Ming J, Gilchick RA, Bender SJ. Accident Analysis & Prevention, 2006;38(1):128–134
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. 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. 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. However, the study did not confirm that helmet legislation alone significantly reduced head injury rates in San Diego County during the study period.
This paper provides nothing in the way of comparable pedestrian or motor vehicle occupant head injury data, nor does it address anything along the lines of exposure data, either pre- or post-law. Indeed, the authors acknowledge a number of limitations with their study, including the relatively large number of unknown helmet use cases (~26% of 1116 bicyclists found in the Trauma Registry maintained by San Diego County from 1992-96), but perhaps the most interesting conclusion they draw is the following:
"Despite the increase of helmet use, we were not able to show significant reduction of serious head injury rates over the study period."
Indeed, the percentage of cyclists with serious head injuries actually rose from 27.2% in the pre-law period to 28.2% post-law, despite a (modest) increase in percent helmet wearing.
Helmet use among juvenile (<18) San Diego cyclists apparently increased from 1% in 1992 to 10% in 1993, 25% in 1994 (the year California's MHL for juvenile cyclists went into effect), peaked at 39% in 1995, and then went down to 24% in 1996. Meanwhile, helmet use among adult (18+) cyclists is said to have jumped from 5% in 1992 to 20% in 1993, and then drifted up to 22% in 1994 and 26% in 1995-96. While the authors have no explanation for the big drop in helmet use among juvenile cyclists in 1996, it would be a natural outcome of a failure to effectively enforce the mandatory helmet law..
Likewise, while the post-1993 data seem to indicate an appropriately inverse relationship between helmet use and the number of "serious" (ASI>=3) head injuries, the relatively small number of such serious head injuries (~60 per year), and the fact that serious head injuries actually rose from 59 in 1992 to 71 in 1993 when helmet use was growing fastest among both juvenile and adult cyclists negate any statistical significance for that apparent inverse relationship.
It would have been interesting to see how this relationship between serious head injuries and helmet use progressed after 1996 (not to mention an appropriate control group of pedestrian and/or motor vehicle occupant head injury cases), but the authors evidently limited themselves to a simple before and after exercise to study just the effects of the law rather than examine the larger question of helmet effectiveness in general.
The study had serious limitations, but the authors acknowledge the essentially null result they found.