The number of people injured or killed on Britain's roads in 20mph zones rose by 24% in 2011, it has emerged.
Councils were given powers to designate the zones to improve safety in 2009.
Some 2,262 people were road casualties in the zones last year - 1,966 of them minor injuries - according to the Department for Transport figures.
But this is only a fraction of the number of casualties on 30mph roads where more than 125,000 were reported in 2011, a drop of 1% on 2010.
Safety campaigners have suggested lower speed limits make crashes less likely and less severe when they do happen but the figures have triggered a debate on how useful the restrictions are.
Local transport minister Norman Baker said: "It's vital that speed limits are suitable for local conditions and councils are best placed to determine what these limits are, based on local knowledge and the views of the community," he said.'Jury out'
Neil Greig, director of policy at the Institute of Advanced Motorists, said evidence on 20mph areas "now seems very mixed and contradictory".
"The IAM has always expressed concern that such areas were being seen as a magic bullet to stop all accidents when this had never been clearly proven...
"In our view the main benefits of 20mph zones are health and environmental improvements. The jury is still out on their wider road safety success."
Figures released in July show that in 2010-11, 1,901 people were killed on Britain's roads. That is 51 more than the year before and the first rise since 2003.
According to the figures, there were seven deaths in 20mph zones last year, a 17% rise on 2010 while there were 636 deaths in 30mph zones, up 13%.
There were 289 serious injuries in 20mph zones last year, a 39% rise year-on-year.
However, the Department for Transport could not give figures for how many 20mph zones existed in 2011 compared with 2010, which could put the rise in casualties in more context.
The Royal Society for the Prevention of Accidents said the increase in 20mph casualties was "worrying" but represented small numbers compared with accidents on 30mph roads where 13,168 people were seriously injured in 2010.
Kevin Clinton, head of road safety at the society, said: "Road deaths and serious injuries on Britain's roads as a whole increased in 2011 after consistently falling for many years.
"We need to understand why and to ensure that sufficient resources are devoted to road safety to make sure that one year's increase does not turn into a long term trend."
BBC Update 17 August 2012: This story has been amended to clarify that there are no official figures showing if the number of 20mph zones has increased significantly.
Source : http://www.bbc.co.uk/news/uk-19226144