For years, cyclists who ride on city streets have cherished an unusual superstition: if they wear a helmet, they are more likely to get hit by a car. “I belong to an e-mail list for cyclists, and they complain about this all the time,” says Ian Walker, a psychologist at the University of Bath who rides his bike to work every day. But could this actually be true?

Walker decided to find out — putting his own neck on the line. He rigged his bicycle with an ultrasonic sensor that could detect how close each car was that passed him. Then he hit the roads, alternately riding with a helmet and without for two months, until he had been passed by 2,500 cars. Examining the data, he found that when he wore his helmet, motorists passed by 8.5 centimeters (3.35 inches) closer than when his head was bare. He had increased his risk of an accident by donning safety gear.

Why? You might suspect that cyclists wearing helmets are more prone to take risks. But studies have found otherwise. The real answer, Walker theorizes, is that helmets change the behavior of drivers. Motorists regard a helmet as a signal that the cyclist is experienced and thus can be approached with less caution. “They see the helmet and think, Oh, there’s a serious, skilful person,” Walker says. “And you get hit.”

Walker will publish his findings next year in the journal Accident Analysis & Prevention. After a preprint of the article was circulated to colleagues, he was flooded with angry e-mail. Wasn’t he giving cyclists a green light to doff their headgear? Walker takes a more diplomatic approach. The mere fact that drivers are responding to a helmet means that they are making judgments, however unconscious, about cyclists. That means a government advertising campaign could retrain drivers to make the opposite judgment: a helmeted rider deserves just as much space.