Twenty’s Plenty

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.

This entry was posted on Tuesday, December 22nd, 2009 at 9:41 am and is filed under Traffic Enforcement, Traffic Engineering, Traffic Laws, Traffic safety, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

9 Responses to “Twenty’s Plenty”

  1. David Hembrow Says:

    I have to point out two things related to this issue. 20 mph zones are a great idea in themselves, and I support the “20’s plenty” campaign.

    However, Britain actually has very few areas with this low speed limit. The Netherlands now has over 30000 km of 30 km/h zones so has performed a much larger test of the concept. The results have been interesting. Over time, the effectiveness of the lower speed limits have reduced. You could put this down to motorists being more used to the lower limit, so more likely to ignore it.

  2. Yokota Fritz Says:

    In California: local authorities are forbidden to set the speed limit less than 25 mph! A few towns have 15 mph speed limits through heavy pedestrian downtown zones, but those limits are unenforceable.

  3. SteveL Says:

    As one of the people who pushed for 20mph in Bristol, england, I am pleased that a big chunk of our city has got. What is critical is that nearly all the main roads in the zones will be covered. Without that you’d only have the residential back roads broken up into roads you can’t get to from other parts of the city without hitting 30-mph roads, which are still the problem.

  4. Andy Says:

    Why not design roads that allow one to commute in a reasonable time and separate the driver from the pedestrian?

    I accept that travelling at high speed has its risks. so should you.

  5. John Says:

    How about 10 mph?

    Seriously, 20 mph will be commonly exceeded by human powered vehicles in the not so distant future, and they will be precisely where these speed limits are. I think 30 mph is longer-term, and the safety, especially for children, will come from other venues.

    I have not yet written of cycling in my blog Best Driver In The World, but I am an experienced cyclist who has led bike expeditions, raced, rode a recumbent, and all-weather commuted. The key here, as it is with driving, is awareness of what is around you. You should feel safe on any road. So the basic solution is training.

    Many writers overlook the idea that the future will bring higher not lower speeds. It is inevitable.

  6. fred_dot_u Says:

    Just because a vehicle, even a human powered one can exceed 20 mph does not mean it’s advisable. The purpose of the reduced speed limits is aimed at increasing safety. An HPV at 30 mph is more dangerous to others than one at 20 mph.

    Part of the solution being training is reasonable to believe and part of that training should include consideration for other road users, pedestrians included.

    I can and do operate at speeds greater than 20 mph in my velomobile, but not when pedestrian traffic is a concern.

    The USA would do well to emulate many things from overseas, and reduced speed limits is one. Enforcement is not impossible.

  7. John Says:

    Yes, granted, the US can learn from the safer driving of other nations. At the same time, I believe a responsible individual who is surrounded by lawbreaking drivers, as I am, can become that much better. But I really, really would like a writer to take on the challenge of explaining how the world is going to get to higher speed limits. I do feel this is inevitable.

  8. Ian Turner Says:

    Andy, why should I, as a pedestrian, be subject to the consequences of your risky behavior?

  9. John W. Says:

    Some of those “safer driving” nations also have strict liability laws. Auto accidents create serious problems for those involved, and, in the Netherlands at least, the driver is almost always at fault when involved in a vehicle vs. pedestrian/cyclist accident (the big exception being when the pedestrian/cyclist runs into the stopped/parked car).

    And, John, as far as your idea for “training” goes, increasing one’s situational awareness doesn’t do squat for the cyclist or pedestrian about to get run over or off the road by a motorist. Or has the cyclist been made safer for being more aware of his life flashing before his eyes?

    Besides, what’s the hurry?

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