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Glossaries
- A&A Traffic (A to Z)
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Highway Capacity Manual (HCM)
Glossary of Traffic Terms
(A to H )
Acceleration Lane
A paved auxiliary lane, including tapered areas, allowing vehicles to accelerate when entering the through-traffic lane of the roadway.
Access Point
An intersection, driveway, or opening on the right-hand side of a roadway. An entry on the opposite side of a roadway or a median opening can also be considered as an access point if it is expected to influence traffic flow significantly in the direction of interest.
Access-Point Density
The total number of access points on a roadway divided by the length of the roadway and then averaged over a minimum length of 5 km.
Accuracy
The degree of a measure's conformity to a standard or true value.
Adjustment
An additive or subtractive quantity that adjusts a parameter for a base condition to represent a prevailing condition.
Adjustment Factor
A multiplicative factor that adjusts a parameter for a base condition to represent a prevailing condition.
Aggregate Delay
The summation of delays for multiple lane groups usually aggregated for an approach, an intersection, or an arterial route.
Alighting Time
The time required for a passenger to leave a transit vehicle, expressed as time per passenger or total time for all passengers.
All-Way Stop-Controlled
An intersection with stop signs at all approaches. The driver's decision to proceed is based on the rules of the road (e.g. the driver on the right has the right-of-way) and also on the traffic conditions of the other approaches.
Analysis Period
A single time period during which a 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 initial queue from the previous analysis period. Also referred to as time interval.
Analytical Model
A model that relates system components using theoretical considerations that are tempered, validated, and calibrated by field data.
Angle Loading Area
A bus bay design, similar to an angled parking space, requiring buses to back up to exit and allowing more buses to stop in the given linear space. Typically used when buses must occupy berths for a long period of time (e. g. at an intercity bus terminal).
Annual Average Daily Traffic
The total volume of traffic passing a point or segment of a highway facility in both directions for one year divided by the number of days in the year.
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.
Area Type
A geographic parameter reflecting the variation of saturation flows in different areas.
Arrival Rate
The mean of the statistical distribution of vehicles arriving at a point or uniform segment of a lane or roadway.
Arrival Type
Six assigned categories for determining the quality of progression at a signalized intersection.
Arterial
A signalized street that primarily serves through-traffic and that secondarily provides access to abutting properties, with signal spacings of 3. 0 km or less.
Articulated Bus or Articulated Trolleybus
An extra-long, high-capacity bus or trolleybus, with a rear body section or sections, flexibly but permanently connected to the forward section. The vehicle can bend for curves but does not require an interior barrier between its sections.
Auxiliary Lane
An additional lane on a freeway to connect an on-ramp and an off-ramp.
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.
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.
Base Condition
The best possible characteristic in terms of capacity for a given type of transportation facility, that is, further improvements would not increase capacity. It is a condition without hindrances or delays.
Base Saturation Flow Rate
The maximum steady flow rate, expressed in passenger cars per hour per lane, at which previously stopped passenger cars can cross the stop line of a signalized intersection under base conditions, assuming that the green signal is available and no lost times are experienced.
Basic Freeway Segment
A length of freeway facility whose operations are unaffected by weaving, diverging, or merging.
Berth
A position for a bus to pick up and discharge passengers, including curb bus stops and other types of boarding and discharge facilities.
Bicycle
A vehicle with two wheels tandem, propelled by human power, and usually ridden by one person.
Bicycle Facility
A road, path, or way specifically designated for bicycle travel, whether exclusively or with other vehicles or pedestrians.
Bicycle Lane
A portion of a roadway designated by striping, signing, and pavement markings for the preferential or exclusive use of bicycles.
Bicycle Path
A bikeway physically separated from motorized traffic by an open space or barrier, either within the highway right-of-way or within an independent right-of-way.
Bicycle Speed
The riding speed of bicycles, in kilometers per hour or meters per second.
Boarding Time
The time for a passenger to board a transit vehicle, expressed as time per passenger or total time for all passengers.
Body Ellipse
The space provided per pedestrian on a pedestrian facility, expressed as square meters per pedestrian.
Bottleneck
A road element on which demand exceeds capacity.
Breakdown
The onset of a queue development on a freeway facility.
Breakdown Flow
Also called forced flow, this occurs either when vehicles arrive at a rate greater than the rate at which they are discharged or, when the forecast demand exceeds the computed capacity of a planned facility.
Bus
A self-propelled, rubber-tired road vehicle designed to carry a substantial number of passengers (at least 16) and commonly operated on streets and highways.
Bus Lane
A highway or street lane reserved primarily for buses during specified periods. It may be used by other traffic under certain circumstances, such as making a right or left turn, or by taxis, motorcycles, or carpools that meet the requirements of the jurisdiction's traffic laws.
Bus Platoon
A convoy of several buses, with each bus following the operating characteristics of the one in front.
Bus Stop
An area in which one or more buses load and unload passengers. It consists of one or more loading areas and may be on line or off line.
Busway
A right-of-way restricted to buses by a physical separation from other traffic lanes.
Calibration
The process of comparing model parameters with real-world data to ensure that the model realistically represents the traffic environment. The objective is to minimize the discrepancy between model results and measurements or observations.
Capacity
The maximum sustainable flow rate at which vehicles or persons reasonably can be expected to traverse a point or uniform segment of a lane or roadway during a specified time period under given roadway, geometric, traffic, environmental, and control conditions; usually expressed as vehicles per hour, passenger cars per hour, or persons per hour.
Captive Riders
Transit riders, such as people with disabilities, the elderly, young adolescents, and adults without driver's licenses, who do not have alternative means of travel.
Change Interval
The yellow plus all-red interval that occurs between phases of a traffic signal to provide for clearance of the intersection before conflicting movements are released.
Circulating Flow
The volume of traffic on the principal roadway of a roundabout at a given time.
Circulating Roadway
The continuous-flow section of a roundabout that requires other vehicles entering the roadway to yield.
Circulation Area
The portion of a sidewalk street corner used by moving pedestrians passing through the area, expressed in square meters.
Clearance Lost Time
The time, in seconds, between signal phases during which an intersection is not used by any traffic.
Clearance Time
The time loss at a transit stop, not including passenger dwell times. This parameter can be the minimum time between one transit vehicle leaving a stop and the following vehicle entering and, can include any delay waiting for a sufficient gap in traffic to allow the transit vehicle to re-enter the travel lane.
Climbing Lane
A passing lane added on an upgrade to allow traffic to pass heavy vehicles whose speeds are reduced.
Collector Street
A surface street providing land access and traffic circulation within residential, commercial, and industrial areas.
Commuter Rail
The portion of passenger railroad operations that carries passengers within urban areas or between urban areas and their suburbs. Unlike rapid rail transit, the passenger cars generally are heavier, the average trip lengths are usually longer, and the operations are carried out over tracks that are part of the area' s railroad system.
Composite Grade
A series of adjacent grades along a highway that cumulatively has a more severe effect on operations than each grade separately.
Compound Left-Turn Protection
A signal phasing scheme that provides both a protected and permitted phase in each cycle for a left turn. See also protected plus permitted and permitted plus protected.
Conflicting Approach
The approach opposite the subject approach at an all-way stop-controlled intersection.
Conflicting Flow Rate
The flow rate of traffic that conflicts with a specific movement at an unsignalized intersection.
Conflicting Movements
The traffic streams in conflict at an unsignalized intersection.
Congested Flow
A traffic flow condition caused by a downstream bottleneck.
Constrained Operation
An operating condition in a weaving segment, involving geometric and traffic constraints, that prevents weaving vehicles from occupying a large portion of the lanes available to achieve balanced operation.
Control Condition
The traffic controls and regulations in effect for a segment of street or highway, including the type, phasing, and timing of traffic signals, stop signs, lane use and turn controls, and similar measures.
Control Delay
The component of delay that results when a control signal causes a lane group to reduce speed or to stop. It is measured by comparison with the uncontrolled condition.
Corridor
A set of essentially parallel transportation facilities designed for travel between two points. A corridor contains several subsystems, such as freeways, rural (or two-lane) highways, arterials, transit, and pedestrian and bicycle facilities.
Coverage
The geographical area that a transit system serves, normally based on acceptable walking distances from loading points.
Crawl Speed
The maximum sustained speed that can be maintained by a specified type of vehicle on a constant upgrade of a given percent. Expressed in kilometers per hour.
Critical Density
The density at which capacity occurs for a given facility, usually expressed as vehicles per kilometer per lane.
Critical Gap
The minimum time, in seconds, between successive major-stream vehicles, in which a minor-street vehicle can make a maneuver. Also see Pedestrian critical gap.
Critical Lane Group
The lane groups that have the highest flow ratio for a given signal phase.
Critical Speed
The speed at which capacity occurs for a facility, usually expressed as kilometers per hour.
Critical Volume-to-Capacity Ratio
The proportion of available intersection capacity used by vehicles in critical lane groups.
Cross Flow
A pedestrian flow that is approximately perpendicular to and crosses another pedestrian stream. The smaller of the two flows is the cross-flow condition.
Crosswalk
A marked area for pedestrians crossing the street at an intersection or designated mid-block location.
Crown Line
A lane marking that connects from the entrance gore area directly to the exit gore area.
Crush Load
The maximum number of passengers that can be accommodated on a transit vehicle.
Cycle
A complete sequence of signal indications.
Cycle Length
The total time for a signal to complete one cycle.
Deceleration Lane
A paved auxiliary lane, including tapered areas, allowing vehicles leaving the through-traffic lane of the roadway to decelerate.
Default Value
A representative value that may be appropriate in the absence of local data.
Delay
The additional travel time experienced by a driver, passenger, or pedestrian.
Demand
The number of users desiring service on the highway system, usually expressed as vehicles per hour or passenger cars per hour.
Demand-Responsive Service
Passenger cars, vans, or buses with fewer than 25 seats, dispatched by a transit operator in response to calls from passengers or their agents.
Demand Starvation
A condition when portions of the green time at a downstream intersection cannot be used because conditions at an upstream intersection prevent vehicles from reaching the stop line downstream at an interchange ramp terminal.
Demand to Capacity Ratio
The ratio of demand flow rate to capacity for a traffic facility.
Density
The number of vehicles on a roadway segment averaged over space, usually expressed as vehicles per kilometer or vehicles per kilometer per lane. Also see Pedestrian density.
Departure Headway
The average headway, in seconds, between two consecutive vehicles departing from a lane at an all-way stop-controlled intersection.
Descriptive Model
A mathematical model that applies concepts or theoretical principles to represent the behaviour of a system.
Design Application
Using capacity analysis procedures to determine the size (number of lanes) required for a specified level of service.
Design Category
A type of urban street defined by geometric features and roadside environment.
Design Hour
An hour with a traffic volume that represents a reasonable value for designing the geometric and control elements of a facility.
Design Hour Factor (K-factor)
The proportion of the 24-h volume that occurs during the design hour.
Design Speed
A speed used to design the horizontal and vertical alignments of a highway.
Deterministic Model
A mathematical model that is not subject to randomness. The result of one analysis can be repeated with certainty.
Diamond Interchange
An interchange that results in two or more closely spaced surface intersections, so that one connection is made to each freeway entry and exit, with one connection per quadrant.
Directional Design Hour Volume
The traffic volume for the design hour in the peak direction of flow, in vehicles per hour.
Directional Distribution
A characteristic of traffic, that volume may be greater in one direction than in the other during any particular hour on a highway.
Directional Flow Rate
The flow rate of a highway in one direction.
Directional Segment
A length of two-lane highway in one travel direction, with homogeneous cross sections and relatively constant demand volume and vehicle mix.
Directional Split
The directional distribution of hourly volume on a highway, expressed in percentages.
Diverge
A movement in which a single lane of traffic separates into two lanes without the aid of traffic control devices.
Double-Stream Door
A transit vehicle door, generally 1. 14 to 1. 37 m wide, that permits two passengers to board, alight, or board and alight simultaneously.
Downstream
The direction of traffic flow.
Downtown Street
A surface facility providing access to abutting property in an urban area.
Drive-Through (Pull-Through) Loading Area
A bus bay design for compact areas, providing several adjacent loading islands, between which buses stop, drive through, and then exit.
Driver Population
A parameter that accounts for driver characteristics and their effects on traffic.
Duration of Congestion
A measure of the maximum amount of time that congestion occurs anywhere in the transportation system.
Dwell Time
The time a transit unit (vehicle or train) spends at a station or a stop, measured from stopping to starting.
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.
Effective Red Time
The time during which a given traffic movement or set of movements is directed to stop; it is equal to the cycle length minus the effective green time.
Effective Walkway Width
The width, in meters, of a walkway usable by pedestrians, or the total walkway width minus the width of unusable buffer zones along the curb and building line.
85th-Percentile Speed
A speed value that is less than 15 percent of a set of field measured speeds.
Empirical Model
A model that describes system performance based on the statistical analysis of field data.
Entrance Ramp
A ramp that allows traffic to enter a freeway.
Equilibrium Distance
The distance between the next upstream ramp and the subject ramp, or between the next downstream ramp and the subject ramp, that produces a PFM or PFD value indicating that the subject ramp is isolated.
Event
A meeting or a passing on a bicycle facility.
Event-Based Model
A simulation model that advances from one event to the next, skipping over intervening points in time when no event occurs.
Exclusive Bus Lane
A highway or street lane reserved for buses.
Exclusive Turn Lane
A designated left or right turn lane or lanes used only by vehicles making those turns.
Exit Ramp
A ramp for traffic to depart from a freeway.
Extension of Effective Green Time
The amount of the change and clearance interval at the end of the phase for a lane group, usable for movement of its vehicles.
Extent of Congestion
The maximum geographic extent of congestion on the transportation system at any one time.
Facility
A length of highway composed of connected sections, segments, and points.
Failure Rate
The probability that a bus will find all available loading areas occupied by other buses at a bus stop.
Far-Side Stop
A transit stop that requires transit units to cross an intersection before stopping to serve passengers.
Fixed Obstruction
Obstructions along a roadway, including light poles, signs, trees, abutments, bridge rails, traffic barriers, and retaining walls.
Fixed Route Service
Service provided by transit vehicles on a repetitive, fixed schedule along a specific route, picking up and delivering passengers to specific locations. Each fixed route serves an assigned origin and destination.
Flared Approach
A shared right-turn lane that allows right-turning vehicles to complete their movement while other vehicles are occupying the lane.
Flow Rate
The equivalent hourly rate at which vehicles, bicycles, or persons pass a point on a lane, roadway, or other trafficway. It is computed as the number of vehicles, bicycles, or persons passing the point, divided by the time interval (usually less than I h) in which they pass, and, expressed as vehicles, bicycles, or persons per hour.
Flow Ratio
The ratio of the actual flow rate to the saturation flow rate for a lane group at an intersection.
Follow-up Time
The time between the departure of one vehicle from the minor street and the departure of the next vehicle using the same gap under a condition of continuous queuing, expressed in seconds.
Free Flow
A flow of traffic unaffected by upstream or downstream conditions.
Free-Flow Speed
(I) The theoretical speed of traffic, in kilometers per hour, when density is zero. That is, when no vehicles are present.
(2) The average speed of vehicles over an urban street segment without signalized intersections, under conditions of low volume.
(3) The average speed of passenger cars over a basic freeway or multilane highway segment under conditions of low volume.
Freeway
A multilane, divided highway with a minimum of two lanes for the exclusive use of traffic in each direction and full control of access without traffic interruption.
Freeway Facility
An aggregation of sections comprising basic freeway segments, ramp segments, and weaving segments.
Fully Actuated Control
A signal operation in which vehicle detectors at each approach to the intersection control the occurrence and length of every phase.
Functional Category
An urban street defined by the traffic service it provides.
Functional Class
A transportation facility defined by the traffic service it provides.
Gap
The time, in seconds, for the front bumper of the second of two successive vehicles to reach the starting point of the front bumper of the first.
Gap Acceptance
The process by which a minor-street vehicle accepts an available gap to maneuver.
Gate
A point at which a major facility crosses the boundary of a corridor.
Gate Tree
A list of segments connected to the entry gate of a corridor.
General Terrain
A classification used for analysis in lieu of a specific grade.
Geometric Condition
The spatial characteristics of a facility, including approach grade, the number and width of lanes, lane use, and parking lanes.
Geometric Delay
The component of delay that results when geometric features cause vehicles to reduce their speed in negotiating a facility.
Gore Area
The area located immediately between the left edge of a ramp pavement and the right edge of the roadway pavement at a merge or diverge area.
Green Time
The duration, in seconds, of the green indication for a given movement at a signalized intersection.
Green Time Ratio
The ratio of the effective green time of a phase to the cycle length.
Group Critical Gap
The minimum time during which a platoon of pedestrians will not attempt to cross a stop-controlled intersection, expressed in seconds.
Growth Factor
A percentage increase applied to current traffic demands to estimate future demands.
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 (for example, 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.
Heavy Rail
A transit system using trains of high-performance, electrically powered rail cars operating in exclusive right-of-way.
Heavy Vehicle
A vehicle with more than four wheels touching the pavement during normal operation.
High Occupancy Vehicle (HOV)
A vehicle with a defined minimum number of occupants (>I); HOVs often include buses, taxis, and carpools, when a lane is reserved for their use.
Hindrance
A concept related to the comfort and convenience of bicyclists. Used to derive level of service for a bicycle facility. Often, the number of events is used as a surrogate for hindrance.