Drivers pass closer when overtaking cyclists wearing helmets than when overtaking bare-headed cyclists, increasing the risk of a collision, the research has found.
Dr Ian Walker, a traffic psychologist from the University of Bath in the UK, used a bicycle fitted with a computer and an ultrasonic distance sensor to record data from over 2,500 overtaking motorists in Salisbury and Bristol.
Dr Walker, who was struck by a bus and a truck in the course of the experiment, spent half the time wearing a cycle helmet and half the time bare-headed. He was wearing the helmet both times he was struck.
He found that drivers were as much as twice as likely to get particularly close to the bicycle when he was wearing the helmet.
Across the board, drivers passed an average of 8.5 cm (3 1/3 inches) closer with the helmet than without
The research has been accepted for publication in the journal Accident Analysis & Prevention.
"This study shows that when drivers overtake a cyclist, the margin for error they leave is affected by the cyclist's appearance," said Dr Walker, from the University's Department of Psychology.
"By leaving the cyclist less room, drivers reduce the safety margin that cyclists need to deal with obstacles in the road, such as drain covers and potholes, as well as the margin for error in their own judgements.
"We know helmets are useful in low-speed falls, and so definitely good for children, but whether they offer any real protection to somebody struck by a car is very controversial.
"Either way, this study suggests wearing a helmet might make a collision more likely in the first place."
Dr Walker suggests the reason drivers give less room to cyclists wearing helmets is down to how cyclists are perceived as a group.
"We know from research that many drivers see cyclists as a separate subculture, to which they don't belong," said Dr Walker.
"As a result they hold stereotyped ideas about cyclists, often judging all riders by the yardstick of the lycra-clad street-warrior.
"This may lead drivers to believe cyclists with helmets are more serious, experienced and predictable than those without.
"The idea that helmeted cyclists are more experienced and less likely to do something unexpected would explain why drivers leave less space when passing.
"In reality, there is no real reason to believe someone with a helmet is any more experienced than someone without.
"The best answer is for different types of road user to understand each other better.
"Most adult cyclists know what it is like to drive a car, but relatively few motorists ride bicycles in traffic, and so don't know the issues cyclists face.