Geometric Design Consistency Model for Evaluating Safety at Horizontal Curves on Two-Lane Rural Highways Under Mixed Traffic Conditions

Horizontal curves are considered critical sections, where traffic safety is of at most concern due to sight distance restriction. In this study, the free flow speed data of each vehicle type were collected on multiple zones along 18 horizontal curves for which sight distance was restricted. A trap length of 15 m was marked in the field at five zones along each curve, and by using videographic survey, each vehicle was traced on these five zones and free flow speed data were extracted. A total of 3042 free-flowing vehicles (i.e. headway > 8 s) speeds were extracted and used in the analysis. The 85th percentile speed reduction model for each vehicle type from tangent section to curved sections is developed for evaluating design consistency as a surrogate measure of safety. Results show that the significant parameters obtained are radius, 85th percentile tangent speed, and minimum available sight distance. This study also suggested the procedure to adopt variable safe speed limits for each vehicle type based on design consistency model developed, which can be used as a prior warning information to enhance safety on horizontal curves.