Why a 20mph limit is plenty
Posted on 19 December 2023
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In this blog, Professor Adrian Davis, Edinburgh Napier University and chair of the Data and Evidence subgroup of the Public Health and Sustainable Transport partnership, discusses why a 20mph speed limit on road in Scotland can help save lives.
We are seeing far too many people being killed or suffering life-changing injuries because of road traffic collisions, and the high rate of traumatic incidents on the road is stubbornly resilient to efforts to end it. In fact, casualties have been increasing in recent years.
Vehicle speeds impacts all our lives
Vehicle speeds are a major public health issue, not only in terms of the high number of deaths and injuries, but also due to negative impacts on quality of life and health and wellbeing. We are burdened with road traffic noise, air pollution due to the higher fuel consumption the faster we drive, and a lack of physical activity with our reliance on cars as a means of travel.
Speeds of over 30mph are likely to cause deaths and life-changing injuries
We know that most UK pedestrian and cyclist injuries and deaths occur on 30mph roads in urban areas. Increasingly we are seeing the introduction of 20 miles per hour (mph) speed limits on some roads, as we know that deaths and life-changing injuries to vehicle occupants occur with greater frequency at speeds above 30mph.
A person is around five times more likely to be killed when hit by a vehicle travelling at around 30mph than they are from a vehicle travelling around 20mph. If a driver needs to stop urgently, in the distance a 20mph car can stop, a 30mph car will still be doing 24mph.
Ways to tackle speeding vehicles
In the 1990s traffic calming measures including speed humps (vertical) and chicanes (horizontal) were introduced across the UK, supported by the 1992 Traffic Calming Act. During consideration of the Bill, the then Minister for Transport stated that eight out of every 10 urban roads would potentially be eligible to be part of a 20mph speed limit. However, relatively few have been implemented. This is due mainly to costs, meaning that only small residential geographic areas were treated with physical measures. As a result, it would take many decades to achieve the predicted 80% coverage.
An expansion of the 20mph limit
With a greater understanding of the effectiveness of 20mph speed limits, supported by signs, there has been a significant expansion of the schemes across European cities. Portsmouth City Council was the first UK city to introduce it, when signs-only 20mph speed limits were installed on 94% of its road network. When comparing the number of recorded casualties in the three years before the implementation of the scheme with the two years after, the number fell by 22% from 183 per year to 142 per year. This was greater than the 14% reduction in casualties in comparable areas elsewhere in the UK.
Research shows that reductions can save lives
Previous research shows that an average speed reduction of 1mph leads to a 6% reduction in collisions and injuries. There have been robust studies of Bristol and Edinburgh, as well as from elsewhere in Europe and North America, to demonstrate the effects of speed reduction. This includes our recent study on the effect of 20mph limits in rural settlements across the Scottish Borders, which saw average speed reductions of 3mph.
All of the above suggests that a further roll-out of 20mph speed limits in settlements will help meet the National Road Safety Framework’s Scotland’s Road Safety Framework to 2030 | Transport Scotland. This includes a 50% reduction in the number of people killed and seriously injured by 2030, and a 40% reduction in the number of pedestrians killed or seriously injured.
What's next for 20mph limits?
The Scottish Government has stated that all appropriate roads in built up areas will have a safer speed limit of 20mph by 2025. With the aim of preventing all deaths and life-changing injuries on the roads, Public Health Scotland needs to be a lead agency on road safety by working with other groups, such as spatial planning and housing to help keep trip length short with urban density.
There are some useful resources available to those seeking more support for speed management, and not just for 20mph speed limits. These include: