The Peltzman Effect is the hypothesized tendency of people to react to a safety regulation by increasing other risky behavior, offsetting some or all of the benefit of the regulation.
I had always thought of this as driven by the behavior of the people 'protected' -- they take extra risks. Turns out in the case of bicyclists, drivers nearby may take more risks when seeing a cyclist in a helmet.
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.
Update: I just noticed that the researcher's title is traffic psychologist. I wonder: does he find psychoanalysis or pharmacotherapy better at controlling traffic? Can we get 'failure to signal' into DSM-IV? When I dream I'm in a flying car, what does that mean?