Bike HelmetA triathlete acquaintance of mine was recently injured in a cycling incident. Her most serious injury seems to be a Traumatic Brain Injury (TBI). She sustained this injury even though she was wearing a helmet, the crash was not at a high speed, and the impact to her helmet was with the ground. An injury under these circumstances, plus the fact that the “plastic” shell of her helmet apparently cracked pretty good in the accident, prompted me to look into what we know about bike helmets and how (or if) they protect a cyclist from injury.

I was surprised to find, even among qualified experts, a significant difference of opinion as to whether wearing a bike helmet is even a good idea. The basic argument for the use of bike helmets is that studies show a decrease in injuries and death among cyclists who wear a helmet when compared to cyclists who were not (See, i.e., . The basic argument against helmet use is that the helmets were not the reason the cyclists in the statistical studies survived the crash, or were less injured–that the cyclist would have survived (or avoided head injury) even without the helmet and the riders that died without helmets would have died even with a helmet. In addition, the opponents claim that the use of helmets increases the risk of accidents and cause additional injuries themselves (see, i.e., the second section of

After trying to interpret the studies, and mostly succeeding, I think the “pro-helmet” camp has the better evidence.  They assert that wearing a helmet decreases the chance of death or serious head injuries following a cycling accident by 66 to 85%. Even if these percentages are wrong to some degree, it still looks like strapping on the helmet makes sense. The debate reminds me of the statistics on car airbags. As a personal injury lawyer, I can confirm what the numbers show on airbag safety—they cause more injuries than they prevent. The catch is that the injuries they prevent are often life-threatening. In other words, the airbag kept someone alive who would have died. So, our society deems the tradeoff justified. Bike helmets seem a good idea for the same reasons: Even if there is some evidence that certain injuries would not have happened if the cyclist was helmetless, the injuries the helmets prevent constitute a net gain.

So, how does a bike helmet decrease the chance of head injury? I will leave out some of the technical (eyes glazing over) explanation in the information I read on bike helmets, but here are some of the highlights:

  • Bike helmets are designed to decrease the gravitational force to the head to a level below 300 Gs (units of gravitational force) upon impact (, when the cyclist hits the opposing force at a speed of 14 miles per hour (see,
  • The reason the magic number is 300 Gs, under U.S. safety standards, is that the average skull can take 500 Gs before fracturing (I don’t want to know how “they” tested that one)(
  • The reason the impact speed is 14 miles per hour is that cyclists are most often hitting the ground with their head, rather than another object. The speed at which a cyclist hits the ground is not as affected as much by the speed she was traveling, or what may have hit her, but rather by her distance from the ground when she left the bike, and her speed traveling through the air before she lands. In multiple tests, the speed at which a cyclists hit the ground was at or under 10 miles per hour ( .
  • The helmet decreases the amount of gravitational force transferred to the head through the use of the shell (the outer plastic part) and the polystyrene layer between the shell and the head (the Styrofoam-looking stuff). It is really the polystyrene that is offering the protection. If, for example, the helmet breaks its outer shell (as in the case of my friend), the helmet still probably did its job if the polystyrene “compressed.” This material will not fully rebound after an impact, so it can be visually checked. If the helmet took a good hit, but the polystyrene did not compress, it probably did little to protect the cyclist (See,
  • If the impact is with anything that causes the speed of impact to be greater than 14 miles per hour, then the helmet may be overwhelmed, because the speed will change the G force coming through the helmet (
  • Under circumstances when the cyclist is impacting with something other than the ground, and consequently exceeding the 10 m.p.h/300 G standard, the cyclist is probably running right into something (like a truck, tree, barrier, etc.), and lots of other bad things are going to happen besides just impact with the helmet/brain. So, the helmet is not going to save the cyclist. (
  • Yes, this all means that the main thing a bike helmet is designed to do is to protect the cyclist from a head injury she may have sustained from falling off her bike and hitting the ground, regardless of what caused the cyclist to leave her bike. But, statistically, that is the most likely scenario.

So a helmet can clearly offer some protection under some circumstances. But it is a fact that a cyclist can crash, hit her head, have the helmet work perfectly, and she can still sustain a traumatic brain injury. Again, taking the medical terminology out of the explanation, it doesn’t take nearly 300 G’s of force to create an impact between the brain and the inside of the skull.  In fact, that impact can happen with changes in direction of the head, without any impact of the head to an outside object, because (think about it), what causes an injury to the brain is actually an impact with the skull surrounding it, not with the ground, car, tree, barrier, etc. (except if the skull does open up). Consequently, brain injuries, including those resulting in long-term cognitive losses, can happen in many kinds of accidents, not just severe impacts of the head with an opposing force (See, .

All activity carries some risk. Life itself is a dangerous activity. Seatbelts, airbags, and bike helmets are not perfect solutions some of those dangers, but they are significantly better than nothing. I hope the information I have shared helps you make a better risk/benefit analysis.  If you want to dedicate the rest of your life reading articles on bike helmets, you may want to visit  If you have any other questions, please feel free to contact Attorney Kevin Fine with Davis Miles McGuire Gardner at (480) 733-6800 or via email