Risk homeostasis--for drivers

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Economists love to talk about risk homeostasis, the theory that when activities are made safer, people respond by acting in riskier ways. The classic example here is the Munich taxicab study, in which it was found that drivers of taxis equipped with ABS had the same accident rate as those without ABS (see here for a discussion of the experiment by Gerald Wilde, the main proponent of the theory), and drove more aggressively--with the implication being that they expected the ABS to help them recover.

There's a lot of controversy over the extent to which risk compensation is actually a factor in people's behavior, but here's an interesting result from Ian Walker at U. Bath: when riders are wearing helmets, drivers are less careful:

To carry out the research, Dr Walker used a bike fitted with a computer and an ultrasonic distance sensor to find drivers were twice as likely to get close to the bicycle, at an average of 8.5cm, when he wore a helmet.

The experiment, which recorded 2,500 overtaking motorists in Salisbury and Bristol, was funded by the Engineering and Physical Sciences Research Council.

Dr Walker, a traffic psychologist from the University's Department of Psychology, said: "This study shows that when drivers overtake a cyclist, the margin for error they leave is affected by the cyclist's appearance.

"By leaving the cyclist less room, drivers reduce the safety margin that cyclists need to deal with obstacles in the road, such as drain covers and potholes, as well as the margin for error in their own judgements.

"We know helmets are useful in low-speed falls, and so definitely good for children, but whether they offer any real protection to somebody struck by a car is very controversial.

"Either way, this study suggests wearing a helmet might make a collision more likely in the first place," he added.

Dr Walker thinks the reason drivers give less room to cyclists wearing helmets is because they see them as "Lycra-clad street warriors" and believe they are more predictable than those without.

I don't know of any end-to-end controlled trials of mortality with and without bicycle helmets. Looks like I need to spend some time with pubmed.

4 Comments

I remember this New York Times article from a few years ago.

Synchronicities are interesting — I just heard an item related to this on the radio this morning. Since it was just in the local segment, not the NPR portion, I have no reference to point to, but the announcer said that 97% of the bicycle fatalities in NYC in the last three years were of cyclists that were not wearing helmets. As a result of this, they said, the city is giving out free helmets — I didn't get the details, but I presume that's for kids, probably low-income kids.

From my experiences with the drivers in Bath, I'm amazed there are any cyclists alive at all...

I don't know if you can get to Science Direct, but the full text is here

Bicycle helmet legislation: Can we reach a consensus?
Robinson DL.

c/o Prof JSF Barker Building, Trevenna Rd, Armidale, NSW 2351, Australia.


Accid Anal Prev. 2006 Aug 17; [Epub ahead of print]


Debate continues over bicycle helmet laws. Proponents argue that case-control studies of voluntary wearing show helmets reduce head injuries. Opponents argue, even when legislation substantially increased percent helmet wearing, there was no obvious response in percentages of cyclist hospital admissions with head injury-trends for cyclists were virtually identical to those of other road users. Moreover, enforced laws discourage cycling, increasing the costs to society of obesity and lack of exercise and reducing overall safety of cycling through reduced safety in numbers. Countries with low helmet wearing have more cyclists and lower fatality rates per kilometre. Cost-benefit analyses are a useful tool to determine if interventions are worthwhile. The two published cost-benefit analyses of helmet law data found that the cost of buying helmets to satisfy legislation probably exceeded any savings in reduced head injuries. Analyses of other road safety measures, e.g. reducing speeding and drink-driving or treating accident blackspots, often show that benefits are significantly greater than costs. Assuming all parties agree that helmet laws should not be implemented unless benefits exceed costs, agreement is needed on how to derive monetary values for the consequences of helmet laws, including changes in injury rates, cycle-use and enjoyment of cycling. Suggestions are made concerning the data and methodology needed to help clarify the issue, e.g. relating pre- and post-law surveys of cycle use to numbers with head and other injuries and ensuring that trends are not confused with effects of increased helmet wearing.


PMID: 16919590 [PubMed - as supplied by publisher]

My perception (when I cycled a lot) was that drivers treated me better when I was in "proper" cycling gear on a fast bike. I assumed this was because they put me in a different category from the kids and foreign students (who were notorious for ignoring all rules of the road). I think it also occurred to them that I might actually be travelling at a noticeable speed - many drivers seem to regard cyclists as one of those abstract physics concepts ("a mass with zero width and zero speed"), leading to them getting too close or pulling out in front of you.

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