What about substitute activities, they can’t ride so they walk, probably a definite cost benefit both in fitness and reduced accidents. And riskier behaviour because you wear a helmet, sounds suspicious. Theoretically maybe, in reality, seems like a very minor effect. Now a reliable protection against gravel rash and you might be onto something.

There is an argument that helmets give riders a false sense of security when they cycle. As a result, riders with helmets may engage in more risky riding behaviours than they would otherwise, which in turn leads to a higher incidence of accidents among helmet wearers.