Drivers get more than 3.1in (8cm) closer to cyclists wearing helmets than they do to bare-headed riders and female cyclists are given more room on the road than male riders, according to a survey from the University of Bath.
Dr Ian Walker, a traffic psychologist, used a bicycle fitted with an ultrasonic distance sensor to record data from more than 2,500 overtaking motorists in Salisbury and Bristol.
He said drivers were twice as likely to get close to his bicycle when he was wearing the helmet.
Walker also wore a long wig for the experiment, and found that motorists gave him more room. He inferred that motorists give women cyclists more room. But it might be that motorists are more courteous to cyclists in drag.