The effectiveness of a 20 mph speed limit intervention on vehicle speeds in Bristol, UK: A non-randomised stepped wedge design

Abstract

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 on motorised vehicle speeds is a crucial first step in any logic model that seeks to associate 20 mph speed limits to improved health and wellbeing outcomes. However, little is known about how the introduction of 20 mph limits affects speeds. This paper presents the findings from a novel comprehensive academic evaluation of the adjusted effects of a 20 mph sign-only city-wide intervention on vehicle speeds. This is based on a natural experiment that took place in Bristol, UK. Applying 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. Results showed an unadjusted speed reduction of 4.7 mph (7.41 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, with speed reductions being larger during night time, at weekends, and in summer months. The roads that did not receive the 20 mph intervention also saw a small reduction in speed (0.03 mph). The findings indicate that the 20 mph intervention was successful in lowering individual vehicle speeds. 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.

Effects of city-wide 20 mph (30km/hour) speed limits on road injuries in Bristol, UK

“…The Bristol 20 mph speed limit policy was implemented between 2010 and 2015 9. The 20 mph limit scheme was introduced in phases corresponding to seven areas (for details on the intervention, refer to Bornioli et al 10), enabling a stepped-wedge design analysis. This is a pragmatic design suited to interventions that are implemented in steps across several clusters (in this case, geographical areas).…”

Factors related to road accidents in Palembang, South Sumatera, Indonesia

“…It need regulation for speed especially in motorcycle. Twenty mph (32.2 km/h) speed limits across urban areas are becoming a widespread tool for public health and road danger reduction globally [22]. However, the human behaviour factors are not the only indicators of the road safety level because it enhances the multidimensional system approach of defining the road safety [23].…”

Vision Zero: Speed Limit Reduction and Traffic Injury Prevention in New York City

“…Using various methodologies, casualty crash measures, and where available, vehicle speed measures, the results show broadly that casualty crashes and speeds declined with the treatment. 8 Three further pertinent studies examined speed limit reductions implemented (in contrast to New York City's) by changing the speed limit signs on sections of roadways at a small number of selected sites in the localities: Bristol, England (Bornioli et al 2018;30-20 mph), Edmonton, Canada (Islam et al 2014;30-25 mph), and Columbia and Springfield, Missouri (Rossy et al 2012;30-25 mph). These studies did not have casualty crash data, but found that average vehicle speeds declined significantly.…”