Trends in helmet use and head injuries in San Diego County: the effect of bicycle helmet legislation.

The purpose of this study is to evaluate the effect of California's bicycle helmet law on bicycle-related head injuries in San Diego County with a year 2000 population of approximately 3 million people. The study design is an ecological trend design based on observational data from a Trauma Registry. Outcome measures include helmet use compliance, site/severity of injury and abbreviated injury scale (AIS). There were 1116 bicycle trauma patients recorded in the San Diego County Trauma Registry between 1992 and 1996. The percentages of pre-law and post-law helmet use were 13.2 and 31.7%, respectively. Over the whole study period, the overall helmet use increased by an average of 43% per year with an averaged 84% rate increase in helmet use among children. Only 16.1% of patients with serious head injury used helmets, compared to 28.2% in those who did not have serious head injury. The odds ratio of helmet use against serious head injuries is 0.43 (95% CI 0.28-0.66) after adjusting for age, ethnicity and time. The p-values for comparing pre- and post-legislation serious head injury rates are p=0.764, 0.4 and 0.194 for the overall, adult and child populations, respectively. Helmet legislation increased helmet use in the targeted child population and the effect was carried over to the adult population. Helmet use has a protective effect against serious head injury. Probably due to several of its limitations, the current study did not confirm that helmet legislation alone significantly reduced head injury rates in San Diego County during the study period.