Department for Transport figures for Great Britain show that comparing 2011 to 2010, the number of casualties on 30mph streets dropped by about 1,000 and the number on 20 mph streets rose by about 400. This is exactly what you would expect because many 30 roads have been reclassified as 20 mph roads, so the casualties are now reported as 20 mph casualties as opposed to 30 mph casualties. The Department for Transport does not record how many roads are 20 mph, so it is not possible to calculate the rate or the risk.

Data from the DfT shows that deaths on 20mph roads for the whole of Great Britain were 6 in 2010 and 7 in 2011. This was reported on in the Sun newspaper as a 17% rise and evidence that 20 mph ‘is not working’. However, the really worrying fact is that deaths on 30 mph roads in Great Britain had increased from 545 to 612 between 2010 and 2011 but this was not mentioned in the news article. For Bristol, the casualty numbers are small and it is impossible to draw conclusions about the impact on casualty rates in Bristol, without careful analysis of long term trends.

What is certain is that the speed at which vehicles travel is directly linked to the severity of injuries sustained in the event of a collision. A pedestrian, if struck by a vehicle driving at 20mph, is likely to suffer slight injuries. At 30mph they would be severely hurt and at 40mph or above are likely to be killed. Reducing the speed limit to 20mph will have a direct impact on pedestrian safety and is one of the principal reasons for introducing the scheme throughout the city. It is also certain that children living in the most deprived areas are up to 5 times more likely to be killed as a pedestrian than children living in affluent areas. London data (see above) over 20 years has shown that 20 mph limits there were associated with a reduction in risk and severity of casualties.