This paper presents the findings from the public health evaluation of the 20?mph sign-only city-wide intervention in Bristol, UK, on vehicle speeds and collision data. Twenty mph (32.2?km/h) speed limits across urban areas are becoming a widespread tool for public health and road danger reduction globally. Determining the effectiveness of these interventions is a crucial first step in any logic model that seeks to associate 20?mph speed limits to improved health and wellbeing outcomes. Speed analysis was conducted with a quasi-stepped wedge design. Speeds of 36,973,090 single vehicles, recorded by Automatic Traffic Counts before and after the 20?mph intervention, were analysed. Generalized linear mixed models were used to control for confounding variables. Changes in collisions were analysed with Poisson regression. Results showed an unadjusted speed reduction of 4.7 mph (7.56?km/h) and an adjusted speed reduction of 2.66 mph (4.28?km/h) over two to three years. Some variability due to time variables was detected. The roads that did not receive the 20?mph intervention also saw a small reduction in speed (0.03?mph). A preliminary descriptive analysis of collision data shows a lower number of collisions in the post-intervention period. The findings indicate that the sign-only 20?mph intervention was successful in lowering individual vehicle speeds, and highlight potential public health benefits. Policy makers are encouraged to implement a careful monitoring of the effects of 20?mph speed limit interventions on vehicle speeds in order to enable a meaningful evaluation of potential public health benefits.
Pilkington, P., Bornioli, A., Bray, I., & Bird, E. Public health evaluation of the 20mph speed limit policy in Bristol, UK