Researchers defend 20 mph speed limits despite rise in casualties

BMJ 2012; 345 doi: (Published 16 August 2012) Cite this as: BMJ 2012;345:e5580
  1. Matthew Limb
  1. 1London

Researchers are standing by evidence suggesting that 20 mph (32 km/h) roads improve road safety despite new figures showing casualties have increased substantially.

Data from the Department for Transport show that 2262 people were injured in 2011 on roads in built up areas with 20 mph speed restrictions—up 24% from 2010.1

Most of these—1966 people—had slight injuries, while the number of deaths—seven—was a 17% increase on the previous year. There were 289 serious injuries in 2011, up 39% on 2010.

The figures have intrigued researchers who have studied 20 mph roads and say they prevent casualties.

Chris Grundy, lecturer in geographical information systems at the London School of Hygiene and Tropical Medicine led research, published in the BMJ in 2009, which found the introduction of 20 mph zones in London was associated with a 41.9% reduction (95% confidence interval 36.0% to 47.8%) in road casualties.2

Commenting on the Department for Transport figures, Grundy told the BMJ. “We can’t tell anything from the raw numbers alone.”

He said the rise in casualties might simply reflect the fact that there were now many more 20 mph roads.

He said, “To know whether roads are more or less dangerous now we need to know the number of roads that have been introduced as 20 mph roads in that year, the length of the roads and the number of collisions that occur per kilometre. Councils have this information and can provide it to the Department for Transport.”

Grundy added, “The evidence we have so far shows that slowing speeds down from 30 to 20 mph reduces the number of collisions, there is no doubt about that. We need further studies and we need to look at the effect of speed limits over a long period of time.”

Vastly more casualties occur on 30 mph roads, according to the Department of Transport figures. There were 125 494 casualties in 2011—a fall of 1% on the previous year. This includes 636 deaths, which was a 13% increase on 2010.

Local transport minister Norman Baker cited the BMJ study and said evidence suggested “carefully implemented” 20 mph zones could contribute to an improvement in road safety.

He said, “That is why we believe 20 mph speed limits are useful in certain residential areas and support their introduction where it can be shown that they benefit road safety and quality of life.

“However, this is a decision that should be taken locally by councils who know the needs of their area, not in Westminster.”

Kevin Clinton, head of road safety at the Royal Society for the Prevention of Accidents, said 20 mph roads, especially in 20 mph zones that have traffic calming to make them “self-enforcing,” were very effective at protecting people, especially children, pedestrians, and cyclists, from being killed or injured.

He said, “The 24% increase in casualties on 20 mph roads in 2011 is clearly worrying, but it represents small numbers, especially when compared with casualties on 30 mph roads, and is probably due to an increase in the number of 20 mph roads. Most of the increase was slight injuries, again showing that when accidents do occur on 20 mph roads, they are less severe.”

The number of people reported killed or seriously injured on Britain’s roads increased by 2% to 25 023 from 24 510 in 2010—the first annual increase since 1994.


Cite this as: BMJ 2012;245:e5580