Adaptive Control Strategy of Variable Speed Limit on Freeway Segments under Fog Conditions

Fog has a critical impact on traffic safety on freeways. Variable speed limit (VSL) control can effectively harmonize vehicle speed and improve safety. However, most existing weather-related VSL controllers are limited in their ability to adapt to the dynamic traffic environment. This study developed an adaptive VSL control strategy under fog conditions with consideration of macroscopic factors that affect traffic safety risks. The optimal factors of VSL control were obtained by solving an optimization problem that coordinated safety and mobility using a genetic algorithm. The results showed that the proposed VSL control under fog conditions effectively improved safety without significantly decreasing mobility; risks were reduced by as much as 37.15% with only a 0.48% increase of total travel time. The VSL control strategy in this study can help transportation managers design VSL control systems to enhance freeway safety under fog conditions.