Statistics and Helmets

bike_helmet_head.jpgThere’s an old saying: "There’s lies, damn lies and then there’s statistics". But it’s not the data that lie, if properly collected, it’s how you interpret the data that matters.

Overall, the report issued by the city on cycling deaths and injuries is a pretty good piece of research and epidemiology. But make no mistakes about it, there are a lot of places that people might make wrong conclusions without better understanding some basic principles of statistics and data interpretation. It’s important to make sure data is interpreted correctly or you face prescribing the wrong remedies.

Let’s start with the helmet issue since this report has restarted the debate over mandating helmets for all cyclists. The report states that 97% of New York City cyclists who died between 1996 and 2005 were not wearing a helmet (page 16). This is a classic example of a high association between two facts, but the data stop short of determining causation between death and lack of a helmet.

Consider these highly assocated sets of facts:

  1. 100% of pedestrian fatalities from automobiles were not wearing a helmet.
  2. 100% of motorists survived their crashes with cyclists and pedestrians
  3. 100% of airline crash passenger fatalities were wearing a seatbelt.

The first example shows that in a similar situation, a person getting hit by a car, the lack of a helmet was highly associated with death, but it doesn’t prove that a helmet would have prevented death. The second example shows that being wrapped in a steel cage is highly associated with surviving a collision with another human being that is not. The last example shows that some safety measures clearly do not prevent death. Whereas seatbelts have proven to prevent deaths in car crashes, they are completely useless in an airplane disaster.

So, do helmets prevent bicycle fatalities? We don’t know.

The data is insufficient to make a conclusion. Based on the data presented in the report (and the laws of physics) one could just as easily hypothesize that the key factor in bike fatalities is the mass and velocity of automobile, rather than the presence of a fairly lightweight piece of safety equipment attached to the cyclist’s head. While this study does not provide data on the speed of the automobile, it does give us a clue about the mass of the vehicle: Trucks represented 32% of cyclist deaths while they only represent 17% of vehicles on the road. (Page 17).

The best way, in my opinion, to determine causation between helmets and cyclist deaths would be to compare the results of crashes between automobiles of similar size and similar speed and see if there is a statistically significant difference in severity of injury or death rate. Frankly, I think helmets help, but are probably not the most important deciding factor between life and death.


Photo by Hannoflickr

  • Sean

    There are all sorts of interesting issues raised by the helmet numbers. For instance, do people who take the risk of riding without a helmet take other risks that put them at increased danger of collision with trucks and cars?

    In the absence of any hard conclusions on the efficacy of helmets in preventing death and injury, it still seems prudent to wear a helmet. It’s logical that wearing one adds a layer of protection. The downside of wearing one is minimal. (But, see the findings on how closely cars travel based on helmet wearing.)

  • Of course it’s prudent to wear a helmet, but I believe there are many (both cyclists and not) that believe a helmet is like a magic force-field, protecting the wearer from harm and/or death. This is, of course, not so.

    A helmet, when worn properly, is very effective at protecting your head in a very particular sort of crash. Many fatalities result from internal injuries, which are not prevented by wearing helmets.

    It makes great sense to wear a helmet while riding, but as there are many other ways of being killed on your bicycle than just cranial injuries, I think that there cannot be a causal relationship between helmets and cycling fatalities.

    That said, ride safe.

  • Fleiboy

    This discussion seems off-key, as if we’re in 1951 discussing smoking and lung injuries. The report says that 74% of the studied deaths involved head injuries, and in half the head was the key recorded injury. Even if one can’t document that helmet-wearing is a proven correlate to surviving a crash, it is unlikely that we’ll find out helmet use is irrelevant. Proper interpretation of statistics matters, but fussing over the helmet statistics distracts from a clear public health imperative; increase helmet wearing among riders.

    Meanwhile, I bet only a small percentage of riders really believe that the helmet provides immunity, and an even smaller percentage take risks based on that belief. Anyone know of data addressing this question? Presumably Bell or some other manufacturer has looked into this.

    I do agree that bicycle advocates would benefit from knowing some data not just on the question of whether helmetless riders engage in other risky behaviors, but also whether those killed/injured were frequent or occasional riders.

  • Fleiboy

    oh, and sean, where is this:

    “see the findings on how closely cars travel based on helmet wearing.”

  • Fleiboy

    ok, i trust you were referring to the 9/15 post on the UK research about car proximity. point taken. does engwicht or anyone else offer bicycle lane safety studies that would illuminate the DOT’s conclusions?

  • Fleiboy –

    It’s very important to make sure that these statistics are interpreted the right way. A misinterpretation of the data would cause a policy analyst to incorrectly conclude that simply requiring helmets would dramatically reduce cyclist deaths.

    This is not like smoking. Cycling is perhaps the least polluting, least noisy, most healthy and economical way to travel around the city. It’s something that should be encouraged and promoted, not something that should be eliminated if proven dangerous. Instead it should be approaches as something like drinking water that should be kept safe to encourage health everyday living and a better society.

    I think helmets are good, but their value should not overestimated to think that they alone can prevent deaths. Bike lanes, education campaigns, good training for motorists and cyclists on proper road behavior (I would target males especially).

  • Anon

    Bike advocates complaining about helmet laws just detract from the cause. The complaint contributes to the image of bikers as reckless law-breakers. It is just as silly as drivers complaining about seat belt laws. A decent helmet costs about $40. If it might save your life, why not wear it? The more advocates talk about safety, the better they sound. The more they talk about being exempt from safety laws, the worse they sound.

  • podsednik

    The thing is, seat belts don’t discourage drivers from driving. But helmet laws probably will cause some cycilsts to stop cycling, or to cycle less.

    Since the health benefits of regular cycling have been shown to outweigh the risks of injury, such laws are counterproductive.

    Add in the fact that each cyclist is safer the more cyclists overall are on the road, and it’s clear that a mandatory helmet law is a double whammy.

  • d

    Comparing car accidents involving bikes to those involving pedestrians doesn’t work, as the risk factors are completely different.


    – A pedestrian might have better use of his arms and hands when falling after being hit (for a better, safer landing).

    – A pedestrian walks at a slower speed than a cyclist bikes (2 – 3 mph versus 10 – 20 mph). The force of impact with a car at any speed is greater for cyclists than it is for pedestrians.

    – A pedestrian spends a minimal amount of time walking on exactly the same space as cars. Cyclists ride with high-speed traffic at all times, meaning the chances of coming in contact with a fast moving vehicle is greater for cyclists.

    100% of pedestrians who died after being hit by a car weren’t wearing helments, you say? Big deal. 100% of people who died on plane crashes were probably wearing seatbelts, but that doesn’t mean I shouldn’t wear one while I’m in my car.

    I agree with the commenter who suggested that arguments about these statistics tend to obscure the reality. In the focus on death, what’s missing from these statistics are figures on minor injuries that would have become major injuries or even fatalities, had it not been for helmets.

  • Greenway enthusiast

    Perhaps a good approach would be to separate fatalities from collisions as we review the data. It seems from the data that helmets do help prevent fatalities. That is, if you get hit by something like a car or truck, your chances of dying from a head injury (a major cause of death in bike accidents) is reduced by wearing a helmet. Makes intuitive sense and is supported by the data.
    What helmets probably do NOT do is to reduce the number of collisions. To reduce the number of collisions will likely take a combination of things: changing the design of the roads to make it safer for bicyclists, encouraging bike riding alot (interesting data from Europe suggests that you can reduce the number of accidents betw motor vehicles and bikes by having more bikers on the road), and driver/biker education campaigns to make us all respect the rules of the road and look out for each other better.
    The problem becomes if something to increase helmet wearing (ie mandatory helment laws) would actually decrease the number of cyclists on the roads then we might not increase overall safety at all. Does anyone have information/data showing what the impact of mandatory helmet laws actually has been in places that have done this on the numbers of people who ride??

  • bartonfink

    Does anyone have information/data showing what the impact of mandatory helmet laws actually has been in places that have done this on the numbers of people who ride??

  • Greenway enthusiast

    interesting info on helmet laws and bicycle usage. thanks for the website with references.

  • Glenn, that link did not work. Can someone check that URL?


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