The introduction of 20 mph traffic speed zones in London has reduced road injuries by over 40%. The greatest benefits were found among younger children and in the numbers killed or seriously injured, finds new research published on bmj.com today.
The authors estimate that 20 mph zones prevent 203 casualties each year, and they support the case for extending 20 mph zones in London with the potential of preventing a further 692 casualties each year.
Road injuries are among the leading causes of death and disability worldwide. It is well known that reducing the speed of traffic in urban areas using traffic calming measures such as speed humps and chicanes can reduce injury rates. There is some evidence to suggest that 20 mph zones are effective in reducing traffic speed, but their impact on road casualties is not clear.
To address this issue, researchers from the London School of Hygiene and Tropical Medicine assessed the effects of introducing 20 mph (32 km an hour) traffic speed zones on road collisions, injuries, and deaths in London over 20 years.
They linked geographically coded police data on road casualties in London between 1986 and 2006 to a detailed database of road segments. After adjusting for background reductions in road injuries, they found that relative to other roads, 20 mph zones were associated with a 40% reduction in casualties and collisions.
Casualties as a whole were reduced by 41.9%, with killed or seriously injured casualties in children reduced by half. Pedestrian injuries were reduced by a little under a third and cycling casualties by 16.9%.
Reductions were greatest in younger children (0-5 and 6-11 years), and higher for the category of killed or seriously injured casualties than for minor injuries.
The authors found no evidence of casualty migration to areas bordering the 20 mph zones. Indeed, in areas adjacent to the zones casualties also fell by an average of 8%.
"The additional effect of the 20 mph zones was that of a step reduction in casualties and collisions by an amount that has taken over 20 years to achieve on roads without 20 mph zones," say the authors.
They conclude: "This evidence supports the rationale for 20 mph zones not just in major cities in Britain but also in similar metropolitan areas elsewhere. Indeed, even within London, there is a case for extending the currently limited provision of such zones to other high casualty roads."
Speed management is key, says Associate Professor Shanthi Ameratunga from the University of Auckland in an accompanying editorial. This includes setting and enforcing speed limits, "engineering treatments" (such as road humps and roundabouts), and public education. She believes that, as witnesses to the consequences of road injuries, clinicians have a responsibility to evaluate the links between decision making in transportation and the effects of these choices on public health and equity.