Pop quiz. Do you wear a helmet when you ride? Spandex or normal clothes? Are you female or male?

Though they may seem unrelated, your answers to those questions affect how much deference motorists give you when you set off down the street on a bicycle. That’s according to a number of studies outlined by Sam Ollinger on Network blog Bike San Diego.

A classic post from Traffic author Tom Vanderbilt on How We Drive, detailing the findings from a UK study on helmet use and motorist behavior, serves as the starting point:

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), [Ian] 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).

New research has identified similar effects. Ollinger writes:

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.”

The gender factor, at least, appears to be noticeable to the general public. In fact, it has a name: the Mary Poppins Effect.

All of which raises the question, what’s a cyclist to do?

Ollinger says: “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 helmet usage, a cyclist is still probably safer with a protective shield over his or her skull, but it does seem to offer support for those who choose to go helmetless.

Elsewhere on the Network: Cyclelicio.us refutes the argument that buses will increase traffic in Santa Cruz. Stop and Move puzzles over the familiar and outrageous remark that parking is hard to find in downtown Fresno, or really any city. And Grid Chicago announces that CDOT is beginning a pedestrian safety campaign.