View source for 902.23 Traffic Signal Phasing and Operation

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==902.23.1 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.

===902.23.1.1 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. 

===902.23.1.2 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 [[#902.23.12.1 Controller Unit|EPG 902.23.12.1 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 [[#902.23.12.1 Controller Unit|EPG 902.23.12.1 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 [[#902.23.12.1 Controller Unit|EPG 902.23.12.1 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 [http://sp/sites/sl/programdelivery/design/SignalsandLighting/Shared%20Documents/Signal_Phasing_and_Layout_Examples.pdf#search=signal%20phasing%20layout 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 [[#902.5.7 Detectors|EPG 902.5.7]] and [[902.15 Designing a Traffic Signal#902.15.3 Detectors|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.

===902.23.1.3 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. 

===902.23.1.4 Comparison of Pre-timed and Actuated Control===

'''Support.''' Each principal type of traffic signal control, pre-timed and actuated, possesses 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.

===902.23.1.5 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. 

===902.23.1.6 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.

=={{SpanID|902.23.2}}902.23.2 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 [https://epg.modot.org/forms/general_files/TS/Signal_Phasing_and_Layout_Examples.pdf#search=signal%20phasing%20layout 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: 
 
[[File:NEMA Ring.gif|frame|center|<center>'''NEMA Ring'''</center>]]

A general [[media:902.5.2.1.1 Fully-Actuated Controller.xls|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.)

=={{SpanID|902.23.3}}902.23.3 Left Turn Phasing==
'''Support.''' Guidelines are available to aid in determining the proper left turn phasing for signalized intersections. 

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[https://epg.modot.org/forms/general_files/TS/Left_Turn_Phasing.xlsx '''Left Turn Phasing Warrants''']
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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 [[#902.23.5|EPG 902.23.5]] 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. 

==={{SpanID|902.23.3.1}}902.23.3.1 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 phasing<sup>'''1'''</sup>. 

:'''cpp''' = The cycle length (in seconds) when those volumes occur using protected/permissive phasing<sup>'''1'''</sup>. 

:'''gp''' = The green time (in seconds) common to both VLT and VO during that cycle using permissive-only phasing<sup>'''1'''</sup>. 

:'''gpp''' = The green time (in seconds) common to both (VLTLT)pp and VO during that cycle using protected/permissive phasing<sup>'''1'''</sup>. 

:'''TP''' = The time allocated to the protected left turn movement using protected/permissive phasing<sup>'''1'''</sup>. 

:<sup>'''1'''</sup> 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'' = <math>\frac{T_p (sec/cycle) \times 3600 (sec/hr)}{2(sec/vehicle) \times c_{pp}(sec/cycle)}</math>

:'''Safety Criteria'''

::'''Protected Only Left Turns'''

::Protected-Only left turns shall be provided full-time when the number of opposing lanes ≥ 4.

::Note: If the number of opposing lanes = 3, protected/permissive should be considered using engineering judgment.

::The following factors should be considered when counting the number of opposing lanes crossed by left-turning traffic:
:::Through Lanes. Any lane in which through traffic is permitted shall be counted, even if turns are also permitted from that lane.
:::Left-Turn Lanes. Opposing exclusive left-turn lanes should usually not be counted, because typically opposing left turns do not conflict with each other.
:::Right-Turn Lanes. It may be acceptable to exclude opposing right-turn lanes. Omitting right-turn lanes is particularly appropriate where the right-turn movement is physically channelized from opposing through lanes and not under signal control. It may be desirable to include right-turn lanes in the count of opposing lanes where right-turn volume is heavy or where conflicts with left-turns are unusually high.  

:::Protected-Only left turns should be provided full-time when any one of the following criteria are satisfied:
::::A. Sight Distance: 
:::::< 200 ft. for 25 mph 
:::::< 240 ft. for 30 mph 
:::::< 280 ft. for 35 mph 
:::::< 320 ft. for 40 mph 
:::::< 360 ft. for 45 mph 
:::::< 400 ft. for 50 mph 
:::::< 440 ft. for 55 mph 

::::B. Number of Correctable Crashes By Upgrading to Protected Only Phasing > 5 over 12 months 
:::::Note: The correctable crashes should involve the SAME Left Turn approach. Only those approaches satisfying that criteria should be upgraded. 

::::C. 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. 

::::D. 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 

::::E. ≥ 2 left turn lanes.
:::::Note: 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. 

::::F. 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 Cycle<sup>'''1'''</sup> 

::::VO < 100 Vehicles per Hour 

:::<sup>'''1'''</sup> 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 Cycle<sup>'''1'''</sup> AND VO > 100 Vehicles per Hour 

::::VLT + VO > 600 x (gp/cp) 

:::<sup>'''1'''</sup> 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)pp + VO < 1200 x (gpp/cpp) 
::::(VLT)pp x VO < 50,000 

'''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.

===902.23.3.2 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. 

===902.23.3.3 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. 

===902.23.3.4 Alternate Sequences===

'''Support.''' When needed, the normal NEMA ring sequence can be altered to fit operating conditions, with restrictions as detailed in [[#902.23.3 Signal Phasing|EPG 902.23.3]]. The most common application is to provide lead-lag left turns. Assume this phasing assignment for the following intersection: 
 
[[File:Alternate Sequence.gif|frame|center|<center>'''Alternate Sequence'''</center>]]
	
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: 
 
[[File:Alternate Sequence Ring.gif|frame|center|<center>'''Alternate Sequence Ring'''</center>]]

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.

==902.23.4 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 [http://onlinepubs.trb.org/onlinepubs/nchrp/esg/esg.pdf 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 [[620.2 Pavement and Curb Markings (MUTCD Chapter 3B)#620.2.8 Extensions Through Intersections or Interchanges (MUTCD Section 3B.08)|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.
[[File:Figure 902.23.1.jpg|left|100px]]

=={{SpanID|902.23.5}}902.23.5 Guidelines for Use of the Flashing Yellow Left Arrow (FYA)==
{|style="padding: 0.3em; margin-left:15px; border:1px solid #a9a9a9; text-align:center; font-size: 95%; background:#f5f5f5" width="180px" align="right" 
|- 
|<center>'''Video'''</center>
|-
|[[media:902.11.28 FYA animation.wmv|Flashing Yellow Arrow animation]]
|-
|<center>'''Worksheet and Brochure'''</center>
|-
| [https://epg.modot.org/forms/general_files/TS/Left_Turn_Phasing.xlsx Left Turn Phasing Warrants]
|-
|[[media:902.5.29 Flashing Yellow Arrow brochure.pdf|Flashing Yellow Arrow brochure]]
|-
|<Center>'''Studies'''</center>
|-
|[http://onlinepubs.trb.org/Onlinepubs/nchrp/nchrp_rpt_493.pdf NCHRP Study 493]
|-
|[http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_w123.pdf NCHRP Study 123]
|-
|[https://spexternal.modot.mo.gov/sites/cm/CORDT/cmr23-005.pdf Safety Evaluation of Permissive Flashing Yellow Arrows for Left-Turn Movements in Missouri]
|}
'''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 ensure 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. 

===902.23.5.1 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. 

====902.23.5.1.1 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. 

====902.23.5.1.2 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

===902.23.5.2 Equipment for Reconstructed Signals===

'''Support.''' 

:'''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. 

===902.23.5.3 Equipment for New Construction===

'''Support.'''

:'''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 [http://www.modot.org/business/standards_and_specs/SpecbookEPG.pdf#page=14 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. 

'''Support.'''
:'''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.

===902.23.5.4 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: 
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|[[media: 902.11.28.3.6 D6 Contract Submittal Example.pdf|St. Louis District contract submittal example]]
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:* 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 

:* Signing 

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.

===902.23.5.5 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.

==902.23.6 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:  

[[File:Simple Overlap Application.gif|frame|center|<center>'''Simple Overlap Application'''</center>]]

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: 

[[File:Practical Application for Signalizing a Right Turn.gif|frame|center|<center>'''Practical Application for Signalizing a Right Turn'''</center>]]

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.

==902.23.7 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.

===902.23.7.1 Green Interval===

====902.23.7.1.1 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.

====902.23.7.1.2 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 [[#902.23.5 Coordination|EPG 902.23.5 Coordination]] for more discussion on cycle lengths for all types of control.

===902.23.7.2 Yellow Change and Red Clearance Intervals===

'''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. 
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|[[media:902.5.3.2 Change Interval Timing.xls|Change Interval Timing Worksheet]]
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'''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. 

Change Period: 
 
::<math>CP = t + \frac{V}{2a + 64.4g} + \frac{W + L}{V}</math>

Guidance:
:CP = nondilemma change period (yellow plus all red), seconds 

:t = perception-reaction time, recommended as 1.0 second 

:V = approach speed (recommended as 85<sup>th</sup> or 15<sup>th</sup> percentile speed), ft/sec 

:g = percent grade (positive for upgrade, negative for downgrade), expressed as a decimal 

:a = deceleration rate, recommended as 10 ft/sec<sup>2</sup> 

: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 85<sup>th</sup> 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 85<sup>th</sup> percentile speed to be used in the equation. It is important, however, to also consider slower traffic going through the intersection at the 15<sup>th</sup> 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 85<sup>th</sup> and 15<sup>th</sup> percentile speeds. Engineering judgment is used to determine whether the 85<sup>th</sup> or 15<sup>th</sup> 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/sec<sup>2</sup> 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/sec<sup>2</sup>) 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 902.23.11 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 [[903.6 Warning Signs#903.6.28 Advance Traffic Control Signs (W3-1, W3-2, W3-3, W3-4) (MUTCD Section 2C.36)|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.

===902.23.7.3 Pedestrian Signal Timing===
'''Guidance.''' If a pedestrian signal head is used that does not have a concurrent vehicular phase (exclusive pedestrian movement), the pedestrian change interval (flashing UPRAISED HAND) should be set to be approximately 4 seconds less than the required pedestrian clearance time and an additional clearance interval (during which a steady UPRAISED HAND is displayed) should be provided prior to the start of the conflicting vehicular phase. See [[902.9 Pedestrian Control Features (MUTCD Chapter 4I) #fig902.9.6|Figure 902.9.6]].

Every effort is to be made to display the WALK indications with a green phase or interval. This "phase-associated" pedestrian operation lessens the overall delay to drivers. Using a fully actuated intersection as shown:

[[File:Phase-Associated Pedestrian Operation.jpg|frame|center|<center>'''Phase-Associated Pedestrian Operation'''</center>]]


The pedestrian WALK and flashing DON'T WALK indications for northbound-southbound on the east side of the intersection would be displayed only during phase 6. Likewise, the indications for eastbound-westbound on the south side would be displayed only during phase 4.

'''Standard.''' Under no circumstance will a pedestrian WALK or flashing DON'T WALK indication be active during a phase or interval which leads vehicles into the crosswalk.

'''Guidance.''' Using the previous example, the pedestrian indications for northbound-southbound cannot be active during phase 5, or any phase for the east-west direction of travel. Pedestrian indications are allowed in conjunction with the phase 6 right turn (using the permissive indication) and/or the phase 5 southbound yielding left turn, if allowed, since the pedestrian movement has legal right-of-way over the northbound right turn or southbound yielding left turn.

Under rare circumstances, an exclusive movement might be needed for the pedestrian indications.

Other options should be considered prior to the installation of an exclusive pedestrian movement.

==902.23.8 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. 

===902.23.8.1 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.

===902.23.8.2 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 85<sup>th</sup> 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 85<sup>th</sup> 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. 
 
[[File:Gap reduction settings and added initial time.gif|center|thumb|700px|<center>'''Gap reduction settings and added initial time'''</center>]]

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 ensure 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.

===902.23.8.3 Detection on High-Speed Approaches===

'''Support.'''	With signals installed on high-speed roadways (85<sup>th</sup> 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 85<sup>th</sup> percentile). Minimum gap is set to three seconds using gap reduction control. 

This allows for a vehicle approaching the intersection at the 85<sup>th</sup> percentile speed to hit the first detector and extend their call for at least three seconds once gap reduction is finished. At the 85<sup>th</sup> 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 85<sup>th</sup> 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.  

[[File:Figure 902.23.8.3.gif|thumb|550px|center|<Center>'''Figure 902.23.8.3'''</center>]]


Table 902.23.8.3 is a reference for detector placement. 

====<center>''Table 902.23.8.3 Vehicular Distance Traveled''</center>====
{| border="1" class="wikitable" style="margin: 1em auto 1em auto"
|+ 
! style="background:#BEBEBE" colspan="2"|Speed !! style="background:#BEBEBE" colspan="14"|Time, in seconds
|-
! style="background:#BEBEBE" rowspan="2"|mph!! style="background:#BEBEBE" rowspan="2"|ft./sec|| align="center"|1||align="center"|5||align="center"|8||align="center"|10||align="center"|15||align="center"|20||align="center"|25||align="center"|30||align="center"|35||align="center"|40||align="center"|45||align="center"|50||align="center"|55||align="center"|60
|-
!style="background:#BEBEBE" colspan="14" align="center"|Distance Traveled in Feet
|-
|align="center"|'''1'''||align="center"|'''1.5'''||align="center"|1.5||align="center"|7.3||align="center"|11.7||align="center"|15||align="center"|22||align="center"|29||align="center"|37||align="center"|44||align="center"|51||align="center"|59||align="center"|66||align="center"|73||align="center"|81||align="center"|88
|-
|align="center"|'''2'''||align="center"|'''2.9'''||align="center"|2.9||align="center"|15||align="center"|23||align="center"|29||align="center"|44||align="center"|59||align="center"|73||align="center"|88||align="center"|103||align="center"|117||align="center"|132||align="center"|147||align="center"|161||align="center"|176
|-
|align="center"|'''3'''||align="center"|'''4.4'''||align="center"|4.4||align="center"|22||align="center"|35||align="center"|44||align="center"|66||align="center"|88||align="center"|110||align="center"|132||align="center"|154||align="center"|176||align="center"|198||align="center"|220||align="center"|242||align="center"|264
|-
|align="center"|'''4'''||align="center"|'''5.9'''||align="center"|5.9||align="center"|29||align="center"|47||align="center"|59||align="center"|88||align="center"|117||align="center"|147||align="center"|176||align="center"|205||align="center"|235||align="center"|264||align="center"|293||align="center"|323||align="center"|352
|-
|align="center"|'''5'''||align="center"|'''7.3'''||align="center"|7.3||align="center"|37||align="center"|59||align="center"|73||align="center"|110||align="center"|147||align="center"|183||align="center"|220||align="center"|257||align="center"|293||align="center"|330||align="center"|367||align="center"|403||align="center"|440
|-
|align="center"|'''10'''||align="center"|'''14.7'''||align="center"|15||align="center"|73||align="center"|117||align="center"|147||align="center"|220||align="center"|293||align="center"|367||align="center"|440||align="center"|513||align="center"|587||align="center"|660||align="center"|733||align="center"|807||align="center"|880
|-
|align="center"|'''15'''||align="center"|'''22.0'''||align="center"|22||align="center"|110||align="center"|176||align="center"|220||align="center"|330||align="center"|440||align="center"|550||align="center"|660||align="center"|770||align="center"|880||align="center"|990||align="center"|1100||align="center"|1210||align="center"|1320
|-
|align="center"|'''20'''||align="center"|'''29.3'''||align="center"|29||align="center"|147||align="center"|235||align="center"|293||align="center"|440||align="center"|587||align="center"|733||align="center"|880||align="center"|1027||align="center"|1173||align="center"|1320||align="center"|1467||align="center"|1613||align="center"|1760
|-
|align="center"|'''25'''||align="center"|'''36.7'''||align="center"|37||align="center"|183||align="center"|293||align="center"|367||align="center"|550||align="center"|733||align="center"|917||align="center"|1100||align="center"|1283||align="center"|1467||align="center"|1650||align="center"|1833||align="center"|2017||align="center"|2200
|-
|align="center"|'''30'''||align="center"|'''44.0'''||align="center"|44||align="center"|220||align="center"|352||align="center"|440||align="center"|660||align="center"|880||align="center"|1100||align="center"|1320||align="center"|1540||align="center"|1760||align="center"|1980||align="center"|2200||align="center"|2420||align="center"|2640
|-
|align="center"|'''35'''||align="center"|'''51.3'''||align="center"|51||align="center"|257||align="center"|411||align="center"|513||align="center"|770||align="center"|1027||align="center"|1283||align="center"|1540||align="center"|1797||align="center"|2053||align="center"|2310||align="center"|2567||align="center"|2823||align="center"|3080
|-
|align="center"|'''40'''||align="center"|'''58.7'''||align="center"|59||align="center"|293||align="center"|469||align="center"|587||align="center"|880||align="center"|1173||align="center"|1467||align="center"|1760||align="center"|2053||align="center"|2347||align="center"|2640||align="center"|2933||align="center"|3227||align="center"|3520
|-
|align="center"|'''45'''||align="center"|'''66.0'''||align="center"|66||align="center"|330||align="center"|528||align="center"|660||align="center"|990||align="center"|1320||align="center"|1650||align="center"|1980||align="center"|2310||align="center"|2640||align="center"|2970||align="center"|3300||align="center"|3630||align="center"|3960
|-
|align="center"|'''50'''||align="center"|'''73.3'''||align="center"|73||align="center"|367||align="center"|587||align="center"|733||align="center"|1100||align="center"|1467||align="center"|1833||align="center"|2200||align="center"|2567||align="center"|2933||align="center"|3300||align="center"|3667||align="center"|4033||align="center"|4400
|-
|align="center"|'''55'''||align="center"|'''80.7'''||align="center"|81||align="center"|403||align="center"|645||align="center"|807||align="center"|1210||align="center"|1613||align="center"|2017||align="center"|2420||align="center"|2823||align="center"|3227||align="center"|3630||align="center"|4033||align="center"|4437||align="center"|4840
|-
|align="center"|'''60'''||align="center"|'''88.0'''||align="center"|88||align="center"|440||align="center"|704||align="center"|880||align="center"|1320||align="center"|1760||align="center"|2200||align="center"|2640||align="center"|3080||align="center"|3520||align="center"|3960||align="center"|4400||align="center"|4840||align="center"|5280
|-
|align="center"|'''65'''||align="center"|'''95.3'''||align="center"|95||align="center"|477||align="center"|763||align="center"|953||align="center"|1430||align="center"|1907||align="center"|2383||align="center"|2860||align="center"|3337||align="center"|3813||align="center"|4290||align="center"|4767||align="center"|5243||align="center"|5720
|-
|align="center"|'''70'''||align="center"|'''102.7'''||align="center"|103||align="center"|513||align="center"|821||align="center"|1027||align="center"|1540||align="center"|2053||align="center"|2567||align="center"|3080||align="center"|3593||align="center"|4107||align="center"|4620||align="center"|5133||align="center"|5647||align="center"|6160
|-
|align="center"|'''75'''||align="center"|'''110.0'''||align="center"|110||align="center"|550||align="center"|880||align="center"|1100||align="center"|1650||align="center"|2200||align="center"|2750||align="center"|3300||align="center"|3850||align="center"|4400||align="center"|4950||align="center"|5500||align="center"|6050||align="center"|6600
|-
|align="center"|'''80'''||align="center"|'''117.3'''||align="center"|117||align="center"|587||align="center"|939||align="center"|1173||align="center"|1760||align="center"|2347||align="center"|2933||align="center"|3520||align="center"|4107||align="center"|4693||align="center"|5280||align="center"|5867||align="center"|6453||align="center"|7040
|-
|align="center"|'''85'''||align="center"|'''124.7'''||align="center"|125||align="center"|623||align="center"|997||align="center"|1247||align="center"|1870||align="center"|2493||align="center"|3117||align="center"|3740||align="center"|4363||align="center"|4987||align="center"|5610||align="center"|6233||align="center"|6857||align="center"|7480
|-
|align="center"|'''90'''||align="center"|'''132.0'''||align="center"|132||align="center"|660||align="center"|1056||align="center"|1320||align="center"|1980||align="center"|2640||align="center"|3300||align="center"|3960||align="center"|4620||align="center"|5280||align="center"|5940||align="center"|6600||align="center"|7260||align="center"|7920
|}

===902.23.8.4 Delay and Extend Detector Settings===

'''Support.''' 

:(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. 

===902.23.8.5 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. 

===902.23.8.6 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.  

'''Support.'''	Example:

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.

===902.23.8.7 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. 

===902.23.8.8 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). 
 
[[File:Figure 902.23.8.8.gif|thumb|400px|center|<center>'''Figure 902.23.8.8'''</center>]]

=={{SpanID|902.23.9}}902.23.9 Power Outages at Signalized Intersections==
'''Guidance.''' Each District should plan for signalized intersection power outages by developing procedures for signalized intersections that include information about the installation, use, and recovery of Temporary Stop Signs (TSS) and, if used, the installation of battery backup systems. These subarticles provide information for these items.

===902.23.9.1 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.''' Utilities or other non-MoDOT parties doing planned permitted work that will cause a power outage leading to a non-functioning signalized intersection(s) shall be responsible for providing the necessary TSS or generator(s) to power the signalized intersection(s) until power at the non-functioning signalized intersection(s) has been restored.

====902.23.9.1.1 Conditions For Use====
'''Option.''' TSS may 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. When a signalized intersection is non-functioning, then TSS may 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.

'''Guidance.''' 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). 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.

'''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 [[#902.23.9.2 Start up from Dark at Signalized Intersections|EPG 902.23.9.2]] for more information regarding Startup from Dark).

If used, TSS signs shall remain at the intersection until power at the non-functioning signalized intersection has been restored ([[#902.23.9.1.4 Recovery|see EPG 902.23.9.1.4 Recovery]]).

'''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.

====902.23.9.1.2 Location and Placement====
'''Standard.''' The signalized intersection locations for installation of TSS shall meet the conditions of use in [[#902.23.9.1.1 Conditions For Use|EPG 902.23.9.1.1]] and shall be at the discretion of the District.

'''Option.''' Each District may develop a list of signalized intersections to establish a priority for TSS installation. 

'''Guidance.''' The installation of TSS should be prioritized as follows (as applicable to each district) or, if a list is developed, should begin at the identified intersections:
# Signals with railroad preemption
# Signals with a speed limit greater than 50 mph
# Signals with a high accident rate
# Intersections difficult to flag or require multiple flaggers (non-routine roadway configurations/geometry, SPUIs, multi-lane approaches, etc.)
# Signals with high volumes (freeway type off-ramps, major roadways, etc.)
# Signals with frequent power outages 
# Signals located at schools.  

If battery backup systems are installed (see [#902.23.9.3 Battery Backup Systems at Signalized Intersections|EPG 902.23.9.3 Battery Backup Systems at Signalized Intersections]]) at signalized intersections, Districts should re-evaluate their list of prioritized intersections, if developed, for the installation of TSS.

'''Standard.''' When used, TSS shall be placed in a location where they are visible to all lanes on all roadways. 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 or at location for best visibility of all approaches.

'''Guidance.''' If the power outage is widespread, additional personnel should be requested to help with the placement of the signs.

====902.23.9.1.3 Storage and Distribution====
'''Guidance.''' Each District should store enough TSS to be deployed at high priority signalized intersections.

'''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.

====902.23.9.1.4 Recovery====
'''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 [[#902.23.9.2 Start up from Dark at Signalized Intersections|EPG 902.23.9.2 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.

'''Option.''' Detailed recovery procedures for each intersection with TSS may be developed by each District at their discretion.

===902.23.9.2 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.

===902.23.9.3 Battery Backup Systems at Signalized Intersections===
====902.23.9.3.1 Installation/Placement====
'''Option.''' Battery Backup Systems (BBS) may be installed at signalized intersections at the District’s discretion. Each District may develop a list of signalized intersections to establish a priority for the installation of BBS.

'''Guidance.''' The installation of BBS should be prioritized as follows (as applicable to each District) or, if a list is developed, should begin at the identified intersections:
# Signals with railroad preemption
# Signals with a speed limit greater than 50 mph
# Signals with a high accident rate
# Intersections difficult to flag or require multiple flaggers (non-routine roadway configurations/geometry, SPUIs, multi-lane approaches, etc.)
# Signals with high volumes (freeway type off-ramps, major roadways, etc.)
# Signals with frequent power outages 
# Signals located at schools.  

If developed, each District’s prioritized installation list for BBS should be based on their traffic conditions and needs. The prioritized TSS installation list, if developed, will need to be reevaluated as BBS are installed.

====902.23.9.3.2 Duration====
'''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.

=={{SpanID|902.23.10}}902.23.10 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 [[#902.5.37.2 Advance Detectors and Gap Reduction|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 [https://epg.modot.org/index.php/902.5_Traffic_Control_Signal_Features_(MUTCD_Chapter_4D)#902.5.37.6_Recalled_Phases EPG 902.5.37.6]. 

=={{SpanID|902.23.11}}902.23.11 Coordination==

'''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. 

===902.23.11.1 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. 

===902.23.11.2 Planning a Coordinated System===
====902.23.11.2.1 Determine System Limits====

'''Guidance.'''

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: 

::CI = V / L

where: 

:CI = 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.

====902.23.11.2.2 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. 
 
====902.23.11.2.3 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.
 
=====902.23.11.2.3.1 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

<center>'''''Minimum Recommended Cycle Lengths'''''</center>
{| border="1" class="wikitable" style="margin: 1em auto 1em auto"
|+ 
! style="background:#BEBEBE" rowspan="2" align="center"|Sum of Critical Lane Volumes !! style="background:#BEBEBE" colspan="3" align="center"|Number of Phases 
|-
! style="background:#BEBEBE" align="center"|2 !! style="background:#BEBEBE" align="center"|3-4 !! style="background:#BEBEBE" align="center"|5-8 
|-
|  align="center"|800 || align="center"|30||  align="center"|45||  align="center"|60
|-
|  align="center"|900 || align="center"|35||  align="center"|55||  align="center"|70
|-
|  align="center"|1000 || align="center"|40||  align="center"|60||  align="center"|80
|-
|  align="center"|1100 || align="center"|45||  align="center"|70||  align="center"|90
|-
|  align="center"|1200 || align="center"|55||  align="center"|85||  align="center"|110
|-
|  align="center"|1300 || align="center"|70||  align="center"|105||  align="center"|120
|-
|  align="center"|1400 || align="center"|90||  align="center"|120||  align="center"|*
|-
|  align="center"|1500 || align="center"|120||  align="center"|*||  align="center"|*
|-
| colspan="4" align="center"| * 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. 

=====902.23.11.2.3.2 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. 

====902.23.11.2.4 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. 

=====902.23.11.2.4.1 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. 

=====902.23.11.2.4.2 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 [[#902.5.27.1 Leading and Lagging Left-Turns|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. 

====902.23.11.2.5 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. 
 
[[File:Figure 902.23.2.gif|center|600px]]


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. 

=====902.23.11.2.5.1 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 85<sup>th</sup> 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. 

=====902.23.11.2.5.2 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.

====902.23.11.2.6 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.

<u>Traffic Responsive</u>
	
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.

<u>Traffic Adaptive</u>

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.
{|style="padding: 0.3em; margin-right:7px; border:1px solid #a9a9a9; text-align:center; font-size: 95%; background:#ffddcc" width="210px" align="right" 
|-
|'''Adaptive Traffic Signals in Lee’s Summit'''
|-
|[https://spexternal.modot.mo.gov/sites/cm/CORDT/or10020_Adv.pdf#search=%27%27%27Adaptive%20Traffic%20Signals%20in%20Lee%E2%80%99s%20Summit Summary]
|-
|'''See also:''' [https://www.modot.org/research-publications 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.

<u>Traffic Adaptive “Light”</u>

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 [http://www.ops.fhwa.dot.gov/publications/fhwahop06006/chapter_3p2.htm#3-8 FHWA’s Traffic Control Systems Handbook].

===902.23.11.3 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. 

====902.23.11.3.1 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.

====902.23.11.3.2 Types of Interconnect====

=====902.23.11.3.2.1 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. 
 
[[File:Figure 902.23.3.jpg|center|500px]]

Controller 2 - Sync Reference: Start of Timing Plan 

At 6 a.m., this controller will begin to time offset breaks every 70 seconds. 
 
[[File:Figure 902.23.4.jpg|center|500px]]

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.

=====902.23.11.3.2.2 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. 

=====902.23.11.3.2.3 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. 

=====902.23.11.3.2.4 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. 

=====902.23.11.3.2.5 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. 

===902.23.11.4 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. 

====902.23.11.4.1 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. 

====902.23.11.4.2 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. 

====902.23.11.4.3 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.

===902.23.11.5 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. 

[[media:902.5.5.5 Diamond Interchange Examples.pdf|Diamond Interchange Examples]] provides examples of phasing configurations for diamond interchanges. 

==902.23.12 Controller Assembly Components==

===902.23.12.1 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 [[#902.5.2 Traffic Signal Operation|EPG 902.5.2 Traffic Signal Operation]]. 

<u>NEMA TS1</u>

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 [http://www.modot.org/business/standards_and_specs/SpecbookEPG.pdf#page=13 Sec 902]. 

<u>NEMA TS2</u>

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. 

<u>Type 170/2070</u>

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.

===902.23.12.2 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.  

===902.23.12.3 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.

===902.23.12.4 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. 

===902.23.12.5 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.  

=={{SpanID|902.23.13}}902.23.13 Detectors==

'''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 [https://www.fhwa.dot.gov/publications/research/operations/its/06108/  FHWA’s ''Traffic Detector Handbook''] (also available through ITE).  

===902.23.13.1 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.  

====902.23.13.1.1 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 [https://www.modot.org/media/16916 Standard Plan 902.50] for more information. 

====902.23.13.1.2 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. 

===902.23.13.2 Probes ===

'''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. 

===902.23.13.3 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.  

===902.23.13.4 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. 

===902.23.13.5 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 [[#902.23.11|EPG 902.23.11 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.

[[Category:902 Signals (MUTCD Part 4)|902.23]]