The Helmet "Facts"

For some reason I've seen these facts out there bouncing around lately, in both the Washington Post and the NY Times

Bicycle helmets have been shown to reduce the risk of head injuries by up to 88 percent and facial injuries by 65 percent, according to a Cochrane Database Systemic Review published in 2000. Bike riders who play against those odds do not fare well in accidents. More than 90 percent of the 714 bicyclists killed in 2008 were not wearing helmets, according to the Insurance Institute for Highway Safety.

There was also a press release

by the American College of Emergency Physicians based on an NHSTA flyer that sort of made the same claims, but to an even greater degree.

A properly fitted helmet can prevent brain injury 90 percent of the time, according to the NHTSA

Bicycle helmets are 85- to 88-percent effective in mitigating head and brain injuries, making the use of helmets the single most effective way to reduce head injuries and fatalities resulting from bicycle crashes.

Which all seems pretty convincing. If only that were accurate. Before we go farther, you can read about my position on bike helmets here (Long story short: I wear one, I think you're probably better off wearing one, but their is scant evidence to support it). I have no idea where the NHTSA got that first fact, but it doesn't even seem to agree with their other statement and appears to be a misstatement of claim 2.

Claim 1: Bicycle helmets have been shown to reduce the risk of head injuries by up to 88 percent

This is based on a 1989 study (repeated in the early 90's) of hospital admissions. The 88% figure comes from the first study and the second study put it at 69% to 74%. Still the "up to 88%" number prevails. But 69% to 74% is pretty good, right? Maybe. There are several problems with the studies.

First those with head injuries wore helmets less than the control group of those who did not, but the "community control" group was not representative of the community.

Of 4,501 child cyclists observed cycling around Seattle, just 3.2% wore helmets. This is not statistically different from the 2.1% of the hospital cases who were wearing helmets.

As well as having a helmet wearing rate 7 times that of the cyclists riding round Seattle, the ‘community control’ group came from higher income households and had parents with higher educational levels. The observational survey of child cyclists riding in Seattle found that helmet wearers were predominantly white, middle class, riding with their parents in parks, whereas the non-wearers were more often black or other races riding alone on busy city streets. The risk profile of these two groups would be quite different.

Also, if you remove forehead lacerations from consideration, it cuts the benefit down to 39%. Avoiding a nasty cut on the forehead is not what most people think of when thinking about helmets and safety.

The study uses "odds ratios" instead of risk ratios and that doesn't really make sense.

McDermott’s data on hospital admissions also illustrates the folly of labeling odds ratios as risk ratios. 28.6% of adult cyclists who wore helmets still had head injuries. If helmets prevented 85% of head injuries, an impossible 191% of non-helmeted cyclists would have head injuries. The actual figure (38%) was higher than for helmet wearers, but the difference due to helmet wearing was not statistically significant.

There is also a self-selection bias. Cyclists who choose to wear helmets are not the same as those who do not.

A combined analysis of 30 studies, all or which compared women who chose to use Hormone Replacement Therapy (HRT) with non-users, estimated that HRT reduced the risk of heart disease by 50%. But when the bias of self-selection was avoided by choosing at random who would take HRT, it was found that HRT may even increase the risk of heart disease! The higher socioeconomic status of women who chose HRT may have been associated with other factors such as better diet and more exercise.

The study of bike/motor vehicle collisions found that helmet wearers also had much less serious non-head injuries.

So while helmets might be effective at preventing serious head injuries (beyond forehead lacerations) it is probably not 88%.

Claim 2: Ninety-one percent of bicyclists killed in 2008 reportedly weren't wearing helmets.

This claim comes from looking at FARS data and also seems damning. Especially when combined with this statement based on a gallop poll.

Half (50%) of bicyclists wear a helmet for at least some trips, with 35 percent using them for all or most trips.

But this data is based on Police Accident Reports and many of these do not include a separate entry for bike helmet use. So the FARS data is based on the crash narrative. If no information is given, it should be listed as unknown.

Unfortunately, it appears that nearly all of these cases that should have been coded as "unknown" (including a considerable number where the bicyclist actually was using a helmet, but such usage was either never noted or overlooked in the narrative) were instead coded as "not used"

One strong indicator that the FARS bicycle helmet use data should not be fully trusted is the fact that the "unknowns" are so few in number. It is simply not credible that a low priority data element such as bicycle helmet use would have a precision associated with it that is a factor of 20 better than that seen for much higher priority data elements such as seat belt or motorcycle helmet use (0.5% "unknowns" vs. 11% or 10%)

In 2008, there were 0 unknowns listed.

And the data reported by states does not match the state's data in the FARS.

California data from the StateWide Integrated Traffic Records System (SWITRS) indicates that at least 13.2% of fatally injured bicyclists were using a helmet during the period 1994-98 (since SWITRS combines "unknowns" and "not used" into a single category, the helmet use rate is actually a minimum estimate and could be much higher, depending upon the relative number of true "unknowns" and how biased the distribution might be), but only 3.4% supposedly were doing so according to FARS.

And some cyclists are listed as wearing seatbelts or using child protective seats.

Even a cursory examination of the data indicates FARS was underestimating actual helmet use among fatally injured bicyclists by up to an order of magnitude or more during the period 1994- 98. Although the situation has improved considerably since then, FARS continues to underestimate overall bicycle helmet use in the US by a factor of two or more as of 2004 (the most recent data available)

And, of course, you have the self-selection bias as well. [Plus, observation of cyclists has never shown anything near the 50% helmet use that people reported to Gallop].

It's hard to say what the efficacy of helmet is, but these numbers 88% effective and 90% of dead cyclists didn't wear one, just seem unreasonable high. Plus they never mention that at least 65% of cyclists who didn't die don't wear helmets.

Photo by Eric Gilliland.

Comments

[contrarian]

Excellent analysis. I'll add one more point: we shouldn't expect helmets to have a dramatic safety effect because they aren't designed to prevent serious injuries.

If you look at the federal standards for bike helmets you'll see that they are designed to provide some cushioning in a slow-speed fall. Unless manufacturers are routinely making helmets that substantially exceed the standards -- and there is no evidence they are -- a reduction in lacerations and contusions is all we should expect. Expecting a bike helmet to prevent serious injury is like expecting your TV remote to control your microwave: it's something it just wasn't designed to do.

I'm particularly disappointed in the American College of Emergency Physicians for feeling they have to mislead the public in order to make their case, I thought they were proponents of science. In their defense, children are particularly succeptible to the type of falls that helmets are most effective against. I'm a firm believer that children should wear helmets.

We want to believe that we can control our own safety, and years of seat-belt education have drummed the idea into us that there are effective steps we can take. Critics of seat-belt education campaigns make the valid point that a far more effective way of saving lives than wearing seatbelts is to operate your vehicle in a safe manner. The same is true of bicycles and helmets.

Obligatory disclaimer: I wear a helmet every time I ride my bike. I make my kids where helmets on their bikes.

[Blue-eye Devil]

Excellent post! Thanks, yet again.

Always refreshing to see critical thinking applied to topics that many consider unassailable.

[D]

Whatever the stats may be I just did a face plant on the Mount Vernon trail on Bike to Work day. It was about 5:30 PM. I was going 20+ grabbed my water bottle with my right hand and a squirrel ran in front of me. I tried to evade it with my left hand only on the brake hood. I inadvertantly turning my wheel all the way to right, the next thing I know I was doing a face plant on the asphault. My lips and my helmet visor hit. I think the visor kept me from breaking my nose, my lips are still swollen. I suffered no head trauma, but would likely have had trauma if I hadn't been wearing a helmet. Stats may be debated, but I will never ride without a helmet. I've been bike commuting for 1.5 yrs/18 mls each way about twice/month (Mar – Oct). This is the first time in my life I've ever needed a helmet. I thank God for His kindness and the helmet for the additional protection that kept this accident from being much worst. I was able to get up, check out the bike and myself and ride the 16 remaining miles of my commute.

[jeff]

Surprisingly, I have found not crashing reduces incidents of brain injury 100%. Whether it's seat belts, air bags or helmet (not arguing against any of these) it always amazes me how little attention is given to addressing the cause of head injury inducing crashes. LaHood, Oprah and others focus on distracted driving has been a welcome change. I'd love to see the numbers from a study that looks at the combined effects of proper facility/roadway and maintenance; cyclist and motorist education and proper traffic law enforcement. Maybe doing an overlay of the indexed crash rate in Portland could demonstrate that? http://www.portlandonline.com/transportation/index.cfm?a=185776&c=34811

[washcycle]

Yeah, that's why a helmet doesn't even make it into my top 3 safety tools. Anything that helps you avoid a crash comes before a helmet. I rank them like this:

1. An aware and skilled cyclist (you are your own best safety feature).
2. A bike in good working order
3. Lights/reflectors
4. A helmet
5. Gloves
6. Sunglasses/eye protection

I might even put sunscreen in between the lights and helmet.

[contrarian]

The field of traffic safety focuses almost entirely on equipment and facility, there is basically a taboo against trying to change driver behavior. When safety officials do talk about changing behavior, it's the behavior of the victims -- pedestrians and cyclists -- they seek to change. For instance, when a MPD officer hit and killed a pedestrian in a crosswalk on Wisconsin Ave recently, DDOT's response was to remove the crosswalk! It's an article of faith that drivers can't be changed.

This is despite ample evidence that behavior dwarfs all other causes of collisions. In "Killed by Auto" Komanoff reports that over 90% of the drivers in collisions that kill pedestrians or cyclists are men. Since men account for only 75% of miles driven this means that men drivers kill people at about three times the rate of female drivers. Just getting the average man to drive like the average woman would reduce the fatality rate for pedestrians and cyclists by a staggering 60%. It's highly likely that there are behaviors associated with the risk of fatal collisions that are present in both men and women, but more common in men. Eliminating those behaviors in all drivers would probably eliminate the preponderance of fatalities.

In this environment the helmet is the ideal cyclist safety device: it's a piece of equipment, and it requires no behavioral change on the part of drivers, only on the part of cyclists. The fact that it is ineffectual is irrelevant.