Bicycle debate: do motorists drive closer to cyclists wearing helmets?

I came to the debate on the mandatory helmet law last year with an agnostic view. Having seen many references to past studies, I decided to read the key source documents that are consistently cited (e.g. see this article by the Institute of Public Affair’s Luke Turner) in support of the argument to repeal the law .

I started with the before-and-after study done at the time the law was introduced in Victoria in the early 90s. Then I looked at four before-and-after studies undertaken in NSW when the law was introduced at much the same time there.

Next on my list is a UK study which Luke Turner says shows “that some motorists drive closer to helmeted cyclists, than unhelmeted ones.” Although he doesn’t say who did it, he undoubtedly means the frequently quoted 2006 study by Dr Ian Walker from the Department of Psychology, University of Bath.

Like the other ‘foundation’ studies, I suspect this one’s quoted by many more people than’ve actually read it. I’m now one of the apparently select few who has.

Dr Walker’s prime interest was in how closely motorists came to cyclists when they overtook them. He was also interested in how the leeway overtaking motorists gave to cyclists varied by vehicle type, cyclist’s gender and whether or not helmets made any difference.

In order to collect the data, he personally rode 320 km in daytime on a bicycle equipped with an ultrasonic distance sensor and video camera, using a range of street types in Bristol and Salisbury. He rode at a range of fixed distances from the kerb, both with and without a helmet.

In total he recorded 2,355 overtaking events. In roughly half the events he wore a helmet.

Dr Walker also undertook a supplementary exercise to determine the effect of gender. He rode a 1.25 km stretch of road at a fixed 0.75 metres from the kerb, alternating between wearing and not wearing “a long feminine wig.”

His key findings are that overtaking drivers pass closer to cyclists “when the rider wears a helmet, rides away from the edge of the road, is male, or when the vehicle concerned is a bus or heavy goods vehicle.”

Having read the published journal article, I’m not persuaded the study lends convincing support to the argument that cyclists in Australia would be safer if they didn’t wear helmets. That’s for a number of reasons.

This is only one study and, moreover, it applies to a different country. It should be treated with caution until the findings are duplicated in other contexts.

In fact there’s a US researcher who found distance from the kerb made no difference at all to overtaking distances in Los Angeles. Whether either is ‘right’ or not, contrary findings illustrate the dangers of making sweeping generalisations on the basis of a single study in a specific context.

Dr Walker’s study also suffers from the fact the researcher is himself the key participant. That opens up all sorts of opportunities for bias and contravenes a prime rule of serious scientific method.

The idea of wearing a “long female wig” in order to “look plausibly female to motorists approaching from behind” doesn’t give me a lot of confidence in the research either. I’m not at all surprised drivers gave him a wider berth in this get-up – men in drag on bicycles probably aren’t a common daytime sight in Bristol and Salisbury.

Dr Walker’s findings also give some pause for thought.

The exhibit shows the differences are actually quite small. For example, when Dr Walker rode 0.25 metres from the kerb, drivers gave him on average 1.46 metres clearance when he was bare-headed and 1.38 metres when he wore a helmet.

That’s an average difference of 80 mm, or about three inches. It’s a reduction of just 5%. Further, whether with or without a helmet, drivers gave him a considerably wider berth than the one metre minimum overtaking distance cycling organisations in Australia are seeking to have enshrined in law.

Dr Walker reports the distribution of overtaking distances is bell-shaped. It’s strongly clustered around the mean (around 1.5 metres) with much smaller numbers in the tails.

The inner tail, though, is where most accidents are likely to happen. Dr Walker says 23% more vehicles came within one metre of him when he wore a helmet.

His data indicates that figure is right, but the numbers are very small. Only 5% of vehicles came within one metre when he wore a helmet and 4% when he didn’t.

That comes down to 60 overtaking events versus 49. It’s a difference of 11 out of a total of 2,355! That’s hardly a compelling argument.

The convergence of both curves at one metre from the kerb (see exhibit) also suggests that too much shouldn’t be read into the apparent difference. Dr Walker speculates it might be because that’s the distance where motorists have to cross/straddle the white lane to overtake, but why that would so decisively eliminate the helmet effect isn’t explained.

The author suggests the overall difference in overtaking distances could be due to drivers thinking either that helmeted riders are less vulnerable, and/or that they’re more experienced and predictable. The UK differs from Australia however, with only around a fifth of riders on major roads in the UK wearing a helmet, according to a source cited by Dr Walker.

That’s not the case in Australia. Helmets are normalised here – the vast bulk of riders comply with the law. If riding without a helmet were also normalised and popular, it can’t be assumed the two classes of riders would necessarily be treated differently by overtaking drivers.

So I’m not persuaded that this study by itself provides much support for the argument against mandatory helmets in Australia. The other two ‘foundation’ studies I reviewed previously (here and here) didn’t live up to all of the extravagant claims made on their behalf either.

The most interesting findings have nothing to do with helmets. Although they’re also subject to some of the caveats raised above, Dr Walker found buses and trucks pass closest to cyclists and, contrary to popular belief, ‘occupying the road’ encourages drivers to pass nearer. The effects are relatively small though.