Novel Freeway Traffic Control with Variable Speed Limit and Coordinated Ramp Metering

Freeway corridor traffic flow is limited by bottleneck flow. If the section upstream of a bottleneck is congested, the bottleneck flow will drop well below its capacity. A logical approach to maximizing recurrent bottleneck flow is to create a discharge section immediately upstream of the bottleneck. This paper proposes a control strategy for combining variable speed limits (VSL) and coordinated ramp metering (CRM) design to achieve this objective when the bottleneck can be represented as a lane (or virtual lane) reduction. At each time step, VSL is designed first, with mainline flow, on-ramp demand and length, and driver acceptance taken into account. VSL values are determined for three locations: (a) the critical VSL to regulate the discharge section flow to match bottleneck capacity, (b) the VSL for the potentially congested section (mainline storage), and (c) the VSL upstream of the congestion tail. With the selected VSL, the first-order mainline flow model is linearized and then used for CRM design with model predictive control to minimize the difference between scaled total travel time (or vehicle hours traveled) and the total travel distance (equivalent to vehicle miles traveled). Microscopic simulation shows that this control strategy should improve bottleneck throughput significantly.