Wasserman RC, Waller JA, Monty MJ, Emery AB, Robinson DR.
American Journal of Public Health, 1988 Sep;78(9):1220-1.
516 bicyclists over the age of 10 were interviewed regarding helmet use and head injuries. Although 19% owned helmets, only 8% were wearing them when interviewed. Riders wearing helmets were more highly educated and reported higher car seat belt use. Nearly 4% of the bicyclists reported striking their heads in a cycling mishap during the previous 18 months; those wearing helmets at the time were less likely to have sustained head injuries.
At the time of questioning by the researchers, 40 (7.8%) of the cyclists were wearing helmets and 476 were not. Of the 21 cyclists who reported striking their heads in the previous 18 months, eight had been wearing helmets at the time of the mishap. This is 20% of the 40 helmeted cyclists questioned. The 13 other, unhelmeted, cyclists who had struck their heads represented 2.7% of the 476 unhelmeted cyclists who were questioned in the study. Thus helmet wearers were 7 times more likely to hit their heads than those who cycled bare-headed.
Towner E, Dowswell T, Burkes M, Dickinson H, Towner J, Hayes M, 2002. Bicycle helmets - a review of their effectiveness: a critical review of the literature. Department for Transport Road Safety Research Report 30.