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.
Statistics are fun.