Thursday, August 20, 2009

The fallacies of shared space

On my recent skeptical post about "shared space"—the principle that many roads are actually safer when traffic rules are removed—commenter Dan points out a even more significant flaw in advocates' analysis:
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.
This is an incredibly important point. The relevant consideration is not whether shared space decreases accidents on the roads where it is implemented; it is whether shared space decreases accidents on these roads per passenger mile. If the slow, harrowing experience of driving on a road without well-defined rules causes most drivers to seek an alternate route, it's not surprising at all that we see a measured decrease in accidents. The accidents are simply redistributed elsewhere. To make a real case for shared space, you'd have to show that accidents decrease after you adjust for the much lower level of traffic. Using raw totals instead is one of the most egregious examples of statistical malpractice I've ever seen.

2 comments:

Justin said...

I think your analysis is lacking. If a particular road has particularly high pedestrian traffic and that's leading to increased accidents then moving traffic away from that road will reduce fatalities, even if it's not shared space per se as opposed to slower traffic that has the effect.

Of course that might change the rationale for shared space or still undermine the rationale for shared space past reducing speed. In particular, it would give reason to doubt that city planners knew enough to be implementing shared space for good reasons.

Anonymous said...

so following your train of thought, the drivers who "relocate the accidents somewhere else" are still bad drivers regardless of what type of road or street they are on. So why wouldn't you prefer to have them choose to crash somewhere else and also create a better more economically invigorated place? People in cars don't spend money. You need them out of the car on foot.