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.
"There should definitely be more information on the needs of other road users when people learn to drive, and practical experience would be even better.
"When people try cycling, they nearly always say it changes the way they treat other road users when they get back in their cars."
The study also found that large vehicles, such as buses and trucks, passed considerably closer when overtaking cyclists than cars.
The average car passed 1.33 metres (4.4 feet) away from the bicycle, whereas the average truck got 19 centimetres (7.5 inches) closer and the average bus 23 centimetres (9 inches) closer.
However, there was no evidence of 4x4s (SUVs) getting any closer than ordinary cars.
Previously reported research from the project showed that drivers of white vans overtake cyclists an average 10 centimetres (4 inches) closer than car drivers.
To test another theory, Dr Walker donned a long wig to see whether there was any difference in passing distance when drivers thought they were overtaking what appeared to be a female cyclist.
Whilst wearing the wig, drivers gave him an average of 14 centimetres (5.5 inches) more space when passing.
In future research, Dr Walker hopes to discover whether this was because female riders are seen as less predictable than male riders, or because women are not seen riding bicycles as often as men on the UK's roads.
Notes for editors
11,257 adult cyclists were injured and 109 killed on the UK's roads in 2004, the latest year for which figures are available. However, for each bicycle accident officially recorded there are as many as 14 more which do not go on police records, and so the number injured is certainly an under-estimate. Being struck by an overtaking car is arguably the most dangerous form of collision for a cyclist, with a particularly high mortality rate.
This work was funded by the Engineering and Physical Sciences Research Council. A bicycle was modified for the project by Jeff Brewster from the University of Bath's Department of Mechanical Engineering. It carried an ultrasonic distance sensor which could measure how close vehicles passed to within a millimetre and a video system so that vehicle type could be classified.
This research has been accepted for publication in Accident Analysis & Prevention, the world's top-rated peer-reviewed ergonomics journal.
The University of Bath is one of the UK's leading universities, with an international reputation for quality research and teaching. In 19 subject areas the University of Bath is rated in the top ten in the country. View a full list of the University's press releases: <a href="http://www.bath.ac.uk/news/releases/">http://www.bath.ac.uk/news/releases/</a>