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A Guidebook on Transit-Supportive Roadway Strategies (2016)

Chapter: Appendix E - Glossary

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Suggested Citation:"Appendix E - Glossary." National Academies of Sciences, Engineering, and Medicine. 2016. A Guidebook on Transit-Supportive Roadway Strategies. Washington, DC: The National Academies Press. doi: 10.17226/21929.
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Suggested Citation:"Appendix E - Glossary." National Academies of Sciences, Engineering, and Medicine. 2016. A Guidebook on Transit-Supportive Roadway Strategies. Washington, DC: The National Academies Press. doi: 10.17226/21929.
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Suggested Citation:"Appendix E - Glossary." National Academies of Sciences, Engineering, and Medicine. 2016. A Guidebook on Transit-Supportive Roadway Strategies. Washington, DC: The National Academies Press. doi: 10.17226/21929.
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Suggested Citation:"Appendix E - Glossary." National Academies of Sciences, Engineering, and Medicine. 2016. A Guidebook on Transit-Supportive Roadway Strategies. Washington, DC: The National Academies Press. doi: 10.17226/21929.
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Suggested Citation:"Appendix E - Glossary." National Academies of Sciences, Engineering, and Medicine. 2016. A Guidebook on Transit-Supportive Roadway Strategies. Washington, DC: The National Academies Press. doi: 10.17226/21929.
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Suggested Citation:"Appendix E - Glossary." National Academies of Sciences, Engineering, and Medicine. 2016. A Guidebook on Transit-Supportive Roadway Strategies. Washington, DC: The National Academies Press. doi: 10.17226/21929.
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Suggested Citation:"Appendix E - Glossary." National Academies of Sciences, Engineering, and Medicine. 2016. A Guidebook on Transit-Supportive Roadway Strategies. Washington, DC: The National Academies Press. doi: 10.17226/21929.
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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 delay—delay experienced by vehicles slowing from and subsequently returning to their running speed. access point—an intersection, driveway, or opening on either side of a roadway. active priority—a form of traffic signal priority that adjusts signal timing in reaction to the arrival of a bus. actuated signal control—phase time based on detection. adaptive signal control—an advanced signal system that does not operate with time-of-day plans. alight—to get off or out of a vehicle. approach—a set of lanes at an intersection that accommodates all left-turn, through, and right-turn movements from a given direction. arterial roadway—a signalized street that primarily serves through traffic and secondarily provides access to abutting properties. back of queue—the maximum backward extent of queued vehicles during a typical cycle, as measured from the stop line to the last queued vehicle. 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. board—to go on to or into a vehicle. boarding island—a 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. bunching—a 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 capacity—the 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)—an 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 headway—the scheduled headway between transit unit (vehicle or train) trips; based on even times (e.g., 60, 30, 20, 15, 10, and 7½ min). collector street—a surface street providing land access and traffic circulation within residential, commercial, or industrial areas. concurrent phases—two or more phases in separate rings that are able to operate together without conflicting movements. conflict—the crossing, merging, or diverging of two traffic movements at an intersection. control delay—delay 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. controller—the piece of hardware that determines how a traffic signal responds to calls based on signal timing parameters. coordinated phase(s)—the 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 extension—an 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. cycle—a complete sequence of signal indications. cycle failure—a 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 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). cycle time—the time required for a bus to make a round trip on a route, including layover and schedule recovery time. delay—additional travel time experienced by a driver, passenger, bicyclist, or pedestrian beyond that required to travel at the desired speed. demand—the 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). detector—a 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)—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 traffic flow. dwell time—the 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 variability—the distribution of dwell times at a stop because of fluctuations in passenger demand for buses and routes. 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 non-coordinated phases. effective green time—the 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 time—the 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 stop—a bus stop located beyond an intersection. flow rate—the 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). frequency—the 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 control—a signal operation in which vehicle detectors on each approach to the intersection control the occurrence and length of every phase. general traffic lane—a lane open to any motorized vehicle. green time—the duration of the green indication for a given movement at a signalized intersection. green-time (g/C) ratio—the ratio of the effective green time of a phase to the cycle length. headway—the time interval between successive buses in the same direction. indication—see display (head, signal group). interval—the duration of time during which traffic signal indications (e.g., red, yellow, green, and flashing “Don’t Walk”) do not change state (i.e., red interval, yellow interval, green interval, and flashing “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 (free) operation. lagging left turn—a left-turn phase that occurs toward the end of service to an intersection approach. layover time—time 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 turn—a left-turn phase that occurs at the start of service to an intersection approach. leading pedestrian interval—a 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)—a 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 traveler’s perspective and LOS F the worst. loading area—a curbside space where a single bus can stop, load, and unload passengers. Bus stops include one or more loading areas. 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). 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 the signal system with local controllers. maximum green—the maximum amount of time that a green signal indication can be displayed in the presence of conflicting demand. median—the area in the middle of a roadway separating opposing traffic flows. midblock stop—a bus stop located at a point away from intersections. minimum green—the least amount of time that a green signal indication will be displayed when a signal phase is activated. mode—a transport category characterized by specific right-of-way, technological, and operational features. 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 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. multimodal—the availability of transportation options using different modes within a system or corridor. near-side stop—a bus stop located on the approach side of an intersection. off-peak period (base period)—in 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 stop—a bus stop where buses stop outside the travel lane. offset—the 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)—the defined point that creates an association between a signalized intersection and the master clock. online stop—a bus stop where buses stop in the travel lane.

192 A Guidebook on Transit-Supportive Roadway Strategies 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. oversaturated flow—traffic 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. paratransit—forms 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 priority—A form of traffic signal priority that is pretimed, such as the setting of a street’s signal progression to favor buses. peak period—(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 interval—time provided for pedestrians who depart the curb during the “Walk” indication to reach the opposite curb (or the median). pedestrian phase—time allocated to pedestrian traffic that is typically concurrent with compatible vehicular phase(s). pedestrian recall—a 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 movement—a movement that is allowed to proceed if there are available gaps in the conflicting flow. 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. A phase includes the green, yellow change, and red clearance intervals. phase sequence—(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. platoon—a group of vehicles or pedestrians traveling together as a group, either voluntarily or involuntarily because of signal control, geometrics, or other factors. practitioner—a general term for anyone responsible for signal timing, traffic engineering, or transit operation. pretimed control—a signal control in which the cycle length, phase plan, and phase times are preset to repeat continuously. progression—the 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 movement—a movement that has the right-of-way with no conflicting movements occurring.

Glossary 193 quality of service—the overall measured or perceived quality of transportation service from the user’s or passenger’s point of view rather than from the operating agency’s point of view. queue—a line of vehicles, bicycles, or persons waiting to be served due to traffic control, a bottleneck, or other causes. ramp meter—a 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 interval—a 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 time—the period in the signal cycle during which, for a given phase or lane group, the signal is red. reentry delay—delay experienced by buses leaving a bus stop when they must wait for a gap in traffic before reentering the travel lane. reliability—how often transit service is provided as promised; affects waiting time, consistency of passenger arrivals from day to day, total trip time, and loading levels. ring—a 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 rate—the 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 time—additional 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 span—the 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 delay—delay 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 faces—See display (head, signal group). speed, running—the 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. split—the 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 phase—the sum of the green, yellow change, and red clearance intervals for a phase. start-up lost time—the 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 plans—signal 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 delay—the 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 preemption—the transfer of normal operation of a traffic signal to a special control mode serving a transit vehicle. transit signal priority—adjustments to traffic signal timing to provide more usable green time to transit vehicles. See also active priority and passive priority. uncoordinated (free) operation—a traffic signal not operating as part of a coordinated system of intersections. Free operation can be set by time of day. undersaturated flow—traffic 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 intersection—an intersection not controlled by traffic signals. upstream—the direction from which traffic is flowing. volume-to-capacity (v/c) ratio—the ratio of flow rate to capacity. walk interval—a period of time intended to give pedestrians adequate time to perceive the “Walk” indication and depart the curb before the pedestrian clear interval begins. yellow change interval—the period of time that a yellow indication is displayed to alert drivers to the impending presentation of a red indication. yield point—the 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.

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TRB’s Transit Cooperative Research Program (TCRP) Report 183: A Guidebook on Transit-Supportive Roadway Strategies is a resource for transit and roadway agency staff seeking to improve bus speed and reliability on surface streets, while addressing the needs of other roadway users, including motorists, bicyclists, and pedestrians.

The guidebook identifies consistent and uniform strategies to help improve transportation network efficiency to reduce delay and improve reliability for transit operations on roadways; and includes decision-making guidance for operational planning and functional design of transit/traffic operations on roads that provides information on warrants, costs, and impacts of strategies.

The guidebook also identifies the components of model institutional structures and intergovernmental agreements for successful implementation; and highlights potential changes to the Manual on Uniform Traffic Control Devices (MUTCD) and related documents to facilitate implementation of selected strategies.

In addition to the report, TCRP Web-Only Document 66: Improving Transportation Network Efficiency Through Implementation of Transit-Supportive Roadway Strategies documents the methodology used to develop the report.

A PowerPoint presentation accompanies the report.

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