What's the greatest threat to pedestrians? Drunk drivers? Ornery bicyclists? Rabid beavers? If Jeff Speck, a D.C.-based city planner is to be believed, it's wide lanes. Specifically, the 12-footers that are de rigueur in most parts of the country.

In a ridiculously detailed and well-researched piece over at City Lab, Speck makes the case that while 12-foot-wide lanes are believed to be safer, they're not. And in his words, every state DOT has "blood on its hands" because of this misconception.

The salient point in Speck's exhaustive (and occasionally melodramatic) piece is this:

…higher-speed crashes cause more injuries and deaths than lower-speed crashes. This has been amply demonstrated to apply to all road users, especially pedestrians. According to a broad collection of studies, a pedestrian hit by a car traveling 30 m.p.h. at the time of impact is between seven and nine times as likely to be killed as one hit by a car traveling 20 m.p.h. This tremendously sharp upward fatality curve means that, at urban motoring speeds, every single mile per hour counts.

In a nut, Speck contends – and backs up, mostly – that wider lanes cause people to travel faster. When they travel faster, they do more damage when hitting a pedestrian or cyclist. By reducing the lane width in urban areas from 12 to 10 feet, drivers will slow down because of the perceived lack of space.

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Anecdotal experience in my previous life as a driving instructor bears this out. With new students, I'd have them drive in a rural area where they'd go from a 12-foot-wide lane to a 10-foot-wide lane and finally an 8-foot-wide lane. The speed limit on all these roads was 55 MPH, but by the time they got down to the 8 footer, they'd be crawling along between 25 and 35 mph, just because they didn't feel they had enough space to maintain the posted speed limit.

Speck hits on the larger issue, which is the traffic engineer's bible, the American Association of State Highway and Transportation Officials "Green Book". It serves as the primary source for designing every road imaginable and as such, it blocks civil engineers from lawsuits. But some contend certain aspects need to be revised, and the near-religious adherence to the 12-foot-wide lane might be one aspect that deserves a second look.