Variable speed limit strategy of expressway under foggy conditions considering dynamic sight distance and dynamic friction coefficient

Fog is a weather condition which significantly threaten expressway safety and efficiency by reducing natural visibility and road friction coefficient. Variable speed limit (VSL) strategy is an effective traffic management method for harmonizing vehicle speed and reducing rear-end crash potential under foggy conditions. As two key inputs, driver sight distance and road friction coefficient determine the reliability of VSL calculation. However, driver sight distance in the running vehicle (dynamic sight distance) is significantly lower than that at rest. Additionally, the dust-water coupling film on the expressway surface will also lead to the dynamic friction coefficient. In this paper, based on stopping sight model, a new VSL strategy is proposed, which considers dynamic sight distance, dynamic friction coefficient, alignment, and slope. The results obtained from simulation based on CarSim and Simulink represent that under the foggy conditions with natural visibility is 200 m, the speeds calculated by proposed VSL strategy in straight and curve road segments are 70km/h and 67km/h, 16% and 11% higher than China's legal speed limit respectively. Braking experiment on Wentai Expressway confirm those speeds are safe enough and can improve efficiency. The research results can provide theoretical basis and support for management of expressway under foggy conditions.