Paul Jakma's Blog

This is an edited version of a comment I made elsewhere.

A new study has been published recently on the impact on head injury rates of cyclists by the compulsory helmet laws in New South Wales, Australia. This claims there’s a positive effect on injury rates from such helmet laws, even adjusting for the reported fall in cycling rates due to the introduction of the law. This is being reported by some as evidence that compulsory helmet laws work or even that they lead to a 29% drop in head injuries. However, that seems a misleadingly simplistic view of it.

A careful look at the study  shows there appears to be a significant benefit only over a short-period of time, across the few months where the law is passed. While beyond that the helmet law has managed to turn a decreasing head injury rate into an increasing head injury rate (fig 3). This is despite accident rates themselves being somewhat stable (fig 2). Within a year and a half of the law coming in, the head injury rate is almost back to the same level as before! Given the increasing trend, it is likely that not long after the study period head injury rates would have been worse than before!

So what is happening? There is a divergence between the arm injury rate and the leg rate in fig 3. The arm injury rate has increased, mirroring the head injury rate, while the leg rate stays flat – possibly reflecting the stable-ish overall accident rate. This suggests the nature of accidents may have changed.

The study itself notes there are some possible limitations to its findings:

• Injury rates are seasonal, and they have only very limited amount of data (less than a year) on the pre-law rates.
• They assumed exposure to potential injuries was identical for head, leg and arms. While this seems a reasonable assumption, as they say, their own data shows this assumption may not be entirely safe.
•  They have no data on cyclist types or behaviour, so they can’t factor out things like proportionally more cyclists in riskier environments having given up, or cycling in safer environments having increased (e.g. commuter v recreational cycling).

One possibility is risk compensation, that the extra perception of safety from the wearing of helmets leads to drivers and/or cyclists taking on more risks. E.g. a study on the behaviour of motorists when passing cyclists  has shown that motorists pass closer if the cyclist is wearing a helmet. Another possibility is that injury rates just happened co-incidentally to be at a high prior to the law, and that the law has had little causal effect. Further, the very proposal of the law no doubt lead to a lot of media exposure of the issues around cycle safety. That media exposure no doubt had a strong impact on awareness, even prior to the law’s passing – awareness which faded in time after it was all done and dusted. Or some combination thereof.

That means, so far as the study is accurate, that beyond a short-period, the study actually shows an apparent detrimental impact on safety in terms of head injury rates, and only a transient impact on head injury ratios relative to arm and leg injury rates (the metric they relied on to normalise out changes in cycle use).

Wearing a helmet may well protect your head from serious injury if you get into an accident, the problem is that in wearing one, it you may also be more likely to get into an accident and that accident may even be more serious. Which of those is better is a hard question, but it should probably be an individual choice. Interestingly, some of the safest countries for cycling have very low rates of cycle helmet use – almost no-one wears them in the Netherlands, except for serious sports cyclists.

Update: dg01d linked me to this interesting discussion on cycle helmet safety in the BMJ. Also fine-tuned the language in the conclusion on the possible trade-offs in risk.