In 1980, the Federal Republic of Germany (West Germany) introduced spot fines on motorcyclists not wearing helmets. The primary motivation was to reduce head injuries, but it had an unexpected and dramatic impact in a totally different area: thefts. In the wake of the change, motorcycle thefts fell by 60 per cent, and stayed down.
You might think that if a person intended to steal a bike, this change in the law would not make that much difference: they just had to remember to bring a helmet with them, or to steal one, too. But, it would seem, most offenders did not do this. It was extra hassle, and required forethought. Riders often carried their helmets with them, rather that leaving them on the bike. In short, the requirement to wear a helmet introduced ‘friction’ to the act of stealing a motorbike, with dramatic consequences.