Typically in transportation — and most social arenas, for that matter — laws promoting safety precautions lead to an increase in public health. Legislation on speed limits, drunk driving, and seatbelt are a few of the most obvious examples. Even bans on the relatively new phenomenon of driver-texting seem to be doing the trick, according to early evidence.

arena：舞台，竞技场　　　　precaution：预防，警惕　　　　Legislation：立法，法律　　　　

With bike helmet laws, however, the connection isn't quite so clear.

helmet：头盔，安全帽

Take a recent study published earlier this month in BMJ [ PDF]. The Canadian research team, led by Jessica Dennis of the University of Toronto, analyzed the rate of cycling-related hospital admissions for head injuries across the country between 1994 and 2008 — an enormous research sample of more than 66, 000 people. The size and length of the study allowed Dennis and collaborators to track the injuries against the emergence of bike helmet laws in various provinces.

collaborators：合作者

What they found initially seemed to suggest that this legislation improved public safety. In provinces with helmet laws, the rate of head injuries among young people decreased 54 percent and the rate among adults decreased 26 percent. At the same time, in provinces without the laws, the rate among youth riders dropped only 33 percent and among adults remained constant. (It bears mention that the study was the first to examine the effects of helmet laws on adults.)

But upon closer inspection, according to Dennis and company, this positive effect failed to stand. On the contrary, the researchers concluded that head injuries were decreasing across the country at a rate that wasn't "appreciably altered" by the new helmet laws. Other rider health initiatives — namely, public safety campaigns and the introduction of better bike infrastructure — rendered the contribution of helmet laws "minimal":

appreciably：明显地，相当地　　　　infrastructure：基础设施，公共建设

Reductions were greatest in provinces with helmet legislation. Rates of admissions for head injuries, however, were decreasing before the implementation of provincial helmet legislation and did not seem to change in response to legislation.

The study is just the latest to highlight the paradox of bike helmet laws. At the foundation of the puzzle is the fact that wearing a helmet, without question, reduces a rider's risk of injury. ( Recent work estimates an 88 percent reduction in brain injuries and an 85 percent decrease in head injuries.) Add in the fact that legislation does increase helmet usage among riders, and it would seem to follow that these laws should produce a net public safety benefit.

paradox：悖论，自相矛盾的人或物　　　　　　　

But what's clearly good for the individual rider appears oddly neutral (or worse) for riders at large. Public education and infrastructure upgrades, as the aforementioned works shows, protect riders considerably even before helmets come into play. Both efforts increase the overall amount of cycling, which provides safety in numbers. Mandatory helmet laws, meanwhile, may discourage riding to the point where public safety as a whole suffers from the relative decrease in physical exercise.

neutral：中立的　　　　

The bike safety researcher Piet de Jong framed the situation nice

ly in a paper published last spring [ PDF]. In places where riding is already safe, helmet laws are likely to have a "large unintended negative health impact" for some of the reasons stated above. In places where it's unsafe, the laws may make riding a little safer, but are also likely to distract attention from initiatives, such as infrastructure upgrades, that would be even more effective.

"Resolution of the issue for any particular jurisdiction requires detailed information on the bicycling environment, " De Jong writes.

jurisdiction：司法权，审判权　　　　

Of course, none of this should be seen as an argument against wearing a bike helmet. Quite the opposite, really, if you're a lone reader thinking of taking a ride today. What the helmet paradox does suggest, however, is that there's a progression toward rider safety that each city must pursue according to its own situation. And that if helmet laws do have a place along this path, it's toward the end rather than the beginning.

progression：前进，连续