A Systematic Review of Traffic Conflict-Based Safety Measures with a Focus on Application Context

Relative to safety assessment using data from observed crashes, conflict-based road safety assessment can potentially provide additional insights into crash causation processes. Despite numerous review studies on this topic, the application context of conflict measures has been generally overlooked. This study conducts a systematic review of conflict-based safety measures with a specific focus on the context of their applications. This study employs the Preferred Reporting Items for Systematic Reviews and Meta-Analyzes (PRISMA) guidelines of systematic review and meta-analysis to review conflict measures used for the safety assessment of intersections over the past ten years (2010-19). A total of 386 studies are systematically reviewed to identify conflict measures used for various contexts, including intersection types, traffic operating conditions, study types, and the purpose of the study.

The systematic review indicates that temporal proximity measures, specifically time-to-collision and post-encroachment time, are the most widely used conflict measures regardless of the application context. Other families of conflict measures such as spatial proximity, kinematic, mixed and combinations of measures have also been applied depending on the context. Using the extracted data from relevant studies, linear regression models were developed for time-to-collision and post-encroachment time thresholds at signalized intersections and time-to-collision thresholds at unsignalized intersections. The thresholds are found to be associated with traffic environment types, sources of conflict data and the application purpose of conflict measures. The findings of this study identify several critical gaps in the literature that can help guide future research directions in the conflict-based safety assessment of transport facilities. Critical gaps include the scarcity of validation studies for conflict measures, the lack of suitable techniques to estimate crash risk by severity types, the primary focus on signalized intersections (leaving studies of other facility types underrepresented), and the lack of suitable conflict measures for vulnerable road users.