ONE spectacular Sunday in Paris last month, I decided to skip museums and shopping to partake of something even more captivating for an environment reporter: Vélib, arguably the most successful bike-sharing program in the world. In their short lives, Europe's bike-sharing systems have delivered myriad benefits, notably reducing traffic and its carbon emissions. A number of American cities - including New York, where a bike-sharing program is to open next year - want to replicate that success.
So I bought a day pass online for about $2, entered my login information at one of the hundreds of docking stations that are scattered every few blocks around the city and selected one of Vélib's nearly 20,000 stodgy gray bikes, with their basic gears, upright handlebars and practical baskets.
Then I did something extraordinary, something I've not done in a quarter-century of regular bike riding in the United States: I rode off without a helmet.
I rode all day at a modest clip, on both sides of the Seine, in the Latin Quarter, past the Louvre and along the Champs-Élysées, feeling exhilarated, not fearful. And I had tons of bareheaded bicycling company amid the Parisian traffic. One common denominator of successful bike programs around the world - from Paris to Barcelona to Guangzhou - is that almost no one wears a helmet, and there is no pressure to do so.
In the United States the notion that bike helmets promote health and safety by preventing head injuries is taken as pretty near God's truth. Un-helmeted cyclists are regarded as irresponsible, like people who smoke. Cities are aggressive in helmet promotion.
But many European health experts have taken a very different view: Yes, there are studies that show that if you fall off a bicycle at a certain speed and hit your head, a helmet can reduce your risk of serious head injury. But such falls off bikes are rare - exceedingly so in mature urban cycling systems.
On the other hand, many researchers say, if you force or pressure people to wear helmets, you discourage them from riding bicycles. That means more obesity, heart disease and diabetes. And - Catch-22 - a result is fewer ordinary cyclists on the road, which makes it harder to develop a safe bicycling network. The safest biking cities are places like Amsterdam and Copenhagen, where middle-aged commuters are mainstay riders and the fraction of adults in helmets is minuscule.
"Pushing helmets really kills cycling and bike-sharing in particular because it promotes a sense of danger that just isn't justified - in fact, cycling has many health benefits," says Piet de Jong, a professor in the department of applied finance and actuarial studies at Macquarie University in Sydney. He studied the issue with mathematical modeling, and concludes that the benefits may outweigh the risks by 20 to 1.
He adds: "Statistically, if we wear helmets for cycling, maybe we should wear helmets when we climb ladders or get into a bath, because there are lots more injuries during those activities." The European Cyclists' Federation says that bicyclists in its domain have the same risk of serious injury as pedestrians per mile traveled.
Yet the United States National Highway Traffic Safety Administration recommends that "all cyclists wear helmets, no matter where they ride," said Dr. Jeffrey Michael, an agency official.
Recent experience suggests that if a city wants bike-sharing to really take off, it may have to allow and accept helmet-free riding. A two-year-old bike-sharing program in Melbourne, Australia - where helmet use in mandatory - has only about 150 rides a day, despite the fact that Melbourne is flat, with broad roads and a temperate climate. On the other hand, helmet-lax Dublin - cold, cobbled and hilly - has more than 5,000 daily rides in its young bike-sharing scheme. Mexico City recently repealed a mandatory helmet law to get a bike-sharing scheme off the ground. But here in the United States, the politics are tricky.
SHAUN MURPHY, the bicycling coordinator of Minneapolis-St. Paul - which inaugurated its "Nice Ride" bike-sharing program this year - has been pilloried for riding about without a helmet. "I just want it to be seen as something that a normal person can do," Mr. Murphy explained to the local press this past summer. "You don't need special gear. You just get on a bike and you just go."
In New York, where there were 21 cyclist fatalities last year, the transportation commissioner, Janette Sadik-Khan, is always photographed on a bike and wearing a helmet. The administration of Mayor Michael R. Bloomberg has nonetheless rejected calls by Comptroller John C. Liu for a mandatory helmet law when New York's 10,000-cycle bike-share program rolls out next year, for fear it would keep people from riding. Still, the mayor says helmets are a "good idea," and the city promotes helmet use through education and with giveaway programs.
In the United States, cities are struggling to overcome the significant practical problems of melding helmet use with bike-sharing programs - such as providing sanitized helmet dispensers at bike docking stations, says Susan Shaheen, director of the Transportation Sustainability Research Center at the University of California, Berkeley.
But bicycling advocates say that the problem with pushing helmets isn't practicality but that helmets make a basically safe activity seem really dangerous.
"The real benefits of bike-sharing in terms of health, transport and emissions derive from getting ordinary people to use it," said Ceri Woolsgrove, safety officer at the European Cyclists' Federation. "And if you say this is wonderful, but you have to wear armor, they won't. These are normal human beings, not urban warriors."
In fact, many European researchers say the test of a mature bike-sharing program is when women outnumber men. In the Netherlands, 52 percent of riders are women. Instead of promoting helmet use, European cycling advocates say, cities should be setting up safer bike lanes to slow traffic or divert it entirely from downtown areas. "Riding in New York or Australia is like running with the bulls - it's all young males," says Julian Ferguson, a spokesman for the European Cyclists' Federation. And that's in part what makes it dangerous. (Many European countries do require helmet use for children.)
In London, where use of a new bike-share program is exceeding all expectations, the number of riders in suits and dresses is growing, Mr. Woolsgrove says. And more Londoners seem to be leaving helmets at home.
We may follow a similar pattern. In her study of nascent bike-sharing programs in North America - including Montreal, Washington and Minneapolis - Dr. Shaheen found that the accident rate was "really low." A large majority of participants strongly agreed that they got more exercise since the program started. And helmet use in bike programs tended to be far lower than among the general public.
Another study this summer found that only 30 percent of local riders using Washington's Capital Bikeshare program wore helmets, compared with 70 percent of people on their own bikes, said John Kraemer of Georgetown University, the study's author, who supports helmet use.
Before you hit the comment button and tell me that you know someone whose life was probably saved by a bike helmet, I know someone, too. I also know someone who believes his life was saved by getting a blood test for prostate specific antigen, detecting prostate cancer. But is that sense of salvation actually justified, for the individual or society? Back in New York I strapped on my helmet for a weekend bike ride in Central Park. But I'm not sure I'll do the same two years from now if I'm commuting to work on a mature Citi Bike system.
Mr. De Jong, who grew up in the Netherlands, observes of Amsterdam: "Nobody wears helmets, and bicycling is regarded as a completely normal, safe activity. You never hear that 'helmet saved my life' thing."
A reporter and blogger on environmental issues for The New York Times.
