The proposed bike helmet law in Maryland came as a bit of a surprise, so we have less time to learn the facts than we would have preferred. Your comments were very helpful. Thanks.
Here I am going to drill down into one aspect of the issue, not because it is the only important argument, but because it's potentially the most confusing. Please let me know what I got wrong within the narrow confines of this part of the story.
In Maryland, there are 500-600 bicycle injuries per year, with head injuries about 40% of the time and about 10 fatalities. There are also about 100 pedestrian fatalities per year. Because the proposed law applies to cyclists but not pedestrians, by design it is already omitting 90% of the potential fatality reduction in bike-ped fatalities. What's the most that we might expect the law to accomplish?
There are three ways people assess helmet effectiveness: By considering the design, case-control studies, and population studies.
Bike helmets do two things: Like all helmets, they spread the shock around the skull, so if a rock hits your helmet, it will feel like I slapped the top of your head. And like a motorcycle helmet, they spread the shock out over time when the foam compresses. So if your helmet hits a brick wall at 20 mph, it will feel as if your head hit some very rigid Styrofoam at 20 mph.
Bike helmets are a compromise between a motorcycle helmet and nothing. They are designed to work best for the force from simply falling without colliding from a stationary position. So they are most effective for children, pedestrians, and low speeds. But if you want to be protected at high speeds or collisions, you probably need something else.
Motorcycle helmets cover your whole head and face, while bike helmets only go down to the middle of the forehead. So if the force strikes lower in the head or face, motorcycle helmets protect you while bike helmets do not. More importantly, motor cycle helmets stay on your head, while bike helmets often fall or move during an accident, both because they rely more on their chin straps, and because many adults and most children mount them incorrectly. Bike helmets often break, which may limit their effectiveness in an accident involving two hits to the head.
Finally, bike helmets increase the risk of neck injuries by increasing the radius of your head by 25-50%. That increases the probability that your head will be twisted by something that would have missed an unhelmeted head, and it also provides greater torque to whatever force is applied.
The best way to know the effectiveness of a drug or helmet is to get a representative sample of the population, and then randomly assign them to the test group or the control group. Such an experiment allows one to reliably estimate effectiveness subject to a statistical margin of error. But we don’t know who will be involved in a crash, and if we did, it would be unethical to tell them whether to wear a helmet. Instead, researchers collect data after the fact.
In 1989, Thompson et al. obtained data from Seattle hospitals for two groups of cyclists who went to the hospital after a crash. Only 7% of the first group wore a helmet, and they all had head injuries. But 24% of the second group wore helmets, and none of them had head injuries. Assuming that both groups were the same except for the type of injury they experienced, these results imply that helmets reduced head injuries by 75%. Thompson et al. realized that the two groups were different, ran regression analysis on the data to isolate the effects of helmets and found that helmets were even more effective: 85%.
That study led the researchers to start saying two things that have almost become mantras among many public safety advocates: “Helmets reduce head injuries by 85%” and “The most important thing you can do to be safe on a bike is wear a helmet.”
In the last 24 years, similar studies have found that helmets reduce head injuries, but to a less extent than in the Seattle Study. A comprehensive synthesis of all studies in 2001 estimated 53--63% effectiveness, but because helmets increase neck injuries, the net effectiveness is 41--50%. Studies in the last decade have estimated that helmets only prevent 20-40% of potential head injuries, so the most recent synthesis of all studies ever published finds the helmets reduce head injuries by 30--50% and total injuries by 10--20%, when you include the increased neck injuries. But we still hear the refrain “helmets reduce injuries up to 85%!”
Why have the estimates come down? Mainly because none of these studies are truly controlled experiments: On average, the people that go to the hospital without helmets were more likely than the people with helmets to be drunk, riding at night without lights, have lower income and education, to be riding on the streets rather than a trail, and to have collided with an automobile. The researchers have gotten increasingly sophisticated about trying to isolate some of those factors, but they usually don’t have data for all of them. So the actual effectiveness is probably at the low end of the uncertainty range, especially for adults.
Studies of entire populations are less precise than case-control studies, but they are also less likely to have a biased data set. If we had data on miles biked and total injuries by state or city, an estimate of helmet usage, and decent injury data, we could compare injury rates in cities or states with high helmet use to those with low use---or areas with and without mandatory helmet laws. Unfortunately, while there is some data on injuries, states generally do not keep annual data on miles biked, so valid population studies on helmet effectiveness are not possible.
Such studies have been done in Australia and New Zealand, in response to mandatory helmet laws. Those studies did not detect a significant reduction in injuries attributable to helmets. In one case, there was a large reduction in injuries when the helmet law went into effect, but there was also a proportional reduction in cycling at the same time.
A rough comparison across nations is possible as shown in the figure. The United States has high rates of fatalities and helmet use, while the Netherlands has low rates of both. This does not mean that helmets increase the risk of injuries. But it does show the helmets are not the most important thing one can do to promote bicycle safety.
If the proposed law successfully increases the use of helmets by 20%, it is likely to prevent about 10-20 injuries per year, and possibly save someone’s life every year or two. I leave it to others to assess whether that would outweigh the premature deaths the law would cause due to decreased cycling.
(Jim TItus is a bicycle advocate from Prince George County. The opinions expressed here do not represent the official view of any organization with which he is affiliated.)