Variable speed limit in foggy conditions considering dynamic sight distance and friction

Abstract

Fog is a weather condition that can significantly threaten expressway safety and efficiency by reducing natural visibility and the road friction coefficient. The variable speed limit (VSL) strategy is an effective traffic-management method for harmonising vehicle speed and reducing rear-end crash potential under foggy conditions. Two key inputs – driver sight distance and the road friction coefficient – determine the reliability of VSL calculation. However, a driver's sight distance in a running vehicle (i.e. the dynamic sight distance (DSD)) is significantly lower than that at rest. Additionally, a dust–water coupled film on the expressway surface will also affect the dynamic friction coefficient (DFC). Based on a stopping sight distance model, a new VSL strategy that considers the DSD, DFC, alignment and slope is proposed. The results obtained from simulations based on the CarSim software program and the Simulink programming environment showed that, in foggy conditions with a natural visibility of 200 m, the speeds calculated by the proposed VSL strategy in straight and curved road segments were 70 km/h and 67 km/h, respectively, 16% and 11% higher than China's legal speed limit. A braking experiment on Wentai expressway in Zhejiang province, China confirmed that these speeds are safe and could improve efficiency. The research results provide a theoretical basis and support for the management of expressways in foggy conditions.