Scuffham P, Alsop J, Cryer C, Langley JD. Accident Analysis and Prevention, 2000 Jul;32(4):565-73. 2000.
The purpose of this study was to examine the effect of helmet wearing and the New Zealand helmet wearing law on serious head injury for cyclists involved in on-road motor vehicle and non-motor vehicle crashes. The study population consisted of three age groups of cyclists (primary school children (ages 5-12 years), secondary school children (ages 13-18 years), and adults (19+ years)) admitted to public hospitals between 1988 and 1996. Data were disaggregated by diagnosis and analysed using negative binomial regression models. Results indicated that there was a positive effect of helmet wearing upon head injury and this effect was relatively consistent across age groups and head injury (diagnosis) types. We conclude that the helmet law has been an effective road safety intervention that has lead to a 19% (90% CI: 14, 23%) reduction in head injury to cyclists over its first 3 years.
The surveys were based on approx 10,000 cyclists in each of the three age groups. No direct account was taken of exposure (cycle use). The ratio of head injuries to non-head injuries was used as a proxy.
Scuffham claimed that the NZ law reduced head injuries by 20%, but he included scalp lacerations in the definition of head injury (scalp lacerations are not truly serious head injuries) and he omitted to emphasise that the cyclists head injury trend had drifted well above the control trend before the law, so the law merely returned the cyclist trend to what it would have been without helmets.
Nigel Perry (Perry, 2001) obtained the original data from the authors, and produced the following graph directly from this. It shows information not presented in the paper.
The graph shows the change in the ratio of head injuries to all injuries, relative to the reference date, March 1988. This is shown for two groups: cyclists, and a control group of the whole population –motorists, pedestrians, ladder climbers – everyone who sustained an injury. Also shown is the percentage of cyclists wearing helmets. Cycle helmets became mandatory in New Zealand from January 1994.
This graph shows that:
The New Zealand Household Travel Survey (LTSA, 1993-7) shows that cycling hours decreased by 34% from 1989 to 1997, or approx 22% since the helmet law. This is greater than the 19% reduction in head injuries suggested by Scuffham et al.
Notwithstanding the previous critique, head injuries to cyclists have not fallen by more than the decline in cycle use.
New Zealand Household Travel Survey. Land Transport Safety Authority.
Perry N, . The bicycle helmet legislation, curse or cure?. University of Canterbury. Presented to Cycling 2001, Christchurch..