Motorway network control via nonlinear optimization

The on-line application of nonlinear optimization methods for control in motorway networks is presented. The considered control measure is route diversion by Variable Message Signs (VMS). The control task is formulated as a dynamic, nonlinear, discrete-time optimal control problem with constrained control variables and is solved by a gradient-based search. A modified algorithm for the heuristic search of optimal discrete (binary value) control sequence is presented. Feedback control is realized by solving the optimization problem for each control interval over a sufficiently long future time horizon (certainty equivalent open-loop feedback control). As a prerequisite, a state-space description of the network traffic dynamics is derived from a macroscopic model as used also in the simulation program METANET. Simulation results for route diversion on a hypothetical example network are given. The required computational effort is quantified and the control benefits are given in terms of network performance and the switching frequency of VMS.