Optimizing Networks of Traffic Signals in Real Time-The SCOOT Method

Dennis I. Robertson and R. David Bretherton

Abstract-This paper describes bow the SCOOT Urban Traffic Con- trol system evolved from research by the British Government’s Trans- port and Road Research Laboratory (TRRL) on the TRANSYT method of optimizing networks of 6 x 4 time signals. The key principles of the SCOOT on-line tratlic model and real-time signal optimlzers are ex- plained. The results of surveys are reviewed, applications are summa- rized, and some new developments are outlined.

INTRODUCTION HERE adjacent junctions in a network of urban road are W less than a mile apart and are controlled by traffic signals,

major benefits can be obtained by installing an urban traffic control (UTC) system to coordinate the operation of the signals. Such systems use a central computer to control the signal sequences and to monitor their operation; the signals and com- puter are usually connected by voice grade data transmission lines. The method of controlling the signal sequences can have a significant effect on levels of urban congestion, on fuel econ- omy, and on exhaust emissions. This is the main topic of this paper.

UTC SYSTEMS The first UTC systems came into operation in the mid-l960’s,

and since then their use has been growing rapidly. In 1987, the U.S. and Canada had in operation or under construction over 300 UTC systems controlling 20 000 signals [7]. The largest is in New York with 300(! signaled junctions under central control. In Japan, 34500 signals out of a total of 122000 signals are controlled by UTC systems in 74 cities [6]. The largest UTC system in the world is in Tokyo where 5500 signals are con- trolled centrally. France has about 50 UTC systems in operation; this covers almost all cities with more than 80000 inhabitants. Similar numbers of systems are in operation in Britain, a few more in Germany, and about 10 in Australia. Many other countries have UTC systems in their major cities and in some smaller towns.

This rapid growth in the use of UTC systems stems from their success in optimizing traffic flow through urban networks. Expe- rience suggests that, depending on the prior method of signal control, traffic flows, and road layout, coordination can reduce delays and stops by between 10 and 40%. Since the cost of delay at an average signaled junction is likely to be a few hundreds of thousands of U.S. dollars each year, it is understandable that many UTC systems have recovered their capital cost well within the first year of operation.

COORDINATION ON FIXED TIME PLANS Signals are usually coordinated on a common cycle time so

that the platoons of vehicles that leave one signal arrive at the

Manuscript received May 1990. The authors are with the Transport and Road Research Laboratory, Old

IEEE Log Number 9040535. Wokingham Road, Crowthome, Berkshire RGll 6AU, U.K.

Fig. 1 . A time-distance diagram.


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