We're not sure which is more absurd, the ongoing unwarranted belief in the effectiveness of bicycle helmets at preventing injury/death or the exaggerated claims regarding the cost that bicyclist head injuries pose to society
Whenever someone states that "helmets reduce the incidence of serious head injuries by 85%" or something similar, they are basing this claim on a series of studies that gathered data from Seattle-area emergency rooms in the late 1980's and early 1990's. Even the authors of these studies now acknowledge that these studies suffer from serious methodological flaws. The same data, for example, can be used to "show" that bicycle helmets significantly reduce the risk of leg injuries, too. All such studies are hopelessly compromised by confounding variables, hidden factors which affect the outcome, such as the fact that helmeted riders tend to be more cautious by nature than riders who refuse to wear a helmet, hence will be less likely to have an accident in the first place.
There is no statistically significant evidence suggesting that increased helmet use decreases the rate of bicyclist injuries/fatalities. On the contrary, in the US, increased helmet use is actually correlated with an increase in head injuries! According to an article published in the New York Times July 29, 2001, from 1991 to 2000, at the same time that voluntary helmet use in the United States went from 18% to 50%, the number of bicyclist head injuries increased by 10%. However, during this period bicycle use actually declined by 21%, so that the effective increase in head injuries was 51% -- a strong linear correlation between increased helmet use and increased head injuries.
G.B. Rogers [Rodgers, G.B., Reducing bicycle accidents: a reevaluation of the impacts of the CPSC bicycle standard and helmet use, Journal of Products Liability, 11, pp. 307-317, 1988] studied over 8 million cases of injury and death to cyclists over 15 years in the USA. He concluded as follows: "There is no evidence that hard shell helmets have reduced the head injury and fatality rates. The most surprising finding is that the bicycle-related fatality rate is positively and significantly correlated with increased helmet use."
When one combines this with the fact that the entirely unhelmeted bicyclists in Holland, Denmark, and Japan have per capita (or per billion km. traveled) fatality rates that are 6-12 times lower than bicyclists in the US, the evidence is rather clear: bicycle helmets play at best a negligible role in increasing bicyclist safety, and compelling evidence exists that they might actually reduce safety.
Additional statistics and other useful information can be found here:
http://www.cyclehelmets.org/mf.html?1052
