Using telematics data to evaluate safety policies: a case study of Chicago’s red-light camera programme

Mobile telematics offers a promising new data source for evaluating safety interventions, providing detailed information about driving behaviour and safety events. We examined whether telematics data could effectively evaluate the impact of red-light cameras on driver behaviour and crash risk. We analysed mobile telematics data from over 770 000 users in Chicago to assess how the presence of a red-light camera at an intersection approach affected the likelihood of collisions and harsh braking. We matched intersection approaches with and without cameras on the number of lanes, speed limit, traffic volume and segment length. We used negative binomial regression models to evaluate the impact of cameras on collisions and harsh braking by time of day and season. Harsh braking events occurred 24 times more frequently than collisions and showed remarkably similar patterns of association with environmental factors. Both showed higher frequency during rush hour (11% and 23% increases, respectively), lower at night (73% and 80% decreases) and increasing frequency with more lanes. These effects were consistent across seasons and time of day. Cameras reduced both collisions (25% reduction; 95% CI 15% to 34%) and harsh braking events (21% reduction; 95% CI 12% to 28%). Telematics data show effects of cameras that are consistent with past evaluations. Furthermore, there was close correspondence between collision and harsh braking patterns. Together, these suggest that telematics-reported data provide a surrogate measure for road safety and can provide richer information for safety evaluation in settings where crash data are sparse, though the inability to distinguish injury severity remains a limitation.