Proactive Strategy for Variable Speed Limit Operations on Freeways Under Foggy Weather Conditions

Adverse weather conditions are a critical external factor that threaten traffic safety on freeways. Variable speed limit (VSL) operations can be used as a traffic management tool to harmonize vehicle speed and reduce the likelihood of crashes during adverse weather conditions. This paper proposes a novel VSL operations strategy based on the prediction of weather and traffic conditions, referred to as a proactive VSL operations strategy. The k-nearest neighbors (k-NN) statistic was used to predict weather and traffic conditions. VSL operations logic, which used visibility distances and safe stopping distances obtained from road weather information systems and average speeds obtained by vehicle detection systems, was developed. The proposed methodology is focused on the restricted visibility distance resulting from foggy weather conditions on freeways. A microscopic traffic simulator, VISSIM, was used to simulate a freeway traffic stream and collect vehicle-maneuvering data. An external application program interface, the VISSIM COM interface, was used to implement the proposed VSL operations strategy. A surrogate safety assessment model was used to derive indirect safety measures to evaluate the effectiveness of the proposed methodology. Total conflicts in the proposed VSL algorithm were reduced in comparison with a system lacking VSL by 19.10% and 27.27% under moderately and severely foggy weather conditions, respectively. These outcomes are expected to be of great use in the development of more effective VSL systems to enhance freeway safety during adverse weather conditions.