The first major study of bicycle helmet effectiveness, and probably the most famous and most often quoted is the 1989 case-control study of the effectiveness of bicycle safety helmets by Thompson, Rivara and Thomas. This is the study that concluded that "riders with helmets had an 85 percent reduction in their risk of head injury... and an 88 percent reduction in their risk of brain injury." These numbers have been repeated ever since by a variety of medical and insurance organizations and government agencies, despite the fact that "later efforts to replicate those results found a weaker connection between helmets and head injuries." In fact, in 2013, in response to a petition from WABA, the CDC and NHTSA agreed to remove these estimates from their website.
Now, in 2015, [Update: this was re-printed in 2015, but the study actually dates to 1997. I apologize for the confustion] Thompson, Rivara and Thompson have a new study out did another study in 1997 that showed no connection between helmet use and serious injury. In a review of questionnaires filled out by 3390 cyclist injured over a three year period, they determined that "Risk for serious injury was not affected by helmet use (OR=0.9)...[and]...neck injury was not affected by helmet use." Instead they determined that
Prevention of serious bicycle injuries cannot be accomplished through helmet use alone, and may require separation of cyclists from motor vehicles, and delaying cycling until children are developmentally ready.
Their other conclusions (looking at just the abstract, because I don't have access to the full article) include:
- 51% of injured cyclists wore helmets at the time of crash.
- Only 22.3% of patients had head injuries and 34% had facial injuries.
- Risk of serious injury was increased by collision with a motor vehicle (duh), biking faster than >15 mph, young age (<6 years), and age >39 years.
- Risk of neck injury was increased in those struck by motor vehicles, hospitalized for any injury, and those who died.
Update: The study does seem to have more support for the efficacy of helmets. It points out that only 1 out of 14 (7.1%) fatal crashes involved a helmeted cyclist, while 50.9% of non-fatal injury crashes did. And it notes that
Head injuries constituted a much lower proportion of all injuries than in prior reports, including our previous case-control study...We have previously documented a more than two thirds reduction in population based rates of emergency department treated head injuries among children from a large health maintenance organization over this period of time associated with increases in community wide helmet wearing rates.We believe these data provide further support for the effectiveness of community based helmet promotion programs.
Of course, it is possible that un-helmeted cyclists (who already demonstrate different behavior than others) are more likely to behave in ways that result in fatal crashes, like riding dangerously in traffic. While helmeted cyclists are more likely to be injured on trails. And they also show that helmets do nothing to prevent neck injuries and that those with neck injuries are 15 times more likely to die.
So there's something for everyone here.