Here's something strange: turns out, bicycle helmets are car magnets!
Last September a plucky psychologist at the University of Bath in England announced the results of a study in which he played both researcher and guinea pig. An avid cyclist, Ian Walker had heard several complaints from fellow riders that wearing a helmet seemed to result in bike riders receiving far less room to maneuver—effectively increasing the chances of an accident. So, Walker attached ultrasonic sensors to his bike and rode around Bath, allowing 2,300 vehicles to overtake him while he was either helmeted or naked-headed. In the process, he was actually contacted by a truck and a bus, both while helmeted—though, miraculously, he did not fall off his bike either time.
His findings, published in the March 2007 issue of Accident Analysis & Prevention, state that when Walker wore a helmet drivers typically drove an average of 3.35 inches closer to his bike than when his noggin wasn't covered. But, if he wore a wig of long, brown locks—appearing to be a woman from behind—he was granted 2.2 inches more room to ride.
I guess being a mountain biker (ride on the road with cars? no thanks) I think; there are no cars, yet you still wear a helmet. Why? Because it doesn't matter WHY you crash, it's the fact that when you do crash (and you will someday) the helmet keeps your brain on the inside. You don't have to do a study to know that helmets don't prevent crashes, but they do prevent more serious injuries.
Andy is kinda right , 2300 vehicles may not be enough to draw such conclusions, depending on the accuracy of the sensors and the locales he rode in. It would be more convincing with more people, in the test and more 'samples' to base a result on.