A study published on 11th December , 2009 shows the accident rate could be almost halved on roads with the limit. Experts say an extension would prevent an extra 692 casualties a year. 

It comes as eight councils  revealed they are introducing 20 mph limits on residential roads. 

The London School of Hygiene and Tropical Medicine findings were based on analysing the impact of introducing 20 mph traffic speed zones in the capital over 20 years. 

They linked police data on road casuaities and deaths in London between 1986 and 2006. 

The researchers found 20 mph zones were associated with a 40 % reduction in casualties and collisions. The greatest reduction in accidents was among children and those seriously injured. 

Pedestrian injuries were reduced by nearly a third and cycling casualties by 17 %.  Reductions were greatest in children aged under 11. 

There was no evidence of casualty migration to areas bordering the 20 mph zones.  Injuries actually fell by an average of 8% in areas adjacent to zones.