Cyclists who wear safety helmets are more likely to be hit by overtaking vehicles, according to a study from the University of Bath which suggests that drivers give bare-headed riders a wider berth because they assume that they are less experienced on the road.

The research found that motorists get more than 8cm (3.1in) closer to cyclists wearing helmets than to other riders. Surprisingly, female cyclists are given more room on the road than their male counterparts.

In a hands-on survey, Dr Ian Walker, a psychologist who specialises in road safety, used a bicycle fitted with an ultrasonic distance sensor to record data from more than 2,500 motorists who overtook him in Salisbury and Bristol.

He covered a total of 320 km (200 miles) during the study and was hit twice, once by a lorry and once by a bus. Both times he was wearing a helmet, but says: "As a scientist, I don't think you can read too much into two incidents."

Dr Walker said that drivers were twice as likely to get close to his bicycle when he was wearing the helmet. "This was something I had suspected, as many cyclists had told me of similar experiences. The perception is that those wearing helmets are experienced and more predictable," he said.

"Drivers think, ‘He knows what he’s doing, he won’t do anything surprising’. But that’s really quite a dangerous thought, particularly as so many cycling novices are told to wear helmets."

Dr Walker, whose research is to be published in the journal Accident Analysis & Prevention, said that buses and trucks were the worst offenders. While the average car gave cyclists 1.33m of room, the average truck got 19cm closer and the average bus 23cm closer.

There was no evidence of sports utility vehicles or 4x4s getting any closer to bikes than ordinary cars did, although an earlier finding from the study was that drivers of white vans give cyclists 10cm (4in) less space to cyclists than car-drivers.

To test another theory, Dr Walker wore a long wig to see if there was any difference in passing distance when vehicles thought they were overtaking a female cyclist.

Vehicles gave him an average of 14cm more space when he was wearing the wig. Dr Walker said that this may be because women are seen as less predictable than men on the roads, or because female cyclists are more rare and so are treated with more caution.

Dr Walker said that he hoped his research would raise awareness of the dangers facing cyclists on busy roads. More than 11,000 cyclists were injured and 109 killed on UK roads in 2004, the latest year for which figures are available.

"The fundamental message is that drivers need to be more understanding of the people that they share the roads with and cannot make judgments based on their appearance," he said.

But the message to cyclists is less clear. Dr Walker, himself a keen cyclist who rode from John O'Groats to Land's End earlier this year, without mishap, is unconvinced as to how much protection helmets give cyclists and says previous studies on it are mixed.

He is more confident of the benefits of cycling, the risks of which are far outweighed by the health benefits, and says that those countries that have made helmets compulsory have seen the number of people who use bicycles fall.

"The message to cyclists is a really difficult one," he told Times Online. "The evidence on (the usefulness of) helmets in a collision is very mixed. Evidence that it might make a collision more likely will be added to mix and people will have to make their mind up."

Dr Walker says that he "goes through phases" of wearing or not wearing helmets but if he had children would insist that they wear them.

"My personal belief is that a bit of polystyrene is not going to help much when I'm hit by a car, but when children come off it's usually because they all over, low-speed impacts, and they definitely help then."

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