Adaptation of road users to fixed-location speed enforcement cameras : a difference-in-differences analysis of Ring Road I in Helsinki

In this study, the long-term impact of fixed-location speed enforcement cameras on driver behavior is analyzed using a difference-in-differences (DiD) approach on data from Ring Road I in Helsinki. The study reveals that speed cameras lead to significant, immediate reductions in speed at camera locations (by about 10 km/h) and also at most camera-free locations (by about 5 km/h), supporting their role as a key tool in road safety. However, drivers gradually return to higher speeds over time, indicating behavioral adaptation. Eight years post-installation, speeds remain about 5 km/h lower at camera sites, while nearby locations show mixed effects, with some returning to pre-treatment speeds. The study underscores the need for continuous monitoring and dynamic enforcement strategies to sustain the benefits of speed cameras. These insights inform future road safety policies, particularly regarding long-term compliance with speed limits and the design of adaptive enforcement mechanisms.