Over half of all head injuries occur in motor vehicles and more people were hospitalized after walking down the street than riding on a bicycle. Consider another statistic: According to a 2006 French study, pedestrians are 1.4 times more likely to receive a traumatic brain injury than unhelmeted cyclists. We can also approach it from the perspective of injuries per million hours from a 1996 Australian study looking at head injury risk before the beginning of any helmet laws:

Risk of head injury per million hours travelled

Cyclist - 0.41

Pedestrian - 0.80

Motor vehicle occupant - 0.46

Motorcyclist - 7.66

In each of these three examples we see that cyclists are not the group at highest risk for serious head injury.

Let's be clear. I am NOT trying to say that studies definitively show that cycling is safer than driving or walking. The studies that are out there give us mixed messages about the relative safety of the different modes of transport. What I am saying is that these statistics raise an interesting question: If we're so concerned about head injuries, why aren't we wearing helmets all the time? Why do places that have mandatory helmet laws for cyclists not have them for drivers or pedestrians? The same 1996 Australian study suggests that a mandatory helmet law for motor vehicle occupants could save seventeen times more people from death and serious head injury than a similar law for cyclists.

Yet, despite the clear threat of fatal head trauma from these other activities, virtually nobody insists that people wear helmets in these situations. In fact, doing so is openly mocked. Consider a sentence from this recent article from Forbes magazine that reports that vehicle accidents are the number one cause of fatal head injuries among teenagers :

Short of suggesting all teen drivers and their passengers wear helmets, the survey determined that states which maintain the strictest graduated driver licensing laws (GDL) are the most effective in reducing both brain injuries and fatalities among young motorists.

Did you catch that? Despite the fact that car accidents are the number one cause of all fatal head trauma among teenagers, the suggestion that teens wear helmets when they drive is simply brushed off. The passage treats the idea of mandatory driving helmets as completely preposterous. Yet we insist that children wear bike helmets (in fact, in some places, it's the law) despite data that shows kids are more likely to die of head injuries riding in a car than riding on a bike. Children and toddlers on foot are far more likely to receive traumatic brain injuries than cyclists, yet parents who place protective headwear on their walking toddlers are openly ridiculed.

In other words, if the reason we are supposed to wear helmets while biking is to prevent serious head injury on the off-chance we get into an accident, then why is it socially acceptable for pedestrians and drivers to go about bare-headed? Why has cycling been singled out as an activity in need of head protection?

There's an important caveat to the results of that 1989 New England medical study: It shows that bike helmets may reduce the risk of head and brain injury by 85-88%—but only for those who get into accidents.

If we take a closer look at the article we see that both the experiment and the control groups studied are those who have already been hospitalized for bike injuries. If one were to examine the medical and epidemiological literature on bike helmet effectiveness, you'll find the exact same condition over and over: Studies show that helmeted cyclists who are hospitalized are far less likely to have serious head trauma than bare-headed cyclists that have been hospitalized.

But wouldn't this be true, regardless of the activity? Logically, helmeted drivers should also receive significantly fewer head injuries than bare-headed drivers. Similarly, helmeted pedestrians should be less likely to receive serious head trauma than bare-headed ones.

But studies that compare head injuries for drivers and pedestrians simply don't exist as there aren't enough helmeted drivers or pedestrians to make a comparison. Science, after all, can only be accomplished on observable phenomena. If no one wears a helmet when they walk down the street, how can we measure the effectiveness of helmets on pedestrians? In other words, one of the reasons we think helmeted cyclists are safer than unhelmeted ones may be due to availability of information more than actual levels of head safety.

Maybe that explains why there's no comparable fear of driving or walking without a helmet.

How bike helmets may be harmful

But say you are someone who is concerned enough about head injury to wear a helmet while you're driving or while walking down the street. Is there an argument that says that wearing a helmet actually increases risk of injury?

Turns out that there is. There is some evidence that wearing a helmet may directly increase your chance of getting injured in the first place. In 2001, an article in the New York Times reported that the rate of bicycle head injuries had risen sharply — an increase of 51% — during a ten-year period when bicycle helmet use became widespread. This during a time when statistics showed an overall decrease in bicycling in the United States. No one knows for sure why head injuries among cyclists increased, but there are a few theories.

First, wearing a helmet changes how drivers perceive the cyclist. A University of Bath study showed that drivers, when overtaking cyclists, gave helmeted cyclists significantly less space than they gave cyclists who don't wear head protection. The study found that drivers were twice as likely to pass closely to a helmeted cyclist, and that drivers passed an average of 8.5 cm (3 1/3 inches) closer when the researcher was helmeted than when he was not. Not only does this increase the chance of being clipped by a vehicle, it leaves cyclists with far less maneuvering room to avoid other potentially injurious road hazards like potholes and icy patches.

Second, the design of the helmets themselves may increase the chance of some types of injuries when incidents do occur. Three separate studies have shown that bike helmets may increase the probability of certain types of neck injuries. There's some evidence that having an enlarged piece of plastic and foam on your head increases the probability of hitting an object that you'd be able to avoid in the first place, or that otherwise glancing contact with a surface becomes a full-on blow when the head is helmeted.

Finally, wearing a helmet may create a false sense of security and induce risk-taking that cyclists without head protection might not make. Those wearing helmets may take risks that they wouldn't otherwise take without head protection.

There are even some startling statistics that show helmets may have little to negative effects on the incidence of head injuries outside of the cycling world as well. A recent study from the National Ski Areas Association found that, despite a tripling of helmet use among skiers and snowboarders in the United States since 2003, there has been no reduction in the number of snow-sport related fatalities or brain injuries. On the contrary, and 2012 study at the Western Michigan University School of Medicine found an increase in head injuries between 2004 and 2010 despite an increase in helmet use, while a 2013 University of Washington study concluded that snow-sports related head injuries among youths and adolescents increased 250 percent from 1996-2010, a timeframe that also coincides with the increased use of head protection.