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SCOOT is an adaptive traffic control system which sits "on top" of a standard PEEK or Siemens UTC System – think of it as Microsoft Office sat on top of Windows – and takes information from loops traffic detectors usually located about 150m upstream of the junction and decides, according to database values set by the traffic engineer and the SCOOT model, how much green time to allocate to a particular traffic stage.
How it works
At the core of SCOOT is the main SCOOT kernel; this is a mathematical model which translates information set in by the traffic engineer, against traffic flow information taken from the SCOOT detectors and calculates the most efficient timings for the network according to the amount of traffic. The following article is based on the version of SCOOT as applied to the Siemens UTC system, although the principles are exactly the same for the PEEK version.
SCOOT is split up in to various elements.
For most systems this is the borough or city. There would only be more than one “area” if the system was unusually large, for instance London or Manchester.
A collection of junctions in a common area, for instance on part of an arterial route, where co-ordination between the junctions is advantageous.
This is the individual junction (or in closely associated junctions, a number of streams or junctions).
This is the data SCOOT uses to determine which stages to run in the plan. It has the stage minimum time – the preceding interstage and the UTC stage minimum length – and the maximum length you want to allow the stage to run for. For instance an all round pedestrian stage would not be varied in length so its minimum length would be the same as its maximum length.
This is a particular lane or lanes on an approach to a junction. Information about the number of vehicles on the link and the various settings applied to the link are a major part of how SCOOT calculates its timings.
Setting up SCOOT
For the SCOOT model to be effective it needs to know a number of critical parameters for each link.
This is the journey time for a vehicle travelling over the detector to that vehicle crossing over the stop line in free flow traffic conditions.
This is the saturation occupancy value. This is calculated by the traffic engineer according to how many vehicles are crossing the stop line.
Default offset: the time, in seconds between a vehicle crossing the stop line of a node upstream to it crossing the stop line of a downstream junction.
Start Lag: this is the time from the SCOOT stage starting to traffic on the actual link starting to move (for instance to take in to account of a phase delay).
End Lag: the time from the SCOOT stage ending to traffic on that link actually stopping.
Queue Clearance Max: the time it takes for a queue of vehicles on the link, particularly the vehicle sat on the SCOOT detector, to clear the stop line.
Congestion Importance Factor: the importance of a link in the model. A value of 0 means that no importance is applied to the link. A value of 7 would mean that a link must not become congested at all costs. Typical values would be 2 for a side road and 3 for the main road.
How these influence the SCOOT model
The SCOOT model then calculates the percentage of green time required for that link out of the whole cycle time for that node according to how much traffic it has seen cross over the detector and according to other factors set up by the traffic engineer.
The cycle time for the region will vary up or down between the minimum cycle time (which is set by the junction with the longest minimum cycle time) and the maximum cycle time, usually 120 seconds. The cycle time optimiser will check every five minutes (or 2.5 mins if set by the engineer) to see whether the cycle time needs to rise or fall if traffic flows are rising or falling.
Cycle time options are (in seconds);
32, 36, 40, 44, 48, 52, 60, 66, 72, 80, 88, 96, 104, 112, 120.
Usually, most large junctions with multiple stages will have a minimum cycle time of between 66 and 80 seconds. A small crossroads will have a cycle time much smaller, for instance a simple two stage crossroads will have a cycle time of 32 seconds.
While there is a facility in UTC called a Green Wave, this is usually set for clearing routes absolutely of most traffic for emergency vehicles. In SCOOT, Green Progression is the more appropriate term and is the effect of keeping a platoon of traffic moving through successive junctions.