Design by Petri nets of an intersection signal controller

Currently, traffic responsive control strategies of an isolated intersection gather available informtion about traffic conditions and then, in a real-time context, determine a stage (set of simultaneous green signals) by optimizing an arbitrary criterion. This optimal control is then executed by the intersection controller, which transforms the stage in traffic signal colors. In this paper, an optimal signal control device for any generic intersection is constructed just from the functioning rules: a timed color sequence for each signal, clearing times between conflicting signals and a maximum service delay for any user waiting in the intersection. The design takes into account two constraints: the controller must always respect the specified safety operating rules, and it must give the maximum freedom to the upper level. Petri nets formalism is a graphical and mathematical tool adapted to the modeling of the main features of discrete event systems: it can be used to translate specifications into operational systems. Each of the previous three safety rules can be translated in this formalism, furnishing an exact intersection controller, described by a Petri net. The application of the methodology to an actual intersection is presented. It shows that the intersection controller description is fully achieved by Petri nets, providing both a model for any upper control level and automation for any real implementation.