Motorcycle Helmet Laws Save Lives, but Not Very Many

A new study reported in the American Journal of Public Health finds that the repeal of motorcycle helmet requirements for adults has been associated with an increase in motorcycle fatalities, beyond the general increase that has been seen throughout the country in the last decade or so. Although the researchers argue that their study reinforces the case for universal helmet laws, the impact they found looks pretty modest:

On average, when compared to state experience with no helmet mandate, universal helmet laws were associated with an 11.1% reduction in motorcyclist fatality rates, whereas rates in states with partial coverage statutes [applying only to riders younger than 21] were not statistically different from those with no helmet law. Furthermore, in the states in which recent repeals of universal coverage have been instituted, the motorcyclist fatality rate increased by an average of 12.2% over what would have been expected had universal coverage been maintained. Since 1997, an additional 615 motorcyclist fatalities have occurred in these states as a result of these changes in motorcycle helmet laws.

In terms of fatalities prevented each year, the effect estimated by this study is not very impressive. In 2004, for example, "an estimated 135 (or 5.8%) fewer fatalities would have occurred" in the 31 states without universal helmet laws had those states forced adult motorcyclists to wear head protection. That's just a handful of fatalities per state each year.

Previous research has indicated that helmet laws do substantially increase the percentage of motorcyclists who wear helmets. The fatality numbers probably are so small for two main reasons. First, riding a motorcycle, while much more dangerous than driving a car, is also much less common, so there are only 4,300 or so total motorcycle fatalities each year, one-tenth of all road fatalities. Second, helmets are only partly effective at preventing deaths: The National Highway Traffic Safety Administration estimates that in the event of a crash they cut the chances of a fatality by about 35 percent (compared to a 60 percent reduction for motorists who wear seat belts).

Even the undramatic results of this study may overestimate the impact of helmet laws. To the researchers' credit, instead of doing a simple before-and-after comparison in a single state or a few states, they looked at accident data from all 50 states and D.C. for 1975 through 2004. They took into account the general upward trend in motorcycle deaths since 1996 and several potential confounding variables, including other traffic-related laws, weather patterns, alcohol consumption, population density, and the age breakdown of each state's population. But since the outcome measure they used was fatalities per 10,000 registered motorcycles, they did not take into account miles traveled. That could matter if motorcyclists who hate helmets start riding more often or longer distances once they are no longer required to wear them. In that case, some of the increase in deaths could be due to an increase in miles traveled. Another possible factor: If people who stopped riding motorcycles because they were irked by a helmet requirement suddenly start riding them again once the requirement is repealed, the percentage of motorcyclists who are out of shape and out of practice might increase, which could independently raise the frequency of crashes.

Some anti-helmet-law activists argue that helmets, on balance, decrease motorcycle safety by making riders more reckless, making their heads heavier, or impairing their hearing, peripheral vision, and sensitivity to air pressure changes. There isn't much evidence to support that claim, and I have little doubt that helmet laws reduce fatalities to some extent. It just does not seem to be a very big effect, which is one reason opponents of these laws have been so successful at rolling them back and preventing legislators from reimposing them. In principle, the fatality numbers shouldn't matter: The right to ride without a helmet should not hinge on exactly how big the risk is. But practical politics is rarely about principle, a point illustrated by the contrast between helmet and seat belt laws that I drew in reason a few years ago.