From John Stossel at Real Clear Politics:
Ian Walker, a human-behavior researcher at the University of Bath in England, put a sensor and camera on his bike and rode for miles with and without a helmet. His data showed that when he wore the helmet, 23 percent more cars came within three feet of him.
"[The drivers are] saying, 'He knows what he's doing.' When they see a cyclist who has all the gear, they think it's a sign of someone who's experienced and skillful," Walker surmises.
...And here's another unintended consequence: Now that I wear a helmet, I feel safer, so I ride in traffic more often. Economists call this the Peltzman effect -- people adjust their behavior in ways that counteract the intended safety effect.
It's possible that I'd be safer if I junked the helmet and bought a woman's wig instead. Walker discovered that drivers gave him the most room when he wore a wig and they apparently thought he was a woman.
HT to DOL