902.18 Glossary
85th Percentile Speed - The speed at which 85% of the traffic is traveling at or below.
AASHTO - American Association of State Highway and Transportation Officials. A national organization that develops guidelines, specifications, policies, and standards for transportation design, construction and operation.
Acceptance Test Period - Time period in which the acceptance of and payment for a new device is delayed pending satisfactory performance.
Actuated - Indications controlled by vehicle or pedestrian detection. Also refer to Semi-Actuated and Full-Actuated.
Actuated, Fully- - All indications at a signal controlled by detection.
Actuated, Semi- - Some, but not all, indications at a signal controlled by detection.
Actuation - The action of a vehicle or pedestrian causing a detector to create a call in that phase or movement to request right of way.
Adaptive Split Control - A means of local intersection split selection based on vehicular activity.
Added Initial - An increment of time added to the minimum initial portion in response to vehicle actuation.
Address - The identification of specific intersections for transmission of commands or the receipt of data.
Advance Call Detector - A detector located a considerable distance ahead of an intersection which calls the green to that approach.
Advance Warning - A per movement output used to give advance notice of an upcoming yellow or red indication. Typically used at hidden intersections with “prepare to stop” indicators.
Agreement - A concord of understanding and intention between two or more parties that addresses the rights and responsibilities of each party.
Algorithm - A procedure, process, or rule for the solution of a problem. An algorithm may be a set of computational rules for the solution of a mathematically expressed problem.
All Red - (1) An optional feature in intersection controls which provides adjustable time intervals for a red clearance interval following any yellow clearance interval. (2) All red flashing intersection signalization used during normal control off periods.
Amplifier (Detector) - An electrical device used to sense electrical load changes on associated sensing equipment (e.g. inductive loops) and provide an output to an intersection controller for vehicle detection.
Approach - Section of roadway in advance of an intersection. The remaining traffic lanes minus any exclusive turns or parking lanes.
Arterial - A main street generally considered to be a thoroughfare with preferential right of way.
Asynchronous - A non-synchronized condition. Free running without any specific relationship in operation to any other condition.
Attestation - The act of witnessing an instrument in writing and recording the name of the witness.
Auto/Manual Switch - A cabinet switch, when operated, discontinues normal signal operation and permits manual operation.
Average Day - The day that best represents the normal flow of traffic.
Background Cycle - Cycle length run at the master controller that, once at the start of the cycle, outputs the system reference to the local controllers. May also be run at a controller operating TBC, which supervises the actuated operation of the intersection.
Back Panel - A board within the controller cabinet upon which are mounted field terminals, fuse receptacles or circuit breakers, and other portions of the controller assembly not included in the controller unit or auxiliary devices.
Backplate - A strip of thin material extending outward parallel to the signal face on all sides of a signal housing to provide a suitable background for the signal indications.
Band (Green Band) - Through or green elapsed time between the first and last possible vehicle permitted through an intersection on a progressive coordination system.
Barrier (Compatibility Line) - A reference point in the preferred sequence of a dual-ring controller at which both rings are interlocked. Both rings cross the barrier simultaneously to select and time phases on the other side.
Basic Lighting - Roadway lighting provided only at potential conflict areas of an intersection or interchange
Baud Rate - The input rate of data transmission to a communications channel, usually expressed in bits per second.
Buffer - A device used to make two other devices or systems compatible, in particular, a device or routine that compensates for differences in times of occupance or rates of flow when data is transmitted between devices.
Buss Interface Unit (BIU) - A device in a controller cabinet that converts analog inputs to serial communications data and serial data to analog outputs to interface with various cabinet components.
Cabinet - An outdoor enclosure for housing the controller unit and associated equipment.
Call - A registration of a demand by a vehicle or pedestrian for right of way at a controller unit.
Calling Detector - A detector installed in a selected location to detect vehicles which may not otherwise be detected, and whose output may be modified by the controller unit. When the signal is green, the detector is disconnected so that extensions of the green can come only from the detector located upstream of the calling detector.
Capacity - The maximum rate of flow at which persons or vehicles can be reasonably expected to traverse a point or uniform segment of a lane or roadway during a specified time period under prevailing roadway, traffic, and control conditions, usually expressed as vehicles per hour or persons per hour.
Carrier Frequency - A single frequency that is modulated by the lower frequency signals being communicated or carried, each carrier frequency provides an independent communications channel.
Cars Waiting - Also known as a Queue. Term used in sophisticated volume-density controls to apply more stringent requirements to traffic having the right of way based on the number of vehicles waiting in a conflicting movement. (Also see Time Waiting).
Change Interval - The "yellow" plus "all red" intervals that occur between phases of a traffic signal to provide for clearance of the intersection before conflicting movements are released.
Channelization - Physical separation of traffic movements by positive barrier or markings.
Clear Zone - An unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles.
Clearance Interval - The "all red" interval that permits traffic to clear the intersection before conflicting traffic movements are released.
Closed Loop System - Coordinated signal system that allows for remote access and data communication to and from all controllers within the system via the master controller.
Collision Diagram - Symbolic representation of number and type of crashes occurring in an area.
Combination Lane - See Shared Lane.
Commission - Missouri Highway and Transportation Commission, a group appointed by the governor of Missouri to oversee MoDOT operations. The term may also indicate MoDOT as a whole.
Compatibility Line - The dividing line crossing both rings, in dual ring operation, that separates compatible phase combinations. Usually it divides phases associated with North/ South versus East/West.
Conditional Service - A dual-ring feature which allows re-service to an odd phase (left turn) once the opposing through phase has gapped out. The service is conditioned by the time remaining in the adjacent through phase’s MAX time.
Conduit - Pipe used to house stretches of cable.
Conflict Monitor - A device used to continually check for the presence of conflicting signal indications and other malfunctions and to provide an output in response to conflict or malfunction.
Conflict Monitor Program Card - Card on which compatible movements, or channels, are programmed for use with the conflict monitor.
Conflicting Phases - Two or more traffic phases which cause interfering traffic movements if operated concurrently.
Continuous-Presence Mode - A mode in which the detector output continues if any vehicle (first or last remaining) remains in the field of influence.
Contract - A concord of understanding and intention between two or more parties that addresses the rights and responsibilities of each party.
Controller - The portion of a controller assembly that is devoted to the selection and timing of signal displays.
Controller Assembly - A complete electrical mechanism mounted in a cabinet for controlling the operation of a traffic control signal.
Coordinated Phase(s) - The phase(s) designated under coordinated operation to allow for the platoon to proceed through the intersection.
Coordinated System - Set of interconnected signals in an area of intersecting roads that provides for coordinated flow in more than one direction on different roads.
Coordination - The establishment of a definite timing relationship between adjacent traffic signals.
Critical Intersection(s) - The intersection(s) in a coordinated system which is most congested.
Crosstalk - The adverse interaction of any channel of a detector unit with any other detector channel in that unit or another unit. It is the mutual coupling of magnetic fields that produces an interaction between two or more detector units in the same cabinet when the units are operating at similar frequencies. Crosstalk results in a detector outputting an actuation in the absence of a vehicle.
Crosswalk - The marked crossing area for pedestrians crossing the street at an intersection or designated mid-block location.
Cycle - Any complete sequence of signal indications.
D-Plug - Plug which attaches to the controller from the interconnect panel and provides for the reception and/or transmission of interconnect and pre-emption signals.
Dallas Left Turn Phasing – Left turn phasing option which uses a visibly-limited exclusive indication to display a circular green.
Delay - Additional travel time experienced by a driver, passenger, or pedestrian beyond what would reasonably be desired for a given trip.
Delay (Stopped) - Time lost by vehicles due to being stopped by a traffic control device.
Demand Control - A mode of operation in which the service provided at an intersection reflects the presence of demand for that service often without regard for background cycling.
Density - Signifies the number of vehicles in a given distance such as vehicles per mile. Density = volume divided by speed.
Density Control - An advanced actuated model of detection that can count waiting vehicles beyond the first vehicle.
Detection - The process used to identify the presence or passage of a vehicle at a specific point or to identify the presence of one or more vehicles or pedestrians in a specific area.
Detection Zone - The area of the roadway within which a vehicle will be detected by a vehicle detector.
Detector - An electrical or electronic device which when actuated signifies the passing or presence of a vehicle or pedestrian desiring the right of way. (See specific detectors: vehicle or pedestrian.
Detector Amplifier - A device that is capable of intensifying the electrical energy produced by a sensor. An example is a magnetic detector amplifier. A loop detector unit is commonly called an amplifier, although its electronic function actually is different.
Detector Failures - The occurrence of detector malfunctions, such as non-operation, chattering, or other erroneous signaling as well as occupancy errors and false alarms.
Detector Memory (On) - The retention of an actuation for future utilization by the controller assembly.
Detector Unit - The portion of a detector system other than the sensor and lead-in cable, consisting of an electronic assembly.
Diagnostics - A program that facilitates computer maintenance by detection and isolation of malfunctions or mistakes.
Diamond Interchange - Freeway interchange where the four entrances and exit ramps to and from the freeway run directly into the intersecting roadway. The four ramps form a diamond pattern with the intersecting roadways.
Digital Timing - Pertaining to a method of timing that operates by counting discrete units.
Dilemma Zone - Area approaching an intersection where a driver first sees a yellow indication, but is too far away to proceed through the intersection, and is too close to stop comfortably.
Dimming - This feature allows selected signal indication to be dimmed during night time operation.
Doppler Effect - A change in the frequency with which waves from a given source reach an observer, when the source and the observer are in rapid motion with respect to each other, by which the frequency increases or decreases according to the speed at which the distance is decreasing or increasing.
Downstream - The direction to which traffic is flowing.
Dual Entry - A mode of operation (in a dual-ring controller unit) in which one phase in each ring must be in service. If a call does not exist in a ring when it crosses the barrier, a phase is selected in the ring to be activated by the controller in a predetermined manner.
Dual Left Turns - Two left turn movements on the same approach.
Dual-Ring Controller Unit - A controller unit containing two interlocking rings which are arranged to time in a preferred sequence and to allow concurrent timing of both rings, subject to the restraint of the compatibility line (barrier).
Dummy Phase - Phase programmed in a controller which does not directly control traffic by changing signal indications, but allows for other phases or overlaps to time out during its time.
Duplex - Two-way communication over a single communication link.
Dwell - A steady indication whereby no timing device is in control of the time the signal will be illuminated. Usually the condition where no traffic exists to place a request for right of way. Applicable to traffic actuated equipment and normally assigned to the artery.
Easement - Agreement between MoDOT and property owner to allow MoDOT to construct or maintain roadway facilities off of the normal right of way.
Electromechanical Device - A device characterized by electrical circuits utilizing relays, step switches, motors, etc.
Electronic Device - A device which is characterized by electrical circuits utilizing vacuum tubes, resistors, capacitors, and inductors, and which may include electromechanical and solid-state devices.
Emission - Portion of vehicle exhaust considered hazardous to the environment.
Engineering Judgment - A decision based on available data and information using engineering knowledge and experience.
Engineering Study - An Engineering Study documents the collection and analysis of data for a specific location, a set of locations or an area. The criteria used to determine the data collected and analyzed is based upon engineering judgment and accepted practices. Some sources for accepted practices are: MoDOT Manuals, FHWA and AASHTO Manuals, the MUTCD, the Highway Capacity Manual and publications of ITE, TRB and other organizations with expertise in transportation engineering.
Envelope - The glass outer enclosure of an incandescent lamp. The envelope seals the gas around the filament and protects the filament. (not used in document)
Exclusive Movement - A movement at a signal that runs without any other movements being allowed to proceed.
Exclusive Pedestrian Phase - A traffic phase allocated to pedestrian traffic only, provides a right of way pedestrian indication to the exclusion of all vehicular phases (all red).
Exclusive Turn Lane - Lane at an intersection where only one movement is allowed.
Execution of Contract or Execution of Document - The performance of all acts (signing, sealing, notarizing, etc.) necessary to render the contract complete.
Extendible Recall - A form of recall where the associated pahse will always service MIN green and further green time is subject to detector extension.
Extension (or Passage) - The timing interval during the extendible portion that is resettable by each detector actuation. The green right of way of the phase may terminate on expiration of the unit extension time.
Fail Safe - A primary safety consideration in equipment and system design. Precludes dangerous signalization when power or equipment malfunctions occur.
Feature - The selected functions of equipment which perform other than elementary signalization control.
Feedback - A system or circuit path which is provided to verify or limit the response to a command. Provides an indication that a command has been executed properly.
FHWA - Federal Highway Administration. A section of the federal government, Department of Transportation, that oversees the nation's highways.
Fiber Optic Cable - Cable that transmits data by light instead of electricity through finely woven glass wires.
Field Terminals - Devices for connecting wires entering the controller assembly.
Filament - The part of an incandescent lamp that produces light. It is constructed of a wire material that converts electricity to light (i.e. tungsten).
Flash Switch - A cabinet switch, when operated, discontinues normal operation and causes the flashing of any predetermined combination of signal lights.
Flasher - A device used to open and close signal circuits at a repetitive rate.
Flasher (Installations) - The equipment, including heads, poles, and supports, that provides a flashing indication for incoming traffic.
Flashing Beacon - Single indication flashing circular yellow or red.
Flashing Indication or Beacon - An indication that flashes at a rate of neither less than 50 nor more than 60 times per minute. The illuminated period of each flash is not less than one-half and not more than two-thirds of the total cycle. For beacons with more than one section or for traffic signal installations, indications may be flashed alternately.
Flashing Operation - Signal plan where indications flash either red or yellow instead of cycling solid red-yellow-green.
Flashing Yellow Arrow – Signal indication that tells the driver they are allowed to drive in the direction the arrow is pointing (left or right) but are required to wait for an adequate gap in the opposing traffic prior to making their turn.
Force Off - A command that will force the termination of the right of way. Used in preemption and coordination.
Free Operation - Signal operating in a non-coordinated mode.
Free Right Turn - A right turn lane that is separated from the adjacent through and/or left turn movements by a triangular shaped island.
Free Turns - Turns that are not under signal or stop control.
Full-Actuated - Identifies a type of intersection control with vehicle detector input capability in all phases.
Gap (Passage) - The maximum time on volume density timed controllers allotted for vehicles to proceed through the intersection.
Gap, Minimum - The lower limit to which the extendable portion or teminating gap may be decreased on volume density time controllers.
Gap-Out - Expiration of gap timer that causes advancement of the phasing sequence.
Gap Reduction - A feature whereby the "unit extension" or allowed time spacing between successive vehicle actuations on the phase displaying the green in the extendible portion of the interval is reduced.
Gaps - Distance or time between two moving vehicles.
Geometrics - (see Intersection Geometrics)
Grades - Slope of roadway in direction of travel.
Green Band - Graphical representation in a time-space diagram that shows the amount of time available to a platoon traveling through a coordinated system.
Guy Cable - Steel cable used to anchor signal poles in span-wire installations.
Hardwire Interconnection - Interconnection method which uses cable strung between controllers in a signal system to send timing plans, system reference, and other data.
HCM - Highway Capacity Manual. Publication by the Transportation Research Board (TRB)
Hertz - (Hz) Cycles per second.
Highway Advisory Radio - A low powered AM radio transmitter used to inform motorists of local roadway or traffic conditions.
Hold - A command that retains the existing right of way.
Inbound - The traffic condition wherein an imbalance exists with a heavier flow towards central points. Also described as the a.m. peak period.
Infrared Detector - With infrared detectors the detection zone is illuminated with low power infrared light. As vehicles pass in the detection zone, the infrared light is reflected from vehicles and is focused by an optical system onto a sensor matrix. A real time signal processing technique analyzes the received signal and determines the presence of a vehicle. Infrared detectors are two types: Active and Passive. Active infrared detectors are most applicable to stop line presence and approach presence detection. Passive infrared detectors detect moving vehicles only, normally up to 300 ft. from the detector unit.
Inhibit - An action or signal that prevents a normal change in the operating sequence of a control. It is used to obtain coordination or other special condition upon command from an external source.
Input File - An assembly in a Type 170 controller cabinet that contains input devices such as detector cards, isolators, etc.
Interchange - Crossing of two or more roads where grades are separated and connections are provided.
Interconnect - Method of communication between controllers that allows for synchronized operation and/or data transfer.
Interconnect Panel - Panel mounted inside a signal cabinet which interfaces with the controller and incoming interconnect wire.
Intersection - At-grade crossing of two or more roads.
Intersection Geometrics - Physical layout of driving surfaces and other features.
Interval - The time period allotted to a specific control function. Does not necessarily result in a signal change but does indicate a transfer of control within the unit to another circuit. Initial interval, extension interval, etc.
Interval, Signal - A period of time in a signal cycle during which all signal indications remain constant.
Isolated Control - Signal not operating in coordinated mode.
ITE - Institute of Transportation Engineers. A national organization made up of traffic and transportation engineers primarily focused on traffic engineering.
Jumper Wire - Any wire used to electrically connect one point to another.
Lagging Left Turn - A protected left turn phase which starts after the beginning of that direction's through phase, or immediately after the completion of that direction's through phase.
Lamp - That part of the optical unit which, when energized, electrically provides the optical unit light source.
Lane Reduction - Closing of a lane on a multi-lane roadway.
Lane Usage - Distribution of vehicle volume by lane or allowable movements in a lane.
Last Car Passage - A feature option on many controllers providing full passage time increment for the vehicle crossing the detector after the allowable gap has timed out.
Lead-In Wire - (Loop Lead-In) The portion of the loop wire between the physical edge of the loop and the pull box; for a magnetic detector and magnetometer it is the wire runnig from the sensor (probe) to the pull box.
Lead-In Cable - (Feeder Cable, Home-Run Cable, Transmission Line) The electrical cable connecting the lead-in wire to the input of the loop detector unit.
Lead-Lag - A phasing scheme where opposing leading and lagging left turns are used for a road. This feature makes it possible to reverse the phase sequence on a phase pair basis. When the phase pairs 1-2, 3-4, 5-6 and 7-8 are reversed, the odd phases lag the even phases instead of leading, as normal.
Leading Left Turn - A protected left turn phase which starts along with or immediately before that direction's through phase.
Lens, Signal - That part of the optical unit through which light from the light source and reflector passes and, in so doing, is redirected into a prescribed pattern and is filtered to a prescribed color.
Level of Service (LOS) - A qualitative measure describing operational conditions within a traffic stream; generally described in terms of such factors as speed and travel time, freedom to maneuver, traffic interruptions, comfort and convenience, and safety. The six LOS grades are defined as:
- A - free flow
- B - reasonably free flow
- C - stable flow
- D - approaching unstable flow
- E - unstable flow
- F - forced or breakdown flow
Light Center Length (LCL) - The dimension from the center of the filament of a light bulb to the tip of the base (including solder on the base eyelet).
Light Emitting Diode - An electronic semiconductor device that converts electrical energy to light.
Limited-Presence Mode - Operation of a detector where output continues for a limited period of time if vehicles remain in the field of influence.
Liquidated Damages - A monetary penalty assessed as part of a contract that penalizes the contractor for late completion or delivery.
Load Switch - A device used to switch power to the signal lamps.
Local Controller - Controller within a coordinated system that is dependent on the master controller for offset reference and timing plan commands.
Locking Memory - Controller setting that "remembers" a vehicle call even if the call from the detector is removed. The vehicle call is locked in the controller's memory until that phase is serviced.
Logic Circuit - Electrical circuit that makes additional decisions by comparing inputs.
Loop - The sensor element imbedded in a roadway that senses a vehicle's passage or presence. It is field fabricated by laying a continuous wire in a roadway slot that is generally rectangular in shape.
Loop Detector - A detector that senses a change in inductance of its inductive loop sensor by the passage or presence of a vehicle near the sensor.
Loop Detector Unit - An electronic device capable of energizing the sensor loop(s), of monitoring the sensor loop(s) inductance and of responding to a predetermined decrease in inductance with an output that indicates the passage or presence of vehicles in the zone of detection. It is the electronics package, exclusive of the loop(s) and lead-in cable.
Louver - A device that is used to limit the view of a signal indication. It is usually a vane-type assembly installed in the signal head visor.
Main Street Green (MSG) - An electrical signal transmitted from the local controller to the central computer during the period when the local controller is displaying green for the coordinated phase.
Magnetometer Detector - A detector that measures the difference in the level of the earth's magnetic forces caused by the passage or presence of a vehicle near its sensor. (not used in article)
Maintenance - The upkeep of signal equipment to keep it functioning properly, including prevention, repair, and record keeping.
Major Street - Through street with highest level of importance.
Malfunction Management Unit (MMU) - A device used to continually check for the presence of conflicting signal indications and other malfunctions and to provide an output in response to conflict or malfunction.
Manual Control - (1) The control used by authorized personnel at an intersection to terminate an active phase on the basis of observations of traffic conditions. Does not permit alteration of clearance intervals. (2) Occasionally supplied in master control systems to permit programming by an operator.
Manual Operation - The operation of a controller assembly by means of a hand-operated device(s).
Manual Pushbutton - An auxiliary device for hand operation of a controller assembly.
Manual Reset - A reset by which it is possible to manually establish the offset.
Master - A control device for supervising and monitoring a system of secondary (local) controllers, maintaining definite time interrelationships and/or accomplishing other supervisory functions. In the case of traffic responsive operation, the master generally includes computation equipment and recording equipment.
Master Controller - Controller within a coordinated system responsible for outputting offset references and timing plans to local controllers. May also serve as a local controller.
Maximum 1 (Max 1) - Usually refers to a time limit applied to traffic actuated controls to terminate the right of way and prevent massive delays to opposing vehicles during heavy traffic. Termination by maximum usually results in a recall placement to prevent trapping of vehicles.
Maximum 2 (Max II) - An optional feature usually associated with coordination methods. Provides two possible maximum time settings for cycle or right of way interval total time before termination begins.
Maximum Green - The maximum green time after an opposing actuation, which may start in the initial (minimum green) portion.
Maximum Initial - A feature of Density Control. The maximum amount of initial green that a phase may receive.
Maximum Recall - A continuous request for service for maximum green timer on selected phases.
Mercury Relay - Relay used to remove power from the load switches when a signal is switched from normal operation to a flashing operation.
Microwave - High-frequency radio data transmission option for system interconnection that does not require wires between controllers.
Microwave Detector - Microwave energy is beamed toward an area of roadway from an antenna mounted overhead or in a sidefire position on a pole. The antenna is angled toward traffic to create a dopplar effect on the reflected signal. When a vehicle passes through the beam, the energy is reflected to the sensing unit (antenna) at a different frequency. The detector, electronic unit, senses the change in frequency which is an indication of the passage of a vehicle. The operating frequency is usually either K-band (24gHz) or Xband (10gHz).
Mid-Block - Location between two intersections.
Minimum Gap - The minimum amount of space between cars, measured in seconds, at the end of the time to reduce (TTR) period.
Minimum Green - The shortest green time of a phase.
Minimum Recall - A continuous request for service for minimum green time on selected phases.
Minor Phase - A vehicle movement requiring protection but does not necessarily require full sophistication as provided for other movements at the intersection.
Minor Street - Street or entrance intersecting the major street.
MIST - Management Information System for Traffic.
Mode - (1) In detection, refers to the pulse width generated by a vehicle actuation. (2) Also used to identify certain logic condition in controls that permit predetermined operation.
Module - A packaged circuit or device easily removed without tools and contains a specified function. Frequently is duplicated to handle similar functions in different areas such as phase modules, load switch modules, etc.
MOV - Metal Oxidized Varistor. Component used for lightning protection.
Movement - Term to identify travel direction and the approach of a vehicle at an intersection.
Multiplexer - A device that combines signals from several communications sources into one communications line.
Multiway Stop - All approaches at an intersection controlled by a STOP sign.
MUTCD - Manual on Uniform Traffic Control Devices. Publication issued regularly by the National Committee on Uniform Traffic Control Devices which is used as a standard for traffic control for most public agencies.
NEMA - National Electrical Manufacturers Association. Commonly refers to standards for traffic controllers that define requirements covering the following areas: functional, environmental, interface levels and physical characteristics. Objective is to attain compatibility and interchangeability of equipment manufactured in conformance with these standards while providing for the safe installation, operation and performance of the equipment.
Non-Actuated - A control type generally in the pretimed category, but can be one phase in any control that does not employ a vehicle call. Right of way timing for the non-actuated phase is fixed regardless of traffic volume except for possible dwell.
Nonlocking Memory - A mode of actuated-controller-unit operation that does not require detector memory.
Occupancy - Indicate a relationship between the number of vehicles in a roadway as compared with the estimated capacity of the roadway. It is a measurement based upon presence time versus gap time averaged over a time period.
Offset - The time difference or interval in seconds or percentage of cycle length between the start of the green indication of the coordinated phase and the system reference signal.
Offset Break - See System Reference.
Omit, Phase - A command causing ommission of a phase.
Optically Limited Signal Head - A signal head designed to limit the view of a signal indication by internal optical masking.
Orientation - Signalization at first turn on or recovery after lengthy power failures of an intersection control.
Output File - An assembly in a Type 170 controller cabinet that serves to control high voltage power to signal lamps. The output file contains load switches, transfer relays and the conflict monitor.
Outside Party - Organization, entity, agency, etc. other than the Commission or MoDOT.
Overlap - A right-of-way indication that allows traffic movement when the right-of-way is being assigned to two or more traffic phases.
Parameter - A definable characteristic of an item, device, or system.
Passage Detection - The ability of a vehicle detector to detect the passage of a vehicle moving through the detection zone and to ignore the presence of a vehicle stopped within the detection zone.
Passage Time - Amount of time it takes a car to leave the loop and get into the intersection.
Peak Hour - Hour during a day with highest volume.
Pedestrian Clearance Interval - The first clearance interval following the pedestrian WALK indication, i.e. flashing DON'T WALK.
Pedestrian Detector - A detector capable of sensing manual operation by a pedestrian.
Pedestrian Indications - Signal displays that control pedestrian movement.
Pedestrian Permissive Period - Provides an independent permissive period for ped timing, usually requiring more guaranteed time than the vehicle.
Pedestrian Phase - A traffic phase allocated to pedestrian traffic that may provide a right-of-way pedestrian indication either concurrently with one or more vehicular phases, or to the exclusion of all vehicular phases.
Pedestrian Recall - With the control activation, pedestrian walk and clearance intervals for the phase are timed once during each cycle without the necessity of a push-button actuation.
Percentage - Used in lieu of expressing time in seconds since many intervals are a portion of the total cycle. The total cycle length can be changed due to coordination. 100 percent is used to indicate the total cycle length and the splits and intervals are a percentage of that total.
Percentile Speed - Information obtained from intersection warrants relating to vehicle speeds in an approach. Example: 80th percentile speed is the speed above which 20% and below which 80% of the speeds occurred in the approach.
Permissive Period - Relates to selected periods during coordinated operation in which a call from the opposing phase can be acknowledged.
Permit - Agreement between MoDOT and an outside party that modifies the roadway at part or all of the outside party's expense.
Permitted Turn - Turning movement where the driver must yield to opposing traffic before completing the movement.
Phase - The part of the signal cycle allocated to any combination of traffic movements receiving the right-of-way simultaneously during one or more intervals. Symbol used: 0.
Phase Diagram - Illustrates the sequence of phases at an intersection, with movement arrows indicating each phase and showing overlaps, concurrent timing, etc.
Phase-Associated Pedestrian Indications - Indications that display WALK and DON'T WALK messages during the green and yellow signal indications for that direction.
Platoon - A group of vehicles or pedestrians traveling together as a group, either voluntarily or involuntarily due to signal control, geometrics or other factors.
Posted Speed - Speed designated by regulatory signing.
Power Distribution Assembly (PDA) - A unit installed in a Type 170 controller cabinet that distributes power, provides circuit protection and surge protection for cabinet equipment.
Preemption - The term used when the normal signal sequence at an intersection is interrupted and/or altered in deference to a special situation such as the passage of a train, bridge opening, or the granting of the right of way to an emergency vehicle.
Presence Detection - The ability of a vehicle detector to sense that a vehicle, whether moving or stopped, has appeared in its field.
Pressure Sensitive Detector - Detector capable of sensing the pressure exerted by a vehicle passing over the surface of its sensor.
Pre-timed Control - Traffic signal control in which the cycle length, phase plan, and phase times are preset, and are repeated continuously according to the preset timing plan.
Printed Circuits - Circuitry constructed on a special insulation material by the formation of a copper conductor directly onto the material in lieu of using a wire to interconnect to points.
Projected Values - Anticipated traffic volumes based on historic data from similar locations.
PROM - Program Read-Only Memory. Device that stores data that cannot be altered by computer instructions.
Progression - Coordination method based upon offsets between intersections along an artery. Determined by time required to travel from one controlled intersection to the next and adjusted to give artery right of way without delay.
Protected Turn - Turning movement that is indicated on the signal faces with a green arrow in the desired direction of travel.
Protected-Permissive Turn Movement - A movement that is protected during part of the signal cycle and permitted during another part of the signal cycle.
Pull Box - In-ground box which houses cable runs connected by conduit and is accessed by removing a heavy cover plate that protects the contents from damage. Splice between lead-in cable and loop lead-in wire are located here.
Pulse Detection - Detection type that produces a short output pulse upon detection, even if the vehicle remains in the detection zone.
Push Button Detector - A push button device used to place a call into the signal controller. Typically used with actuated pedestrian indications.
Quadrapole - A detector loop configuration that adds a longitudinal sawcut along the center of the rectangle, so that the wire can be installed in a figure-8 pattern thereby producing four electromagnetic poles instead of the normal two. The design improves the sensitivity to small vehicles and also minimizes calls from adjacent or nearby loops.
Queue - A line of vehicles or persons waiting to be served by a system.
Radar Detector - A detector that is capable of sensing the passage of a vehicle through its field of emitted microwave energy.
Ramp Metering - A traffic signal installation placed on a freeway on ramp used to control the rate of flow of traffic entering the freeway traffic stream.
Ramp Terminal - The end of a ramp that intersects with a grade separated crossroad.
Random Access Memory (RAM) - A storage device with both read and write capabilities allowing random access to stored data.
Rated Initial Lumens - The average amount of luminous flux (light) produced by a statistically acceptable sample of lamps on operation at rated voltage after having been seasoned by one half to one percent of rated life
Read-Only Memory (ROM) - A storage device not alterable by computer instructions; e.g., magnetic core storage with a lock-out feature.
Rear-End Crashes - Crash involving one vehicle striking the back of the vehicle in front.
Recall - An operational mode for an actuated controller whereby a phase, either vehicle or pedestrian, is displayed each cycle whether demand exists for it or not. Usually in a temporary or emergency situation. See MIN Recall, MAX Recall, PED Recall, Soft Recall and Extendable Recall.
Red Clearance Interval - A clearance interval that may follow the yellow change interval, during which both the terminating phase and the next right of way phase display red.
Red Rest - A feature whereby, in the absence of demand, the controller will return to all red instead of resting in green of the last serviced phase.
Remote Flash - An input, when energized, causing normal signal operation to be discontinued and specified signal light to be operated in a flashing manner.
Rest - The interval portion of a phase when present timing requirements have been completed.
Revision marking - Using specific type styles to indicate proposed additions and deletions to a document.
Right Angle Crash - Collision/crash where a vehicle traveling in one direction collides with a vehicle approaching from the left or right.
Right of Way (ROW)- (1) Period of time where it is lawful for a vehicle to proceed. (2) MoDOT-owned property associated with roadway.
Ring - Two or more sequentially timed and individually selected conflicting phases so arranged as to occur in an established order.
ROW & Construction Program - Long-term list of highway projects proposed by MoDOT.
RTOR - Right Turn On Red. Where right turns are permitted on a steady red indication after a stop.
Saturation Flow Rate - The maximum rate of flow that can pass through a given lane group under prevailing traffic and roadway conditions, assuming that the lane group has 100 percent of real time available as effective green time. Serial Communications - Transmission of digital data over a single line in a sequential fashion.
Secondary Controller Operation (slave) - A method of operating the traffic signals under the supervision of a master controller.
Semi-Actuated - Identifies type of intersection control with one or more phase lacking vehicle detector input capability.
Sensor - The device(s) located in a roadway that is acted upon directly by a vehicle to create a usable pulse to an intersection control.
Sequence - The order that signal intervals are displayed around an intersection on a PRE-TIMED control or with calls all around on an actuated control.
Shared Lane - A lane at an intersection that is shared by two or three movements.
Sideswipes - Crashes where vehicles traveling in the same or opposite direction collide on the sides or side of vehicles.
Sight Distance - Distance of a driver's first perception of an object or feature.
Signal Face - A section or combination of sections, each capable of displaying its indication in one directions.
Signal Head - An assembly containing one or more signal faces.
Signal Indication - The illumination of a signal lens (or an equivalent device) whereby the movement of vehicular or pedestrian traffic is controlled.
Signal Installation - The traffic signal equipment, signal heads, supports, and electrical circuitry necessary to control vehicle and/or pedestrian traffic at an intersection.
Signal Load Switch - A device used to switch power to the signal lamps.
Signal Mast Arm - A structural support over the roadway, extending from a pole, for the purpose of supporting the signal head(s).
Signal Pedestal - A vertical support on top of which the signal head or controller assembly is mounted.
Signal Plan - The set of parameters that define the sequence, timing, and other operational aspects of signal control.
Signal Progression - See Coordination.
Signal Section - The optical unit and housing capable of displaying one indication.
Signal Sequence - The order of appearance of signal indications during successive intervals of a cycle.
Signals - Any power-operated traffic control device, other than a barricade warning light or steady burning electric lamp, by which traffic is warned or directed to take some specific action.
Signals, Permanent - Signal installation constructed using steel poles and mast arms with underground wiring.
Signals, Temporary - Signal installation constructed using poles with signal heads hung from span wires and external wiring.
Skip Phase - A feature of actuated intersection controls that omits signalization in a phase or movement that does not have any vehicle or pedestrian calls. Reduces delay by progressing through sequence without timing inactive movements.
Soft Recall - Recall option that allows the recalled phase to be skipped provided there is no demand on the recalled phase and a conflicting call to another phase is detected by the controller.
Soft Flash - Flash plan where the flashing indications are programmed in the controller instead of wired in the cabinet.
Solid-State Device - A device that is characterized by electrical circuits, the active components of which are semi-conductors, to the exclusion of electromechanical devices or tubes.
Span Wire - One or more cables used as a means of support for traffic control devices.
Special Event - Event occurring outside of normal operating hours that produces traffic.
Split - A division of the cycle length allocated to each of the various phases (expressed in seconds or percent).
Split Phasing - Serving an intersection one approach at a time.
State Highway System - All roadways owned and operated by MoDOT.
Steady Signal Indication - Indication that is constantly illuminated, generally for 3 or more seconds.
Stop Bar - Thick white line that designates the appropriate stopped location for the first
Stop Time - A feature in many intersection controls permitting the application of an external signal to stop any one or all interval timers from timing further.
Storage - Ability of a lane to keep waiting vehicles out of the way of moving traffic, expressed in either feet or number of vehicles.
Storage Area - The area, usually expressed in terms of the number of vehicles, between the stop line and the detector located upstream. In pre-timed control it may be the number of vehicles arbitrarily used to determine green time.
Synchronization - (1) Motors or cam switches designed to remain in step or in the correct rotational relationship at all times. (2) Internal circuitry used to be sure several circuits begin actions from the same reference point.
System Reference - An output or signal during coordinated operation that starts the offset timer.
System, Signal - Network of two or more signals in coordination.
System Status - The operational condition of each monitored remote control unit or data sensing station in the system as indicated bya display or a print-out.
TBC - Time Based Coordination. Coordination where controllers in a system are synchronized by a clock and not a master controller.
TBR - Time Before Reduction. Time period that begins when the phase is green and there is a serviceable conflicting call. After this period expires, the TTR begins to time down.
Tee Intersection - An intersection with three approaches in which one approach meets the other two at approximately right angles.
Telemetry - A specific level or value of a parameter above or below a change of activity will occur.
Terminate - Applies most frequently in traffic control to the end of a timing interval. Termination of right of way begins in an active phase when a call is received from an inactive phase in a full-actuated control; right of way termination must always include adequate clearance intervals.
Tether Cable - Cable used to stabilize overhead signal heads in a span-wire installation.
Threshold - A specific level or value of a parameter that above or below a change of activity will occur.
Through Movement - A movement at an intersection that continues in a straight line and across the intersection; does not turn in any direction.
Time Base Coordination - Coordinated operation in response to internally generated time clock commands selecting cycle, split and offset.
Time Clock - A device for the automatic selection of modes of operation of traffic signals in a manner prescribed by a predetermined time schedule.
Time Out - The state of an interval timer that was timed for the preset period and has produced a signal so indicating. At time out, the timing device ceases to be in control since another action is initiated by the timing out of the timer.
Time/Space Diagram - A semi-pictorial presentation of two or more intersections being coordinated that shows the offset, cycle and distance relationships.
Time Waiting - Related to cars waiting in volume-density control methods, but represents the time vehicles must wait in an opposing phase before termination of the phase having right of way begins. Imposing restrictive action upon the phase having the right of way to decrease delay in opposing phases.
Timing - Values, usually in seconds, which give lengths of signal indications and other functions.
Tone Interconnection - Inefficient interconnection method that transmits timing plans and offset breaks.
Traffic Actuated - A type of traffic control signal where the intervals are varied according to the demands of traffic as registered by detector actuation.
Traffic Adjusted - Term used in master supervisory systems where vehicle actuations and other data are fed to the master for effecting signalization changes at several intersections rather than at each intersection independently.
Traffic Count - Summary of traffic volumes by approach and/or time period.
Transfer Relay - Relay used to transfer control between normal signal operation and flashing operation. When the relay is energized, the signal operates off the load switches and when the relay is de-energized, the signal operates in a flash mode.
Traffic Responsive System - A system where a master controller specifies cycle/split/offset on the real-time demands of traffic as sensed by vehicle detectors.
Trailing Green - Usually an overlap phase coupled with a through phase that permits a turn movement to continue a fixed interval after the through movement has been stopped.
TRB - Transportation Research Board. Research organization sponsored by AASHTO and the Federal Highway Administration.
TTR - Time To Reduce. Time period by which the measured gap between vehicles is reduced from the passage time to a value equal to the minimum gap. A measured gap greater than the instantaneous calculated gap shall cause the phase to be terminated due to gap out.
Turning Radius - Designed radius that allows for the turning of a certain type of vehicle through an intersection.
Two-Way Stop - Two approaches at an intersection controlled by a STOP sign, with the major movements allowed to continue through.
Type 170 - Alternate signal controller design that stresses interchangeability and simplified maintenance by building all components to the same hardware specification.
Ultrasonic Detector - Ultrasonic detectors use the same principle as the microwave detectors: a transducer transmits a beam of energy into an area and receives a reflected beam from a vehicle. The sonic detector transmits pulses of ultrasonic energy (20 to 50 kHz at 20 to 25 times/sec) through a transducer. The passage of a vehicle causes a beam to be reflected to the transducer at a different frequency. The transducer senses the change in frequency and converts it to electrical energy. This energy is relayed to a transceiver, that then sends an impulse to the controller unit to denote passage of a vehicle.
Upstream - The direction from which traffic is flowing. Vehicles are to obey the red indication or STOP message.
Visor - The part of a signal section that protects the lens face from direct ambient light.
Voice Grade Lines - A channel suitable for transmission of speech, digital, analog or facsimile data, generally in the frequency range of 300 to 3000 Hz.
Volume - The number of persons or vehicles passing a point on a lane, roadway, or other trafficway during some time interval, often taken to be 1 hour, expressed in vehicles or persons.
Volume Classification - Summary of vehicle types (i.e. trucks, buses, cars) by percentage of volume.
Volume Density - A process used with detectors located at, or at sufficient distance in advance of, the intersection that makes use of vehicle actuation quantities and time-of-waiting of the vehicles to vary green interval portions for increased capacity and minimized delays.
VPH- Vehicles Per Hour, the total number of vehicles passing a given point per hour.
Warrants - Criteria used to determine the need for traffic signals or other traffic control devices that presents quantitative procedures to solving many traffic engineering problems.
Yellow Change Interval - The first interval following the green right-of-way interval in which the signal indication for that phase is yellow.
Yielding Movement -Turn movement that is required to yield to conflicting movements that have higher priority.
Zone of Detection - (Area of Detection, Effective Loop Area, Field of Influence, Sensing Zone) The area of the roadway within which a vehicle is to be detected by a vehicle detector system.
