Car clinic: Will lowering speed limit from 30mph to 20mph really affect accident rates?

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20mph speed limit resized

Q. The speed limit is currently being cut from 30mph to 20mph on many roads in Cambridge. Will this really make much difference to the number of accidents? Is there anywhere else in the UK that has tried a 20mph limit and possesses the data to back up the need to make this change?
MH, Cambridge

A. Cutting the speed of a car from 30mph to 20mph reduces its stopping distance by almost half, from 75ft to 40ft, according to the Highway Code. And if you can’t stop in time, a lower speed will reduce the force of impact. There is a 2.5% chance of a collision being fatal at 20mph, compared with 20% at 30mph, based on analysis by one study.

The first widespread evaluation of 20mph zones in the UK, conducted by the Transport Research Laboratory in 1996, found that the number of accidents involving death or injury dropped by 60%, and child injury accidents were reduced by 67% (compared with when those roads had 30mph limits). The evaluation did not find evidence that accidents increased on surrounding roads because of drivers changing their route — a common criticism made by opponents of 20mph zones.

A study of 20mph zones in Hull between 1994 and 2001 found the total number of reported accidents dropped by 56%. A large-scale review of road casualties in London between 1986 and 2006 found that 20mph zones reduced the number of fatal or serious accidents by about 42% and the drop for child casualties was about 50%.

The number of 20mph roads has shot up in the past decade (in Portsmouth, for example, 94% of previously 30mph roads have been cut to 20mph) and it is hoped that lower traffic speeds will also encourage more cycling and walking — one of the stated aims of the Cambridge scheme.

Cambridge city council says it will publish before-and-after measurements of traffic speed, accident statistics and the number of people walking and cycling to show whether the scheme has been effective. 

Sunday Times Driving car clinic expert: Tim Shallcross

Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motoring


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