Rates per 100,000 population of bicycle related head and arm hospitalisations. Source: Olivier et al, 2012

There’re two key arguments in the debate on the mandatory helmet law (MHL). On the one hand, it’s argued compulsory helmets deter cycling. On the other, it’s contended they reduce head injuries. Both claim a social benefit in lower health costs – more exercise vs fewer head injuries.

I’ve looked at the deterrent argument before (here and here). Now, a new paper analysing bicycle-related head injuries provides an opportunity to look at the claimed benefits of helmets (you can download the full paper here).

The paper is by three researchers from UNSW, led by Dr Jake Olivier from the School of Mathematics and Statistics. The authors examined hospital admissions data in NSW from 1990, the year before the mandatory helmet law was introduced, until 2010.

To isolate the impact of helmets, they compared hospitalisations for bicycle-related head injuries with those for bicycle-related arm injuries. The logic is that, while there’s no reason they should be the same, they should change over time at the same rate, unless an intervention is directed at one and not the other.

In 1990, just prior to the introduction of the MHL, head injury rates were higher than arm injury rates (see exhibit). However following enactment of the law, they fell 29% relative to the change in arm injuries.

That was a substantial and dramatic benefit. It’s equivalent to around 170 avoided hospitalisations in the first year alone. It’s compelling evidence of the benefits conferred by wearing a helmet.

The authors go further, though, and attribute the social benefit to the mandatory helmet law, because it drastically increased the rate of helmet wearing. Prior to the law, 10% of children and 25% of adults wore helmets, but this jumped to around 80% within two years.

We should expect the helmet law to provide a one-off, permanent lowering of the head injury rate. But what’s very interesting about the exhibit is the two variables didn’t run in parallel over the period from 1991 to 2010 but diverged sharply.

The number of arm injuries increased by 145% over the period while head injuries increased by only 20%. The latter figure is broadly in line with the increase in population, but it’s much, much lower than other indicators of the growth in cycling cited by the authors.

For example, a survey of NSW residents found the number who had cycled within the previous 12 months increased 50% between 2001 and 2010. Bicycle imports grew 145% between 2000 and 2009 and the number of cyclists counted in the Sydney CBD increased 156% between 2002 and 2010.

Thus head injury hospitalisations fell massively in ‘real’ terms over the 20 years from 1990 to 2010.

The exhibit also shows there was a distinct turning point in the trend at 2006. Thereafter the number of both arm and head injury hospitalisations declined, although the reduction in head injuries was even stronger.

The authors attribute the difference between the two variables up to 2006 as entirely due to the protective effect of wearing helmets. Dr Olivier says:

We found that the overall benefit of mandatory helmet legislation in lowering head injuries was larger than previously reported and has been maintained over the past two decades. Before the law commenced in 1991, bicycle-related head injury rates exceeded those of arm injuries. By 2006, head injuries were 46% lower than arm injuries.

On the other hand, they ascribe the post-2006 decline to the combined effect of the mandatory helmet law and the construction of safer cycling infrastructure.

Infrastructure matters because head injuries are strongly correlated with collisions between motorised vehicles and cyclists. These usually occur at higher speeds than other bicycle accidents.

While much cycling infrastructure is only a painted line on the road, it nevertheless increases the separation between bicycles and heavier, faster-moving vehicles.

Why head injuries fell continuously and sharply in ‘real’ terms over the entire 20 year period isn’t clear. It’s possibly helmet-related e.g. improved helmet fitting or fewer ‘risk-takers’ going helmetless.

It might possibly reflect the dramatic decline (mostly for reasons unrelated to helmets) in high school children cycling to school over the last 20 years. Or perhaps the rudimentary cycling infrastructure of the 1990s and early 2000s made a greater contribution to safety than it’s usually given credit for.

We can be pretty confident the big jump in rates of helmet wearing accounted for the initial spectacular reduction in head injuries. Their role in the subsequent equally remarkable further improvement is less clear.

That doesn’t mean however that mandating helmets was necessarily good policy. It’s possible rates of voluntary helmet wearing might’ve improved significantly without the law, perhaps driven by education, better helmet design, and rising concern about the dangers of cycling on roads.

And of course the injury avoidance benefits have to be assessed against the costs. Some cyclists are convinced the cost to the community in foregone exercise due to the deterrent effect of helmets far outweighs the benefits from avoided head injuries.

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