Scientific evaluations of 20 mph zones in the UK that have demonstrated significant benefits in terms of reduced casualties, especially among vulnerable groups such as children. In 1996, the Transport Research Laboratory reviewed 20 mph zones in Great Britain (cited in the review by Grundy et al, 2008).
The uncontrolled study included 72 such schemes and used five years of before data and at least 1 year of after data (the average was 30 months). The researchers found that overall collision rates decreased 61%, pedestrian collision rates decreased 63%, child pedestrian collision rates decreased 70% and overall child casualty rates decreased 67%.
Grundy and colleagues (2008) also cite an evaluation of 20 mph schemes in Kingston upon Hull (population c. 250,000). Hull has extensive 20 mph zones, covering 25% of its roads in 2003. An uncontrolled before and after study found remarkable declines in casualties. Overall, Hull’s road casualties decreased 14% from 1994-2001.
Comparatively, road traffic casualties increased 1.5% over the same time period in the nearby areas of Yorkshire and Humberside. In the 20 mph zones, total collisions decreased 56%, the numbers killed or seriously injured decreased 90%, pedestrian casualties decreased 54%, child casualties decreased 64% and child pedestrian casualties decreased 74%.
In December 2009 the British Medical Journal published a study of the effect of introducing 20 mph zones on road collisions, injuries and fatalities in London, based on analysis of geographically-coded police road casualty data collected over twenty years (Grundy et al, 2009). Overall, the introduction of 20 mph zones was associated with a 41.9% reduction in road casualties, after allowing for underlying time-trends.
The percentage reduction was greatest in younger children, and was greater for people killed or seriously injured in collisions. Pedestrian injuries were reduced by a third with a greater reduction in children aged 0-15 years. The reduction was smaller in cycling casualties (17%) but again this was higher in children. The researchers concluded that “this evidence supports introducing 20 mph zones in major British cities and also in similar metropolitan areas elsewhere.”