Oct 24, 2012

Do motorists drive closer to cyclists wearing helmets?

Critics of the helmet law say an often-cited UK study found overtaking motorists drive closer to cyclists who wear helmets. But does the study provide convincing evidence of the need for change?

Alan Davies — Editor of The Urbanist

Alan Davies

Editor of The Urbanist

Average distance left by drivers overtaking cyclists in the UK (Source: Walker, 2006)

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.

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33 thoughts on “Do motorists drive closer to cyclists wearing helmets?

  1. St Etienne

    Just responding to Alan’s comment #30. I’ve always been a keen observer of helmetless riders and where and when they’re likely to show up. Work commuters during a peak hours have a very high compliance rate and I rarely ever come across anyone without a lid during this time. Same for recreational riders I see on the various trails. The most lidless cyclists I see are mainly around the inner-northern suburbs (particulalry the Yarra district) and outside of peak hours, usually just doing basic transport trips to shops, cafes, etc. And trust me, the stereotypical lidless hipster is just a myth; people who make up the helmetless riding community are a very diverse lot in terms of age, sex and class. Totally unscientific observations, but I thought it might interest you.

    I should also dispute your observation that helmetless riders at night are more likely to be in dark clothes and without lights. I do a lot of riding at night and I haven’t noticed this trend whatsoever.

  2. SBH

    I’ll take that as a concession

  3. RidesToWork

    Perhaps the Melbourne police have too much time on their hands?

    Enforcement used to be pretty strict here, but I think the police are less likely to bother nowadays. Wearing rates here appear to be falling, especially for children. AFAIK, the police can’t fine children directly, so enforcing the helmet law is likely to be troublesome. suggests that the WA police have an even more lax attitude and rarely bother enforcing the law – anyone have recent data on the number of fines in each state?

    In Ontario, Canada, helmet wearing increased for a couple of years after the introduction of helmet law for children, but there wasn’t a great deal of enforcement, so wearing rates returned to pre-law levels. Fortunately, children’s head injuries continued to decline.

    I know Alan doesn’t think much of the evidence that helmet laws discourage cycling or encourage risk taking, but at least we can see a clear effect of helmet wearing at virtually all distances from the kerb. Saying that the relationship “converges” at 1 metre seems a bit hasty – the consistency at all other distances suggest it might be sampling variation.

    It also seems hasty to dismiss the difference between 60 vs 49 overtaking events (out of 2355) as unimportant, given the number of times the average cyclist is overtaken in a year.

    All it takes is one mistake for a close call to become a collision. This happened twice to Ian Walker during the course of his research. He was hit twice – by a truck and by a bus, both times when wearing a helmet.

    Whether or not it’s due to the way Australian drivers perceive cyclists who don’t wear helmets, there appears to be a difference in passing distance here as well – though I don’t suppose anyone here will be game enough to repeat Dr Walker’s experiment to confirm or deny it, or whether the effect would be more pronounced for non-helmeted female cycles.

    The amount of room left by passing motorists critically affects the perception of safety. I certainly notice the difference and so feel more at ease when not wearing a helmet. Although pure speculation, many female cyclists might have similar experiences and so be more inclined to cycle if the helmet law were relaxed. Would this be a bad thing?

    Many readers of this blog appear keen to dismiss the arguments than the difference in passing room might make a difference. Yet they don’t apply the same critical thinking to claims about helmet laws – such as Figure 2 of The effect, if there is one, is nothing like as clear as the effect of helmet wearing on passing distance.

    Allowing cyclists the choice would therefore seem to represent the best of both worlds – let cyclists do what they consider safest for their individual circumstances and riding conditions.

  4. Alan Davies

    RidesToWork #27, SBH #28, Dylan Nicholson #29:

    There must be some enormous geographical differences in helmet-wearing then. I hardly ever see anyone, adult or child, cycling without a helmet in my neck of the woods, which includes the main Yarra Trail.

    I’ve seen riders without helmets in the inner city in areas like Nth Fitzroy and Brunswick, but only very occasionally (and interestingly, usually at night with no lights either, as well as dark clothing).

  5. Dylan Nicholson

    “Unscientific” is a being polite…I’d suggest “knowingly false”. I live next door to a park very popular with kids on bikes, and at most I might see 3 or 4 kids without helmets every week, unless you count the 2 and 3 yos on those bizarro pedal-less bikes.
    But even though I wouldn’t propose repealing MHL for under 12yos, I will admit I’ve let my 7yo ride there without one more than once.

  6. SBH

    RTW would you be prepared to concede that “about half of all the kids I see on bikes are not wearing helmets,” is simply unscientific and can’t therefore be used in an argument based on statistical analysis?

  7. RidesToWork

    Yes, Olivier et al provide some information in Table 4.

    Is it enough to tell you whether the change since 2006 relates to bike/motor vehicle collisions (as might be expected if the the provision of infrastructure reduced the risk of collisions with motor vehicles)?

    Also, how do you interpret the fact that for adults, non-motor vehicle crashes appear to be increasing faster than vehicle crashes throughout the whole time period?

    As I said above, about half of all the kids I see on bikes are not wearing helmets, so it is interesting that children’s head injuries are falling (despite the slippage in helmet wearing) and this appears to have been offset by an increase in adult head injuries.

  8. Alan Davies

    Cyclesnail #20:

    I presume you’re referring to the Olivier et al paper I discussed recently?

    RidesToWork # 25:

    Olivier et al do appear to distinguish between motor vehicle collisions (MVC) and other causes. On page 4 of their paper the authors say:

    The analysis was repeated separately for each combination of children/adults and MVC/non-MVC with the results presented in Table 4…..

  9. RidesToWork

    Cyclesnail: “From there on it shows a gradual increase in arm injuries, and a smaller, nearly flat rate of increase in head injuries up to 2006. After that year both types of injuries clearly decline.”

    The graph is interesting, especially the big increase in arm injuries from 1991 to 2006. I wonder if this is a reflection of a move away from transport cycling to mountain biking and sports cycling? Cyclists in bike/motor vehicle collisions tend to have leg and head injuries, but falling off the bike, e.g. in a mountain biking situation, seems to give rise to a high rate of arm injuries.

    The census data show a smaller proportion cycling to work in 2006 (0.84%) than 1986 (1.09%), and I’d guess that other transport cycling followed the same trend. Therefore the big increase in arm injuries could well be due to other factors – I suspect that the risk of injury per hour for mountain biking is much higher than normal transport cycling.

    The idea that you can attribute the change since 2006 to “infrastructure” is pure speculation. I would like to see separate graphs of head and arm injuries for falls and bike/motor vehicle collisions, for which the odds of head injury are 3-5 times greater than bike-only crashes.

    I’d be more convinced about the effect of “infrastructure” the data for bike/motor vehicle collisions showed a change, but not people falling off their bikes. The fact that (despite having different codes for bike/motor vehicle collisions and non-MV-collisions, and the big difference in head injury risks) they provide no information on this makes me wonder.

  10. Dylan Nicholson

    SBH, you might think of it as a “dangerous retreat”, but there’s only so much you can do about human nature. Bicycling WILL become massively safer once a high percentage of car drivers are also cyclists, and there’s just too many cyclists on the road for them to be considered a fringe group. At any rate there are plenty of other reasons why we should want to do everything possible to encourage the uptake of cycling, especially options that have virtually zero upfront monetary cost.

  11. SBH

    Dylan @21 completely agree

  12. SBH

    slow down Dylan and have a read of what I wrote. I’m not attacking you. As I’ve noted previously, I’ve no interest in the MHL debate per se. I do think, like Cyclesnail, that it’s a red herring

    My point about normalisation is behaviour doesn’t have to be ‘normalised’- a term I suspect we have very different definitions of in the first place – for it to be able to be conducted without fear of death and injury and with the protection of the law. To say it does lets the people responsible off the hook and turns the normal legal rights and responsibiities of citizens on their head. It’s a dangerous retreat to say things will improve when the behaviour is ‘normailzed’.

  13. Dylan Nicholson

    Actually the second biggest problem with MHL (after the magpies) is the how much time it causes us all to waste on forums and blogs, when frankly there’s more interesting things to discuss…

  14. Cyclesnail

    I feel that the helmet debate is a red herring. To make cycling safer, infrastructure is more beneficial than helmets, based on a graph published in a recent study.

    The paper headed by Mathematician Jake Olivier looks at cycling related hospitalisations due to head or arm injuries in NSW since 1990. The purpose of the paper is to examine the impact of Mandatory Helmet Legislation (MHL) in Australia.

    Not surprisingly the wearing of helmets has an impact on head injuries. But what struck me is the graph that is part of the paper. It shows a sharp drop in the rate of injuries in the year preceding introduction of the new MHL laws in 19991/92. From there on it shows a gradual increase in arm injuries, and a smaller, nearly flat rate of increase in head injuries up to 2006. After that year both types of injuries clearly decline.

    The authors suggest that this is the result of better cycling infrastructure, and not related to MHL. They say “… there was a five-fold increase from 2006 to 2010 in the number of bicycle strategies implemented by local government where the most common element was increasing bicycle infrastructure significantly”.

  15. Dylan Nicholson

    SBH, I don’t know where you live, but if you think cycling is normalised as a mode of transport in Australia then next time you’re in Europe or Asia (probably other places too, just not the U.S.) have a look around at how many people there are on bicycles. At a very minimum I’d expect at least 25% of the population to be using bicycles as a mode of transport (not just recreation) at least one a week. There’s nothing close to that level of usage in any Australian city I’ve visited.

    And I suspect very few of us who want to see MHL repealed would make a simplistic claim like ‘helmets cause accidents’.

  16. pjrob1957

    I live in the vicinity of Pakenham and have ridden without a helmet quite a bit on the road into town. I am certain it makes a difference to the level of care taken by drivers. In one extreme case a few months ago a driver allowed such a distance for me when unhelmeted that I feared for the helmeted cyclist coming the other way. I have thought since it could have been a tragic illustration of this particular type of risk compensation had he been hit.
    I have also ridden quite alot in Sweden and a bit in Netherlands and Denmark where the main difference is that vehicles always drop a little in engine revs as they come up behind you. In Sweden they are pretty much required by law to show awareness and take care around cyclists and a driving test will be failed if a learner driver does not demonstrate this. It is a much safer place to be a cyclist.
    In the absense of laws like this if we can get drivers to take more care by taking of our helmets then perhaps we should simply have the right to do so.

  17. suburbanite

    The distance drivers pass me varies quite a lot, although there are some reliable patterns. Larger vehicles leave much less space than smaller cars, tradie’s ute’s and vans being third only to buses in how dangerously they overtake the gong for the worst driving goes to Australia post vans that also like to cut me off just for good measure. Drivers give me more space in the rain generally than in dry sunny conditions.

  18. SBH

    Thanks Dylan, my view is that there are many more productive and effective ways to encourage cycling and the argument about MHL veers towards the anti-scientific. In short helmets don’t cause accidents, inattentive and aggressive drivers, bad road surfaces and poor path design do.

    As for normalisation itself, I think cycling is already normalised but even so this approach tends to absolve the culpable. We should expect to be treated as human beings going about a normal, legal, enjoyable and healthy public activity and provided the protection of the law to do so without harrasment (no ridesTW you can’t extrapolate that into support for free-dogging).

  19. Dylan Nicholson

    SBH, I suppose the argument is that without MHL, there’d be a much better chance of normalizing cycling as a mode of travel, hence car drivers would no more think of them as a weird aberration as they do of pedestrians. I don’t think MHL is the primary reason cycling is not a popular mode of travel in Australia, but it sure doesn’t help, and it’s worth keeping the argument for reviewing it alive.

  20. SBH

    I wonder what causal effect my helmet had yesterday on the fuming driver who a)blew his horn to tell me I was occupying his piece of road, b) pulled along side me to scream abuse, c)used his car as a weapon to threaten me, d)blocked the cycle path and swung his door into traffic to block me, and e) continued screaming abuse and offering to fight me as he drove away. (XOB-*&^ you know who you are)

    My guess is that driver education and better enforcement of existing laws would have a significantly greater beneficial effect with much less effort than the on going cursing the night of MHL.

    I’ll avoid saying that I have ABSOLUTELY NO DOUBT because absolutism shuts the door on reason.

  21. Last name First name

    Parker Alan•OAM

    I first got involved in bicycle planing and bicycle education and law enforcemnet in the Geelong Bike plan 35 years ago. And followed and took all the disputes about bicycle helmets since 1974 15 years before the helmet legislation came in. I never had any objections myself but I always wore one and still. Helmets should be worn by male primary and secondary school children because they their risk taking behaviour merits that, but studies have shown that nearly all girls are traffic shy and are scared of trucks and buses.

    the way ensure that boys wear helmets is for schools to mandate male pupils to wear helmets and recommend that parents advise their children to wear them except when girls go to secondary and often refuse to ride their bikes because the Helmet messes up their hair. This why far fewer girls ride to school today. With hire bikes that are easily identifiable By police there no need make tourist wear helmets on Hire bikes.

    In London 50,000 people are riding hire bikes. Hire bike installations in Australia will fail to earn their keep and never grow as they do overseas. We have another failure in Brisbane.

  22. RidesToWork

    hk, all the evidence suggests the opposite – that helmet wearers are over-represented in serious injury rates.

    Post law-surveys showed about 83% of adults in NSW (and 74% of children) wore helmets in 1993. Helmet wearing rates appear to have fallen since then – the police often seem to have more important things on their mind. In the school holidays about half of all the kids I see on bikes are not wearing helmets.

    Yet the study RPA study reported in the MJA said that 84% of seriously injured cyclists (16 years or older) were wearing helmets.

    There are a lot of studies showing helmet wearing rates of injured cyclists are higher than population wearing rates. I’m not aware of any showing the opposite effect, are you?

    The research showing the increased health benefits of cycling outweigh the injury risks are based on population studies that follow people for many years until some of them die. The researcher translate this into mortality rates, which show (even in London, where at the time the traffic used to be notoriously dangerous), people who cycled for transport (nearly all without a helmet) lived longer than non-cyclists.

    Whenever people tell me about the health costs of treating injuries to non-helmeted cyclists, I think of the NZ estimates of the cost of hospital treatment – somewhere between 0 and 13 cents (NZ) per helmet per year (reference )

  23. Dylan Nicholson

    hk, and I’m sure swimming fatality rates are a lot more than 3 or 4 times higher for non-wearers of life jacket and other swimming aids. So if that’s an argument for MHL, there’s just as good an argument for mandatory use of swimming aids.

  24. hk

    Excellent and informative reporting. However would we also not be better informed when we make personal decisions knowing that cycling fatality and serious injury rates are three to four times higher for non wearers of helmets based on nationally aggregated data? Such info if it exists, should be the data presented at every opportunity to those who argue the increased cycling health benefits, attributable to non-helmet wearing outweigh the serious injury rates evidenced by differences in injury rates.

  25. Dylan Nicholson

    Well I wouldn’t be too quick to assume anything about what happens in the UK (or anywhere in Europe) has much relevance here. From everything I’ve read, drivers there are generally far more respectful of cyclists anyway (plus there are a lot more cyclists). UK drivers may even be perfectly rational in giving helmeted cyclists less room, if such cyclists tend to be more experienced ones that are used to sharing the road with traffic and less likely to wobble or veer away from their current trajectory. But the behaviour of a sadly high percentage of Australian drivers around cyclists is anything but rational, and based apparently on the idea that their own vehicle should have priority of every other vehicle*, especially those that in their minds don’t belong on roads anyway.

    * Even the phrase “I’m stuck in traffic” bears this out. If I had my way there’d be billboards everywhere saying ‘Remember: *you* are the traffic too’.

  26. nick

    Well done for actually looking at the study Alan. Men in drag on bikes on the road from Bath to Southampton – with ultrasonic distance sensors and video cameras on their helmets in 2006 – must be an even smaller group.

    I wonder if – given the huge reductions in relatively low cost helmet video cameras – this could now be done using local crowd sourced data…

  27. Alan Davies

    Dylan Nicholson #5,6:

    Walker explicitly states it’s the RHS extremity of the bicycle. Good point about the risk profile of helmeted riders (esp in the UK where helmet wearing rates are low).

  28. Dylan Nicholson

    (Just to confirm, we are measuring from very RHS extremity of your bike – typically the tip of the handlebar – to the very LHS extremity of the vehicle – typically the tip of the wing mirror, right?)

  29. Dylan Nicholson

    Alan, sure, though I could probably count on one hand the number of times anyone’s given me as much as a whole metre clearance. Nor would I typically expect that much.
    The average passing distance to me irrelevant anyway – it’s the number of times cars give obviously far too little clearance (say, less than 25 cm) that matters. I’m willing to bet that happens far more often with helmeted cyclists – however, I wouldn’t assume it was causal: it’s quite plausible that cyclists who choose to wear helmets are more likely to feel confident on busy roads where close passing distances are likely anyway.

  30. Alan Davies

    RidesToWork #2:

    Wearing a helmet is normalised in Australia and it’s unusual not to wear one. I’d say drivers are wary of anyone without one and accordingly give them a lot of room. That might not be the case though if unhelmeted riding ever became ‘normal’.

    Dylan Nicholson #3:

    8 cm is the difference between how close cars overtake helmeted and unhelmeted drivers. It’s not how close they pass – that’s over a metre and a half on average for both helmeted and unhelmeted cyclists.

  31. Dylan Nicholson

    One other thing – if you think a 8 cm difference in passing distance doesn’t matter, you haven’t ridden on roads very much – I’m quite used to being passed (and passing) with no more than 20-25 cm distance, which is closer than I’d like but safe enough. 12-20cm on the other hand is pretty perilous.
    Despite this (and the magpies!) I’d still probably personally choose to ride with a helmet even if it weren’t for MHL.

  32. RidesToWork

    In hot weather, I have to take the helmet off before I can make it comfortably up a steep hill on my way home.

    It’s an awkward road where drivers often pass uncomfortably close.

    There’s ABSOLUTELY NO DOUBT that Australian car drivers leave me more room when passing if I don’t wear a helmet, or that I feel safer riding this road without a helmet than when wearing one.

    If it’s really hot enough to irritate my head, I take the helmet off elsewhere. One another awkward road, one driver stayed behind for about 200 metres until it was safe to pass, something that would never happen with the helmet on.

    This may be just another person’s experience, but it’s based on being overtaken by lots of vehicles in hot weather. It’s also based on my experience when the law was introduced, and having 10 near misses (incidents where I would have been hit if I had not taken avoiding action) in the year after the law was introduced, compared to none in the previous 2 years.

    10 near misses post-law compared to none pre-law is a statistically significant difference. I started to wonder about giving up cycling because of the danger, whereas before the law it felt like a cyclist’s paradise.

    I definitely feel safer and less threatened by motor vehicle drivers when not wearing a helmet.

  33. Dylan Nicholson

    Actually I just came across the most convincing evidence yet that helmets do more harm than good: it turns out the best way to prevent swooping magpies (which have very nearly caused me to come off my bike in traffic more than once) is, yep, you guessed it, take your helmet off!

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