Development of conflict severity index for safety evaluation of severe crash types at unsignalized intersections under mixed traffic

Abstract Existing proximal indicators lack the capability of addressing the severe implications of a possible collision. This study proposes a conflict severity index (CSI) based on crash risk and expected collision severity for severe crash types at unsignalized intersections on intercity highways. CSI is estimated for critical crossing and rear-end conflicts observed at ten unsignalized junctions using Post Encroachment Time (PET), Delta t (Δt) and the expected loss in kinetic energy (ΔKE). Δt is a newly proposed indicator reflecting the rear-end collision probability, which is the difference between Modified Time to Collision (MTTC) and Time-to-Stop (Ts). To get critical crossing and rear-end conflicts, ‘critical speed’ and 1.5 s threshold value of Δt are used respectively. The study concept postulates that with the decreasing PET and Δt or an increasing ΔKE, the CSI value increases. The sensitivity analysis shows that for a specific PET or Δt value, CSI increases with the increasing speed of right-of-way vehicle/follower, vehicular mass difference and conflict angle. For crossing conflicts, angle between involved vehicles plays a major role in increasing ΔKE and resulting CSI. Finally, relationships between CSI values of critical crossing/rear-end conflicts and related severe crash types (for left-hand traffic) demonstrate that the proposed index is an appropriate measure to reflect the probable collision severity. This index will be very useful in network screening for safety improvement and reducing the number of crash victims at hazardous locations.