Archive for December 22nd, 2009

For those looking to explain why the U.K. has made comparatively greater advances in traffic safety than the U.S. over the last few decades, urban speed zones are one good place to look.

An article recently published in the British Medical Journal, “Effect of 20 mph traffic speed zones on road injuries in London, 1986-2006: controlled interrupted time series analysis,” by Chris Grundy, et al., notes that “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 reduction, they also note, was greatest for young children — which brings up the point that it’s not merely children’s risk-taking behavior responsible for their deaths as pedestrians, that addressing driver’s behavior can make a difference — and mattered more for KIAs (killed or serious injuries) than for minor injuries. They also report that “there was no evidence of casualty migration to areas adjacent to 20 mph zones, where casualties also fell slightly by an average of 8.0% (4.4% to 11.5%).” Perhaps driving more slowly on one set of streets even had a carry-over effect. The reductions are impressive and seem beyond what might be explained by some other factor, such as a reduction in pedestrian volumes over that same time period (although other factors, like the presence of enforcement cameras, need to be kept in mind).

About now is where someone usually complains that putting up 20 mph signs is ineffective and won’t change driver behavior. But we’re not talking about mere signage here, we’re talking “self-enforcing roads,” with a variety of engineering and design measures, and as the authors write, some evidence “suggests that the self enforcing 20 mph zones are effective in reducing traffic speeds to an average of 17 mph, an average reduction of 9 mph.”

The benefit wasn’t merely for pedestrians. “A somewhat counterintuitive observation,” they write, “is the apparently large reduction in injuries to car occupants.”

And not surprisingly, given their findings, the authors argue for extending, where justified, the 20 mph zone throughout London, and other metropolitan regions. Which isn’t necessarily an easy task, as Shanthi Ameratunga notes in a reply, also worth reading. “Giving provincial or local agencies the authority to reduce national speed limits is an important step in achieving this vision. Yet the 2009 global survey on road safety reported that only 29% of 174 participating countries set speed limits of 50 km an hour or below on urban roads and allowed local authorities to reduce national speed limits. These findings probably reflect both the lack of evidence on cost effective speed management strategies in low income and middle income countries, and the reticence of most governments to enforce laws that limit driving speeds, possibly because of perceived public opposition.”

But progress is being made, at least in the U.K.