Group-based Safety Constraints Description of an Intersection

Traffic lights have been existing for around 100 years. The first signals were installed for safety reasons for avoiding accident between opposed traffic flows at an intersection. Due to a continually increasing traffic, the traffic lights became a powerful tool for controlling the flows : the stake was to increase the intersection capacity and to decrease the congestion and the overall delay. Even if the point of view is now changing and if the stake is more an harmonization between transport modes, signals remain essential control elements. Primary safety, that is to forbid concurrent traffic flows to enter at the same time in the intersection, is managed by the concept of stage (duration during which all the traffic light colours remain constant). In the 1950s, Webster (Webster, 1958) proposed a methodology for optimising signals at an intersection based on the concept of stage and cycle duration. This method has been the origin of the building of the time plans and remains nowadays the basis of this computation. Even now time plans are very important all over the world : almost every traffic control systems installed in cities use time plan based methods. New type of control systems have been developed for 20 years : their characteristics are to use a high density of sensors for real-time traffic measurements and to have no more reference of plans in their method. One consequence is to abandon the concept of cycle (they are a-cyclic). In the following we call these systems real-time UTC (Urban Traffic Control) systems. The objective of the paper is to propose a new description of an intersection. This description is illustrated by two examples : the first one shows a practical case on an intersection and the second one describes the real-time UTC algorithm CRONOS based on this description.