Optimal Control of Isolated Traffic Signals

Abstract

The efficiency of isolated traffic signal control can be substantially improved by using more advanced control strategies in combination with better methods for traffic detection. The purpose of this study was to develop and test such methods for practical use.A control strategy named TOL(Traffic Optimization Logic) based upon the criterion of minimization of traffic operating costs and disturbance of the environment at a given traffic demand was developed. The method is similar to the on-line method suggested by A.J. Miller in 1963 and calculates at regular intervals if the prevailing phase should be extended or not based on short-term predictions of the resulting benefit and disbenefit to the different categories of traffic in the intersection.TOL was tested and compared with conventional fixed-time and vehicle-actuated (VA) control with or without bus priority by means of simulation and field experiments. Two intersections were equipped with minicomputors, analog inductive-loop detectors for direct detection of the number of cars in each approach and special equipment for selective detection of buses.Both simulation and field experiment results showed that TOL could decrease average delay with 25% for vehicles and 40% for buses as compared with VA. Cost-benefit analysis showed that the TOL system was cost-effective even if only the benefits of reduced gasoline consumption was considered. In some cases TOL was cost-effective even if only the gasoline reduction for the buses were considered.Another control strategy for isolated intersections is also presented in the paper, namely a phase generating strategy which on-line creates new phases depending on the traffic demand in the intersection. The method is particularly useful in large intersections where the number of possible combinations of green signals is large.