In Traffic, Tom Vanderbilt detailed the study performed by U.K. based researcher, Ian Walker, on passing distance and driving behavior around cyclists based on gender. On his blog, Vanderbilt summarized the study,
To briefly summarize, in his study (published as “Drivers overtaking bicyclists: Objective data on the effects of riding position, helmet use, vehicle type and apparent gender,” in the journal Accident Analysis and Prevention), Walker outfitted a bike with a device that measured the distance of passing cars. He found, among other things, that drivers tended to pass more closely when he was wearing a helmet than when not (he was struck by vehicles twice, both while wearing a helmet).
This was a surprising, somewhat controversial finding that generated a lot of news coverage. To my mind, Walker’s findings were more interesting for what they said about interpersonal psychology on the road than safety itself; mostly because I felt, and Walker seems to agree, that the primary question of bicycle safety had less to do with the helmet than other factors.
In a Florida DOT commissioned study [pdf] published last month, researchers reached a very similar conclusion. Although the study didn’t specifically address helmet usage, the researchers found that their data was consistent with Walker’s conclusions when it came to how closely drivers passed bicyclists based on the bicyclist’s gender and attire. The study found that on average, drivers passed cyclists more closely when cyclists were dressed in “bicycle attire” and if the cyclist was male. The study was unable to determine the reasons on this passing behavior and the authors of the study speculated that, “it [was] possible that motorists perceived less risk passing riders who were in [a] bicycle outfit.”
I suppose effective measures that can be made as a result of the Florida study would be to encourage cyclists to ride in casual clothing rather than bicycle-specific attire. As for the conclusion on passing distance based on the cyclist’s gender, perhaps it is for the best given that men typically outnumber women on the roads.