Fewer accidents in capital since 20mph limit

Friday 29 January 2021

The number of road traffic collisions (RTCs) in Edinburgh has decreased since a 20mph speed limit was introduced in 2016, according to new research led by the University of St Andrews.

Dr Valentin Popov of the School of Mathematics and Statistics at the University of St Andrews with collaborators from the universities of Edinburgh, East Anglia and Cambridge introduced a cutting-edge methodology to identify shifts in the trend of RTCs and associate them with possible causes.

The research team investigated the effect of the policy on a city-wide scale, implicitly including spill-over effects.

Published in Environment and Planning B, the research found that around the time of the introduction of the 20mph policy in Edinburgh, the trend transitioned from a “slow-decreasing” state to a “fast- decreasing” state.

The research provided compelling evidence that a structural change occurred leading to a further decline in the rates of RTCs, which can be associated with introduction of the 20mph speed limit. Dr Popov and collaborators are careful to note that they are not arguing for causality and that the decline may be due to a cultural shift rather than directly attributable only to the introduction of the new lower limit.

The research found that the average number of RTCs per month in Edinburgh in 1997 was 165, while in 2018 this number fell to 64. In 2016, the year of the intervention (introduction of 20mph limit), the average RTCs per month was just above 95.

Dr Popov said: “We have reasons to believe that the 20mph policy was effective, at least in the short-term. This shows that 20mph limits are worth considering in cities where there is a concerted will to reduce RTCs and make roads safer for users.”

As part of the research, the team also noted a seasonal pattern, which indicated an RTC peak in the autumn and a trough at the start of the year. There is an isolated peak in August which can be attributed to the major arts festival occurring, prior to the pandemic, at that time of the year in Edinburgh.

The United Kingdom’s Department for Transport (DfT) reported that in the 12 months up to September 2018, there were 27,295 people killed or seriously injured on British roads.

Although deaths and injuries on the road have been declining locally and nationally (170,993 casualties in 2017 compared to 160,597 in 2018), as a preventable cause of death and disability, (RTCs) remain an important policy priority.

The paper, ‘Trend shifts in road traffic collisions: An application of Hidden Markov Models and Generalised Additive Models to assess the impact of the 20mph speed limit policy in Edinburgh’ by Valentin Popov, Glenna Nightingale, Andrew James Williams, Paul Kelly, Ruth Jepson, Karen Milton and Michael Kelly is published in Environment and Planning B and available online.

Please ensure that the paper’s DOI [doi.org/10.1177/2399808320985524] is included in all online stories and social media posts and that Environment and Planning B is credited as the source.

The ‘Is Twenty Plenty?’ study is funded by the National Institute for Health Research (NIHR, PRP 17/149/19).

Issued by the University of St Andrews Communications Office.

Category Research

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