TRAFFIC RESPONSIVE COORDINATION

TRAFFIC RESPONSIVE COORDINATION In spite of the success of fixed time UTC systems, experience

has revealed several limitations that are becoming more impor- tant as congestion grows. Fixed time plans are seldom kept up to date, mainly because it takes about 1 man-year of work to produce a set of optimized timing plans for a network of 30-40 signals. Bell [2] estimates that old plans deteriorate to cause an extra 3% of delay a year. Further, even if the plan is up to date, it cannot cope satisfactorily with random variations in flows, for example, after an accident occurs. Finally, no information is available to the traffic manager on the current traffic situation (unless closed circuit television or vehicle detectors are installed). To overcome these limitations, the TRRL cooperated with British Industry (Ferranti, GEC, and Plessey) to develop SCOOT. Research started in the early 1970’s.

  • I

Principles of SCOOT The three key principles are:

1) measure CFP’s in real time; 2) update an on-line model of queues continuously; 3) incremental optimization of signal settings.

Principles 1 and 2 have been outlined above. The traffic data for the CFP’s are collected, usually every second, from inductive- loop sensors located well upstream of signal stoplines, prefer- ably just downstream from the previous junction. In this posi- tion, installation costs are reduced and the earliest possible direct prediction is obtained of arrivals at the downstream stopline. Further, the sensor can anticipate “gridlock,” which may occur if the queue extends back into the upstream junction. The SCOOT optimizer takes special action when vehicles queue over the sensors.

Incremental Optimization The third key principle is that the coordination plan should be

able to respond to new traffic situations in a series of frequent, but small, increments. This is necessary because research has shown that it is very difficult to predict traffic flows in the next few minutes-hence any “fixed” coordination plan may be out of date before it is calculated or inappropriate after it is imple- mented (and implementation is likely to cause extra delay during the transition from the old timings to the new).

SCOOT uses an “elastic” coordination plan that can be stretched or shrunk to match the latest situation recorded by the CFP’s. This is achieved by optimizing the splits, offsets, and cycle time shown in Fig. 2 in the following way. A few seconds before every phase change, the SCOOT split optimizer calcu- lates whether it is better to advance or retard the scheduled change by up to 4 s, or to leave it unaltered. Then, once a cycle, the offset optimizer assesses whether the PI on streets around each junction can be reduced by altering the offset to be 4 s earlier or later. Favorable split and offset alterations are imple-

TABLE I REDUCTION IN DELAY FROM THE USE OF SCOOT

Percent Reduction in Delay Time

Previous AM Off PM Location Control Peak Peak Peak Glasgow Fixed-time – 2 14* 10* Coventry Fixed-time

Coleshill Road 23 33* 22* Spon End 8 0 4

Worcester Fixed-time 11 7* 20* IsolatedV-A 32* 15* 23*

Southampton IsolatedV-A 39* 1 48* London Fixed-time (Average 8 % less

journey time)

*Results significant at the 95 % confidence level.

mented immediately. In a similar manner, the cycle time of a group of junctions may be increment4 up or down by a few seconds every few minutes.

So SCOOT makes a large number of small optimization . xisions-typically over loo00 per hour in a network of 100 iunctions. A few decisions may be wrong, but this is uninrpor- tant provided the large majority are correct. The effect of these optimization decisions is to vary the signal timings in the. manner shown in the lower part of Fig. 2.

SURVEYSOFSCOOT The effectiveness of the SCOOT strategy has been assessed by

major trials in five cities. The results from the trials are summa- rized in Table I. The trials in Glasgow and Coventry were conducted by TRRL and those in Worcester, Southhampton, and London by consultants, a university, and the local traffic author- ity, respectively. In most cases, comparisons were made against a good standard of fixed time coordination usually based on TRANSYT. The table shows that the largest benefits are achieved in comparison with isolated vehicle actuation but, of course, part of this benefit could be achieved by a good fixed time system.

The relative effectiveness of SCOOT varies by area and time of day but overall it is concluded that SCOOT achieved an average saving in delay of about 12% compared with good fixed time plans. Since SCOOT does not “age” in the way typical of fixed time plans, it follows that SCOOT should achieve savings in many practical situations of 20% or more depending on the quality and age of the previous fixed time plan and on the rapidity with which flows change.

APPLICATIONS The research on SCOOT was performed in Glasgow, Scot-

land, with software not suitable for general use. The develop- ment of SCOOT for general application was carried out in Coventry, England, using the CORAL high-level real time computer language. It is this version of SCOOT, with subse- quent enhancements, that has come into use since the late 1970’s in over 40 cities, some eight of which are outside Great Britian. In London, SCOOT controls 250 signals installed in several independent “cells” and is being expanded progressively to replace the older fixed time system, which had about 1200 sets of signals under fixed time computer control.

On the basis of the surveys and subsequent experience, SCOOT is likely to be of most benefit where vehicular flows are heavy, complex and vary

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