But in the world's most popular biking cities, particularly in Europe, very few bikers wear helmets. And there are good reasons for that: biking, it turns out, isn't an especially dangerous form of transportation in terms of head trauma. And the benefits of helmets may be overstated. While they do protect your head during accidents, there's some evidence that helmets make it more likely you'll get in an accident in the first place.
I bike most days, and feel conspicuous when most of the other bikers I see are wearing helmets. This morning I counted 5 riders without helmets out of 58 total (not counting myself and an unhelmeted woman walking her bike).
Anecdotes are compelling, but don't give a clear idea of the big picture. How great is the risk? Should we consider the number of injuries per trip, per mile, or per hour? What do we compare to?
The studies are clear in a certain narrow way, with conclusions like "Wearing a helmet dramatically reduces the risk of head and facial injuries for bicyclists involved in a crash, even if it involves a motor vehicle." But the same could probably be said of pedestrians and people in cars. Why don't we wear helmets all the time?
See also mk's link to a paper by two authors who "dread questions about bicycle helmets."