Speed limit study set to reveal if twenty is plenty on city streets

The impact of 20mph speed limits is to be evaluated in a major study in two British cities.

Casualties and traffic accident rates will be measured to determine whether 20mph limits improve road safety, say researchers.

Rates of cycling and walking will be measured to assess the impact on transport use.

Experts say the project—the largest of its type in the UK—will also look at wider effects on residents and the local area and will inform other cities planning to introduce lower speed limits.

Speed limits have been dropped from 30mph to 20mph in parts of Edinburgh and Belfast with the aim of improving safety, but the move remains controversial. The new study—which will run until 2020—will shed light on its widespread effects, say scientists.

The impact on will be measured by establishing rates of traffic collisions and fatalities before—and after—the speed limit was lowered.

The study team—led by the University of Edinburgh and the Scottish Collaboration for Public Health Research and Policy - will also assess how the change has affected the number of people walking and cycling.

Local residents' attitudes towards the lower limit—and how it has impacted on their quality of life - will be measured.

The study is funded by the National Institute for Health Research and involves partnership with other UK universities, NHS Health Scotland, and the charity Sustrans.

Dr Ruth Jepson from the University of Edinburgh's Usher Institute of Population Health Sciences and Informatics, said: "We are excited to launch this major project, which we hope will provide very important insights into the public health effects of such initiatives.

"We anticipate that our broad focus will generate a wealth of evidence and learning that will be invaluable for informing future roll-outs of similar schemes in the UK and around the world."

Andy Cope, Director of Insight, Research and Monitoring at Sustrans, said: "This study will provide vital intelligence on the impact of 20mph on safety and levels of physical activity. We await its findings with keen interest."

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Citation: Speed limit study set to reveal if twenty is plenty on city streets (2017, June 14) retrieved 16 September 2019 from https://phys.org/news/2017-06-limit-reveal-twenty-plenty-city.html
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Jun 14, 2017
Average traffic speed in London is 10mph

Jun 14, 2017
Note that when speed drops traffic density increases. If, for instance, you halved the speed limit then it will take twice as long for cars to traverse, say, a city block and as just as many cars enter and exit that block and they spend twice as long in transit then the density also doubles.

On roads where cars can actually travel at the speed limit and not crawl between traffic lights the density increases and the number of traffic accidents also increases as the probability of hitting another car or being hit by another car increases. As the speed limit falls, people pay less attention and this also increases traffic accidents.

Thus there is going to be a sweet spot where traffic accidents are minimal, and that will not necessarily occur at the lower speed. This observation is far more relevant on the open road than in the city where the traffic density often can not increase, especially where the streets are short and thee are traffic lights at both ends.

Jun 15, 2017
Thus there is going to be a sweet spot where traffic accidents are minimal, and that will not necessarily occur at the lower speed.

Fuel consumption and pollution also increases surprisingly quickly and dramatically the slower people drive. The minimum fuel consumption in modern small cars is achieved somewhere around 40 mph. Halving the speed will more than double the emissions.

20 mph is starting to be at the limit of what is "driveable", i.e. where the traffic doesn't turn into an accordion because it's difficult to maintain a steady speed because cars are made to go a lot faster..

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