Thousands are injured on the roads each year
UK cities should have more 20mph speed zones, as they have cut road injuries by over 40% in London, a study claims.
In particular the number of children killed or seriously injured has been halved over the past 15 years, the British Medical Journal reported.
The London School of Hygiene and Tropical Medicine study estimates 20mph zones have the potential to prevent up to 700 casualties in London alone.
At 20mph, it is estimated only one in 40 pedestrians is killed in a crash.
This compares with a one in five chance for someone hit at 30mph.
The researchers compared data on road collisions, injuries and deaths in London between 1986 and 2006, with speed limits on roads.
After adjusting for a general reduction in road injuries in recent years, they found that the introduction of 20mph zones were associated with a 41.9% drop in casualties.
The greatest reduction was seen in children under the age of 11 years and in the numbers of all ages killed or seriously injured.
Cyclist injuries fell by 17% once 20mph zones came in, and injuries in pedestrians have been cut by almost a third.
There was also no evidence of a higher rate of casualties in areas bordering the 20mph zones, as in areas adjacent to 20mph zones casualties fell by an average of 8%.
Study leader Dr Chris Grundy, a lecturer at the London School of Hygiene and Tropical Medicine, said: "This evidence supports the rationale for 20mph zones, not just in major cities in Britain, but also in similar metropolitan areas elsewhere.
"Indeed, even within London, there is a case for extending the currently limited provision of such zones to other high casualty roads."
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The drivers who disobeyed the 30mph limit will still disobey the 20mph limit
He estimated that 20mph zones in London save 200 lives a year, but this could increase to 700 if plans to extend the zones were implemented.
A spokesman for the Department of Transport said the study backed their own research showing that 20mph zones help to reduce accidents and casualties.
"Our road safety strategy consultation recommends that local authorities introduce, over time, 20mph zones or limits into streets around schools, and which are primarily residential in nature, to protect pedestrians and cyclists.
"This will save lives and make people feel more secure in walking and cycling on those streets."
Kevin Clinton, head of road safety at the Royal Society for the Prevention of Accidents, said: "This research confirms that one of the most effective ways of protecting vulnerable road users, especially children, is the introduction of 20mph zones.
"It lends weight to calls for an expansion of 20mph zones, which RoSPA strongly supports and which we hope will become a crucial part of the new road safety strategy for the next 10 years."