Comparison of Local Feedback Controllers for the Mainstream Traffic Flow on Freeways Using Variable Speed Limits

Mainstream traffic flow control (MTFC) on freeways, enabled via variable speed limits (VSL), is a traffic management tool capable of avoiding or, at least, postponing congestion formation on freeway bottlenecks, and may complement other existing measures. Two simple feedback controllers for local MTFC via VSL are proposed. Both controllers are evaluated and compared to a previously developed local MTFC feedback cascade controller and to a sophisticated optimal control approach. All feedback controllers rely on readily available real-time measurements, are simple and robust, and take into account a number of practical and safety restrictions; therefore, they are suitable for implementation in the field as a new or as part of an existing intelligent transportation systems infrastructure. The evaluations are conducted for a hypothetical freeway network by use of a validated second-order macroscopic traffic flow model. The results show that the feedback controllers exhibit a satisfactory control behavior and, indeed, approach the optimal control results. Recommendations for the operation of the feedback controllers are given.