A multi-zone differential variable speed limit strategy for on-ramp bottlenecks

Freeway on-ramp merging zones are critical bottlenecks where frequent lane changes and decelerations often lead to congestion and capacity drops. Although differential variable speed limit (DVSL) strategies have shown potential in mitigating these problems, their effectiveness significantly declines under high-density conditions due to speed limit failure, a situation in which drivers are unable to comply with posted speed limits because of excessive congestion. To address this challenge, this study proposes a multi-zone DVSL (MDVSL) control strategy within a multi-lane cell transmission model framework. The main innovation lies in the introduction of a dynamic upstream buffer zone that is activated when potential speed limit failure is detected. By harmonising speeds across lanes in this buffer zone, the strategy facilitates anticipatory lane changes from the merging lane, improving traffic smoothness and reducing congestion in the primary control zone. A predictive control algorithm is employed to dynamically optimise lane-specific speed limits, aiming to minimise total travel time (TTT). Simulation results demonstrate that the proposed MDVSL strategy effectively reduces the occurrence of speed limit failure, enhances traffic flow stability and achieves a 25.6% reduction in TTT compared to the uncontrolled strategy.