National Academies Press: OpenBook

Signal Timing Manual - Second Edition (2015)

Chapter: Appendix A - Glossary

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Suggested Citation:"Appendix A - Glossary ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Appendix A. Glossary A-1 APPENDIX A. GLOSSARY The following is a collection of terms used in this edition of the Signal Timing Manual. Some terms may have different deinitions in other references, but the deinitions below are consistent with the usage of terms in this manual. Actuated Signal Control—Phase time based on detection. See also Fully-Actuated Control and Semi-Actuated Control. Adaptive Signal Control—An advanced signal system that does not operate with time- of-day plans. Analysis Period (or Time Interval)—A single time period during which capacity analysis is performed on a transportation facility. If the demand exceeds capacity during an analysis period, consecutive analysis periods can be selected to account for the initial queue from the previous analysis period. Analytical Model—A model that relates system components using theoretical considerations (tempered, validated, and calibrated by ield data). Approach—A set of lanes at an intersection that accommodates all left-turn, through, and right-turn movements from a given direction. Approach Grade—The grade of an intersection approach, expressed as a percentage (with positive values for upgrade and negative for downgrade). Arterial—A signalized street that primarily serves through trafic and that secondarily provides access to abutting properties, with signals spaced 2 miles (or less) apart. Average Speed—The average distance a vehicle travels within a measured amount of time. Typically, average speed is measured over a short distance. See also Average Travel Speed. Average Travel Speed—The length of the highway segment divided by the average travel time of all vehicles traversing the segment, including all stopped delay times. See also Average Speed. Back of Queue—The distance between the stop line of a signalized intersection and the farthest reach of an upstream queue, expressed as a number of vehicles. The vehicles previously stopped at the front of the queue are counted even if they begin moving. See also Queue. Bandwidth—The maximum amount of green time for a designated coordinated movement as it passes through a corridor at an assumed constant speed, typically measured in seconds. Barnes’ Dance—A common term for an exclusive pedestrian phase where pedestrians may cross all intersection legs (and sometimes diagonally). See also Pedestrian Phase. Barrier—A reference point in the sequence where two or more rings are interlocked. Barriers ensure that there will be no concurrent selection and timing of conlicting movements in different rings. Bike—A vehicle type which may require special timing consideration. Cabinet—An all-weather enclosure that houses the intersection ield equipment. Signal Timing Manual, Second Edion

A-2 Appendix A. Glossary Call—A request for service. Some movements may have separate calls for different user types. For example, a transit priority call is one where the normal phase timing may be altered due to a call from a transit vehicle. More typical calls are vehicle calls and pedestrian calls. Capacity—The maximum rate at which vehicles can pass through an intersection under prevailing conditions. Change Interval—The yellow change interval plus red clearance interval that occurs between phases of a trafic signal in order to clear the intersection before conlicting movements are released. Concurrent Phases—Two or more phases in separate rings that are able to operate together without conlicting movements. Congested Flow—A trafic-low condition caused by a downstream bottleneck. Controller—The piece of hardware that determines how a trafic signal responds to calls based on signal timing parameters. Controller Memory—A term that refers to a controller’s ability to “remember” (i.e., retain) a detector actuation or not remember a detector actuation. There are two modes (non-locking and locking). The locking mode remembers the actuation after it is dropped by the detection unit; the non-locking mode does not remember the actuation. Coordinated Phase(s)—The phase (or phases) that are given a ixed minimum amount of time each cycle under a coordinated timing plan. This phase is typically the major through phase on an arterial. Coordinated phase(s) may also have an optional actuated interval following the ixed interval. Coordination (or Coordinated)—The ability to synchronize multiple intersections to enhance the operation of one or more directional movements in a system. Corridor—A set of essentially parallel transportation facilities designed for travel between two points. A corridor contains several subsystems, such as freeways, arterials, transit, and pedestrian and bicycle facilities. Critical Movement Analysis—A simpliied technique used to identify the critical movements at an intersection, estimate whether the intersection is operating adequately, and approximate the amount of green time needed for each critical movement. Crosswalk—A marked area for pedestrians crossing the street at an intersection or designated midblock location. Cycle Length—The duration of a complete sequence of phases in the absence of priority calls. In an actuated controller unit, a complete cycle is dependent on the presence of calls for all non-priority phases. Some indications may be served more than once in a cycle. Occasionally, an indication may not be part of a normal cycle (e.g., a left-turn arrow may only be displayed during railroad preemption). Signal Timing Manual, Second Edion

A-2 Appendix A. Glossary Call—A request for service. Some movements may have separate calls for different user types. For example, a transit priority call is one where the normal phase timing may be altered due to a call from a transit vehicle. More typical calls are vehicle calls and pedestrian calls. Capacity—The maximum rate at which vehicles can pass through an intersection under prevailing conditions. Change Interval—The yellow change interval plus red clearance interval that occurs between phases of a trafic signal in order to clear the intersection before conlicting movements are released. Concurrent Phases—Two or more phases in separate rings that are able to operate together without conlicting movements. Congested Flow—A trafic-low condition caused by a downstream bottleneck. Controller—The piece of hardware that determines how a trafic signal responds to calls based on signal timing parameters. Controller Memory—A term that refers to a controller’s ability to “remember” (i.e., retain) a detector actuation or not remember a detector actuation. There are two modes (non-locking and locking). The locking mode remembers the actuation after it is dropped by the detection unit; the non-locking mode does not remember the actuation. Coordinated Phase(s)—The phase (or phases) that are given a ixed minimum amount of time each cycle under a coordinated timing plan. This phase is typically the major through phase on an arterial. Coordinated phase(s) may also have an optional actuated interval following the ixed interval. Coordination (or Coordinated)—The ability to synchronize multiple intersections to enhance the operation of one or more directional movements in a system. Corridor—A set of essentially parallel transportation facilities designed for travel between two points. A corridor contains several subsystems, such as freeways, arterials, transit, and pedestrian and bicycle facilities. Critical Movement Analysis—A simpliied technique used to identify the critical movements at an intersection, estimate whether the intersection is operating adequately, and approximate the amount of green time needed for each critical movement. Crosswalk—A marked area for pedestrians crossing the street at an intersection or designated midblock location. Cycle Length—The duration of a complete sequence of phases in the absence of priority calls. In an actuated controller unit, a complete cycle is dependent on the presence of calls for all non-priority phases. Some indications may be served more than once in a cycle. Occasionally, an indication may not be part of a normal cycle (e.g., a left-turn arrow may only be displayed during railroad preemption). Signal Timing Manual, Second Edion Appendix A. Glossary A-3 Decision Zone—An area in front of a stop bar where some drivers would choose to stop and others would choose to proceed through the intersection, upon the change from a green to yellow indication. Detection designs can be used to reduce the probability of drivers having to make this type of decision on high-speed approaches. This area is also known as option zone, indecision zone, and Type II dilemma zone. The term decision zone is used in this manual to avoid confusion with dilemma zones that occur due to improper clearance. See also Dilemma Zone. Delay—(1) The additional travel time experienced by a driver, passenger, or pedestrian. (2) A detector parameter typically used with stop-bar presence-mode detection for turn movements from exclusive lanes. Demand—The number of users desiring to use an intersection, approach, or movement during a particular time period. Not to be confused with volume, which is a measure of the number of users accommodated at the intersection (which is limited to the available capacity). Density—The number of vehicles on a roadway segment averaged over space, usually expressed as vehicles per mile or vehicles per mile per lane. See also Volume-Density. Detector (or Detection)—A device used to count and/or determine the presence of a motorized vehicle, bicycle, or pedestrian. See also Setback Detection and Stop-Bar Detection. Detector Card(s)—The detector processor module that is installed in a detector rack within a cabinet. Detector Delay—See Delay. Detector Extend—See Extend. Detector Rack—The hardware module that holds detector cards within a cabinet. Detector Switching—A common detector function in traf­ic signal controllers that allows detectors to extend calls for one phase (extend phase) and then send calls to another phase (switch phase) once the extend phase ends. Detector switching allows the programmed switch phase to be extended after the extend phase terminates. It is typically only effective when the switch phase is green. Also, detector switching does not typically switch phase calls. Dilemma Zone—A condition that occurs when yellow change and red clearance times are too short for a driver to either stop or clear the intersection before the beginning of a con­licting phase. Also known as Type I dilemma zone. See also Decision Zone. Display (or Head, Signal Group)—A combination of indications (e.g., red, yellow, green, green arrow, audible) grouped together for controlling one or more movements. Double Cycle—A cycle length that allows phases at an intersection to be served twice as often as the phases at other intersections in the coordinated system. Downstream—The direction of traf­ic ­low. Generally the discharge side of an intersection. Dual Entry—The parameter used to call vehicle phases that can time concurrently, even if only one of the phases is receiving an active call. For example, if dual entry is active for Phases 4 and 8, and Phase 4 receives a call but no call is placed on Phase 8, Phase 8 will still be displayed along with Phase 4. The most common use of dual entry is to activate the parameter for compatible through movements. Signal Timing Manual, Second Edion

A-4 Appendix A. Glossary Early Return to Green—A term used to describe the servicing of a coordinated phase in advance of its programmed begin time as a result of unused time from uncoordinated phases. Effective Green Time—The time during which a given trafic movement (or set of movements) may proceed; it is equal to the cycle length minus the effective red time. In a practical sense, effective green time is equal to actual green time, as the start-up lost time is approximately equal to the amount of time during the yellow change interval when vehicles are still entering the intersection. Effective Red Time—The time during which a given trafic movement (or set of movements) is not moving into the intersection; it is equal to the cycle length minus the effective green time. Exclusive Pedestrian Phase—A separate phase that is conigured such that no vehicular movements are served concurrently with pedestrian trafic. See also Pedestrian Phase. Exclusive Turn Lane—A designated left- or right-turn lane (or lanes) used only by vehicles making those turns. Extend—A detector parameter that extends a detector actuation by a programmable ixed amount of time. It is typically used with detection designs that combine multiple setback detectors for safe phase termination of high-speed intersection approaches or to provide lane-by-lane detection. Firmware—The software embedded in the trafic signal controller that operates the trafic signal system. Features may vary by irmware version. Fixed Force-Off—A mode of split management used with coordinated operations, where force-off points cannot move. Under this mode, uncoordinated phases can utilize unused time from previous phases. See also Force-Off. Fixed Time Signal Control—See Pretimed Signal Control. Flashing Don’t Walk—An indication warning pedestrians that the walk indication has ended and the don’t walk indication is underway. The pedestrian clearance interval may be longer than the lashing don’t walk interval, as it can include the yellow change and red clearance times. Flashing Yellow Arrow—A type of signal head display that reduces “yellow trap” problems by providing a permitted indication that operates concurrently with the opposing through movement. It is physically separated from the adjacent through movement, minimizing the association of the through yellow with the left-turn yellow. Floating Car Method—A commonly employed technique for travel time runs, which requires the vehicle driver to “loat” with the trafic stream while traveling at a speed that is representative of the other vehicles on the roadway (i.e., pass as many vehicles as pass the loating car). Floating Force-Off—A force-off mode where force-off points can move depending on the demand of previous phases. Under this mode, uncoordinated phases are limited to their deined split times, and all unused time is dedicated to the coordinated phases. Essentially, the split time is treated as a maximum amount for the uncoordinated phases. See also Force-Off. Signal Timing Manual, Second Edion

A-4 Appendix A. Glossary Early Return to Green—A term used to describe the servicing of a coordinated phase in advance of its programmed begin time as a result of unused time from uncoordinated phases. Effective Green Time—The time during which a given trafic movement (or set of movements) may proceed; it is equal to the cycle length minus the effective red time. In a practical sense, effective green time is equal to actual green time, as the start-up lost time is approximately equal to the amount of time during the yellow change interval when vehicles are still entering the intersection. Effective Red Time—The time during which a given trafic movement (or set of movements) is not moving into the intersection; it is equal to the cycle length minus the effective green time. Exclusive Pedestrian Phase—A separate phase that is conigured such that no vehicular movements are served concurrently with pedestrian trafic. See also Pedestrian Phase. Exclusive Turn Lane—A designated left- or right-turn lane (or lanes) used only by vehicles making those turns. Extend—A detector parameter that extends a detector actuation by a programmable ixed amount of time. It is typically used with detection designs that combine multiple setback detectors for safe phase termination of high-speed intersection approaches or to provide lane-by-lane detection. Firmware—The software embedded in the trafic signal controller that operates the trafic signal system. Features may vary by irmware version. Fixed Force-Off—A mode of split management used with coordinated operations, where force-off points cannot move. Under this mode, uncoordinated phases can utilize unused time from previous phases. See also Force-Off. Fixed Time Signal Control—See Pretimed Signal Control. Flashing Don’t Walk—An indication warning pedestrians that the walk indication has ended and the don’t walk indication is underway. The pedestrian clearance interval may be longer than the lashing don’t walk interval, as it can include the yellow change and red clearance times. Flashing Yellow Arrow—A type of signal head display that reduces “yellow trap” problems by providing a permitted indication that operates concurrently with the opposing through movement. It is physically separated from the adjacent through movement, minimizing the association of the through yellow with the left-turn yellow. Floating Car Method—A commonly employed technique for travel time runs, which requires the vehicle driver to “loat” with the trafic stream while traveling at a speed that is representative of the other vehicles on the roadway (i.e., pass as many vehicles as pass the loating car). Floating Force-Off—A force-off mode where force-off points can move depending on the demand of previous phases. Under this mode, uncoordinated phases are limited to their deined split times, and all unused time is dedicated to the coordinated phases. Essentially, the split time is treated as a maximum amount for the uncoordinated phases. See also Force-Off. Signal Timing Manual, Second Edion Appendix A. Glossary A-5 Flow Rate—The equivalent hourly rate at which vehicles, bicycles, or pedestrians pass a point on a lane, roadway, or other traficway. Computed as the number of vehicles, bicycles, or pedestrians passing the point, divided by the time interval (usually less than 1 hour) in which they pass, and expressed as vehicles, bicycles, or pedestrians per hour. Force-Off—A point within a cycle where a phase must end regardless of continued demand. These points in a coordinated cycle ensure that the coordinated phase begins with enough time to maintain the designated offset. However, force-offs cannot override clearance times. See also Fixed Force-Off and Floating Force-Off. Free Flow—A low of trafic unaffected by upstream or downstream conditions. Free Operation—See Uncoordinated. Fully-Actuated Control—A signal operation in which vehicle detectors on each approach to the intersection control the occurrence and length of every phase. See also Actuated Signal Control. Gap—The time separation between vehicles (in seconds). In other words, the time required for the front bumper of the second of two successive vehicles to reach the ending point of the rear bumper of the irst. It is less than the headway between vehicles. Gap Out—The passage timer has expired (timed out). Gap Reduction—This is a feature that reduces the passage time to a smaller value while the phase is active. Green Time—The duration (in seconds) of the green indication for a given phase at a signalized intersection. Hardware—The devices that physically operate the signal timing controls, including the controller, detectors, signal heads, and signal monitor. Hardware-in-the-Loop (HITL)—A means of providing a direct linkage between simulation models and actual signal controllers. Headway—(1) The time (in seconds) between two successive vehicles as they pass a point on the roadway, measured from the same common feature of both vehicles (e.g., the front axle or the front bumper). (2) The time (usually expressed in minutes) between the passing of the front ends of successive transit units (vehicles or trains) moving along the same lane or track (or other guideway) in the same direction. Indication—See Display. Individual Signalized Intersection—An intersection controlled by a trafic signal. They are sometimes operated as an “isolated intersection,” which refers to a mode of operation, rather than a spatial relationship. Individual intersections may also be described as “free” operation, which indicates they are not currently being coordinated. An individual intersection may also be operated as part of a coordinated system for all or part of a day. Inhibit Max—A basic timing parameter that removes maximum green as a phase parameter during coordination and allows a phase to extend beyond its normal maximum green time. Input(s)—The contact closure from detectors that tells the trafic signal controller of the presence of vehicles (in general or by type) and pedestrians. Signal Timing Manual, Second Edion

A-6 Appendix A. Glossary Intersection Delay (Average)—The total additional travel time experienced by users (as a result of control measures and interactions with other users) divided by the volume departing from an intersection. See also Delay. Interval—The duration of time during which trafic signal indications (e.g., red, yellow, green, and lashing don’t walk) do not change state (i.e., red interval, yellow interval, green interval, and lashing don’t walk interval). Isolated Operation—An intersection that is not currently being operated as part of a coordinated system. Also known as free operation. See also Uncoordinated. Lane Assignment—Movements that are permitted from a speciic lane. Lane Utilization—The distribution of vehicles among lanes when two or more lanes are available for a movement. When lane utilization is unequal due to trafic patterns upstream or downstream, additional green time may be necessary beyond what would be needed for uniform lane distribution. Leading Pedestrian Interval—A pedestrian interval option that starts a few seconds before the adjacent through vehicular phase, allowing pedestrians to establish a presence in the crosswalk, and thereby reducing conlicts with turning vehicles. Lead-Lag Left-Turn Phasing—A left-turn phase sequence where one left-turn movement begins with the adjacent through movement and the opposing left-turn movement begins at the end of the conlicting through movement. This option may create a “yellow trap” with permitted signal displays that do not use the lashing yellow arrow. Load Switch (or Switch Pack)—A device that allows the controller, which operates in a 12/24-volt DC environment, to direct a 120-volt AC current to various signal displays. Local Controller—See Controller. Locking Mode—A controller memory mode used to trigger a call for service with the irst actuation received by the controller during the red interval. Typically only used when there is no stop-bar detection. See also Controller Memory. Lost Time—The time per signal cycle during which the intersection is effectively not used by any movement; this occurs during the yellow change and red clearance intervals (clearance lost time) and at the beginning of most phases (start-up lost time). Manual on Uniform Trafic Control Devices for Streets and Highways (MUTCD)— The MUTCD, published by the Federal Highway Administration (FHWA), provides the standards and guidance for installation and maintenance for trafic control devices on roadways. Master Clock—The background timing mechanism within the controller logic to which each controller is referenced during coordinated operations. Master Controller—An optional component of a signal system that facilitates coordination of a signal system with local controllers. Max Out—The maximum green has been reached. Maximum Green—The maximum length of time that a phase can be green in the presence of a conlicting call. Maximum Initial—The maximum period of time for which the added initial can extend the initial green period. This cannot be less than the minimum green time. Signal Timing Manual, Second Edion

A-6 Appendix A. Glossary Intersection Delay (Average)—The total additional travel time experienced by users (as a result of control measures and interactions with other users) divided by the volume departing from an intersection. See also Delay. Interval—The duration of time during which trafic signal indications (e.g., red, yellow, green, and lashing don’t walk) do not change state (i.e., red interval, yellow interval, green interval, and lashing don’t walk interval). Isolated Operation—An intersection that is not currently being operated as part of a coordinated system. Also known as free operation. See also Uncoordinated. Lane Assignment—Movements that are permitted from a speciic lane. Lane Utilization—The distribution of vehicles among lanes when two or more lanes are available for a movement. When lane utilization is unequal due to trafic patterns upstream or downstream, additional green time may be necessary beyond what would be needed for uniform lane distribution. Leading Pedestrian Interval—A pedestrian interval option that starts a few seconds before the adjacent through vehicular phase, allowing pedestrians to establish a presence in the crosswalk, and thereby reducing conlicts with turning vehicles. Lead-Lag Left-Turn Phasing—A left-turn phase sequence where one left-turn movement begins with the adjacent through movement and the opposing left-turn movement begins at the end of the conlicting through movement. This option may create a “yellow trap” with permitted signal displays that do not use the lashing yellow arrow. Load Switch (or Switch Pack)—A device that allows the controller, which operates in a 12/24-volt DC environment, to direct a 120-volt AC current to various signal displays. Local Controller—See Controller. Locking Mode—A controller memory mode used to trigger a call for service with the irst actuation received by the controller during the red interval. Typically only used when there is no stop-bar detection. See also Controller Memory. Lost Time—The time per signal cycle during which the intersection is effectively not used by any movement; this occurs during the yellow change and red clearance intervals (clearance lost time) and at the beginning of most phases (start-up lost time). Manual on Uniform Trafic Control Devices for Streets and Highways (MUTCD)— The MUTCD, published by the Federal Highway Administration (FHWA), provides the standards and guidance for installation and maintenance for trafic control devices on roadways. Master Clock—The background timing mechanism within the controller logic to which each controller is referenced during coordinated operations. Master Controller—An optional component of a signal system that facilitates coordination of a signal system with local controllers. Max Out—The maximum green has been reached. Maximum Green—The maximum length of time that a phase can be green in the presence of a conlicting call. Maximum Initial—The maximum period of time for which the added initial can extend the initial green period. This cannot be less than the minimum green time. Signal Timing Manual, Second Edion Appendix A. Glossary A-7 Maximum Recall— A recall mode that places a continuous call on a phase. See also Recall. Measure of Effectiveness (MOE)—A quantitative parameter indicating the performance of a transportation facility or service. Memory Mode(s)—See Controller Memory. Minimum Gap—The volume-density parameter that speciies a minimum time between detector actuations (that is applied with the gap reduction feature). See also Gap Reduction. Minimum Green—The minimum length of time that a phase must be green. It should be set based on driver expectancy and the storage of vehicles between the setback detectors and the stop bar (if stop bar presence detection is not used). Minimum Recall—A recall parameter that times the minimum green for a phase, regardless of the demand on that movement. See also Recall. Modiier Phase—A phase associated with an overlap that causes the overlap to be red when the modiier phase is green. See also Overlap. Movement—A term used to describe the user (e.g., vehicle or pedestrian) action taken at an intersection (e.g., vehicle turning movement or pedestrian crossing). Two different types of movements include those that have the right-of-way (protected/exclusive) and those that must yield (permitted/permissive), consistent with the rules of the road or the Uniform Vehicle Code. See also Permitted Movement and Protected Movement. Movement Priority—Each movement may be assigned a relative or absolute priority depending on the operating environment and locally desired outcomes. Non-Locking Mode—A controller memory mode that does not retain an actuation in the controller after the actuation is dropped by the detection unit. See also Controller Memory. Occupancy—The time that a detector indicates a vehicle is present. May also be expressed as a percent. Offset—The time relationship between the coordinated phase(s) based on the offset reference point and a deined master reference (i.e., master clock or sync pulse). Offset Reference Point (or Coordination Point)—The deined point that creates an association between a signalized intersection and the master clock. Operating Environment—An area with similar characteristics that would have similar signal timing objectives. Operating System—The controller processor system on which the irmware runs. Operational Objectives—The desired signal timing outcomes for each operating environment and user group. Output—The voltage from a load switch that powers a signal indication. Overlow Queue—Queued vehicles that are present at the beginning of green and that are not served during the green interval at a signalized intersection, so must be served by a subsequent cycle (or cycles). Overlap—A timing process that provides a way to operate a particular movement with one or more phases. It is a separate output that can use special logic to improve operations. Signal Timing Manual, Second Edion

A-8 Appendix A. Glossary Oversaturation—A trafic condition in which the arrival low rate exceeds capacity. Parent Phase—A phase (or phases) used to determine whether an overlap should be active. Parent phases may be in different rings and/or may be on different sides of a barrier. See also Overlap. Passage Time (or Vehicle Interval, Vehicle Extension, Gap Time)—A phase timer that ends a phase when the time from the last detector output to the controller exceeds the timer setting. Pedestrian—An individual traveling on foot. Pedestrian Clearance Interval—The time provided for a pedestrian to cross the entire width of an intersection. This interval is longer than the lashing don’t walk interval, as it can include the yellow change and red clearance intervals. Pedestrian Phase—Time allocated to pedestrian trafic that is typically concurrent with compatible vehicular phase(s). See also Barnes’ Dance and Exclusive Pedestrian Phase. Pedestrian Recall—A recall mode where there is a continuous call for pedestrian service, resulting in the pedestrian walk and clearance intervals timing every cycle. See also Recall. Pedestrian Scramble—See Barnes’ Dance. Pedestrian Walk Interval—An indication that allows pedestrians to begin crossing the intersection. Pedestrian Walking Speed—The average walking speed of pedestrians (in feet per second). Performance Measures—Quantiiable means that are used to assess whether a signal system has achieved its operational objectives. Examples include stops, arrival on green, phase failure, arterial travel time, vehicle delay, and existence of spillback queuing from turn bays or between closely spaced intersections. Permissive Movement—See Permitted Movement. Permissive Period—A period of time during the coordinated cycle in which calls on conlicting phases will result in the coordinated phase transitioning to an uncoordinated phase. Permitted Movement—A movement that is allowed to proceed if there are available gaps in the conlicting low. Also known as a permissive movement (MUTCD term). See also Movement. Phase—A timing unit associated with the control of one or more movements. Phases are often assigned to vehicular and pedestrian movements. Phase Failure—The occurrence of one or more stopped vehicles that cannot proceed through a signalized intersection on a single green indication. Phase Pair—A combination of two phases allowed within the same ring and between the same barriers (i.e., 1+2, 5+6, 3+4, and 7+8). Phase Sequence—The order of phases in a ring. Phase Table—The timing parameters associated with a phase. Signal Timing Manual, Second Edion

A-8 Appendix A. Glossary Oversaturation—A trafic condition in which the arrival low rate exceeds capacity. Parent Phase—A phase (or phases) used to determine whether an overlap should be active. Parent phases may be in different rings and/or may be on different sides of a barrier. See also Overlap. Passage Time (or Vehicle Interval, Vehicle Extension, Gap Time)—A phase timer that ends a phase when the time from the last detector output to the controller exceeds the timer setting. Pedestrian—An individual traveling on foot. Pedestrian Clearance Interval—The time provided for a pedestrian to cross the entire width of an intersection. This interval is longer than the lashing don’t walk interval, as it can include the yellow change and red clearance intervals. Pedestrian Phase—Time allocated to pedestrian trafic that is typically concurrent with compatible vehicular phase(s). See also Barnes’ Dance and Exclusive Pedestrian Phase. Pedestrian Recall—A recall mode where there is a continuous call for pedestrian service, resulting in the pedestrian walk and clearance intervals timing every cycle. See also Recall. Pedestrian Scramble—See Barnes’ Dance. Pedestrian Walk Interval—An indication that allows pedestrians to begin crossing the intersection. Pedestrian Walking Speed—The average walking speed of pedestrians (in feet per second). Performance Measures—Quantiiable means that are used to assess whether a signal system has achieved its operational objectives. Examples include stops, arrival on green, phase failure, arterial travel time, vehicle delay, and existence of spillback queuing from turn bays or between closely spaced intersections. Permissive Movement—See Permitted Movement. Permissive Period—A period of time during the coordinated cycle in which calls on conlicting phases will result in the coordinated phase transitioning to an uncoordinated phase. Permitted Movement—A movement that is allowed to proceed if there are available gaps in the conlicting low. Also known as a permissive movement (MUTCD term). See also Movement. Phase—A timing unit associated with the control of one or more movements. Phases are often assigned to vehicular and pedestrian movements. Phase Failure—The occurrence of one or more stopped vehicles that cannot proceed through a signalized intersection on a single green indication. Phase Pair—A combination of two phases allowed within the same ring and between the same barriers (i.e., 1+2, 5+6, 3+4, and 7+8). Phase Sequence—The order of phases in a ring. Phase Table—The timing parameters associated with a phase. Signal Timing Manual, Second Edion Appendix A. Glossary A-9 Phasing Diagram—A graphical representation of a sequence of phases, typically in the form of a ring-and-barrier diagram. Platoon—A group of vehicles or pedestrians traveling together as a group, either voluntarily or involuntarily because of signal control, geometrics, or other factors. Power Supply—An electrical device that converts AC to correct DC voltage for various devices in the signal cabinet. Practitioner—A general term for anyone responsible for signal timing. Presence Mode—A detection mode where a signal is sent to the controller for the duration of time a vehicle is inside a detection zone. See also Pulse Mode. Pretimed Control—A mode of operation where every phase is on recall every cycle, regardless of changes in trafic conditions. Typically only used in closely spaced grid- like networks. Protected Movement—A movement that has the right-of-way, and there are no conlicting movements occurring. See also Movement. Protected-Permitted Left-Turn Phasing—Compound left-turn protection that displays the permitted phase before or after the protected phase. Pulse Mode—A detection mode where vehicle detection is represented by a single brief “on” pulse to the controller. Typically only used for counting trafic, as presence detection protects against premature gap out due to vehicles queued over the detection area. See also Presence Mode. Queue—A line of motorized vehicles, bicycles, or pedestrians waiting to be served by the system (i.e., stopped) at the beginning of the green/walk interval. Slowly moving vehicles or pedestrians joining the rear of the queue are usually considered part of the queue. Queue Discharge—A low in which queued vehicles start to disperse (with high density and low speed). Queue Spillback—A term used to describe the vehicles stopped at an intersection that exceed the available storage space for a particular movement. Recall (or Phase Recall)—A call is placed for a speciied phase each time the controller is serving a conlicting phase. This ensures that the speciied phase will be served again. Types of recalls include maximum, minimum, pedestrian, and soft. See also Maximum Recall, Minimum Recall, Pedestrian Recall, and Soft Recall. Red Clearance Interval—The period of time following a yellow change interval, indicating the end of a phase and allowing additional time before the beginning of conlicting trafic. Red Rest—All phases may rest in red state when no serviceable calls exist in the ring and no other recall modes are used. Red Time—The period of time (expressed in seconds) in the cycle during which the indication for a given phase is red. Ring—A sequence structure consisting of two or more sequentially timed and individually selected conlicting movements, arranged to allow lexibility between compatible movements in different rings. Signal Timing Manual, Second Edion

A-10 Appendix A. Glossary Saturation Flow Rate—The equivalent hourly rate at which vehicles can traverse an intersection approach under prevailing conditions, assuming a constant green indication at all times and no lost time (in vehicles per hour or vehicles per hour per lane). Saturation Headway—The average headway between vehicles occurring after the fourth vehicle in the queue and continuing until the last vehicle in the initial queue clears the intersection. Semi-Actuated Control—A type of signal control where detection is provided for the minor movements only, and the signal timing returns to the major movement because it has no detection and is placed in recall. This is typical for coordinated operations without coordinated phase detection. See also Actuated Signal Control. Setback Detection—Detection located upstream of the stop bar. Allows for more eficient phase termination and is used for decision zone protection. If stop-bar detection is not present, setback detection will use locking memory mode. See also Detector. Signal Monitor—A safety device that monitors signal outputs for conlicting or improper outputs. Types include a simple conlict monitor; however, modern signal monitors perform many additional functions beyond looking for conlicts. Other types include malfunction management units (MMUs) and conlict monitoring units (CMUs), possibly in combination with auxiliary monitor units (AMUs). Simultaneous Gap Out—This parameter requires all currently green phases to concurrently “gap out” prior to crossing the barrier. Generally only recommended for high-speed rural-intersection through phases. See also Gap Out. Soft Recall—The recall parameter that causes the controller to place a call for vehicle service on a phase in the absence of a serviceable conlicting call. See also Recall. Software-in-the-Loop (SITL)—A means of providing a direct linkage between simulation models and software emulations of controllers. Speed—A rate of motion expressed as distance per unit of time. Split—The time assigned to a phase (green and the greater of the yellow change plus red clearance or the pedestrian walk plus clearance times) during coordinated operations. May be expressed in seconds or as a percentage. Split Failure—See Phase Failure. Start-Up Lost Time—The additional time (in seconds) consumed by the irst few vehicles in a queue at a signalized intersection above and beyond the saturation headway, because of the need to react to the initiation of the green phase and to accelerate. See also Lost Time. Steady Don’t Walk—The period of time after the walk and lashing don’t walk have completed timing. Stop-Bar Detection—Detection located at the stop bar that is generally used to discharge the queue. Generally uses non-locking memory mode in combination with large area detection design, so that vehicles making permitted turns do not extend or call the phase. See also Detector. Switch Pack—See Load Switch. Signal Timing Manual, Second Edion

A-10 Appendix A. Glossary Saturation Flow Rate—The equivalent hourly rate at which vehicles can traverse an intersection approach under prevailing conditions, assuming a constant green indication at all times and no lost time (in vehicles per hour or vehicles per hour per lane). Saturation Headway—The average headway between vehicles occurring after the fourth vehicle in the queue and continuing until the last vehicle in the initial queue clears the intersection. Semi-Actuated Control—A type of signal control where detection is provided for the minor movements only, and the signal timing returns to the major movement because it has no detection and is placed in recall. This is typical for coordinated operations without coordinated phase detection. See also Actuated Signal Control. Setback Detection—Detection located upstream of the stop bar. Allows for more eficient phase termination and is used for decision zone protection. If stop-bar detection is not present, setback detection will use locking memory mode. See also Detector. Signal Monitor—A safety device that monitors signal outputs for conlicting or improper outputs. Types include a simple conlict monitor; however, modern signal monitors perform many additional functions beyond looking for conlicts. Other types include malfunction management units (MMUs) and conlict monitoring units (CMUs), possibly in combination with auxiliary monitor units (AMUs). Simultaneous Gap Out—This parameter requires all currently green phases to concurrently “gap out” prior to crossing the barrier. Generally only recommended for high-speed rural-intersection through phases. See also Gap Out. Soft Recall—The recall parameter that causes the controller to place a call for vehicle service on a phase in the absence of a serviceable conlicting call. See also Recall. Software-in-the-Loop (SITL)—A means of providing a direct linkage between simulation models and software emulations of controllers. Speed—A rate of motion expressed as distance per unit of time. Split—The time assigned to a phase (green and the greater of the yellow change plus red clearance or the pedestrian walk plus clearance times) during coordinated operations. May be expressed in seconds or as a percentage. Split Failure—See Phase Failure. Start-Up Lost Time—The additional time (in seconds) consumed by the irst few vehicles in a queue at a signalized intersection above and beyond the saturation headway, because of the need to react to the initiation of the green phase and to accelerate. See also Lost Time. Steady Don’t Walk—The period of time after the walk and lashing don’t walk have completed timing. Stop-Bar Detection—Detection located at the stop bar that is generally used to discharge the queue. Generally uses non-locking memory mode in combination with large area detection design, so that vehicles making permitted turns do not extend or call the phase. See also Detector. Switch Pack—See Load Switch. Signal Timing Manual, Second Edion Appendix A. Glossary A-11 Time before Reduction—This volume-density timing period begins when the phase is green and there is a serviceable call on a conlicting phase. When this period is completed, the linear reduction of the passage time begins until the minimum gap is reached or the phase terminates due to gap out. See also Time to Reduce. Time-of-Day Plans—Signal timing plans associated with speciic hours of the day (i.e., associated with luctuations in demand), days of the week, or days during the year (e.g., holidays, seasons). Time-Space Diagram—A chart that plots the location of signalized intersections along the vertical axis and signal timing along the horizontal axis. This is a visual tool that illustrates coordination relationships between intersections. Time to Reduce—This volume-density timing period begins when time before reduction ends and controls the linear rate of reduction until the minimum gap is achieved. See also Time before Reduction. Trafic Management Center (TMC)—An optional physical component of a signal system, which contains the operational database that stores controller data, allows monitoring of the system, and allows timing and other parameters to be modiied. Trailing Overlap—A signal indication that ends after the parent phase. An example application of a trailing overlap is providing interior clearance at two “offset-T” intersections. See also Overlap. Travel Time (Average)—The total elapsed time spent traversing a speciied distance. The average travel time represents an average of the runs for a particular link or corridor. Uncoordinated (or Free Operation)—A trafic signal not operating as part of a coordinated system of intersections. Free operation can be set by time of day. Uninterruptible Power Supply (UPS)—A backup battery power supply to operate a trafic signal during power outages Unit Extension—See Passage Time. Upstream—The direction from which trafic is lowing. User—A person (pedestrian) or speciic type of vehicle (e.g., bike, transit, or truck) that uses a trafic signal. User Priority—A user may be assigned a relative or absolute priority based on operating environment and locally desired outcomes. These priorities may vary by movement. Variable Initial—An interval that times concurrently with the minimum green interval and increases by each vehicle actuation received during the initial period. This time cannot exceed the maximum variable initial. Vehicle—Every device in, upon, or by which any person or property is or may be transported or drawn upon a highway, excepting devices used exclusively upon stationary rails or track (Uniform Vehicle Code, 2000). Volume—The number of pedestrians or vehicles passing a point on a lane, roadway, or other traficway during some time interval (often 1 hour), expressed in vehicles, bicycles, or pedestrians per hour. Signal Timing Manual, Second Edion

A-12 Appendix A. Glossary Volume-Density—A phase timing function that uses parameters (e.g., variable initial, minimum gap, time before reduction, and time to reduce) to provide appropriate minimum green time to clear intersection queues when stop-bar detectors are not used, and/or it is desired to adjust the passage time. See also Density. Volume-to-Capacity Ratio (or Degree of Saturation)—A ratio of demand volume to the capacity for a subject movement. Walk Interval—An indication providing initial right-of-way to pedestrians during a pedestrian phase and prior to the pedestrian clearance interval. Yellow Change Interval—An indication warning users that the green, lashing yellow, or lashing red indication has ended, and the red indication will begin. Yellow Trap—A condition that leads the left-turning user into the intersection believing the opposing user is seeing a yellow. Yield Point—The earliest point in a coordinated signal operation that the controller can decide to terminate the coordinated phase(s). It is typically followed by one or more permissive periods that allow the controller to yield to uncoordinated phases later in the cycle, yet still return to the coordinated phase(s) in time to remain in coordination. Permissives are primarily beneicial during lower trafic volumes, allowing uncoordinated phases to be served if they arrive later than the initial yield point. Signal Timing Manual, Second Edion

Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S.DOT United States Department of Transportation

TRA N SPO RTATIO N RESEA RCH BO A RD 500 Fifth Street, N W W ashington, D C 20001 A D D RESS SERV ICE REQ U ESTED N O N -PR O FIT O R G . U .S. PO STA G E PA ID C O LU M B IA , M D PER M IT N O . 88 Signal Tim ing M anual: Second Edition N CH RP Report 812 TRB ISBN 978-0-309-30888-5 9 7 8 0 3 0 9 3 0 8 8 8 5 9 0 0 0 0

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 812: Signal Timing Manual - Second Edition, covers fundamentals and advanced concepts related to signal timing. The report addresses ways to develop a signal timing program based on the operating environment, users, user priorities by movement, and local operational objectives.

Advanced concepts covered in the report include the systems engineering process, adaptive signal control, preferential vehicle treatments, and timing strategies for over-saturated conditions, special events, and inclement weather.

An overview PowerPoint presentation accompanies the report.

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