Monday, August 17, 2009

Is shared space really safer?

Matthew Yglesias points us to the shared space concept, the idea that cities should abolish the traditional separation between vehicles and other road users, in addition to removing traffic signals and signs. This innovation has led to suggestions that a road with fewer rules is actually safer, like this one from the Guardian:
Aesthetics were partly behind the changes - the street is now clad in York stone and granite - but public safety was the other motivation, and accident figures out today seem to justify the council's initiative. Figures for 1998 to 2000, before the changes, show there were 70 casualties on the high street, including eight people killed or seriously injured and 62 suffering slight injuries. In the two years from September 2003 to August 2005 there have been 40 casualties (four killed or seriously injured) and 36 slight injuries - a 43%-plus decrease.
It's easy to see the appeal: the idea is deliciously counterintuitive, and it appeals to the widespread sentiment that cars receive too much special treatment.

Yet I'm skeptical. Certainly I can see how a limited implementation of this idea, for a limited period of time, reduces accidents. Drivers are shocked by the sudden change and take special care to avoid accidents. Since better attention can make an enormous difference in road safety, we see a initial decline in casualties, especially if the new policy is limited to a small set of streets, and drivers are startled every time they enter the rule-free space. It's not clear, however, that this is either sustainable or scalable. Once shared space becomes the norm, motorists will no longer apply the same extreme caution every time they drive, and you'll start to see everyone pushing the limits.

Perhaps this isn't a fair comparison. but some form of "shared space" already exists in the vast majority of third world countries, and the resulting safety record isn't exactly positive...


Gary said...

"...some form of "shared space" already exists in the vast majority of third world countries, and the resulting safety record isn't exactly positive..."

I wish we could get rid of the term "third world countries". It has outlived both its literal meaning and its descriptive usefulness.

In any case, I like your point about potential non-scalability. Also, I've long been curious about relative road safety between countries - do you have any numbers to back up your claim?

Dan said...

I agree that the conclusion is premature, for the above mentioned reasons and others. From later in the article:
I think it's a good thing. Cars go fairly slowly. I think it's safe.
People may be avoiding this road, decreasing the total traffic. Apparently the cars are going slower than before. So the new accident statistics should be compared to the expected decrease due to reduced traffic/slower speeds. Combine that with the decrease in accidents citywide and small N (both mentioned elsewhere in the article) and you have a strong case that the conclusion is overstated.

Matt Rognlie said...


That's a good question. I don't have the statistics about deaths per passenger mile offhand, but I'll look around for them. (My impression comes from visiting a few developing countries with limited traffic rules and observing an extremely high rate of crashes and deaths, although certainly this anecdotal impression isn't as good as hard data.)

I completely agree with you about the term "third world," by the way -- it was a lapse on my part.


You are absolutely right, and this is likely an even more powerful effect than the ones I mentioned. These statistics need to be adjusted for declining traffic to be even remotely meaningful.