Glossary of Road Traffic Analysis Terms
(A to Z)
Actuated Signal Control
A signal control method in which traffic signal phases and their timings are
determined by traffic demands identified by detector actuations subject to
various signal controller settings. See Fixed-Time (Pretimed) Signal Control.
Back of Queue
Maximum extent of the queue relative to the stop line or give-way (yield) line
during a signal cycle or gap-acceptance cycle. The last queued vehicle that
joins the back of queue is the last vehicle that departs at the end of the
saturated part of green interval or the available gap interval.
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.
Circulating Flow
The vehicle flow rate in all lanes of the circulating road in front of a
roundabout entry lane, determined using Stopline Flow Rates.
Control Delay
Sum of Stop-Line Delay and Geometric Delay.
Cost (Operating Cost)
A measure that includes the direct vehicle operating cost (the resource cost of
fuel and additional running costs including tyre, oil, repair and maintenance as
a factor of the cost of fuel) as well as the time cost of vehicle occupants.
Critical Gap (Headway)
The minimum time between successive vehicles in the opposing (major) traffic
stream that is acceptable for entry by opposed (minor) stream vehicles. See
Headway.
Critical Intersection
The intersection in a coordinated signal system that operates with the highest
overall degree of saturation during a given period.
Critical Lane
The lane in a lane group or approach that has the highest degree of saturation
and places the highest demand on green time.
Critical Movements
The set of movements that determine the capacity and timing requirements of a
signalised intersection.
Cycle
A complete sequence of signal phases.
Cycle-Average Queue
The average queue length that incorporates all queue states including zero
queues as counted at regular intervals (e.g. every 5 seconds).
Cycle Length (Cycle Time)
Time required for one complete sequence of signal displays (sum of phase green
and intergreen times). For a given movement, cycle time is the sum of the
durations of red, yellow and green signal displays, or sum of Effective Green
and Red Times. In gap-acceptance analysis, this is the equivalent average cycle
time corresponding to the block and unblock periods in the opposing traffic
stream.
Degree of Saturation
The ratio of arrival (demand) flow rate to capacity during a given flow period.
Also known as the volume to capacity ratio.
Delay
The additional travel time experienced by a vehicle or pedestrian with reference
to a base travel time (e.g. the free-flow travel time).
Demand Flow (Demand Volume)
The number of vehicles or pedestrians arriving during a given period as measured
at the back of queue (as distinct from departure flows measured in front of the
queue). See Stopline Flow Rate.
Density
The number of vehicles per unit distance along a road segment as measured at an
instant in time.
Design Life
The number of years into the future while the intersection operates
satisfactorily considering increases in traffic demand volumes.
Detector
A device by which vehicle or pedestrian traffic registers its presence. The most
common detectors are the inductive loop detectors for vehicles and the
push-button detectors for pedestrians.
Downstream
In the direction of the movement of traffic.
Effective Green and Red Times
The movement green and red times for capacity and performance analysis purposes,
which are determined by adjusting the displayed green and red times for Start
Loss and End Gain effects.
Effective Intersection Capacity
An aggregate measure of intersection capacity determined as the ratio of total
intersection demand flow to the intersection degree of saturation, where the
intersection degree of saturation is the largest lane degree of saturation
considering all lanes of the intersection.
End Gain
Duration of the interval between the end of the displayed green period and the
end of the effective green period for a movement. This is used in signal timing
and performance analysis to allow for additional departures after the end of
green period.
Equivalent Stop Value
Value of a deceleration-acceleration cycle in terms of a major stop-start cycle.
See Major Stop.
Exclusive Pedestrian Phase
The phase at an intersection during which all pedestrian displays are green and
all vehicle displays are red, allowing all pedestrian movements to operate
simultaneously while all vehicle movements are stopped. Also see
Scramble-Crossing Phase.
Exclusive Lane
A lane (or length of lane) allocated for use only by a particular movement or a
type of vehicle, e.g. left-turn lane, through lane, right-turn lane, bus lane,
as opposed to a Shared Lane.
Filter Turn
A turning movement that must give way to and find safe gaps in conflicting
(opposing) vehicle or pedestrian traffic before proceeding, e.g. filter
right-turn, slip-lane left turn, left turn on red. Also see Opposed Movement.
Fixed-Time (Pretimed) Signal Control
A signal control method that allows for only a fixed sequence and fixed duration
of displays.
See Actuated Signal Control.
Flow Rate
Number of vehicles or pedestrians per unit time passing (arriving or departing)
a given reference point.
Flow Ratio
The ratio of arrival (demand) flow rate to saturation flow rate.
Follow-up Headway
The average headway between successive opposed (minor) stream vehicles entering
a gap available in the opposing (major) traffic stream. The Follow-up Headway
(seconds) is a saturation (queue discharge) headway, and the corresponding
saturation flow rate (vehicles per hour) in gap-acceptance analysis is 3600 /
Follow-up Headway. See Saturation Flow Rate.
Free-Flow Speed
The uninterrupted traffic speed when density is approximately zero, i.e. when
only few vehicles are present in the traffic stream.
Full Control
Control of a turning movement using three-aspect (red, yellow, green) turn
arrows on a six-aspect signal face, where the green arrow indicates that the
vehicle can turn unopposed (with no opposing vehicle or pedestrian traffic) and
the red arrow indicates that the vehicle is not permitted to turn (filter turns
not permitted).
Gap Acceptance
The process by which an opposed (minor) stream vehicle accepts an available gap
in the opposing (major) stream for entering (departing from queue or merging).
See Critical Gap (Headway).
Gap Setting
A controller setting equivalent to a predetermined space time measured between
successive vehicles at the given (approach) speed, detection zone length and
vehicle length values that will cause the signal controller to terminate the
green display. See Space Time.
Geometric Delay
Delay due to physical and basic traffic control factors as experienced by a
vehicle negotiating the intersection in the absence of any other vehicles (due
to a deceleration from the approach cruise speed down to an approach negotiation
speed, travel at that speed, acceleration to an exit negotiation speed, and then
acceleration to the exit cruise speed).
Geometric Stop
The effective value of a slow-down and speed-up manoeuvre associated with
Geometric Delay, which is measured in terms of equivalent Major Stops.
Green Time
Duration of the green display for a phase or a movement.
Headway
The time between passage of the front ends of two successive vehicles. See
Spacing.
Intergreen Time
Duration of the clearance part of the phase corresponding to the period between
the phase change point (the end of running intervals) and the beginning of the
green display for the next phase (end of phase). Normally, it comprises Yellow
Time and All-Red Time.
Intra-Bunch Headway
Average headway between vehicles in a moving queue (minimum headway in a
stream). This is used in order to define moving queues (bunches) of vehicles for
the purpose of modelling headway distribution of vehicles.
Lane Group
A set of lanes with one or two shared lanes (e.g. Lane 1: Left-Turn and Through,
Lane 2: Through) or a set of exclusive turn lanes (e.g. a single Right-Turn
lane).
Lane Utilisation
The distribution of vehicles among lanes when two or more lanes are available
for a movement.
Lane Utilisation Ratio
Ratio of the lane degree of saturation to the highest (critical) lane degree of
saturation in a Lane Group.
Level of Service
An index of the operational performance of traffic on a given traffic lane,
carriageway, road or intersection, based on service measures such as delay,
degree of saturation, density and speed during a given flow period.
Major Stop
A drive cycle element that involves a deceleration from the approach cruise
speed to zero speed and an acceleration from zero speed to the exit cruise
speed.
Occupancy Time
The time that starts when the front of a vehicle enters the detection zone and
finishes when the back of the vehicle exits the detection zone, i.e. the
duration of the period when the detection zone is occupied by a vehicle.
Off-Peak Period
The periods that have low demand volumes of traffic during the day (24-hour
period).
Offset
The difference between the start or end times of green periods at adjacent
(upstream and downstream) signals.
Opposed Movement
A movement (Filter Turn, Permitted Turn, Minor Movement) that gives way to one
or more opposing traffic streams at a signalised or unsignalised intersection.
Opposing Movement
A movement that conflicts with, and has priority over, another (opposed)
movement.
Overflow
An interrupted traffic condition when a number of queued vehicles are not able
to depart due to insufficient capacity during a traffic signal or gap-acceptance
cycle (also known as cycle failure).
Overflow Queue
Average number of vehicles per cycle left over at the end of green periods at
signals or at the end of acceptable gap (unblock) periods during gap-acceptance
process.
Overlap Movement
A movement that runs in consecutive phases without stopping during the
associated intergreen period(s).
Parallel Pedestrian Movement
A signalised pedestrian movement that runs at the same time as the parallel
vehicle movement (s) that are controlled by circular green displays.
Peak Flow Factor (PFF)
Ratio of the average demand flow rate in the Total Flow Period (e.g. one hour)
to the demand flow rate in the Peak Flow Period (e.g. 15 minutes). This is
equivalent to the more traditional term Peak Hour Factor (PHF) when the Total
Flow Period is one hour.
Peak Period
The period that has the highest demand volume of traffic during the day (peak
hour, peak half hour, etc).
Pedestrian Clearance Period
The Flashing Don't Walk period that immediately follows the termination of
pedestrian Walk display to enable pedestrians, who have just stepped off the
kerb at the commencement of this period, to complete their crossing to the
nearest kerb or refuge.
Pedestrian Crossing
A transverse strip of carriageway marked for the use of pedestrians crossing the
road (mid-block or at intersections) at a place with a pedestrian crossing sign,
and with or without alternating flashing twin yellow lights. Also called Zebra
Crossing where indicated by parallel white stripes on the road surface.
Pedestrian Minimum Green Time
Minimum time required for both Walk and Flashing Don't Walk displays, but
excluding any overlaps with terminating intergreen displays.
Performance Index
A measure that combines several performance statistics such as delay, number of
stops and queue length.
Phase
That part of a signal cycle during which one or more movements receive right of
way subject to resolution of any vehicle or pedestrian conflicts by priority
rules. A phase is identified by at least one movement gaining right of way at
the start of it and at least one movement losing right of way at the end of it.
Phase Sequence
The order of phases in a signal cycle.
Phase Split
Duration of each phase (Green Time and Intergreen Time) within a signal cycle.
It is normally expressed as a percentage of cycle length.
Platoon
A group of vehicles or pedestrians travelling together because of signal
control, geometric conditions or other factors.
Platoon Ratio
Ratio of the average arrival flow rate during the green period to the average
arrival flow rate during the signal cycle.
Practical Degree of Saturation
A target, or maximum, degree of saturation that corresponds to an acceptable
level of traffic performance.
Practical Spare Capacity
The amount of increase possible in the demand flow rate to obtain a degree of
saturation equal to the practical (target) degree of saturation.
Progression
Progression is a time-relationship, between adjacent traffic signals, which
allows vehicle platoons to be given a green signal as they pass through the
sequence of intersections.
Progression Factor Method
A simple technique to determine signal coordination effect on delay, queue
length, stop rate, etc. where detailed platoon patterns generated at upstream
signals are not available.
Proportion Queued
Proportion of traffic that is queued due to the effects of traffic control and
the existence of other vehicles. This is related to the Major Stops or Slow
Downs from the approach cruise speed.
Queue
A line of vehicles or pedestrians waiting to proceed through an intersection.
Slowly moving vehicles or pedestrians joining the back 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. See Back of Queue and Cycle-Average Queue.
Queuing Delay
Part of the Stop-Line Delay that includes the Stopped Delay (while vehicle is
idling at near-zero speed) and the Queue Move-up delay (while a queued vehicle
accelerates towards the stop-line but stops again, e.g. because the signal
display changes to red).
Queue Storage Ratio
The ratio of the queue length to the available queue storage distance.
Red Time
Duration of the red signal display for a phase or a movement.
Saturation Flow Rate
The maximum departure (queue discharge) flow rate achieved by vehicles departing
from the queue during the green period at traffic signals. Saturation Headway
(seconds) is 3600 / Saturation Flow Rate (vehicles per hour). The Follow-up
Headway parameter used in gap-acceptance analysis is a saturation (queue
discharge) headway. See SCATS Maximum Flow (MF) and Follow-up Headway.
SCATS Maximum Flow (MF)
A maximum departure flow rate during a fully saturated green period averaged
over the green and intergreen times as a special measure of saturation flow
rate. See Saturation Flow Rate.
Scramble-Crossing Phase
An Exclusive Pedestrian Phase at an intersection where pedestrians are allowed
to cross in any direction including diagonally within the limits of the
crosswalk lines.
Shared Lane
A lane allocated for use by two or more movements, e.g. shared through and
right-turn lane, as opposed to an Exclusive Lane.
Short Lane
A lane of limited length, e.g. a turn bay or part of a lane available downstream
of parked vehicles.
Signalised Crossing
An area of the road used by pedestrians when crossing the road with the guidance
of pedestrian signals at a mid-block or intersection location, and can be used
by cyclists if bicycle signals are provided.
Signal Phasing
Sequential arrangement of separately controlled groups of vehicle and pedestrian
movements within a signal cycle to allow all vehicle and pedestrian movements to
proceed.
Slip Lane
A turning movement lane separated from an adjacent lane by a triangular island.
Slow Down
A drive cycle element that involves a deceleration from the approach cruise
speed to a non-zero intermediate speed and an acceleration from the intermediate
speed to the exit cruise speed.
Space Length (Gap Distance)
The following distance between two successive vehicles as measured between the
rear end of one vehicle and the front end of the next vehicle in the same
traffic lane (spacing less vehicle length).
Space Time
The time between the detection of two consecutive vehicles when the presence
detection zone is not occupied.
Spacing
The distance between the front ends of two successive vehicles in the same
traffic lane.
Speed
Distance travelled per unit time. In a time-distance diagram, the slope of the
time-distance trace of a vehicle is its speed. Approach Speed is the
uninterrupted (mid-block) cruise speed of vehicles before being affected by
traffic signals. This can be represented by the speed limit. Negotiation Speed
is the safe speed of a vehicle moving through the controlled area of the
intersection. For turning vehicles, this can be determined as a function of the
negotiation radius. Running Speed is the average speed including the effect of
delays due to interrupted conditions but not including any stopped (idling)
times. Travel Speed is the average speed including the effect of all delays. See
Free-Flow Speed.
Staged Signalised Crossing
A system by which a long signalised crossing is divided or “staged” into several
time-separated sections, each being a separate group controlled by individual
signals.
Start Loss
Duration of the interval between the start of the displayed green period and the
start of the effective green period for a movement. This is used in signal
timing and performance analysis to allow for queue discharge time losses at the
start of green period due to vehicles accelerating to saturation speed, or due
to giving way to opposing vehicle or pedestrian movements.
Stop-Line Delay
Delay determined by projecting vehicle time-distance trajectories from the
approach and exit negotiation speeds to the stop line (or give-way / yield
line), which includes the Queuing Delay and the deceleration and acceleration
delay associated with the negotiation speeds.
Stopline Flow Rate
Departure flow rate measured at the stop line (or give-way / yield line), which
is the same as the demand (arrival) flow rate for undersaturated cases, and is
limited to the capacity rate for oversaturated cases. See Demand Flow.
Stop Rate
Average number of all acceleration-deceleration manoeuvres including the queue
move-ups, partial stops and geometric stops, expressed in terms of equivalent
Major Stops.
T-Intersection
An intersection where two roads meet (whether or not at right angles) and one of
the roads ends.
Total Delay
Sum of delay experienced by all vehicles or pedestrians (vehicle-hours per hour
or pedestrian-hours per hour). Obtained as the product of average delay per
vehicle or pedestrian and the flow rate.
Total Travel Distance
Sum of distance travelled by all vehicles (vehicle-kilometres per hour or
vehicle-miles per hour). Obtained as the product of travel distance per vehicle
and the flow rate.
Traffic-Actuated Control
A control method that allows a variable sequence and variable duration of signal
displays depending on vehicle and pedestrian traffic demands.
Traffic Delay
Delay that results when the interactions between vehicles cause drivers to
reduce speed below the free-flow (desired) speed. See Free-Flow Speed.
Traffic Volume
The number of vehicles or pedestrians passing a given point on a lane or
carriageway during a specified period of time.
Uninterrupted Flow
A condition in which vehicles travelling in a traffic stream do not have to stop
or slow down for reasons other than those caused by the presence of other
vehicles in that stream.
Unopposed Turn
A left-turn or right-turn movement at a signalised intersection that is made
with no opposing or conflicting vehicular or pedestrian flow allowed.
Upstream
In the direction opposite to the movement of traffic.
Walk Time
Duration of the Walk display (steady green person) for pedestrians.
Yellow Time
Duration of the yellow display for a phase or a movement.
References
AKCELIK & ASSOCIATES (2007). SIDRA INTERSECTION User Guide. Akcelik and
Associates Pty Ltd, Melbourne, Australia.
AUSTROADS (1993). Roundabouts. Guide to Traffic Engineering Practice, Part 6.
Association of Australian State Road and Transport Authorities, Sydney.
AUSTROADS (2003). Traffic Signals. Guide to Traffic Engineering Practice, Part
7. Association of Australian State Road and Transport Authorities, Sydney.
TRB (2000). Highway Capacity Manual. Transportation Research Board, National
Research Council, Washington, D.C., U.S.A.
Prepared by: R. Akçelik, Akcelik & Associates Pty Ltd
(see the references above for direct sources used).
Please let us know your comments to help us improve this
glossary.
Last updated: 10 July 2007