The evidence for the efficacy of cycle helmets is extremely poor. There are a great many people with strong opinions on helmets, but we simply do not have the evidence and it is likely that they are a relatively unimportant factor. While we know that helmets moderately reduce head injuries, we do not have good evidence that they improve rider safety overall. We do have some evidence of risk compensation, with both drivers and riders taking more risks when helmets are used, based on the belief that the helmet provides safety. Head injuries are an important class of injury, particularly in the most severe incidents, but they represent only a minority of the serious injuries suffered by cyclists.
The best available evidence shows that one factor completely overwhelms all others - the number of cyclists on the road. Most Americans believe that the numbers of cyclists will increase when action is taken to improve safety, but in fact the inverse is true. An increase in the number of cyclists invariably leads to a decrease in the number of accidents per km. That is the key message and everything else is just noise. Meaningful improvements in cycle safety are wholly reliant on increasing the number of cyclists and normalising cycling. Motorists cannot be blamed for struggling to predict the movements of a type of vehicle which they encounter rarely and do not understand.