Variable Speed Limit Control Design for Relieving Congestion Caused by Active Bottlenecks

Abstract

Variable speed limit (VSL) can be used on freeways to manage traffic flow with the goal of improving capacity. To achieve this objective, it is necessary that both speed and density dynamics be represented accurately. In this study, to deeply understand the effectiveness of VSL control, an analytical model was developed to represent drivers’ response to updated speed limits and macroscopic speed dynamical change with respect to changeable speed limits. Specifically, to model the freeway links having VSL control, the fundamental diagram (FD) was replaced with the VSL control variable in the relaxation term of the METANET. This modification led to the speed control variable appearing linearly, which is preferable for online computation. The density dynamics are based on the cell transmission model (CTM), which is introduced to estimate the transition flow among successive links with some practical constraints. It also offers flexibility in designing active bottleneck in which there is a capacity drop once feeding flow exceeds its capacity. To exploit this benefit, a modification was introduced in the FD of the density dynamics. A VSL control strategy was proposed that explicitly considers traffic characteristics at active bottleneck and its upstream-downstream segments. It can control traffic flow into any type of active bottleneck. Then, the proposed traffic dynamics with the control strategy are implemented in a freeway corridor using the model predictive control (MPC) approach. The analysis was carried out in the calibrated microsimulation model, VISSIM, within a scenario in which shock waves were present. The microsimulation model functions as a proxy for the real-world traffic system. This study reveals that, in terms of mobility, VSL is mostly effective during congestion periods.