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Glossaries
- A&A Traffic (A to Z)
- HCM (A to H)
- HCM (I to Z)
Highway Capacity Manual (HCM)
Glossary of Traffic Terms
(I to Z)
Impedance
The reduction in the capacity of lower-priority movements. Caused by the congestion of higher-priority movements at a stop-controlled approach.
Incident
Any occurrence on a roadway that impedes the normal flow of traffic.
Incident Delay
The component of delay that results from an incident, compared with the no-incident condition.
Incremental Delay
The second term of lane group control delay, it accounts for non-uniform arrivals and temporary random delays as well as delays caused by sustained periods of oversaturation.
Influence Area
(1) An area that incurs operational impacts of merging vehicles in Lanes I and 2 of the freeway and the acceleration lane for 450 m from the merge point downstream.
(2) An area that incurs operational impacts of diverging vehicles in Lanes I and 2 of the freeway and the deceleration lane for 450 m from the diverge point upstream.
Initial Queue
The unmet demand at the beginning of an analysis period, either observed in the field or carried over from the computations of a previous analysis period.
Initial Queue Delay
The third term of lane group control delay refers to the delay due to a residual queue identified in a previous analysis period and persisting at the start of the current analysis period. This delay results from the additional time required to clear the initial queue.
Intelligent Transportation System (ITS)
A transportation technology that enhances the safety and efficiency of vehicles and roadway systems.
Intensity of Congestion
A measure of the total number of person-hours of delay and mean trip speed or mean delay per person-trip.
Interchange Density
The average number of interchanges per kilometer. Computed for 10 km of freeway including the basic freeway segment.
Interchange Ramp Terminal
A junction with a surface street to serve vehicles entering or exiting a freeway.
Internal Link
The segment between two signalized intersections at an interchange ramp terminal.
Internal Zone
A diamond-shaped area identified in a corridor analysis for each arterial street segment that lies between intersections. An internal zone represents the geographic area likely to generate trips to each segment.
Interrupted Flow
A category of traffic facilities characterized by traffic signals, stop signs, or other fixed causes of periodic delay or interruption to the traffic stream.
Intersection Delay
The total additional travel time experienced by drivers, passengers, or pedestrians as a result of control measures and interaction with other users of the facility, divided by the volume departing from the corresponding cross section of the facility.
Interval
A period of time in which all traffic signal indications remain constant.
Isolated Intersection
An intersection at least 1. 6 km from the nearest upstream signalized intersection.
Jam Density
The density at which congestion stops all movement of persons or vehicles, usually expressed as vehicles per kilometer per lane or pedestrians per square meter.
Kiss and Ride
An access mode to transit allowing passengers (usually commuters) to be driven to a transit stop to board a transit unit and then to be met after their return.
Lane 1
The highway lane adjacent to the shoulder.
Lane 2
The highway lane adjacent and to the left of Lane 1.
Lane Balance
The number of lanes leaving a diverge point is equal to the number of lanes approaching it, plus one.
Lane Distribution
A parameter used when two or more lanes are available for traffic in a single direction, and the volume distribution varies widely, depending on traffic regulation, traffic composition, speed and volume, the number of and location of access points, the origin-destination patterns of drivers, the development environment, and local driver habits.
Lane Group
A set of lanes established at an intersection approach for separate capacity and level-of-service analysis.
Lane Group Delay
The control delay for a given lane group.
Lane Utilization
The distribution of vehicles among lanes when two or more lanes are available for a movement; however, as demand approaches capacity, uniform lane utilization develops.
Lane Width
The arithmetic mean of the lane widths of a roadway in one direction, expressed in meters.
Lateral Clearance
(1) The total left and right side clearance from the outside edge of travel lanes to fixed obstructions on a multilane highway.
(2) The right side clearance distance from the rightmost travel lane to fixed obstructions on a freeway.
Level of Service
A qualitative measure describing operational conditions within a traffic stream, based on service measures such as speed and travel time, freedom to maneuver, traffic interruptions, comfort, and convenience.
Level Terrain
A combination of horizontal and vertical alignments that permits heavy vehicles to maintain approximately the same speed as passenger cars; this generally includes short grades of no more than 1 to 2 percent.
Light Rail Transit (LRT)
A metropolitan electric railway system operating single cars or short trains along exclusive rights-of-way at ground level, on aerial structures, in subways, or occasionally in streets. An LRT also can board and discharge passengers at track or car floor level.
Linear Loading Area
A bus bay design in which buses stop directly behind each other, so that the bus in front must leave its bay before the following bus can exit; often used when buses occupy a bay only for a short time (e.g. at an on-street bus stop).
Link
A segment of highway ending at a major intersection on an urban street or at a ramp merge or diverge point on a freeway. Links have a node at each end.
Load Factor
The number of passengers occupying a transit vehicle, divided by the number of seats on the vehicle.
Loading Area
(1) A branch from, or widening of, a road that permits buses to stop, without obstructing traffic, while laying over or while passengers board and alight.
(2) A specially designed or designated location at a transit stop, station, terminal, or transfer center at which a bus stops to allow passengers to board and alight.
(3) A lane in a garage facility for parking or storing buses, often for maintenance.
Loading Island
(1) A pedestrian refuge within the right-of-way and traffic lanes of a highway or street, designated for transit stops, to protect transit passengers from traffic while awaiting boarding or alighting.
(2) A protected spot for the loading and unloading of passengers within a rail transit or bus station.
(3) A passenger loading platform in the middle of the street, level with the street or more usually raised to curb height, for streetcar and light rail systems.
Local Bus
A bus that stops for passengers within 80m of the stop line of an intersection approach.
Loop Ramp
A ramp requiring vehicles to execute a left turn by turning right, accomplishing a 90-degree left turn by making a 270-degree right turn.
Lost Time
The time, in seconds, during which an intersection is not used effectively by any movement; it is the sum of clearance lost time plus start-up lost time.
Low Floor Bus
A bus without steps at its entrances and exits.
Macroscopic Model
A mathematical model that employs traffic flow rate variables.
Mainline
The primary through roadway as distinct from ramps, auxiliary lanes, and collector-distributor roads.
Major Diverge Segment
A segment in which one freeway segment with multiple lanes diverges, to form two primary freeway segments.
Major Merge Segment
A segment in which two primary freeway segments, each with multiple lanes, merge to form a single freeway segment.
Major Street
The street not controlled by stop signs at a two-way stop-controlled intersection.
Major Weaving Segment
A weaving segment with at least three entry and exit legs, each with two or more lanes.
Maximum Load Point
The point on a transit line or route at which the passenger volume is the greatest. There is one maximum load point in each direction.
Measure of Effectiveness
A quantitative parameter indicating the performance of a transportation facility or service.
Meeting
An encounter of bicycles or pedestrians moving in the opposite direction of the subject bicycle flow.
Merge
A movement in which two separate lanes of traffic combine to form a single lane without the aid of traffic signals or other right-of-way controls.
Mesoscopic Model
A mathematical model for the movement of clusters or platoons of vehicles, incorporating equations to indicate how these clusters interact.
Microscopic Model
A mathematical model that captures the movement of individual vehicles.
Midblock Stop
A transit stop located at a point away from intersections.
Minor Arterial
A functional category of a street allowing trips of moderate length within a relatively small geographical area.
Minor Movement
A vehicle making a specific directional entry into an unsignalized intersection from a minor street.
Minor Street
The street controlled by stop signs at a two-way stop-controlled intersection; also referred to as a side street.
Mixed-Traffic Bus Facility
Buses operating in mixed traffic with automobiles.
Mountainous Terrain
A combination of horizontal and vertical alignments causing heavy vehicles to operate at crawl speeds for significant distances or at frequent intervals.
Movement Capacity
The capacity of a specific traffic stream at a stop-controlled intersection approach, assuming that the traffic has exclusive use of a separate lane, in passenger cars per hour.
Multilane highway
A highway with at least two lanes for the exclusive use of traffic in each direction, with no control or partial control of access, but that may have periodic interruptions to flow at signalized intersections no closer than 3. 0 km.
Multimodal
A transportation facility for different types of users or vehicles.
Multiple Weaving Segment
A segment formed when one merge is followed by two diverge points, or two merge points are followed by one diverge point.
Near-Side Stop
A transit stop located on the approach side of an intersection. The transit units stop to serve passengers before crossing the intersection.
No-Passing Zone
A segment of a two-lane, two-way highway along which passing is prohibited in one or both directions.
Node
The endpoint of a link; also used interchangeably with point.
Non-weaving Flow
The traffic movements in a weaving segment that are not engaged in weaving movements.
Normative Model
A mathematical model that identifies a set of parameters that provide the best system performance.
Off-line Loading
A transit unit (vehicle or train) stops outside the main track or travel lane, so that its passengers can board and alight and other units can pass.
Off-line Model
A mathematical model in which the output neither directly nor immediately influences traffic operations.
Off-line Stop
A location outside the main track or travel lane at which a transit unit (vehicle or train) stops, so that passengers can board and alight and other units can pass.
Off-Ramp
See Exit ramp.
Off-Street Path
A path physically separated from highway traffic for the use of pedestrians, bicycles, and non-motorized traffic.
Offset
The difference, in seconds, between the start of green time at the two signalized intersections of a diamond interchange for through traffic on the internal link or the time between the start of individual green times and a specified time datum in a system of signalized intersections.
On-line Loading
A station stop for transit units on the main track or travel lane.
On-line Model
A model that influences the control system operation in real time.
On-line Stop
A transit unit stop in the main track or travel lane.
On-Ramp
See Entrance Ramp.
Open Fare Collection System
A system for collecting transit fares that does not have turnstiles or fare gates.
Operating Margin
The amount of time that a train can run behind schedule without interfering with following trains.
Operational Application
A use of capacity analysis to determine the level of service on an existing or projected facility, with known or projected traffic, roadway, and control conditions.
Opposing Approach
The approach approximately 180 degrees opposite the subject approach at an all-way stop-controlled intersection.
Opposing Flow Rate
The flow rate for the direction of travel opposite to the direction under analysis.
Overflow Queue
Queued vehicles left over from a green phase at a signalized intersection.
Oversaturation
A traffic condition in which the arrival flow rate exceeds capacity.
Paid Area
(1) An area that a passenger may enter only after paying a fare or showing credentials.
(2) A station area set off by barriers or gates to restrict access to transit only to those who have paid fares or secured passes.
Paratransit
Transportation services that are more flexible and personalized than conventional fixed-route, fixed-schedule services, however, such exclusive services as charter bus trips are not considered paratransit. The vehicles usually are low or medium capacity highway vehicles and the service is often adjustable to individual users requirements.
Parclo
See Partial Cloverleaf Interchange.
Park and Ride
An access mode to transit in which patrons drive private automobiles or ride bicycles to a transit station, transit stop, or carpool or vanpool waiting area, parking in the areas provided.
Partial Cloverleaf Interchange
Also called a parclo, it is an interchange with one or two loop ramps.
Partial Diamond Interchange
A diamond interchange with fewer than four ramps so that not all of the freeway-street or street-freeway movements are served.
Passenger Car Equivalent
The number of passenger cars displaced by a single heavy vehicle of a particular type under specified roadway, traffic, and control conditions.
Passenger Service Time
The time required for a passenger to board or alight from a transit vehicle, expressed in seconds per passenger.
Passing
An encounter with a bicycle or pedestrian moving in the same direction as the subject bicycle flow on a bicycle facility.
Passing Lane
A lane added to improve passing opportunities in one direction of travel on a conventional two-lane highway.
Passing Sight Distance
The visibility distance required for drivers to execute safe passing maneuvers in the opposing traffic lane of a two-lane, two-way highway.
Peak-Hour Factor
The hourly volume during the maximum-volume hour of the day divided by the peak 15-min flow rate within the peak hour; a measure of traffic demand fluctuation within the peak hour.
Pedestrian
An individual traveling on foot.
Pedestrian Critical Gap
The minimum time during which a single pedestrian will not attempt to cross an intersection, expressed in seconds.
Pedestrian Density
The number of pedestrians per unit of area within a walkway or queuing area, expressed as pedestrians per square meter.
Pedestrian Effective Green Time
The minimum effective green time required to serve a given pedestrian demand. It is expressed in seconds.
Pedestrian Flow Rate
The number of pedestrians passing a point per unit of time. usually expressed as pedestrians per 15 min or pedestrians per minute.
Pedestrian Queuing Area
Places such as elevators, transit platforms, and street crossings in which pedestrians stand temporarily while waiting to be served.
Pedestrian Space
The average area provided for pedestrians in a moving pedestrian stream or pedestrian queue, in square meters per pedestrian.
Pedestrian Start-up Time
The time for a platoon of pedestrians to get under way following the beginning of the Walk interval, expressed in seconds.
Pedestrian Walking Speed
The average walking speed of pedestrians, expressed in meters per second.
Percent Time-Spent-Following
The average percent of total travel time that vehicles must travel in platoons behind slower vehicles due to inability to pass on a two-lane highway.
Performance-Based Planning
A way of relating agency planning and project implementation to public benefits.
Performance Measure
A quantitative or qualitative characteristic describing the quality of service provided by a transportation facility or service.
Period of Unmet Demand
The length of time within an analysis period during which the unmet demand is greater than zero.
Permitted Plus Protected
Compound left-turn protection that displays the permitted phase before the protected phase.
Permitted Turn
Left or right turn at a signalized intersection that is made against an opposing or conflicting vehicular or pedestrian flow.
Person Capacity
The maximum number of persons, in persons per hour, that reasonably can be expected to be carried past a given point on a highway or transit right-of-way during a given time period, under specified operating conditions, without unreasonable delay, hazard, or restriction.
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.
Planning Application
A use of capacity analysis to estimate the level of service, the volume that can be accommodated, or the number of lanes required, using estimates, HCM default values, and local default values as inputs.
Platoon
A group of vehicles or pedestrians traveling together as a group, either voluntarily or involuntarily because of signal control, geometrics, or other factors.
Platoon Ratio
A parameter useful in quantifying arrival type, platoon ratio is calculated by dividing the proportion of all vehicles arriving during green by the g/C ratio of the subject movement.
Point
A boundary between segments, usually places at which traffic enters, leaves, or crosses a facility.
Point-Deviation Service
Public transportation service in which the transit vehicle arrives at designated stops on a prearranged schedule but does not follow a specific route.
Potential Capacity
The capacity of a specific movement at a stop-controlled intersection approach, assuming that it is unimpeded by other movements and has exclusive use of a separate lane. Expressed in vehicles per hour.
Precision
The range of accurate and acceptable numerical answers.
Prepositioning
When one or more turning movements are necessary to occupy a lane of the lane group.
Pretimed Control
A signal control in which the cycle length, phase plan and phase times are preset to repeat continuously.
Prevailing Condition
The geometric, traffic, and control conditions during the analysis period.
Principal Arterial
A major surface street with relatively long trips between major points, and with through-trips entering, leaving, and passing through the urban area.
Progression Adjustment Factor
A factor used to account for the effect of signal progression on traffic flow; applied only to uniform delay.
Protected Plus Permitted
Compound left-turn protection at a signalized intersection that displays the protected phase before the permitted phase.
Protected Turn
The left or right turns at a signalized intersection that are made with no opposing or conflicting vehicular or pedestrian flow allowed.
Quality of Service
A performance indicator of a traveler's perceived satisfaction with the trip.
Quantity of Service
A measure of the utilization of the transportation system.
Queue
A line of vehicles, bicycles, or persons waiting to be served by the system in which the flow rate from the front of the queue determines the average speed within the queue. Slowly moving vehicles or people joining the rear of the queue are usually considered part of the queue. The internal queue dynamics can involve starts and stops. A faster-moving line of vehicles is often referred to as a moving queue or a platoon.
Queue Carryover
The queued vehicles left over from the analysis period due to demand exceeding capacity.
Queue Discharge
A flow with high density and low speed, in which queued vehicles start to disperse. Usually denoted as Level of Service F.
Queue Discharge Flow
A traffic flow that has passed through a bottleneck and is accelerating to the free-flow speed of the freeway.
Queue Storage Ratio
The parameter that uses three parameters (back of queue, queued vehicle spacing, and available storage space) to determine if blockage will occur.
Ramp
A short segment of roadway connecting two traffic facilities.
Ramp Junction
A short segment of highway along which vehicles transfer from an entrance ramp to the main roadway or from the main roadway to an exit ramp.
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.
Ramp Roadway
See Ramp.
Ramp Segment
See Ramp.
Ramp-Freeway Terminal
The roadway segment over which an entrance or an exit ramp joins the mainline of a freeway.
Ramp-Street Terminal
The roadway segment over which an entrance or an exit ramp joins with a surface street.
Ramp-Weave Segment
A weaving segment formed by a one-lane entrance ramp followed by a one-lane exit ramp joined by a continuous auxiliary lane.
Random Positioning
Through vehicles can use any lane of the subject lane group.
Rank
The hierarchy of right-of-way among conflicting traffic streams at a two-way stop-controlled intersection.
Rapid Bus
A bus that operates on an exclusive or reserved right-of-way permitting higher speeds. On limited access roads it can include reverse lane operations.
Rapid Transit
Rail systems operating on exclusive right-of-way, i.e. heavy rail or metro.
Real-Time Model
A model that keeps pace with actual time.
Recreational Vehicle
A heavy vehicle generally operated by a private motorist for transporting recreational equipment or facilities. Examples include campers, boat trailers, and motorcycle or jet-ski trailers.
Red Time
The period, expressed in seconds, in the signal cycle during which, for a given phase or lane group, the signal is red.
Residual Queue
The unmet demand at the end of an analysis period, resulting from operation while demand exceeded capacity.
Roadside Obstruction
An object or barrier along a roadside or median that affects traffic flow, whether continuous (e.g. a retaining wall) or not continuous (e.g. light supports or bridge abutments).
Roadway Characteristic
A geometric characteristic of a street or highway, including the type of facility, number and width of lanes (by direction), shoulder widths and lateral clearances, design speed, and horizontal and vertical alignments.
Roadway Occupancy
The proportion of roadway length covered by vehicles, used to identify the proportion of time a roadway cross section is occupied by vehicles. Because it is easier to measure in the field, roadway occupancy is used as a surrogate for density in control systems.
Rolling Terrain
A combination of horizontal and vertical alignments causing heavy vehicles to reduce their speed substantially below that of passenger cars but not to operate at crawl speeds for a significant amount of time.
Roundabout
An unsignalized intersection with a circulatory roadway around a central island with all entering vehicles yielding to the circulating traffic.
Route-Deviation Service
A public transportation service that operates along a public way on a fixed route but not on a fixed schedule. It is a form of paratransit.
Running Speed
The distance a vehicle travels divided by running time, in kilometers per hour.
Running Time
The portion of the travel time during which a vehicle is in motion.
Rural
An area with widely scattered development and a low density of housing and employment.
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 no lost times are experienced, 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.
Sawtooth Loading Area
A bus bay design with the curb indented in a sawtooth pattern, allowing buses to enter and exit bus bays independently of other buses. Often used at transit centers.
Segment
A portion of a facility on which a capacity analysis is performed; it is the basic unit for the analysis, a one-directional distance. A segment is defined by two endpoints.
Semi-Actuated Control
A signal control in which some approaches (typically on the minor street) have detectors, and some of the approaches (typically on the major street) have no detectors.
Service Area
(1) The jurisdiction in which a transit property operates.
(2) The geographic region in which a transit system either provides service or is required to provide service.
Service Coverage
See Coverage.
Service Flow Rate
The maximum hourly rate at which vehicles, bicycles, or persons reasonably can be expected to traverse a point or uniform segment of a lane or roadway during a given time period (usually 15 min) under prevailing roadway, traffic, environmental, and control conditions, while maintaining a designated level of service; expressed as vehicles per hour or vehicles per hour per lane.
Service 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; see also Headway.
Service Measure
A specific performance measure used to assign a level of service to a set of operating conditions for a transportation facility or service.
Service Time
The average time that a vehicle on the subject approach is serviced at an all-way stop-controlled intersection, depending on arrival rates of the opposing and conflicting approaches.
Service Volume
The maximum hourly rate at which vehicles, bicycles, or persons reasonably can be expected to traverse a point or uniform segment of a roadway during an hour under specific assumed conditions while maintaining a designated level of service.
Shared-Lane Capacity
The capacity of a lane, in vehicles per hour, at an unsignalized intersection that is shared by two or three movements.
Shock Wave
The compression wave that moves upstream through traffic as vehicles arriving at a queue slow down abruptly, or the decompression wave of thinning traffic that moves downstream from the point of a capacity reduction on a freeway.
Shoulder
A portion of the roadway contiguous with the traveled way for accommodation of stopped vehicles, emergency use, and lateral support of the sub-base, base, and surface courses.
Shoulder Bypass Lane
A portion of the paved shoulder opposite the minor-road leg at a three-leg intersection, marked as a lane for through traffic to bypass vehicles that are slowing or stopped to make a left turn.
Side Street
See Minor street.
Signalization Condition
A phase diagram illustrating the phase plan, cycle length, green time, change interval, and clearance time interval of a signalized intersection.
Simple Left Turn Protection
A signal phasing scheme that provides a single protected phase in each cycle for a left turn.
Simple Weaving Segment
A segment formed by a single merge point followed by a single diverge point.
Simulation Model
A computer program that uses mathematical models to conduct experiments with traffic events on a transportation facility or system over extended periods of time.
Simulation Model Descriptor
A fundamental descriptor (state variable, event, time step logic, and processing logic) used in combination with others to represent a unified and consistent simulation model.
Single-Point Diamond Interchange
A diamond interchange that combines all the ramp movements into a single signalized intersection.
Single-Stream Door
A door on a transit vehicle that allows passenger flow in only one direction at a time.
Skip-Stop Service
A transit operation in which alternate units stop at alternate sets of stations on the same route. Each set consists of some joint and some alternate stations.
Space
See Pedestrian space.
Space Mean Speed
(1) The harmonic mean of speeds over a length of roadway.
(2) An average speed based on the average travel time of vehicles to traverse a segment of roadway; in kilometers per hour.
Spacing
The distance, in meters, between two successive vehicles in a traffic lane, measured from the same common feature of the vehicles (e.g. rear axle, front axle, or front bumper).
Specific Grade
A single grade of a roadway segment or extended roadway segment expressed in percentage.
Speed
A rate of motion expressed as distance per unit of time.
Split-Diamond Interchange
Diamond interchanges in which freeway entry and exit ramps are separated at the street level, creating four intersections.
Standee
A passenger standing in a transit vehicle.
Start-up Lost Time
The additional time, in seconds, consumed by the first 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.
Static Flow Model
A mathematical model in which the traffic flow rate is constant.
Stochastic Model
A mathematical model that employs random variables for at least one input parameter.
Stop Time
A portion of control delay when vehicles are at a complete stop.
Street Corner
The area encompassed within the intersection of two sidewalks.
Streetcar
An electrically powered rail car that is operated singly or in short trains in mixed traffic on track in city streets.
Study Period
A duration of time on which to base capacity analyses of a transportation facility.
Subject Approach
The approach under study at two-way and all-way stop-controlled intersections.
Suburban
An area with a mixture of densities for housing and employment where high-density, non-residential development is intended to serve the local community.
Suburban Street
A street with low-density driveway access on the periphery of an urban area.
System Level of Service
The quality of service provided by the transportation system.
System Performance Measure
A parameter that measures the efficiency of the transportation system.
System Performance Report Card
A list of measures depicting the use of the transportation system, for decision-making.
System Speed
A space mean speed, in kilometers per hour, of vehicles both in the ramp influence area and in the outer lanes of a 450-m freeway segment.
Taper Area
An area characterized by a reduction or increase in pavement width to direct traffic.
Terrain Type
See General Terrain.
Through Vehicles
All vehicles passing directly through a street segment and not turning.
Time Interval
See Analysis Period.
Time Interval Scale Factor
The ratio of the total freeway entrance demands to the freeway exit counts in each time interval.
Time Mean Speed
The arithmetic average of individual vehicle speeds passing a point on a roadway or lane, in kilometers per hour.
Time-Based Model
A model in which time advances from one point to the next.
Time-Space Domain
A graphical display of a freeway facility with a horizontal scale of distance along the freeway, with traffic moving from left to right, and with the freeway divided into sections.
Time-Varying flow model
A simulation model in which flow changes with time.
Total Delay
The sum of all components of delay for any lane group, including control delay, traffic delay, geometric delay, and incident delay. See also Aggregate Delay.
Total Lateral Clearance
The total width of the left side plus the right side along one direction of a roadway.
Total Lost Time
The time per signal cycle during which the intersection is effectively not used by any movement; this occurs during the change and clearance intervals and at the beginning of most phases.
Traffic Condition
A characteristic of traffic flow, including distribution of vehicle types in the traffic stream, directional distribution of traffic, lane use distribution of traffic, and type of driver population on a given facility.
Traffic Delay
The component of delay that results when the interaction of vehicles causes drivers to reduce speed below the free-flow speed.
Traffic Pressure
A parameter that reflects driver aggressiveness due to heavier volumes or long delays in a confined area.
Transit Accessibility
A measure of pedestrian, bicycle, automobile, and Americans with Disabilities Act accessibility to transit.
Transit Availability
A measure of a transit system's capability for use by potential passengers, including the hours the system is in operation, route spacing, and accessibility for the physically handicapped.
Transit Quality of Service
The overall measured or perceived quality of transit service from the passenger's point of view.
Transit Reliability
A measure of the time performance and the regularity of headways between successive transit vehicles that affect the amount of time passengers must wait at a transit stop as well as the consistency of a passenger's arrival time at a destination.
Transit Stop
An area where passengers await, board, alight, and transfer between transit units (vehicles or trains). It is usually indicated by distinctive signs and by curb or pavement markings and may provide service information, shelter, seating, or any combination of these.
Transit-Supportive Area
An area with sufficient population or employment density to warrant at least hourly transit service.
Travel Speed
The average speed, in kilometers per hour, of a traffic stream computed as the length of a highway segment divided by the average travel time of the vehicles traversing the segment.
Travel Time
The average time spent by vehicles traversing a highway segment, including control delay, in seconds per vehicle or minutes per vehicle.
Traveler Satisfaction
A measure of the quality of a trip from the perspective of the traveler.
Trolleybus
An electrically propelled bus that obtains power from an overhead wire system. The power-collecting apparatus allows the bus to maneuver in mixed traffic over several lanes.
Truck
A heavy vehicle engaged primarily in the transport of goods and materials or in the delivery of services other than public transportation.
Turnout
A short segment of a lane, usually a widened, unobstructed shoulder area, added to a two-lane, two-way highway, allowing slow-moving vehicles to leave the main roadway and stop so that faster vehicles can pass.
Two-Lane Class I Highway
A two-lane highway that generally serves long-distance trips or provides connecting links between facilities that serve long-distance trips.
Two-Lane Class II Highway
A two-lane highway that generally serves relatively short trips, the beginning and ending portions of longer trips, or trips for which sightseeing activities playa significant role in route choice.
Two-Lane Highway
A roadway with a two-lane cross section, one lane for each direction of flow, on which passing maneuvers must be made in the opposing lane.
Two-Sided Weaving Segment
A weaving segment in which vehicles entering the highway approach on the right and vehicles departing the highway depart on the left, or vice versa; weaving vehicles must cross the mainline highway flow.
Two-Stage Gap Acceptance
A process used by drivers entering an unsignalized intersection from the minor street and reaching the median area in a first move, then completing the entry with a second move.
Two-Way Left-Turn Lane
A lane in the median area that extends continuously along a street or highway and is marked to provide a deceleration and storage area, out of the through-traffic stream, for vehicles traveling in either direction to use in making left turns at intersections and driveways.
Two-Way Stop-Controlled
The type of traffic control at an intersection where drivers on the minor street or a driver turning left from the major street wait for a gap in the major-street traffic to complete a maneuver.
Unconstrained Operation
An operating condition when the geometric constraints on a weaving segment do not limit the ability of weaving vehicles to achieve balanced operation.
Uncontrolled Ramp Terminal
A ramp terminal without a traffic control device.
Undersaturation
A traffic condition in which the arrival flow rate is lower than the capacity or the service flow rate at a point or uniform segment of a lane or roadway.
Uniform Delay
The first term of the equation for lane group control delay, assuming uniform arrivals.
Uninterrupted Flow
A category of facilities that have no fixed causes of delay or interruption external to the traffic stream; examples include freeways and unsignalized sections of multilane and two-lane rural highways.
Unit Extension
The minimum gap, in seconds, between successive vehicles moving on a traffic-actuated approach to a signalized intersection that will cause the signal controller to terminate the green display.
Unit Width Flow Rate
The pedestrian flow rate expressed as pedestrians per minute per meter of walkway or crosswalk width.
Unmet Demand
The number of vehicles on a signalized lane group that have not been served at any point in time as a result of operation in which demand exceeds capacity, in either the current or previous analysis period. This does not include the normal cyclical queue formation on the red and discharge on the green phase. See also Initial Queue and Residual Queue.
Unsignalized Intersection
An intersection not controlled by traffic signals.
Upstream
The direction from which traffic is flowing.
Urban
An area typified by high densities of development or concentrations of population, drawing people from several areas within a region.
Urban Street
A street with relatively high density of driveway access located in an urban area and with traffic signals no farther than 3. 0 kIn apart.
Urban Street Class
A category of urban street based on functional and design categories.
Urban Street Segment
A length of urban street (in one direction) from one signal to the next, including the downstream signalized intersection but not the upstream signalized intersection.
Utility
A measure of the value a traveler places on a trip choice.
Utility Equation
A mathematical function for evaluating the use of highway facilities; the numerical values depend on the attributes of the travel options and on the characteristics of the traveler.
Validation
Determining whether the selected model is appropriate for the given conditions and for the given task; it compares model prediction with measurements or observations.
Variability
The probability of congestion or a confidence interval for measures of congestion (intensity, duration, and extent).
Vehicle Capacity
(1) The maximum number of passengers that a transit vehicle is designed to accommodate comfortably, seated and standing; also known as normal vehicle capacity or total vehicle capacity.
(2) The maximum number of vehicles that can be accommodated in a given time by a transit facility.
Volume
The number of persons or vehicles passing a point on a lane, roadway, or other traffic-way during some time interval, often 1 h, expressed in vehicles, bicycles, or persons per hour.
Volume to Capacity Ratio
The ratio of flow rate to capacity for a transportation facility.
Walkway
A facility provided for pedestrian movement and segregated from vehicular traffic by a curb, or provided for on a separate right-of-way.
Wave Speed
The speed at which a shock wave travels upstream or downstream through traffic.
Weave Type
A classification scheme that categorizes weaving configuration into one of the three types (Types A, B, C).
Weaving
The crossing of two or more traffic streams traveling in the same direction along a significant length of highway, without the aid of traffic control devices (except for guide signs).
Weaving Configuration
The organization and continuity of lanes in a weaving segment, which determines lane-changing characteristics.
Weaving Diagram
A schematic drawing of flows in a weaving segment, used in analysis.
Weaving Flow
The traffic movements in a weaving segment that are engaged in weaving movements.
Weaving Length
The length from a point on the merge gore at which the right edge of the freeway shoulder lane and the left edge of the merging lane are 0. 6 m apart to a point on the diverge gore at which the edges are 3. 7 m apart.
Weaving Segment
A length of highway over which traffic streams cross paths through lane-changing maneuvers, without the aid of traffic signals; formed between merge and diverge points.
Weaving Width
The total number of lanes between the entry and exit gore areas, including the auxiliary lane, if present.
Work Zone
A segment of highway in which maintenance and construction operations impinge on the number of lanes available to traffic or affect the operational characteristics of traffic flowing through the segment.
Zebra-Striped Crosswalk
A crosswalk painted with diagonal stripes at an unsignalized intersection, in which pedestrians have the right-of-way.
Zone
A geographic aggregation defined by land use, which generates trips within a corridor.