Effect of 20 mph Traffic Speeds on Injury Rates

January 17th, 2010

One of the most important issues facing cities and towns is traffic safety. For transportation planners and engineers, this has led to the use of traffic calming strategies designed to reduce vehicle speeds.

But the underlying question is the extent to which the reduction in speeds reduces accidents and injuries. A comprehensive study was recently published in the British Medical Journal that provides an important look at this -- by examining 20 years' worth of data in London on the effect of introducing 20 mph zones. We first heard of this study from a blog posting by the New Haven Safe Streets Coalition. Thanks!

Read the full article: "Effect of 20 mph traffic speed zones on road injuries in London, 1986-2006: controlled interrupted time series analysis" (British Medical Journal, BMJ, 10 December 2009).

Excerpts:

Road injuries are among the leading causes of loss of life and disability worldwide, and they are projected to make an increasingly important contribution to public health burdens over the coming decades.

"Road injuries are among the leading causes of loss of life and disability worldwide, and they are projected to make an increasingly important contribution to public health burdens over the coming decades, especially in low and middle income settings. There is good evidence internationally for the effectiveness of reducing the speed and volume of traffic for reducing injury rates.

One strategy for reducing speeds in urban areas is the use of road engineering interventions such as vertical deflections (humps), chicanes, and other physical alterations to prevent motorised traffic travelling at more than 20 miles an hour (32 km an hour). Zones in which traffic is limited to 20 mph are a type of area-wide traffic calming that uses road engineering measures to physically slow traffic. Over the past 15 years or so, 20 mph zones have been established in London and many other areas of the UK.

... By using a geographical information system (GIS), we linked ... casualty data to a detailed road segment database that included the characteristics of all classified and unclassified roads in London. For each financial year (April to March), we classified each segment of road between junctions according to the type of road and whether or not it was in a 20 mph zone or adjacent to a 20 mph zone. ...

Results. The introduction of 20 mph zones was associated with a 41.9% (95% confidence interval 36.0% to 47.8%) reduction in road casualties, after adjustment for underlying time trends. The percentage reduction was greatest in younger children and greater for the category of killed or seriously injured casualties than for minor injuries. There was no evidence of causality migration to areas adjacent to 20 mph zones, where casualties also fell slightly by an average of 8.0% (4.4% to 11.5%)."

See also the following from the American Association of Pediatrics "Policy Statement on Pedestrian Safety" (use link to access footnotes for the statistics noted below).

Pedestrians who are hit by a car traveling 40 mph have a 15% chance of survival, but 85% survive when hit by a car moving at 20 mph.

" ... Vehicle speed is a strong risk factor for pedestrian injuries and is associated with greater injury severity. Pedestrians who are hit by a car traveling 40 mph have a 15% chance of survival, but 85% survive when hit by a car moving at 20 mph. Because a faster-moving vehicle has a longer braking distance, impact with a pedestrian is more likely. Although several factors, such as driver reaction time, vehicle weight, brake quality, and road-surface conditions, play a role, the stopping distance for a vehicle traveling at 30 mph is considerably greater than that of a car traveling at 20 mph (197 vs 112 ft, respectively). In residential neighborhoods, an average vehicle speed of 30 mph, compared with 20 mph, was associated with more than a sevenfold greater risk of children being hospitalized for pedestrian injuries."