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188 This glossary contains transit, traffic engineering, and traffic signal terminology used in this guidebook or that might be used during the course of implementing a transit-supportive roadway strategy. Definitions of the strategies used in this guidebook are provided in Section 2.2 and repeated in the individual strategy write-ups in the toolbox chapters (Chapters 5 through 8). Terms in the glossary are derived from the Transit Capacity and Quality of Service Manual (Kittelson & Associates et al. 2013), the Highway Capacity Manual 2010 (Transportation Research Board 2010), and the NCHRP Report 812: Signal Timing Manual (Urbanik et al. 2015). acceleration/deceleration delayâdelay experienced by vehicles slowing from and subsequently returning to their running speed. access pointâan intersection, driveway, or opening on either side of a roadway. active priorityâa form of traffic signal priority that adjusts signal timing in reaction to the arrival of a bus. actuated signal controlâphase time based on detection. adaptive signal controlâan advanced signal system that does not operate with time-of-day plans. alightâto get off or out of a vehicle. approachâa set of lanes at an intersection that accommodates all left-turn, through, and right-turn movements from a given direction. arterial roadwayâa signalized street that primarily serves through traffic and secondarily provides access to abutting properties. back of queueâthe maximum backward extent of queued vehicles during a typical cycle, as measured from the stop line to the last queued vehicle. bandwidthâthe 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. boardâto go on to or into a vehicle. boarding islandâa pedestrian refuge within the right-of-way and traffic lanes of a highway or street. It is provided at designated transit stops for the protection of passengers from traffic while they wait for and board or alight from transit vehicles; also known as a pedestrian island, loading island, or safety island. bunchingâa situation where two buses on a route arrive together or at much less than the scheduled headway; followed by a long gap in service. A P P E N D I X E Glossary
Glossary 189 capacityâthe maximum sustainable hourly flow rate at which persons or vehicles reasonably can be expected to traverse a point or a uniform section of a lane or roadway during a given time period under prevailing roadway, environmental, traffic, and control conditions. central business district (CBD)âan area with characteristics such as narrow street rights-of- way, frequent parking maneuvers, vehicle blockages, taxi and bus activity, small-radius turns, limited use of exclusive turn lanes, high pedestrian activity, dense population, and midblock curb cuts. clock headwayâthe scheduled headway between transit unit (vehicle or train) trips; based on even times (e.g., 60, 30, 20, 15, 10, and 7Â½ min). collector streetâa surface street providing land access and traffic circulation within residential, commercial, or industrial areas. concurrent phasesâtwo or more phases in separate rings that are able to operate together without conflicting movements. conflictâthe crossing, merging, or diverging of two traffic movements at an intersection. control delayâdelay associated with vehicles slowing in advance of an intersection, the time spent stopped on an intersection approach, the time spent as vehicles move up in the queue, and the time needed for vehicles to accelerate to their desired speeds. controllerâthe piece of hardware that determines how a traffic signal responds to calls based on signal timing parameters. coordinated phase(s)âthe phase (or phases) that are given a fixed minimum amount of time each cycle under a coordinated timing plan. This phase is typically the major through phase on an arterial. A coordinated phase may also have an optional actuated interval following the fixed interval. curb extensionâan extension of the sidewalk into the roadway for passenger loading without the bus pulling into the curb; gives priority to buses and eases reentry into traffic; often land- scaped and fitted with a bus shelter and other passenger amenities. At intersections, also shortens pedestrian crossing distances. Also called a bus bulb, bus bulge, bus nub, or curb bulge. cycleâa complete sequence of signal indications. cycle failureâa condition where one or more queued vehicles are not able to depart an inter- section as a result of insufficient capacity during the cycle in which they arrive. cycle lengthâthe 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). cycle timeâthe time required for a bus to make a round trip on a route, including layover and schedule recovery time. delayâadditional travel time experienced by a driver, passenger, bicyclist, or pedestrian beyond that required to travel at the desired speed. demandâthe number of vehicles or other roadway users desiring to use a given system element during a specific time period. Not to be confused with volume, which is a measure of how many users are accommodated at an intersection (which is limited to the available capacity). detectorâa device used to count or determine the presence of a motorized vehicle, bicycle, or pedestrian.
190 A Guidebook on Transit-Supportive Roadway Strategies display (head, signal group)âa combination of indications (e.g., red, yellow, green, green arrow, audible) grouped together for controlling one or more movements. double cycleâa cycle length that allows phases at an intersection to be served twice as often as the phases at other intersections in the coordinated system. downstreamâthe direction of traffic flow. dwell timeâthe sum of the time required to serve passengers at a transit stop and the time required to open and close the vehicle doors. dwell time variabilityâthe distribution of dwell times at a stop because of fluctuations in passenger demand for buses and routes. early return to greenâa term used to describe the servicing of a coordinated phase in advance of its programmed begin time as a result of unused time from non-coordinated phases. effective green timeâthe time during which a given traffic 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 since 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 timeâthe time during which a given traffic movement (or set of movements) is not moving into the intersection; it is equal to the cycle length minus the effective green time. far-side stopâa bus stop located beyond an intersection. flow rateâthe equivalent hourly rate at which vehicles or other roadway users pass over a given point or section of a lane or roadway during a given time interval of less than 1 h (usually 15 min). frequencyâthe number of transit units (vehicles or trains) on a given route or line, moving in the same direction, that pass a given point within a specified interval of time, usually 1 h. fully actuated controlâa signal operation in which vehicle detectors on each approach to the intersection control the occurrence and length of every phase. general traffic laneâa lane open to any motorized vehicle. green timeâthe duration of the green indication for a given movement at a signalized intersection. green-time (g/C) ratioâthe ratio of the effective green time of a phase to the cycle length. headwayâthe time interval between successive buses in the same direction. indicationâsee display (head, signal group). intervalâthe duration of time during which traffic signal indications (e.g., red, yellow, green, and flashing âDonât Walkâ) do not change state (i.e., red interval, yellow interval, green interval, and flashing âDonât Walkâ interval). isolated operationâan intersection that is not currently being operated as part of a coordinated system. Also known as free operation. See also uncoordinated (free) operation. lagging left turnâa left-turn phase that occurs toward the end of service to an intersection approach. layover timeâtime built into a bus schedule between trips used for operator rest time and to make up delays from the previous trip. See also schedule recovery time.
Glossary 191 leading left turnâa left-turn phase that occurs at the start of service to an intersection approach. leading pedestrian intervalâa pedestrian interval option that starts a few seconds before the adjacent through vehicular phase, thus allowing pedestrians to establish a presence in the cross- walk and thereby reducing conflicts with turning vehicles. level of service (LOS)âa quantitative stratification of a performance measure or measures that represent quality of service; measured on an A through F scale, with LOS A representing the best operating conditions from the travelerâs perspective and LOS F the worst. loading areaâa curbside space where a single bus can stop, load, and unload passengers. Bus stops include one or more loading areas. lost timeâthe 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). master clockâthe background timing mechanism within the controller logic to which each controller is referenced during coordinated operations. master controllerâan optional component of a signal system that facilitates coordination of the signal system with local controllers. maximum greenâthe maximum amount of time that a green signal indication can be displayed in the presence of conflicting demand. medianâthe area in the middle of a roadway separating opposing traffic flows. midblock stopâa bus stop located at a point away from intersections. minimum greenâthe least amount of time that a green signal indication will be displayed when a signal phase is activated. modeâa transport category characterized by specific right-of-way, technological, and operational features. movementâa 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 are 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. multimodalâthe availability of transportation options using different modes within a system or corridor. near-side stopâa bus stop located on the approach side of an intersection. off-peak period (base period)âin transit, the time of day during which vehicle requirements and schedules are not influenced by peak-period passenger volume demands (e.g., between morning and afternoon peak periods). At this time, transit riding is fairly constant and usually moderate in volume when compared with peak-period travel. offline stopâa bus stop where buses stop outside the travel lane. offsetâthe time relationship between the coordinated phase(s) based on the offset reference point and a defined master reference (i.e., master clock or sync pulse). offset reference point (coordination point)âthe defined point that creates an association between a signalized intersection and the master clock. online stopâa bus stop where buses stop in the travel lane.
192 A Guidebook on Transit-Supportive Roadway Strategies overlapâa 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. oversaturated flowâtraffic flow where (1) the arrival flow rate exceeds the capacity of a point or segment, (2) a queue created from a prior breakdown of a facility has not yet dissipated, or (3) traffic flow is affected by downstream conditions. paratransitâforms of transportation services that are more flexible and personalized than con- ventional fixed-route, fixed-schedule service but not including such exclusory services as charter bus trips. The term paratransit originally referred broadly to categories of service that are public (those that are available to any user who pays a predetermined fare [e.g., taxi, jitney, dial-a-ride]) and semi-public (those that are available only to people of a certain group, such as older adults, employees of a company, or residents of a neighborhood [e.g., vanpools, subscription buses]). However, more recently, paratransit has often been used to refer more specifically to ADA- complementary paratransit. passive priorityâA form of traffic signal priority that is pretimed, such as the setting of a streetâs signal progression to favor buses. peak periodâ(1) The period during which the maximum amount of travel occurs. It may be specified as the morning (a.m.) or afternoon or evening (p.m.) peak. (2) The period when demand for transportation service is heaviest. pedestrian clear intervalâtime provided for pedestrians who depart the curb during the âWalkâ indication to reach the opposite curb (or the median). pedestrian phaseâtime allocated to pedestrian traffic that is typically concurrent with compatible vehicular phase(s). pedestrian recallâa form of phase recall where the controller places a continuous call for pedes- trian service on the phase and then services the phase for at least an amount of time equal to its walk and pedestrian clear intervals (longer if vehicle detections are received). permitted movementâa movement that is allowed to proceed if there are available gaps in the conflicting flow. phaseâthe part of the signal cycle allocated to any combination of traffic movements receiving the right-of-way simultaneously during one or more intervals. A phase includes the green, yellow change, and red clearance intervals. phase sequenceâ(1) The sequence of service provided to each traffic movement; (2) A description of the order in which the left-turn movements are served relative to the through movements. platoonâa group of vehicles or pedestrians traveling together as a group, either voluntarily or involuntarily because of signal control, geometrics, or other factors. practitionerâa general term for anyone responsible for signal timing, traffic engineering, or transit operation. pretimed controlâa signal control in which the cycle length, phase plan, and phase times are preset to repeat continuously. progressionâthe act of various controllers providing specific green indications in accordance with a time schedule to permit continuous operation of groups of vehicles along the street at a planned speed. protected movementâa movement that has the right-of-way with no conflicting movements occurring.
Glossary 193 quality of serviceâthe overall measured or perceived quality of transportation service from the userâs or passengerâs point of view rather than from the operating agencyâs point of view. queueâa line of vehicles, bicycles, or persons waiting to be served due to traffic control, a bottleneck, or other causes. ramp meterâa traffic signal that controls the entry of vehicles from a ramp onto a limited-access facility; the signal allows one or two vehicles to enter on each green or green flash. red clearance intervalâa brief period of time following the yellow indication during which the signal heads associated with the ending phase and all conflicting phases display a red indication. red timeâthe period in the signal cycle during which, for a given phase or lane group, the signal is red. reentry delayâdelay experienced by buses leaving a bus stop when they must wait for a gap in traffic before reentering the travel lane. reliabilityâhow often transit service is provided as promised; affects waiting time, consistency of passenger arrivals from day to day, total trip time, and loading levels. ringâa sequence structure consisting of two or more sequentially timed and individually selected conflicting movements arranged to allow flexibility between compatible movements in other rings. saturation flow rateâthe equivalent hourly rate at which previously queued vehicles can traverse an intersection approach under prevailing conditions assuming that the green signal is available at all times and that no lost times are experienced. schedule recovery timeâadditional time built into a bus schedule between trips; used when the time potentially required to recover from delays is longer than the layover time. service spanâthe number of hours during the day between the start and end of service on a transit route; also known as the hours of service. signal delayâdelay experienced by a bus that arrives at a near-side stop during the green interval, serves its passengers during portions of the green and red intervals, and then must wait for the traffic signal to turn green again before proceeding. See also control delay. signal facesâSee display (head, signal group). speed, runningâthe highest safe speed at which a vehicle is normally operated on a given roadway or guideway under prevailing traffic and environmental conditions; the speed between points but not including stopped time. splitâthe segment of the cycle length allocated to each phase or interval that may occur. In an actuated controller unit, split is the time in the cycle allocated to a phaseâthe sum of the green, yellow change, and red clearance intervals for a phase. start-up lost timeâthe additional time consumed by the first few vehicles in a queue at a signal- ized 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. time-of-day plansâsignal timing plans associated with specific hours of the day (i.e., associated with fluctuations in demand), days of the week, or days during the year (e.g., holidays, seasons). traffic delayâthe component of delay that results when the interaction of vehicles causes drivers to reduce speed below the free-flow speed.
194 A Guidebook on Transit-Supportive Roadway Strategies transit signal preemptionâthe transfer of normal operation of a traffic signal to a special control mode serving a transit vehicle. transit signal priorityâadjustments to traffic signal timing to provide more usable green time to transit vehicles. See also active priority and passive priority. uncoordinated (free) operationâa traffic signal not operating as part of a coordinated system of intersections. Free operation can be set by time of day. undersaturated flowâtraffic flow where (1) the arrival flow rate is lower than the capacity of a point or segment, (2) no residual queue remains from a prior breakdown of the facility, and (3) traffic flow is unaffected by downstream conditions. unsignalized intersectionâan intersection not controlled by traffic signals. upstreamâthe direction from which traffic is flowing. volume-to-capacity (v/c) ratioâthe ratio of flow rate to capacity. walk intervalâa period of time intended to give pedestrians adequate time to perceive the âWalkâ indication and depart the curb before the pedestrian clear interval begins. yellow change intervalâthe period of time that a yellow indication is displayed to alert drivers to the impending presentation of a red indication. yield pointâthe earliest point in a coordinated signal operation at which 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 non-coordinated phases later in the cycle yet still return to the coordinated phase(s) in time to remain in coordination. Permissives are primarily beneficial during lower traffic volumes and allow non-coordinated phases to be served if they arrive later than the initial yield point.