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Cook and Sheikh discuss trends in percentages of hospital admissions involving head injury (%HI).1 For pedestrians, %HI declined from 26.9% in 1995/96 to 22.8% in 2000/01 and for cyclists from 27.9% to 20.4%. Did increased helmet wearing (%HW, 16.0% in 1994, 17.6% in 1996 and 21.8% in 1999) cause the larger fall for cyclists?
Another explanation is that more cycle lanes and traffic calming measures (intended to lower the risk of collision with motorised traffic, and hence the proportion of total accidents involving motor vehicles (%MV)), reduced head injuries more than other injuries. Head injuries are 3–5 times more likely in motor vehicle crashes than bike only crashes.2,3 Thus if %MV declines, as in New Zealand (fig 1),4 so should %HI. In South Australia, %HI also declined progressively, as did %MV: 24.6%, 23.6%, 21.3%, 19.7%, and 18.3% over the years 1988 to 1992.5
The risk of head injury decreases with impact speed. When dummies on bikes were hit by imitation vehicles, lowering impact speed from 40 to 30 km/h reduced head injury criterion by 79%, maximum head acceleration by 50%, but maximum chest, pelvis, and knee accelerations by only 30%, 16%, and 21%.6 Traffic calming aims to reduce impact speed, and therefore %HI.
Cyclist injuries contain other trends. In New Zealand, the proportion involving secondary school age children fell from 31% in 1990 to 21% in 1996 (fig 1). Risk of head injury varies with age.7 So %HI will vary with age composition of injured cyclists, within the age ranges (<16, ⩾16 years) considered.
Little can therefore be concluded from datasets with small gradual changes in %HW. The effect cannot be separated from other gradual changes, including overall rider experience, amount of off-road riding, campaigns for drivers to look out for cyclists, or those discussed above.
Differences in %HI of wearers and non-wearers in case-control studies can also be explained by other factors. The two groups often have different riding patterns and attitudes to risk, making it very difficult to correctly adjust for all relevant confounders.
However, when %HW changes dramatically but %HI does not, only one conclusion is possible—that helmets are largely ineffective. In New Zealand, %HI for primary schoolchildren and adults followed almost identical trends, even though adult %HW increased dramatically (43% to 92%) with the law, but not primary schoolchildren (fig 2). Head injury and helmet wearing data have been compiled for New Zealand (fig 2), South Australia,5 Western Australia,8 Victoria,7 Queensland, and New South Wales.9 In every case, helmet laws produced enormous changes in %HW, but little noticeable effect on %HI, just relatively smooth, gradual trends as in fig 2.
The claim that helmets prevent 60% of serious head injuries is simply not plausible if all data (case-control studies, trends in cyclist injuries, and effects of helmet laws) are considered together.