5. Leader–Follower Coordination Control for Urban Traffic

The purpose of this chapter is to describe the leader–follower approach for coordinating many interacting components of a large network. This leader–follower approach is suitable for systems with intermediate load, where only a few critically loaded components are having difficulties to achieve their local specifications. We assume that a supervisor, acting at a hierarchically higher level of the control protocol, has selected assigned to these critical components the role of leader agents. Leader agents send additional specifications to their neighboring follower components so as to ensure that the output of these follower components makes it easier for the leader component to achieve its own specifications. The local control agents generate control inputs so that all specifications are met, including at the followers those specifications imposed by their neighboring leader. Local controllers thus help each other in such a way that the overall system achieves good performance. In this essay, we illustrate this approach by applying it to controlling the switchingtimes of traffic lights in an urban area. Traffic is described in this case study by specifying the arrival times and the size of platoons of vehicles at sensor locations in the network. Control actuation should try to minimize the average vehicle delay by switching traffic lights, as often as possible, from red to green whenever a platoon arrives. The leader–follower approach combines the idea of green waves with local feedback control at each intersection. Each local controller finds an online solution to a local optimization problem, subject to specifications. Coordination is achieved by letting leader intersections specify bounds on the earliest and latest time that platoons are allowed to leave their neighboring follower intersection, in such a way as to minimize the waste of capacity at a leader intersection. The proposed leader/follower approach may be useful for coordination control in directed networks under intermediate load conditions, where heavily loaded leader components can request help from less loaded upstream followers.