From Victoria, Australia, the next in our line of prima facie causes. Thanks to pfh for the link and the best-so-far interpretation: Therefore less feral cyclists with baseball bats taking a swipe at pedestrians
Discussion
Bruce Robinson, the paper's author, was aware that the implied causal interpretation was ridiculous: Indeed, using the same "one-parameter logic" as many of the initial helmet effectiveness reviews, it is possible to "show" in Figure 3. that helmet legislation has substantially reduced pedestrian fatalities in Victoria. But this is clearly impossible. Failure to recognise external factors such as other road safety and usage changes has devalued many Australian evaluations of helmet effectiveness.Using somewhat more thorough analysis, Robinson concludes that helmet legislation had no obvious effect. I'd say he makes a good case, without pushing any particular explanation for the failure. Instead, he makes a case that with no obvious effect, resources could better be spent on preventing crashes:
WA's helmet legislation has diverted resources from preventing bicycle crashes into increasing helmet wearing rates. For instance, about 50% of the WA Police Bicycle Safety Section's time is devoted to enforcing the helmet law. At the same time, about 80% of cyclists on the road at night are riding without lights, with very little chance of getting apprehended.
I've met some of those cyclists on the W&OD trail. One early morning in the dark, I pulled up along side one and said, "You really should get some lights -- there are joggers out here that have no lights on them." He replied, "Yeah I know. I ran into one the other day." I slowed down my ride to stay with him until he left the trail.
Moral Hazard
The most common explanation for the lack of effect is "moral hazard": that people adjust their behavior to compensate for the perceived safety. For example, they ride faster or more dangerously when wearing helmets, or drive faster when wearing seatbelts or using antilock brakes.I'm dubious. This implies first that people can perceive an increased level of safety, and second that they manage to adjust their behavior to precisely compensate for the added safety. It's not clear to me that we can do either. One test would be a randomized trial to have people bike or drive an obstacle course with/without helmets or seatbelts. If you are aware of this or some other test, please email me. I might just be ignorant.
Compare helmet use to sunblock use. A massive study around 2001 showed that there had been zero reduction in skin cancer due to increased use of sunblock. But we know that sunblock does prevent sunburn. Here, a "moral hazard" explanation is quite believable: people wearing sunblock stay out in the sun longer -- that's why they put on the sunblock. Moreover, they can easily adjust their exposure to be precisely the same as it was before: stay out in the sun until just before you burn.
Links
Robinson's paper is avilable from helmets.org.
Ken Kiefer has another good discussion.