Risk compensation and bicycle helmets

Some months ago I left a series on bicycle helmets hanging while I got distracted with other things. We had looked at what the best evidence for the efficacy of helmets in preventing injury in the event of a crash is, and some of the reasons why we should be cautious about that evidence. We found that if you’re unlucky enough to have been hospitalised while riding a bicycle, you’re less likely to be there with a head or brain injury if you were wearing a helmet at the time of the crash. We noted several ways in which this protective effect is exaggerated and used to mislead, we noted that reduction in injury is from a very low level anyway, and that the research so far done fails to provide any sub-analysis of very different riding styles, such as racing cyclists, mountain bikers, and utility cyclists.

We also made careful note of the fact that a reduction in the rate of head injury in the event of a crash is a different finding to a reduction in the rate of injury and death of bicyclists. We briefly began the exploration of what this means by considering the fact that helmets are not much defence against a motor vehicle.

How could a reduction in head injury in cyclists who crash not mean a reduction in injury and death in bicyclists? Well, helmets could be causing other kinds of injury in crashes. Or they could be causing crashes — particularly the worst kinds of crashes.

The latter is a particularly interesting avenue. The idea is risk compensation or risk homeostasis, a phenomenon documented in fine detail by John Adams in the 1985 book Risk and Freedom. Adams showed that advances in road safety — seatbelts, motorcycle helmets, safer vehicle designs and wider, straighter, safer road designs — are never followed by quite the cut in injuries and deaths that is predicted; that while road “safety” has improved crashes are no less frequent, and that bystanders — pedestrians and cyclists — are butchered at an ever increasing rate.  There is a set level of danger that people are willing to tolerate, and so motorists were compensating for the new safety features by driving faster and taking more risks. To put it in Adams’s technical terms, potential “safety benefits” were instead absorbed as “performance benefits”.

James Hedlund reviewed the evidence on risk compensation and came up with a set of rules for when people are likely to compensate for a safety intervention:

  1. They know it’s there.
  2. They know it’s a safety feature.
  3. There is a potential performance benefit to be had.
  4. There is freedom to realise that performance benefit.

Well cyclists know whether or not they’re wearing a helmet, they know that helmets are meant for safety, there are potential performance benefits — riding faster, through smaller gaps, in more hostile traffic, or with less caution in conditions that would otherwise advise it — and cyclists are generally free to ride more furiously if they want to. (Indeed, you may be wanting to cycle faster, in which case go ahead and use a safety feature as a performance benefit if that works for you.)

But that’s only a hypothetical reason to expect risk compensation by cyclists wearing helmets, not evidence that it actually happens. And very little effort seems to have been put into researching that — perhaps because it’s difficult to devise a properly controlled test. A study of cyclists in Spain attempted to test the idea by comparing the rate of helmet wearing in traffic law violators to the rate in non-violators, finding that law breakers were less likely to be helmet wearers, the opposite to what they say should be expected if there is risk compensation. However, this study could not control for all possible differences between the populations (“confounding variables”) — for example, helmet wearers are already a population of safety-conscious conformists, less likely to commit traffic violations, and so a better question to ask would be whether those helmet wearers acted even more cautiously when their helmets were taken away from them, and whether the non-wearers behaved even more recklessly when given a helmet. (This study is, embarrassingly, the British Medical Association’s sole reference for their dismissal of risk compensation.) A more recent study observed a set of participants behaviour with and without a helmet, using speed as an indicator of risk taking and heart rate variability as a proxy for risk perception. This study found that when helmet users had their helmet taken away, the risk taking (i.e. speed) reduced to keep the risk perception stable. However, the study only looked at 35 people, and only looked at proxy variables. Neither study is very convincing — the limitations I describe here are just the tips of the icebergs — and certainly nowhere near strong enough or specific enough to guide policy. We still have a mere plausible hypothesis with no good evidence as to whether or not it’s true.

The authors of the Cochrane review acknowledge the suggestion that risk compensation by cyclists could affect their crash rate, but believe that is unlikely. It’s interesting to see a hypothesis dismissed with the argument from personal incredulity in a Cochrane review.

What is not touched on in the review, and which is potentially far more important (given the fact that crashes with motor vehicles are more likely to kill or seriously injure), is the risk compensation effect not of cyclists themselves but of the other road users around them — i.e., of the motorists. Look again at Hedlund’s rules. Motorists can see whether a cyclist is wearing a helmet; they know that helmets are supposed to be a safety feature; they can potentially find performance benefits — they think they can squeeze through tighter gaps when overtaking against oncoming traffic, or pass more quickly, or shoot in front while turning, because if they hit the cyclist then no harm is done; and there is nothing to stop them realising that performance benefit, since the police, if there even are any, are rarely even aware of the relevant traffic rules, let alone bothered with enforcing them. There is therefore a plausible hypothesis that motorists will take more risks around cyclists who wear helmets than around cyclists who do not.

This hypothesis is made all the more plausible by the fact that, in addition to potentially making cyclists seem less vulnerable, helmets make cyclists look more competent: in surveys of motorists’ beliefs, most assume that cyclists who wear helmets are more experienced and more “responsible“, meaning that they may be driving more carefully around non-helmeted cyclists who they expect to do something silly. And motorists overwhelmingly think that cyclists should be forced to wear helmets — presumably so that the motorists can get the performance benefits of driving more dangerously around them.

The motorist risk compensation theory has famously been tested by @IanWalker in one of the most delightful experiments in the field. Walker rode around Salisbury and Bristol on a bicycle fitted with an ultrasonic distance sensor measuring the effect of a number of variables on passing distance, including rider position in road, type of motor vehicle, and whether he was wearing a helmet. Analysis of over 2,000 passes showed that motorists tended to give on average around 5-10 cm less space when the rider wore a helmet. It’s not much difference, and the effect of motor vehicle type, perceived rider gender, and rider’s distance from the edge of the road were all stronger.

But it’s important to note that there is always a distribution of passing distances — a bell curve. There are a few motorists who give a lot of room, a few who scrape past, and a lot clustered in the middle, giving a little over a metre distance. When wearing a helmet, the bell curve shifts in a little bit. The cautious drivers give a little less space, the average drivers give a little less space, and the dangerous drivers give a little less space.  It’s the latter who are now more likely to drive into you.

Walker’s research, delightful as it is, is itself not without limitations. Most important amongst them is that, when it comes to answering questions of cyclist safety, it suffers the same limitation of measuring only proxy variables: passing distances rather than actual risk of crashes and injuries. But it tells us that there is a very important reason to study more than just the isolated risk of head and brain injury in the event of a crash.

Helmets are a medical intervention, exactly like a drug or surgical procedure. They are a preventative intervention and they are a physical intervention, but neither of those are alien to medicine and to the modern methods of evidence-based medical science. And risk compensation is just a side-effect of this medical intervention, like the side-effects of drugs. The side-effects of drugs that make it to market are by definition outweighed by the beneficial effects; but ten times as many drugs are discarded during development because the research finds that either the side-effects are so big or the beneficial effects are so small that the harm outweighs the help.

The authors of the Cochrane review defend their dismissal of risk compensation by saying “the fundamental issue is whether or not when bicycle riders crash and hit their heads they are benefited by wearing a helmet.” And that’s fine if you’re in the preliminary stages of developing an intervention and you are so far only concerned with whether it has beneficial effects. But the authors go far beyond that early stage in their conclusions, recommending that this intervention be compulsory — despite there being very good reasons to suspect that there are potentially major side-effects of this intervention. They can’t have it both ways. If you haven’t bothered studying the side-effects you can’t license the drug. It might kill people.

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15 responses to “Risk compensation and bicycle helmets

  1. Thank you for this thoughtful post, especially finding that driver risk – compensation study.

  2. Also thanks. Are you planning to look at the relationship between helmet promotion, perception of risk for cyclists and cycle usage rates?

  3. It’s quite a leap to infer bicycle safety from adherence to car laws. It’s far easier to see and hear approaching traffic from a bicycle than from a car, which means that stop lights necessary for safe driving by partly-deaf partly-blind drivers, are not necessary for safe riding by cyclists. That doesn’t mean whiz right on through, but “Idaho rules” (stop-sign=yield, red-light=stop-sign) apparently work both in Idaho, and when put into practice by law-breaking cyclists all over the world.

    A better metric might be to study helmet use on different classes of road; if a measured-dangerous road has a higher fraction of helmet-wearers, I think that allows you to infer risk compensation, and connect it to measured safety instead of a dubious proxy.

  4. Same report on passing distance also said that black cars give more room on average, so surely we should just make everyone drive black cars, problem solved ;-)

    Seriously though aren’t there any stats on actual incidents between bikes and cars and the count of those wearing helmets to those not wearing helmets? This would be a better indicator than speculation.

    • Unfortunately this wouldn’t be much better, for same reason that the Spanish study tells of nothing of any use. People wearing helmets might be less likely to be injured and killed, but that could be despite the helmets rather than because of them. People who make the choice to wear a helmet tend to be safety-conscious conformists (indeed, that’s essentially what the Spanish study shows — they stop at traffic lights). In the terms of evidence-based methods, the study design lacks the randomisation that would control for these “confounding variables”.

      To get around this you would need either to randomise people into helmet wearers and non-wearers, so that naturally cautious and naturally reckless people are distributed evenly, or you would need to compare the behaviour of the same people with and without helmets — perhaps get everybody in the study group to flip a coin every morning to decide whether that day is a helmet or non-helmet day and then compare the crash and injury rate between helmet and non-helmet days.

      Crash and injury rates are low enough that you would need a very large study population, but I don’t think it’s beyond the imagination or the capabilities of modern EBM to design a study that would address this problem. All it needs is the will and the funding.

    • Oh, and, everybody in black cars is a nice idea — like the move towards cigarette packaging having to be plain. Add a maximum engine size and speed limiter at the same time. Indeed, it was Ian Walker who noted at Skeptics in the Pub a while back that cigarette and alcohol advertising is not allowed to promise health, wealth, happiness, sex, and status these days. It’s a rule that certainly needs to be extended to car advertising.

  5. Nice piece.
    If you want to read more on this, with a lot more evidence and discussion to back up the basic thrust of the argument, do go to http://www.cyclehelmets.org .

    You can also go the chapterion helmets in my book “Death on the Streets: Cars and the mythology of road safety”. It doesn’t have any evidence from the last twebty years, but the argument is the same (for more on my book and how to get it, do go to http://rdrf.org.uk/2011/06/death-on-the-streets-cars-and-the-mythology-of-road-safety/ ).

  6. Pingback: How did the BMA get bicycle helmets so wrong? | At War With The Motorist

  7. Pingback: Appendix: Bad Science Bingo in the BMA’s “safe cycling” pages | At War With The Motorist

  8. Excellent blog post. Also, there was a study published recently that looked at head injury rates in cyclists relative to other injuries, in New South Wales around the period of the introduction of their mandatory helmet laws. It’s being touted in some quarters as showing that helmet laws are effective, and I’ve looked at it here.

  9. The drivers that notice us for long enough to decide whether or not we’re wearing a helmet, and who evaluate how close to pass based upon that, are probably not the ones we should be most worried about…

    • Drivers, of course, don’t consciously evaluate how close to pass, and if you stopped them a minute later and asked them “was that cyclist wearing a helmet” they probably couldn’t answer. Driving does not involve rationally contemplating everything you see. Most of these effects are about subtle unconscious responses to visual cues that never make it to the higher functions…

  10. The Ian Walker article link is dead, so here’s another:

    http://www.bath.ac.uk/news/2006/9/11/overtaking110906.html

  11. Pingback: All those helmets posts in one place | At War With The Motorist

  12. Pingback: All those helmets posts in one place | At War With The Motorist

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