A Review of the Optimized Policies for Adaptive Control Strategy (OPAC)

Optimized Policies for Adaptive Control (OPAC) is a real-time demand-responsive traffic signal timing optimization algorithm for individual intersections. It was developed at University of Lowell under the sponsorship of U.S. Department of Transportation in the early 80s. OPAC distinguishes itself from traditional cycle-split signal control strategies by dropping the concept of cycle. In OPAC, the signal control problem consists of a sequence of switching decisions made at fixed time intervals. At each decision point the question is whether to extend or terminate current phase. Dynamic programming techniques are used to calculate optimal solutions. OPAC utilizes on-line data obtained from upstream detectors as well as historical data in the optimization. The objective is to minimize performance measures, such as vehicle delays and stops. Each phase is constrained only by the minimum and maximum phase lengths. Consequently, the duration of a phase is never prespecified. It depends solely on the prevailing traffic flow conditions. The dynamic optimization process is carried out continuously to ensure that the signal control is always up-to-date.