Operation of Closed Loop Signal Systems

In recent years, there have been two parallel research paths for developing advanced traffic signal systems. Real-time traffic adaptive system research, supported largely by the United States Department of Transportation (USDOT) and smaller scale closed loop systems developed primarily by traffic signal system vendors. Simulation models have been developed for evaluating USDOT supported projects, and those results have been reported in the literature. However, even though there are several hundred traffic responsive systems deployed, most of the vendor developed closed loop signal systems have not undergone such rigorous evaluations. This is an area of signficant concern because deployment of efficient closed loop signal systems is one of the most cost effective Intelligent Transportation System (ITS) investment that a small urban area can make. In order to make good deployment decisions, rational quantitative evaluation procedures are required to evaluate feasible options. This paper reports on an evaluation procedure developed for quantifying the impact of traffic responsive operation in modern closed loop signal systems. The paper reviews the concepts of "hardware-in-the-loop simulation," explains the application of this simulation procedure to evaluate closed loop systems, and reports on the observed results. The same procedure can be applied to systems developed outside the United States such as the SCOOT and SCATS systems.