Combined connected vehicles and variable speed limit strategies to reduce rear-end crash risk under fog conditions

Fog is a weather condition that reduces visibility of the driving scene, while slow traffic may be formed due to the bottleneck on freeways. This phenomenon may lead to higher rear-end crash risk when vehicles approach slow traffic, since drivers might not observe the speed reduction ahead of them timely with the reduced visibility and could not have enough time to respond. This study aims to develop a variable speed limit (VSL) control strategy to reduce the rear-end crash risk at freeway bottlenecks under fog conditions. A VSL control algorithm was developed with consideration of the different relationships between the gap and visibility distance. The VSL strategy was also tested in the fully connected vehicles (CV) environment. A feedback control framework was developed to combine the VSL and CV control. The proposed VSL strategy was implemented and tested for a freeway section with a bottleneck through the micro-simulation software VISSIM and the intelligent driver model (IDM) was employed to account for car following in the CV environment. Finally, two measurements, which include time-to-collision at braking (TTCbrake) and total travel time (TTT), were employed to evaluate the effectiveness of the proposed control strategy. The results demonstrated that the VSL control played an important role in reducing rear-end crash risk and the effects of the VSL control could be affected by compliance rates. In addition, it was found that the CV environment could further enhance the safety benefits of VSL control and improve the traffic efficiency.