We use a simple model of drivers' vigilance effort choice to show that drivers' propensity to follow traffic rules has two opposite effects on road safety. On the one hand, it lowers the frequency of dangerous situations. On the other hand, it also reduces drivers' vigilance effort as each driver anticipates that dangerous situations will be less frequent. These two opposite effects may lead to a non-monotonic relationship between compliance with road rules and the incidence of road traffic accidents. We present crosscountry estimates that support the existence of a bell-shaped relationship between norms of compliance with rules and traffic fatalities.