Validation of surrogate measures of safety with a focus on bicyclist-motor vehicle interactions

Surrogate measures of safety (SMoS) can enable quick, pro-active, and detailed safety evaluations by studying near-crashes. A critical concern regarding SMoS is their validity. This study focused on the validity of two commonly used SMoS indicators-minimum time to collision and post-encroachment time-with a specific focus on bicyclist-motor vehicle interactions. The study was divided into two main parts; the first part focused on observations at intersections in Scandinavia, and the second part focused on developing a crash model using data from 166 similar intersections. Nine signalised intersections in Scandinavia were observed for at least 24 h. During this time, each interaction between a bicyclist and a right- or left-turning motor vehicle was identified and trajectories for the road users were created. The corresponding SMoS values were then calculated. Three main results were found when comparing the results of the crash model with the SMoS analysis. First, there is a significant relationship between the expected number of crashes and both indicators. However, the results also suggest that this correlation might originate from the inherent connection between the indicators and the number of interactions between the studied road users. Finally, when the number of interactions is considered, the results show that the minimum time to collision with a threshold of 3-4 s produces the best results.