Module 7 : Traffic Signal Design
Lecture 40 : Area Traffic Control
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Operational models

An operating model is the abstract representation of how an System operates across process. Any system is a complex system consisting of several different interlinked logical components. An operating model breaks this complexity into its logical components in order to deliver better value. Some examples of operational models are SCOOT, SCAT and OPAC which are described below.

SCOOT (Split Cycle Offset Optimization Technique)

The Split Cycle Offset Optimization Technique (SCOOT) is an urban traffic control system developed by the Transport Research Laboratory (TRL) in collaboration with the UK traffic systems industry. It is an adaptive system which responds automatically to traffic fluctuations. Prime objective of this is to minimize the sum of the average queues in the area. It is an elastic coordination plan that can be stretched or shrunk to match the latest traffic situation. Continuously measures traffic volumes on all approaches of intersections in the network and changes the signal timings to minimize a Performance Index (PI) which is a composite measure of delay, queue length and stops in the network. Each SCOOT cell is able to control up to 60 junctions. Handling input data up to 256 vehicle counting detectors on street. Detectors are usually positioned 14 m behind the stop line.

Principles of SCOOT

  1. Cycle Flow Profile’s (CFP) measure in real time
  2. Update an on-line model of queues continuously
  3. Incremental optimization of signal settings
  1. Cyclic Flow Profiles (CFP)
    CFP is a measure of the average one-way flow of vehicles passed at any point on the road during each part of the cycle time of the upstream signal. It records the platoon of vehicles successively within a cycle time during peak flow. It updated in every 4 seconds. CFP’s can be measured easily by hand. Shape of the CFP has to be calculated for each one-way flow along all streets in the area. Accuracy of calculation depends on the accuracy of the data on average Flows, saturation flows, and cruise times.
  2. Queue Estimation
    It is necessary to predict new signal timing due to the queues after alteration according to the situation after knowing CFP, the computer can be programmed to estimate no of vehicles which will reach the downstream signals during red phase. So size of the queue and duration to clear the queue can be calculated. In this calculation it is assumed that the traffic platoons travel at a known cruising speed with some dispersion. Queues discharge during the green time at a saturation flow rate that is known and constant for each signal stop line.
  3. Incremental Optimization
    Incremental Optimization is done to measure the coordination plan that it is able to respond to new traffic situations in a series of frequent, but small, increments. It is necessary because research shows that prediction of traffic flow is very difficult for next few minutes. SCOOT split optimizer calculates whether it is good to advance or retard the scheduled change by up to 4 s, or to leave it unaltered. It is achieved by split optimization, offset optimization, cycle time.
    1. Split Optimizer
      Works at every change of stage by analyzing the current red and green timings to determine whether the stage change time should be advanced, retarded or remain the same. Works in increments of 1 to 4 seconds.
    2. Cycle Time Optimizer
      It operates on a region basis once every five minutes, or every two and a half minutes. Identifies the ``critical node'' within the region and will attempt to adjust the cycle time to maintain this node with 90% link saturation on each stage. It can increase or decrease the cycle time in 4, 8 or 16 second increments according to the current requirement of the traffic flow.
    3. Offset Optimizer
      It works once per cycle for each node. It operates by analyzing the current situation at each node using the cyclic flow profiles predicted for each of the links with upstream or downstream nodes. It assesses whether the existing action time should be advanced, retarded or remains the same in 4 second increments. Fig. 1 is showing the key elements of the SCOOT ATC system which we described in above points.
      Figure 1: Key elements of the SCOOT ATC system (Source: Dennis I. Robertson and R. David Bretherton 1991)
      \begin{figure} \centerline{\epsfig{file=qfAtcScootElements.eps,width=8cm}} \end{figure}

Working Principle of SCOOT

Scoot system consists of a number of SCOOT cells or computers, each cell can control up to 60 junctions and handling input data from up to 256 vehicle counting detectors on street. SCOOT detectors are placed at 14 m from the stop-line, from the approach to the junction as possible. Fig. 2 clearly shows the working principle of SCOOT where the detectors placed upstream sense the occupancy and the information is transmitted to the central computer. SCOOT traffic model and optimizers use this information to calculate signal timings to achieve the best overall compromise for coordination along all links in the SCOOT area. The main aim of the SCOOT traffic signal control system is to react to changes in observed average traffic demands by making frequent, but small, adjustments to the signal cycle time, green allocation, and offset of every controlled intersection. For each coordinated area, the system evaluates every 5 minutes, or 2.5 minutes if appropriate, whether the common cycle time in operation at all intersections within the area should be changed to keep the degree of saturation of the most heavily loaded intersection at or below 90%. In normal operation SCOOT estimates whether any advantage is to be gained by altering the timings. Fig. 2 is showing the working principle of SCOOT.
Figure 2: Working Principle of SCOOT (Source: www.scoot-utc.com)
\begin{figure} \centerline{\epsfig{file=qfAtcScootPrinciple.eps,width=8cm}} \end{figure}
From above fig we can have an idea that vehicle will be detected with the help of vehicle detector. The collected data will be send to intersection controller after that it will be send to the central controller with the help of communication network. There it will be use to estimate the signal timing according to the actual traffic flow needs. Then the central controller will send the signal timing to the intersection controller to implement.

Features of SCOOT

  1. Variable Message Signs
    Scoot display message signs to convey the guidance to the driver which is very helpful for the drive.
  2. Diversions
    This feature is provided to deal with any emergency situation for example if any problem is found out in any lane which is found out with the help of Fault Identification & Management unit then traffic will be diverted from that lane to another lane.
  3. Emergency Green Wave Routes
    This feature is provided to deal with any hazardous situation.
  4. Fixed Time Plan
    This plan is applied when any unit of ATCS stopped working so till the time that unit starts functioning.

Limitations

  1. Inability to handle closely spaced signals due to its particular detection configuration requirements, its require some time to detect vehicle.
  2. Interface is difficult to handle, as this is highly technical so difficult to understand and handle.
  3. Traffic terminologies are different from those used in India.
  4. Primarily designed to react to long-term, slow variations in traffic demand, and not to short-term random fluctuations.