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SCOOT Glossary

advance An advance decision shortens the time to the next stage. It is changed by advancing the time at which it is due.
area Top of the SCOOT hierarchy. The SCOOT Area contains one or more Regions each of which contains a number of nodes. Area data is input as part of the setup of a SCOOT network.
area end lag The normal end delay for traffic, typically 3 seconds. This is the time between the stage terminating (lights turning amber) on-street and the end of effective saturation flow.
area start lag The normal start up delay for traffic, typically 2 seconds. This is the time between the lights turning green on-street and the start of effective saturation flow.
ASTRID Automatic Scoot TRaffic Information Database. ASTRID is a database designed to collect information from a SCOOT traffic control system, or other source of traffic data, and to store it in a database for later retrieval and analysis.
automatic vehicle location (AVL) With AVL, buses are fitted with equipment which determines bus location, usually as part of a bus management system. An indication from the AVL system that a bus is approaching the traffic signals can be used to trigger SCOOT bus priority. Systems that can be set to request priority at specific locations are best suited to SCOOT's needs.
bacfiles (ASTRID) Bacfiles contain a days' ASTRID log data at a medium resolution (15 minute intervals).
bus priority Bus Priority is a facility within SCOOT whereby buses can receive priority benefits at SCOOT controlled traffic signals to reduce delay. Buses can be given extensions where the green is extended allowing the bus to continue, recalls where a bus is detected on a red causing other stage times to be reduced in order to bring forward the green onto that link or stages can be skipped to recall the bus stage even earlier.
congestion SCOOT congestion is defined as the number of 4 second intervals in a cycle during which the detector is continuously occupied. It is expressed as a proportion of the cycle time. With the normal SCOOT detector location at the upstream end of the link, SCOOT congestion indicates a queue that is, or is in danger of, blocking the exit from the upstream junction.
congestion importance factor Is a number between 0 (not important) and 7 (most important) assigned to a link to define how important it is in terms of the impact it will have on the rest of the network if it becomes congested. The congestion importance factor controls SCOOT's response to congestion on the link.
congestion offset Is the offset desired when the link is congested - used in conjunction with congestion weighting. Takes a value in the range -127 to +127 seconds.
cruise speed The average speed vehicles travel at between the detector and the stopline when not constrained by queues or red traffic signals.
cycle / cycle time The length of time in seconds for a complete stage sequence to run before it is repeated.
cyclic flow profile Shows and quantifies the traffic demand over a traffic signal cycle.
delay Is the amount of traffic signal induced delay; the time vehicles are held up at traffic signals and by other traffic queuing at the signals.
demand Is an expression used to represent the number of vehicles on a link. The detector upstream measures demand as vehicles pass creating a cyclic flow profile. The information is passed to SCOOT enabling it to model the demand for green time at the stopline.
demand dependent stages Are stages that only come into play when there is a demand for that stage e.g. a pelican/pedestrian crossing or a side road.
detector A means of detecting vehicles, usually a loop in the road connected to a detector pack which responds to the change in inductance when a vehicle passes over the loop.
dispersion Describes the action of platoons (bunches of vehicles released from the upstream signals) spreading out as they move along a link.
double cycling Facility to allow nodes to operate at half the region cycle time. Reduces delay at lightly loaded junctions within a region that requires a longer cycle time and considerably reduces pedestrian waiting time at Puffins and Pelicans.
double greening When a link is set to have two separate green periods within the cycle.
effective green The time during a green period where traffic can flow at saturation flow through the junction. Effective green starts at:

Area Start Lag + Link End Lag (if any)

after the start of the green aspect.

Effective green ends at:

Appearance of amber + Area End Lag + Link End Lag (if any)

entry link A link bringing traffic into a co-ordinated SCOOT network from outside.
exit blocking A queue on a downstream link blocking the exit from a junction thus not allowing any traffic to proceed on a green.
exit link A link that takes traffic out of a SCOOT network, not specified in the SCOOT data unless a detector is needed on the link.
extension A bus priority method where the green on the link is extended to allow a bus approaching on green to pass through the signal when otherwise the signal would change before the bus reached the stopline.
fast down flag Used to improve the cycle times' response to falling flows. Flows can fall rapidly at the end of an evening peak in which case the fast down should be set by the timetable to allow SCOOT to respond quickly at this time.
filter link A link where the length of green is controlled by information from a detector after the stopline. Usually a right turn.
filter saturation occupancy Is the normal maximum outflow rate of a queue from the stopline on a filter link in LPU/sec.
fixed / biased offsets Fixed and biased offsets favour particular movements. Fixed offsets can be used to maintain a particular value on that link (within a small range). Biased offsets give SCOOT the freedom to optimise about the specific offset.
flow The amount of traffic movement along a link.
gating Action at a distance. It can be used to restrict traffic from entering or leaving a critical area.
incident An incident is an event which reduces the capacity of a network link to carry traffic.
INGRID INteGRated Incident Detection. A computer program for the automatic detection of traffic incidents in urban areas controlled by SCOOT.
intergreen matrix Is a diagram that shows the lengths of intergreen for all the permitted changes between UTC stages.
link Is a length of road from the SCOOT detector to the stopline. More than one link is specified on a stretch of road if their movements are separately controlled at the junction i.e. right turning traffic has different signal requirements to a straight ahead.
link journey time The time taken by an average vehicle to the stopline from a SCOOT detector when not delayed by the traffic signals or queuing vehicles.
link end lag Has a range of -31 to +31 seconds although it is usually set to 0. The link end lag is an adjustment that can be added on to the end of effective green if required. The link end lag is an adjustment in seconds on top of the usual Area End Lag. - see area end lag and effective green.
link profile unit (LPU) All SCOOT traffic behaviour is modelled in terms of link profile units. One vehicle is equivalent to approximately 17 LPU's, but the actual value will vary between links.
link start lag Has a range of -31 to +31 seconds although it is usually set to 0. The link start lag is an adjustment that can be added on to the start of effective green if required. The link start lag is an adjustment in seconds on top of the usual Area Start Lag. - see area start lag and effective green
link suffix The letter used to identify the link e.g. 0315A.
local extensions The decision whether an extension can be given to a bus is always made centrally by the SCOOT computer at the UTC. With local extensions the decision is made in advance so that the local controller can grant the extension when a bus is detected. They usually achieve greater benefits to buses due to avoiding the transmission lag involved with central extensions.
loop Inductive loop used for detecting traffic often used as an alternative word for a detector. See detector.
masking Masking is when parts of two vehicles cross the detector at the same time so that only one vehicle is detected, not two.
minimum practical cycle time (MPCY) Each node in a region has a MPCY which is the cycle time at which the node would be 90% saturated.
network Is the layout of nodes and links in a region; the network of roads controlled by the system.
node Third in the SCOOT hierarchy. A node is a set of traffic signals in a SCOOT region, a junction or signal-controlled pedestrian crossing.
normal link Normal links are those that join one node to another in a network.
offset The offset on a link is the time between the start of green to the main feeder link at the upstream link to the start of green to this link at the downstream node.
offset optimisation The offset optimiser co-ordinates all junctions in a SCOOT region. One junction is considered at a time and the aim is to provide traffic a smooth passage through the network.
offset weighting Offset weighting allows a biasing of specific routes or links. The higher the offset weighting for a link, the more SCOOT will favour the offset of that link, to the detriment of other less weighted links or routes. N.B. offsets are inherently flow-weighted, busy links will automatically be more important in the optimisation than quiet ones.
optimise To provide the best possible signal timings.
Pelican A stand-alone, signal-controlled pedestrian crossing incorporating flashing green man / flashing amber to pedestrians to provide a variable crossing period for pedestrians.
platoon A group of vehicles travelling in 'convoy' along a link.
profiles Are ASTRID log files containing average data per weekday which are stored at medium resolution (15 minute collection periods); see Cyclic Flow Profiles for the cyclic pattern of traffic on a link.
Puffin A stand-alone, signal-controlled pedestrian crossing where the pedestrian crossing period is extended by on-crossing detectors with standard vehicle aspects: no flashing aspects.
queue A number of vehicles delayed at a node.
rawfiles Are ASTRID rawdata files stored at high resolution (data collected per cycle or per 5 minutes).
recall Used in bus priority. If a bus arrives during a red it can be given a recall. Following stages are called early to reduce the delay to the bus.
recovery Recovery is the process of resynchronisation with the normal SCOOT stage timings after bus priority has finished. There are 4 recovery methods available to bus priority in SCOOT these are as follows:
  • DN - do nothing recovery accepts the change which has taken place in the signal timings due to the action of bus optimisation and updates SCOOT with these timings. SCOOT then optimises as normal.
  • MS - minimum stage recovery consists of running stages to a minimum to resynchronise with SCOOT timings.
  • DS - degree of saturation recovery is similar to MS recovery but instead of running stages to a minimum, stages are run short to a length constrained by the degree of saturation. (Default after extensions.)
  • LS - long stage recovery consists of running stages long to resynchronise with SCOOT timings. Stages are still subject to their maximum stage length. (Default following recalls.)
region Second in the SCOOT hierarchy. A SCOOT Area has one or more regions in which there are a number of nodes. All nodes in the region operate at the same cycle time, or possibly half of the region cycle time - see double cycling.
retard A retard decision increases the time to the next stage change by retarding the time at which it is due.
reversion Whenever the split optimiser advances or retards the stage time by -4 or +4 seconds a reversion of 3 seconds takes place the following cycle to give a permanent change of -1 or +1 seconds.
rx lag The receive lag is the time delay between the detector activation on-street and the receipt of the information by the SCOOT computer.
saturation occupancy Is the normal maximum discharge rate of vehicles from a stopline (measured in LPU's). Saturation occupancy validation is the accurate measurement of sat. occ. on-street.
saturation occupancy flow technique (SOFT) Is a method of automatically measuring the saturation occupancy online.
SCOOT The SCOOT (Split Cycle Offset Optimisation Technique) urban traffic control system was developed by TRL in collaboration with the UK traffic systems industry. SCOOT is an adaptive system which responds automatically to traffic fluctuations. It does away with the need for signal plans which are expensive to prepare and keep up to date. SCOOT has proved to be an effective and efficient tool for managing traffic on signalised road networks and is now used in over 200 towns and cities in the UK and overseas. It has demonstrated a range of delay savings averaging about 20% over tpical good fixed-time plans, but much larger against less efficient control systems.
SCOOT hierarchy Data needs to be entered into SCOOT at different levels. The basic SCOOT hierarchy states the different levels available:

Area > Region > Node > Stage > Link > Detector

SCOOT Vega Graphical output showing the operation of the SCOOT Model, developed by TRL.
sources and sinks Refer to specific areas in a network where there can be daily short bursts of heavy traffic. Detectors need to be positioned to account for important sources and sinks. A factory where all staff finish work at the same time is an example.
split The proportion of green time given to a particular stage within a cycle.
split optimisation The split optimiser decides the optimum distribution of green time to minimise delay and congestion. It considers one node at a time.
split weighting Split weighting can be used on links where it is desired that they run at a higher degree of saturation than opposing links (i.e. to give additional green to a link it is necessary to split weight links running in other stages).
stage SCOOT stages consist of an intergreen followed by a green period.
stage skipping Buses can be given extra priority by omitting stages. For instance, if a bus that requires stage 3 is detected during stage 1, the signals can change straight from stage 1 to stage 3. Stages that may be skipped are specified in the SCOOT data. It is NOT recommended to skip pedestrian stages. Not available before SCOOT MC3.
stage suffix This is the single letter or digit used to identify the stage to SCOOT. The full name of the stage is given by appending "/" and then the stage suffix to the node name e.g 1034/2.
stop penalty The Area data value stop penalty equates a vehicle stop to that number of seconds of delay in the SCOOT optimisation. It has a range of 0 to 2000 seconds although the range of 0 to 40 is sensible. The value is usually 20.
traffic model A model is used in SCOOT to estimate the movement of traffic on-street and the delay due to the traffic signals. Demand is measured at a detector and processed into a cyclic flow profile. SCOOT uses this information to model the demand at the stopline and the signal induced delay. The optimisers aim to minimise the modelled delay.
transponder In SCOOT bus priority, transponders can be fitted to buses so that when a bus travels over a bus detector on-street or goes past a beacon SCOOT knows that a bus is approaching a signal. SCOOT can also take information from AVL systems.
TRANSYT TRAffic Network studY Tool. Is a TRRL developed program for calculating the 'best' fixed time plan with which to co-ordinate the traffic signals in any network of roads for which the average traffic flows are known. Demonstrated Journey Time benefits of between 15 and 20%.
trend files Used by ASTRID to contain data per day at low resolution covering peak and off-peak periods as a whole.
trend flag The Trend flag is used to tell SCOOT to respond rapidly at the start of the peak period when flows quickly increase. The trend flag changes the target saturation for the cycle time optimiser forcing cycle times up.
trigger link Is a critical link that is used to trigger the SCOOT gating facility.
tuning The process of ensuring that SCOOT has been correctly set up.
tx lag The transmission lag is the time delay from when SCOOT calls for a stage at its output interface until the amber that terminates the preceding stage appears on-street.
VA Vehicle Actuation is a method of Traffic Signal control in which the duration of the green signal is extended, up to a maximum, according to traffic flow.
validation Is the process in the SCOOT system setup where on-street data is compared with information from the SCOOT model.