Introducing 20mph speed zones reduces road traffic accidents, including those suffered by pedestrians and cyclists. Speed zones use physical obstacles such as road narrowing, speed bumps and chicanes, whereas speed limits only use signs and lines.
The evidence was inconclusive on whether limiting speed to 20mph alone was effective, without the traffic calming. There was scant evidence on whether either speed reduction intervention was linked to increased activity such as walking or cycling.
This NIHR-funded review included 11 quasi-experimental studies from Europe, with seven from the UK. All were from towns or cities, and seven were conducted more than 10 years ago.
Both 20mph zones and limits are supported in current public health guidelines to prevent unintentional injuries on the road in under 15s. The review’s conclusions strengthen the advice on zones.
Why was this study needed?
Though the number of road traffic accidents has been declining since the 1960s, there were still 181,384 casualties in the UK in 2016. The number of fatalities reached 1,792, with 25% of them being pedestrians, 25% motorcyclists and 6% cyclists.
In recent years there have been more 20mph speed zones and limits in urban areas near schools to improve road safety. This has required a consideration of the balance between increasing journey times for vehicles and the needs of pedestrians.
This study sought to establish whether there was a reduction in collisions and casualties in 20mph speed zones and in areas just marked with 20mph limit signs, and if there was any difference in effectiveness.
What did this study do?
This was a systematic review and narrative analysis of quasi-experimental studies looking at the impact of these speed zones or limits on a range of public health outcomes.
Included studies were either controlled before-and-after studies or controlled interrupted time-series where multiple measurements are taken at regular intervals before and after the intervention.
Information on road collisions, such as vehicles colliding with other vehicles, pedestrians, motorcyclists or cyclists and the resulting injuries, came mostly from routinely collected police data.
Only four studies took place in the last 10 years. The impact of 20mph zones or limits may be different now, as the numbers of cars, cyclists and roads have all increased, and the type of zones may have changed. Studies were based in towns and cities in high-income countries, so applicability to rural, and middle- or low-income settings, is not clear.
What did it find?
- Introducing 20mph zones appeared effective at reducing the number and severity of collisions and casualties (nine studies, six from the UK).
- Outcomes and their size of effects differed from study to study, so could not be combined in a meta-analysis. But as an example, one London study from 2009 found 20mph zones were associated with reductions of 41.9% in all casualties (95% confidence interval [CI] 36.0 to 47.8) and 37.5% in all collisions (95% CI 31.6 to 43.4). Outcomes for roads nearby also improved, albeit by a much smaller degree: 8.0% reduction in casualties (95% CI 4.4 to 11.5) and 7.4% reduction in collisions (95% CI 3.8 to 11.0).
- The evidence on whether 20mph limits were effective was inconclusive, coming from just two studies (one from the UK).
- There were few studies on the impact of 20mph zones or limits on other public health outcomes; such as walking, cycling, perceived safety, neighbourhood pleasantness, time spent outside, physical activity, air quality and health inequalities.
What does current guidance say on this issue?
A 2010 NICE public health guideline on Unintentional injuries on the road: interventions for under 15s (updated in 2019) supports both 20mph zones and limits.
Recommendations include introducing engineering measures to reduce speed in streets that are primarily residential or where pedestrian and cyclist movements are high. These measures could include:
- speed reduction features (for example, traffic-calming measures on single streets, or 20mph zones across wider areas)
- changes to the speed limit with signing only (20mph limits) where current average speeds are low enough, in line with Department for Transport guidelines.
What are the implications?
The current NICE public health guideline recommends both 20mph zones and limits to reduce accidents and injuries in under 15s, and so this review supports the evidence base behind zones.
Zones require physical road re-engineering, in addition to signs and paint. So, it is likely that any shift towards increasing the number of zones will need careful consideration by councils.
Citation and Funding
Cleland LC, McComb L, Kee F et al. Effects of 20mph interventions on a range of public health outcomes: a meta-narrative evidence synthesis. J Transp Health. 2019; Oct 3. doi.org/10.1016/j.jth.2019.100633. [Epub ahead of print].
This project was funded by the NIHR Policy Research Programme (project number 17/149/19).
House of Commons Library. Road accident casualties in Britain and the world. Research Briefing. London: House of Commons Library; 2018.
NICE. Unintentional injuries on the road: interventions for under 15s. PH31. London: National Institute for Health and Care Excellence; 2010.
Produced by the University of Southampton and Bazian on behalf of NIHR through the NIHR Dissemination Centre