Surrogate Measures of Safety with a Focus on Vulnerable Road Users : An exploration of theory, practice, exposure, and validity

Sammanfattning: Surrogate measures of safety (SMoS) are meant to function as tools to investigate traffic safety. The term surrogate indicates that these measures do not rely on crash data; instead, they focus on identifying safety critical events (or near-crashes) in traffic, which can be used as an alternative to crash records. The overall aim of this thesis is to explore which SMoS are suitable when analysing the safety of vulnerable road users (pedestrians and cyclists). The thesis attempts to answer this question using two different approaches: 1) a literature review focusing on existing surrogate measures and how well they consider vulnerable road users from a theoretical perspective, and 2) four observational studies which focus on the validity of SMoS and their relation to exposure.The literature review focuses on identifying existing SMoS, and on two main aspects when evaluating their suitability for analysing the safety of vulnerable road users. Firstly, if the indicators theoretically are able to measure both the risk of collision and the potential for injury should a collision occur, and secondly, to what extent vulnerable road users were included in previous validation studies. The findings from the literature review are that the most commonly used indicators (Time to Collision Minimum and Post Encroachment Time) are also the most validated, but that they have several theoretical limitations, mainly that they to do not measure injury potential and that they measure the severity of an event based on the outcome rather than the initial conditions or potential/observed evasive actions. There are also several indicators which theoretically are more suitable but instead lack validation studies.The observational studies, which make up the second part of this thesis, consist of an attempt at a large-scale validation study, followed by several studies which focus on the shortcomings discovered in the first attempt. The large-scale study is based on three weeks of video recordings made at 26 signalized intersections in seven European countries. The analysis of these videos resulted in three major findings. Firstly, the lack of comparable crash records made any large-scale validation attempts impossible. Secondly, the lack of comparability between the critical events identified by human observers and those identified by computer calculations made it infeasible to perform a long-term analysis. Thirdly, there is a significant relationship between meetings and critical events identified using Time to Collision Minimum and Post Encroachment Time, which suggests that some of the benefit of using those (and other indicators) might originate from their inherent connection to simple meetings between road users (i.e. exposure). Following these results, the thesis presents a limited validation study based solely on the Scandinavian intersections followed by a suggestion for how a relative approach to validity might offer a potentially easier way of evaluating SMoS in the future. The results from these studies indicate that Time to Collision Minimum can measure safety to at least some extent, while Post Encroachment Time measures it to a lesser extent. Due to the strong connection between critical events and meetings, the thesis also explores how a meeting between road users can be defined and how understanding what constitutes an opportunity for a crash might help to explain the so-called safety-in-numbers effect, as well as how future SMoS studies should consider meetings.