… it’s not clear that the lack of a helmet should completely deter somebody from biking. “We want to increase people’s ability to get exercise and do things that are environmentally sound,” Fischer [an emergency room doctor at Beth Israel Deaconess Medical Center in Boston] says.
That dilemma is one reason mandatory-helmet laws are so rare. No state requires adults to wear them, and only 21 states require them for younger riders. In Washington, D.C., they’re required for anyone under 16. The argument goes that requiring a helmet doesn’t increase helmet usage so much as decrease bike riding. And studies have shown that the more bikers a city has, the safer biking in that city becomes.