Since the Uxbridge and South Ruislip by-election at the end of July, low emission zones and 20mph speed restrictions, have been in the limelight. The Prime Minister himself declared that he is on the side of the motorist and has called for a review of such schemes.

In any review, scientific evidence should be front and centre in any decisions made about future policy, and although the politics of the issue will no doubt stand out in strong relief, the scientific arguments also deserve an airing. One very important part of the evidence is about traffic speed and road safety - slower traffic saves lives.

A recent study funded by the National Institute for Health and Care Research (NIHR) carried out in the cities of Edinburgh and Belfast, made some important findings of relevance to the debate. In Edinburgh, restricting a city’s speed limits to 20mph reduced road deaths by almost a quarter and serious injuries by a third.

Road accident rates across the city fell without any extra traffic-calming measures or police patrols being put in place. The scheme was highly cost-effective, and the relatively modest cost of replacing speed limit signs not only improved road safety, but also enhanced quality of life for residents.

Prior to the new restrictions, 45 out of 100 cars in Edinburgh travelled faster than 25 mph – one year later, this figure had dropped to 31.

Traffic jams are a common sight on city roads. Photo credit: Getty

The research team observed that the number of collisions in one year fell by 40 per cent to 367, and there were 409 fewer casualties – a drop of 39 per cent. A breakdown of the casualty figures reveals that fatalities dropped by 23 per cent and serious injuries fell by 33 per cent.

The research team members were from the Universities of Edinburgh, Cambridge, East Anglia, St Andrews and Bristol and Queen’s University Belfast. They also collaborated with the walking and cycling charity Sustrans. The researchers worked with local and national traffic authorities to gauge the effectiveness of 20mph restrictions introduced by the City of Edinburgh Council in 2016. The new limits applied in 80 per cent of Edinburgh’s streets.

The main objective of the investigation was to evaluate and understand the processes and effects of developing and implementing 20mph speed limits in Edinburgh and Belfast. The focus was on outcomes (casualties and active travel) that may lead to public health improvements.

An additional objective was to investigate the political and policy factors that led to the decision to introduce the new speed limits. Researchers interviewed residents to assess the overall impact. They also examined official records and data, and studied how decisions were reached and regulations were enforced.

They found that reducing traffic speed also helped to create better quality environments. The researchers measured a metric known as liveability, which includes safety, health, sustainability, education, transport, amenities and living standards, and found it improved in both cities after the introduction of speed restrictions.

In what may be particularly important as politicians position themselves in the debate, is the year after implementation, the number of people who were supportive of the speed limits increased, as did their willingness to obey the limits. The increase in support suggests that the whole idea of speed restrictions is far less contentious than many politicians suppose.

The Belfast scheme, which was restricted to city centre streets, and led to a 2 per cent drop in casualties and collisions. These results reflect the scheme’s narrower reach there, and the fact that it was implemented in an area where traffic speeds were already low prior to the trial – on average less than 20mph.

This was a mixed-methods study that comprised an outcome, process, policy and economic evaluation of the implementation in the two cities.

The Council in Edinburgh expected there would be push back, and there were some vociferous critics early on. But these were from organised pro-motorist pressure groups, not the population as a whole. Taxi drivers didn’t welcome the trial at first, and initially neither did local bus companies. But again in the final analysis there was little outright opposition. There was also some opposition in Belfast, mostly from commercial and delivery people, but again it didn’t seem to amount to much n the end.

The findings suggest that the pre-implementation period was particularly important. Appropriate publicity and information helped frame public and political attitudes.

The scale of implementation in the two cities had a bearing on the impacts. The citywide approach adopted by Edinburgh was effective in reducing speeds and positively affected a range of public health outcomes.

The narrower city-centre approach in Belfast (where speeds were already low) was less effective. However, the main outcome of these schemes was a reduction in road casualties at all levels of severity.

It is to be hoped that as the debate proceeds the participants do not forget what we already know, what the data show, and the public health importance of traffic speed and its consequences.