placed as far as practical from the edge of the traveled way without adversely affecting the visibility of the signal indications.
- C. Where supports cannot be located based on the recommended AASHTO clearances, consideration should be given to the use of appropriate safety devices.
- D. In order to minimize hindrance to the passage of persons with physical disabilities, a signal support or controller cabinet should not obstruct the sidewalk, or access from the sidewalk to the crosswalk.
- E. Controller cabinets should be located as far as practical from the edge of the roadway.
- F. On medians, the minimum clearances provided in Items A through E for signal supports should be obtained if practical.
902.5.46 Use of Signs at Signalized Locations (MUTCD 4D.34)
Support. Traffic signal signs are sometimes used at highway traffic signal locations to instruct or guide pedestrians, bicyclists, or motorists. Among the signs typically used at or on the approaches to signalized locations are movement prohibition signs (see EPG 903.5.11), lane control signs (see EPG 903.5.12 to 903.5.22), pedestrian actuation signs (see EPG 903.5.30), traffic signal signs (see EPG 903.5.30 and EPG 903.6.39), Signal Ahead warning signs (see EPG 903.6.28), Street Name signs (see EPG 903.7.41), and Advance Street Name signs (see EPG 903.7.42).
Traffic signal signs should be located adjacent to the signal face to which they apply.
Support. EPG 903 contains information regarding the use of overhead lane control signs on signalized approaches where lane drops, multiple-lane turns involving shared through-and-turn lanes, or other lane-use regulations that would be unexpected by unfamiliar road users are present.
Guidance. MoDOT does not use illuminated traffic signal signs. But, if they are requested, engineering judgment should be used to determine if installation is appropriate and the contractual process should be followed between MoDOT and the requestor.
Standard. If used, illuminated traffic signal signs shall be designed and mounted in such a manner as to avoid glare and reflections that seriously detract from the signal indications. Traffic control signal faces shall be given dominant position and brightness to maximize their priority in the overall display.
STOP signs shall not be used in conjunction with any traffic control signal operation, except in either of the following cases:
- A. If the signal indication for an approach is a flashing red at all times, or
- B. If a minor street or driveway is located within or adjacent to the area controlled by the traffic control signal, but does not require separate traffic signal control because an extremely low potential for conflict exists.
902.5.47 Use of Pavement Markings at Signalized Locations (MUTCD Section 4D.35)
Support. Pavement markings (see EPG 620 Pavement Marking) that clearly communicate the operational plan of an intersection to road users play an important role in the effective operation of traffic control signals. By designating the number of lanes, the use of each lane, the length of additional lanes on the approach to an intersection, and the proper stopping points, the engineer can design the signal phasing and timing to best match the goals of the operational plan.
Guidance. Pavement markings should be used at traffic control signal locations as provided in EPG 620. If the road surface will not retain pavement markings, signs should be installed to provide the needed road user information.