Modeling traffic signal operations with precedence graphs

A series of examples of traffic signal operations with the precedence graph model is presented to illustrate the interactions among phases, intervals, and overlaps. The examples are built up from the simple operation of a T-intersection by adding more complex behaviors, including advance "Walk," delayed overlap termination indications, and advance flashing warning signals. In each of these extensions the required signal timing behavior is illustrated through the appropriate precedence graph model. The precedence graph approach provides a structured conceptual representation to support the analysis of operational behaviors of controller timing features such as added lost time and required fixed timing intervals that are induced when certain overlap features are enabled. It is believed that the precedence graph modeling approach may provide a mechanism to formalize the often ad hoc interaction between traffic signal controller software development, logic design, and field operations. This improved understanding may ultimately result in a better understood and more robust deployment of innovative signal control logic by utilizing a structure-modeling approach similar to that found in project management techniques such as the critical path method and the program evaluation and review technique.