Investigating the impact of a 20 miles per hour speed limit intervention on road traffic collisions, casualties, speed and volume in Belfast, UK: 3 year follow-up outcomes of a natural experiment
Open Access
- 15 November 2022
- journal article
- research article
- Published by BMJ in Journal of Epidemiology and Community Health
- Vol. 77 (1), 17-25
- https://doi.org/10.1136/jech-2022-219729
Abstract
Background Evidence regarding the effectiveness of 20 miles per hour (mph) speed limit interventions is limited, and rarely have long-term outcomes been assessed. We investigate the effect of a 20 mph speed limit intervention on road traffic collisions, casualties, speed and volume at 1 and 3 years post-implementation. Methods An observational, repeated cross-sectional design was implemented, using routinely collected data for road traffic collisions, casualties, speed and volume. We evaluated difference-in-differences in collisions and casualties (intervention vs control) across three different time series and traffic speed and volume pre-implementation, at 1 and 3 years post-implementation. Results Small reductions in road traffic collisions were observed at year 1 (3%; p=0.82) and year 3 post-implementation (15%; p=0.31) at the intervention site. Difference-in-differences analyses showed no statistically significant differences between the intervention and control sites over time for road traffic collisions. There were 16% (p=0.18) and 22% (p=0.06) reductions in casualty rates at years 1 and 3 post-implementation, respectively, at the intervention site. Results showed little change in mean traffic speed at year 1 (0.2 mph, 95% CI −0.3 to 2.4, p=0.14) and year 3 post-implementation (0.8, 95% CI −1.5 to 2.5, p=0.17). For traffic volume, a decrease in 57 vehicles per week was observed at year 1 (95% CI –162 to −14, pConclusion A 20 mph speed limit intervention implemented at city centre scale had little impact on long-term outcomes including road traffic collisions, casualties and speed, except for a reduction in traffic volume. Policymakers considering implementing 20 mph speed limit interventions should consider the fidelity, context and scale of implementation.Keywords
Funding Information
- Public Health Research Programme (15/82/12)
This publication has 16 references indexed in Scilit:
- Using Ripple Effects Mapping to Evaluate a Community-Based Health Program: Perspectives of Program ImplementersHealth Promotion Practice, 2018
- Developing and refining a programme theory for understanding how twenty mile per hour speed limits impact healthJournal of Transport & Health, 2018
- Natural experiment methodology for research: a review of how different methods can support real-world researchInternational Journal of Social Research Methodology, 2018
- The effect of changing the built environment on physical activity: a quantitative review of the risk of bias in natural experimentsInternational Journal of Behavioral Nutrition and Physical Activity, 2016
- Support and compliance with 20mph speed limits in Great BritainTransportation Research Part F: Traffic Psychology and Behaviour, 2015
- Process Tracing and Contribution Analysis: A Combined Approach to Generative Causal Inference for Impact EvaluationIDS Bulletin, 2014
- Go slow: an umbrella review of the effects of 20 mph zones and limits on health and health inequalitiesJournal of Public Health, 2014
- Layers of complexity in interpreting evidence on effectivenessPublic Health, 2013
- Impact speed and a pedestrian's risk of severe injury or deathAccident Analysis & Prevention, 2013
- Evaluating policy and service interventions: framework to guide selection and interpretation of study end pointsBMJ, 2010