A localized switching ramp-metering controller with a queue length regulator for congested freeways

In this paper, we present a novel switching traffic-responsive ramp-metering controller that adapts to the different traffic dynamics under different congestion conditions - free-flow or congested. The approach of multirate linear quadratic control with integral action (LQI) is employed to compensate for disturbances and to accommodate the difference between the model sampling time and the metering-rate update interval. In addition, a queue length regulator is designed to prevent the queue from exceeding the ramp storage capacity and yield improved performance over the currently used ad hoc "queue-override" scheme. A local ramp-metering control strategy is proposed to achieve the control goal of reducing the spatial and temporal span of the congestion, while satisfying the on-ramp storage capacity constraints, using locally available information. Test results on a calibrated cell transmission model (CTM) based macroscopic traffic simulator have demonstrated the performance and effectiveness of the switching ramp-metering controller, the queue length regulator, and the control strategy. The switching metering controller tracks the desired mainline density profiles in both free-flow and congested conditions. Under the localized control strategy, the total vehicle delay is reduced by 7.7%, while the total vehicle time is reduced by 5.7%, in the CTM traffic simulator.