Abstract
ObjectivesTraffic speed is important to public health as it is a major contributory factor to collision risk and casualty severity. 20mph (32km/h) speed limit interventions are an increasingly common approach to address this transport and health challenge, but a more developed evidence base is needed to understand their effects. This study describes the changes in traffic speed and traffic volume in the City of Edinburgh, pre- and 12 months post-implementation of phased city-wide 20mph speed limits from 2016–2018.MethodsThe City of Edinburgh Council collected speed and volume data across one full week (24 hours a day) pre- and post-20mph speed limits for 66 streets. The pre- and post-speed limit intervention data were compared using measures of central tendency, dispersion, and basic t-tests. The changes were assessed at different aggregations and evaluated for statistical significance (alpha = 0.05). A mixed effects model was used to model speed reduction, in the presence of key variables such as baseline traffic speed and time of day.ResultsCity-wide, a statistically significant reduction in mean speed of 1.34mph (95% CI 0.95 to 1.72) was observed at 12 months post-implementation, representing a 5.7% reduction. Reductions in speed were observed throughout the day and across the week, and larger reductions in speed were observed on roads with higher initial speeds. Mean 7-day volume of traffic was found to be lower by 86 vehicles (95% CI: -112 to 286) representing a reduction of 2.4% across the city of Edinburgh (p = 0.39) but with the direction of effect uncertain.ConclusionsThe implementation of the city-wide 20mph speed limit intervention was associated with meaningful reductions in traffic speeds but not volume. The reduction observed in road traffic speed may act as a mechanism to lessen the frequency and severity of collisions and casualties, increase road safety, and improve liveability.
Highlights
Urban transport is an important determinant of population health, with 1.35 million road traffic deaths being reported in 2016, and road traffic collisions costing most countries around 3% of their Gross Domestic Product [1]
The implementation of the city-wide 20mph speed limit intervention was associated with meaningful reductions in traffic speeds but not volume
The reduction observed in road traffic speed may act as a mechanism to lessen the frequency and severity of collisions and casualties, increase road safety, and improve liveability
Summary
Urban transport is an important determinant of population health, with 1.35 million road traffic deaths being reported in 2016, and road traffic collisions costing most countries around 3% of their Gross Domestic Product [1]. In addition to the direct public health impacts, urban transport indirectly contributes to air and noise pollution, reduced road safety, physical inactivity and sedentary behaviour [3]. Researchers, governments, transport planners, and other stakeholders (e.g., road safety and active travel charities) are seeking ways to reduce the detrimental impact of urban transport on public health, as well as other agendas such as climate change. In 2014 an umbrella review investigating the health implications of 20mph zones and limits concluded that residential and area-level schemes (traffic calming 20mph zones) can reduce road traffic collisions, injuries, traffic speed and volume, improve perceptions of safety, and be cost-effective [7]. The effects on different socio-economic groups and communities are not well understood [7, 8]
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