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If you've ever shaken your head over urban bicyclists' apparent unanimous decision to forgo helmets, you're not far off the mark.

Among users of bike-sharing programs, like Capital Bikeshare in Washington, D.C., the problem is obvious.

Flag down a few helmetless riders on those distinctive firetruck-red cruisers, as we did this week, and you're likely to encounter two kinds of people: tourists, who rented the bikes on a whim and didn't have a helmet with them; or commuters, who'll usually tell you they almost always wear a helmet, but just happen to have forgotten it today.

Of course there's a handful of people who wear helmets religiously. But a study in Boston and Washington, published this week in the Annals of Emergency Medicine, found that helmet wearers account for barely half of all bicyclists and only a tiny minority of riders of shared bikes. In both cities, 4 in 5 of those riders went helmetless.

Study author Christopher Fischer, an emergency room doctor at Beth Israel Deaconess Medical Center in Boston, tells Shots the research is just one piece of the puzzle policymakers need to solve around bike safety. It's a question of balancing risks, such as people riding without helmets, with the rewards from people using a more healthful form of transportation.

"There's been a lot of back and forth about this," Fischer tells Shots. "Wearing a helmet is a good idea because if I'm going to fall off my bike and strike my head, I'd rather have a helmet than not." But 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 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.

Josh Moskowitz, of the District of Columbia's Transportation Department, is a program manager for Capital Bikeshare. He says Bikeshare has a stellar safety record, with only about 20 reported crashes in the program's history. "Which is extraordinary, considering we've seen about 1.9 million trips," resulting in about one-fifth the accident rate in the overall biking population, he says.

Moskowitz says helmet use is just one aspect of bike safety, which also includes infrastructure improvements, such as bike lanes and trails, and awareness campaigns that educate drivers, pedestrians and bikers about safety. The bikes themselves are relatively slow, have a low center of gravity, and have lights that blink automatically. All those features make them safer than some other bikes.

Captial Bikeshare is expanding efforts to make helmets more available and more affordable. New and renewing users can now buy a Bikeshare-branded helmet for $16 through the program's website, and some Washington hotels now stock loaner helmets for guests.

But if there's one thing we learned from talking to Bikeshare riders around town, it's that cyclists can be — pardon the pun — hardheaded about their helmets. Take software developer Dave Craine, who commutes by Metro and Bikeshare a couple of days a week, riding his own bike other days.

He says he took a pretty hard fall near the Washington Monument a few weeks ago, fortunately with his helmet on. "I definitely was glad I had it then," he says. "I ride a little slower now."

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