Do you cringe when a cyclist rides by on one of those urban bike-share bicycles without a helmet? I know I do. One mistake, one distracted motorist, and the worst could happen. As I write this, a young man is spending his 13th day in intensive care in a Washington, D.C., hospital after he was struck by a hit-and-run driver while riding a Capital Bikeshare bicycle on a busy street without a helmet.

The stats on bicycle helmets are abundantly clear: They save lives and prevent head injuries. Between 1994 and 2010, at least 70 percent of the cyclists killed in the U.S. each year weren't wearing helmets, and in many of those years, the proportion was more than 90 percent, according to the Insurance Institute for Highway Safety. Way back in 1989, a study in the New England Journal of Medicine concluded that helmets reduced a bike rider's risk of head injury by 85 percent and the chance of brain injury by 88 percent.

(Update: A reader has pointed out later criticism of the NEJM numbers by others who tried to replicate the study. Subsequent studies found that helmets do reduce head injuries substantially, but not by such large percentages. Read one here.)

"Anybody who’s on a bike should be wearing a helmet," said Kate Douglass, director of the Center for Injury Prevention and Control at the Ronald Reagan Institute of Emergency Medicine, which is part of the George Washington University School of Medicine.

So how can hundreds of thousands of people ride those terrific new public bikes without head protection? This question will be especially important Friday, when tens of thousands across the United States will saddle up for Bike to Work Day.

Like most public policy issues, the answer is complicated. For one thing, the people who run bike share programs don't even agree with one another. Seattle, which will roll out its bike share program in September, will be the first city to have helmets available alongside bikes for anyone who wants to rent one (price: $2). It has to; the law there requires all cyclists, regardless of age, to wear helmets. The Boston area bike share program, Hubway, is preparing to provide helmets in some places after three years and 1.8 million bike trips without them.

"One of the reasons the city, the mayor, promote cycling is because it's good for health," said Nicole Freedman, Boston's director of bicycle programs. "We want to be consistent around that message."

But in Washington and Minneapolis, like most other cities, there are no plans to add helmets. Bill Dossett, executive director of Nice Ride Minnesota, told me that riding the slow, heavy, well-lighted urban bikes "is a fundamentally safe thing to do."  The average rider is not some risk-taking teen-ager, but a cautious adult traveling a few miles to work, or a tourist taking in the city's lakes.

He has a point. In four years, there have been no reports of a major injury to a Nice Ride Minnesota cyclist, Dossett said, and no head injuries at all. Much the same is true for Washington's Capital Bikeshare program, which has had fewer than 100 reported crashes since 2010, despite 6.8 million bike trips, said Kim Lucas, the program manager.

Research backs them up. A 2o11 study of the Barcelona bike sharing program that weighed the increased risk of death from trauma or exposure to air pollution against the benefits of exercise and reduced carbon dioxide output found that the positives outweighed the negatives by 77 to 1. In many European cities, people routinely ride in urban traffic without helmets.

The biggest obstacles to providing helmets are hygiene, (imagine paying to don a sweaty helmet that some stranger has just taken off -- bleah) cost and liability. "It’s expensive , it’s challenging to do in terms of ... the technology and there’s some liability" risk for the bike share program if a cracked helmet fails to prevent a head injury to the next wearer, Lucas said. Instead, most bike share programs in the U.S. encourage riders to use their own helmets, partnering with sponsors to offer big discounts or even give them away. But that means carrying the bulky headgear around, at least until better technology is developed.

But Seattle and Boston have found ways around all those problems. Each is installing helmet vending machines that work just like the bike technology itself. With the swipe of a credit card, a helmet will be available on demand along with the bikes. Riders return the helmets to collection bins, where they are picked up each day, taken to a warehouse, cleaned and inspected before they can be used again. The technology was invented by Massachusetts Institute of Technology students as a class project and later turned into a company.

"You maximize the likelihood that someone will put on a helmet if you make it hyper-convenient and very inexpensive," said Freedman, the Boston bike program director. Holly Houser, executive director of Pronto Cycle Share in Seattle, said the helmet vending machines added about $850,000 to the $4.4 million that will be spent on bicycles, docking stations and other equipment.

"It would be much easier if we did not have the mandatory helmet law," she said. "We all know that."

But "rather than fight it," she said, "we decided to focus our energy on developing the best solution."