When I was a high school senior, I ended up in the hospital after the front wheel of my bicycle fell off when I caught some air on a curb. I landed smack on my face, breaking my jaw, chin and nose. I never recovered any memory of the crash or the 30 minutes or so immediately afterward. I was not wearing a helmet.

My accident happened more than 20 years ago in a state that has since passed a law requiring bike helmets for riders age 17 and younger. I was 16 at the time of my crash, so it’s not a stretch to think that the law might have helped me, had it been in effect back then.

Mandatory helmet laws, like one brought up in the Maryland legislature this winter, might seem like a no-brainer. Yet when the medical journal BMJ polled its readers in 2011, 68 percent of the respondents opposed mandatory helmet laws. The Washington Area Bicyclist Association (WABA) also opposes mandatory helmet laws, and its members testified against Maryland House Bill 339, which never made it out of committee before the legislative session ended in April.

Proponents of helmet laws say that they reduce injuries. But evidence for this claim remains mixed. Two studies published last month came to opposing conclusions. The first, published in BMJ, compared rates of cycling-related head injuries in six Canadian provinces before and after they passed helmet legislation. Researchers analyzed data from 1994 to 2008 and, after accounting for baseline trends, concluded that “the overall rates of head injuries were not appreciably altered by helmet legislation.”

The second study, published in the Journal of Pediatrics, analyzed statistics on U.S. bicyclists who were severely injured or killed between January 1999 and December 2009. The authors compared the injury and death rates among cyclists age 16 and younger in states with mandatory helmet laws for youngsters to rates in states without such laws. They concluded that injury rates were about 20 percent lower in states with helmet laws. No state has passed a universal helmet law, but the District, Maryland and 20 other states do require young riders, generally under 16, to wear one.

How could the two studies come to opposing conclusions? Because it’s inherently difficult to measure whether differences in injury rates are due to helmet legislation or other factors, says Jessica Dennis, an epidemiologist at the University of Toronto and lead author of the BMJ study.

Helmets don’t prevent the things that cause a crash, but they do offer a last line of defense when things go wrong.

“No one would argue that helmets don’t decrease the risk of injury,” says William P. Meehan III, director of the Micheli Center for Sports Injury Prevention and the Sports Concussion Clinic at Boston Children’s Hospital, and author of the Journal of Pediatrics study.

Even most opponents of mandatory helmet laws advocate helmet use, but they say that the negative consequences of helmet laws outweigh the benefits. One of the first things that mandatory helmet laws do is decrease ridership, says WABA board member Jim Titus. “If you’re not allowed to ride a bike without a helmet, often that means you won’t ride a bike.”

Titus says that mandatory helmet laws make it more difficult for people to use public bicycles through programs such as Capital Bikeshare because potential riders would need a helmet handy every time they want to jump on a bike.

Opponents also worry that mandatory helmet laws send a false message — that cycling is an inherently dangerous activity — when, in fact, it’s an activity with well-known health benefits. A study, of 1,504 people, found that those most likely to stop bicycling in response to helmet laws are the ones least likely to be hurt in the first place. Helmet laws, the authors conclude, “disproportionately discourage the safest cyclists.”

Meanwhile, it seemsthat bicyclists wearing helmets may encourage riskier driving by motorists. Traffic psychologist Ian Walker from the University of Bath equipped a bike with a sensor to record the distance between him and passing vehicles. He took more than 2,300 measurements and found that motorists passed him more closely when he wore a helmet. (He was also struck twice, by a bus and a truck, during the study — both times while wearing a helmet.)

The ability of bike helmets to reduce injury has been overblown, Titus says. Helmets are sometimes said to reduce the risk of head injury by 85 percent, but that statistic comes from a 1989 study that has not been replicated. “Studies in the last 20 years have calculated that helmets prevent 10 to 40 percent of head injuries,” says Titus, who compiled the statistics in his testimony against Maryland’s helmet law. Overstating the effectiveness of helmets sends a message that current helmets are good enough, Titus says.

His arguments apparently convinced the Centers for Disease Control and Prevention and the National Highway Traffic Safety Administration. Last month, both agencies removed the 85 percent claim from the information they disseminate to the public.

“We should be talking about other strategies to reduce cycling injuries” beyond helmets, Dennis says. Her study did show that injury rates dropped after helmet laws were implemented, but the effect disappeared when she took into account the rate at which injury rates had been falling before the laws took effect. This suggests that reductions were probably due to other measures, such as traffic calming, designated bike lanes and safety campaigns.

These days, I almost always wear a helmet when I ride, but I recognize that it’s my last line of defense and can provide only limited protection. Not long after my crash, a friend of mine experienced a nearly identical wreck. He was wearing a helmet, yet his injuries were similar to mine. Our wheels flew off because the quick-release mechanism for the front wheel had come undone. A few years after my wreck, bike manufacturers began employing a safety apparatus to prevent this kind of unintended release. A helmet may have lessened my impact, but to prevent the crash in the first place, I needed a safety strategy that looked beyond the helmet.