Social scientists have long been skeptical of the power of the threat of legal sanctions, as invoked by enacted laws, to control behavior. This skepticism has been supported by the failure of empirical studies, especially those based on official records of crimes and of the apprehension of offenders, to provide convincing evidence of the deterrent effect of legal penalties. The possibility that an experimental design might avoid the defects of other studies of deterrence has been suggested, and for this purpose, traffic law and its enforcement are especially salient. This paper reports on such a field experiment, which resulted in the conclusion that an increased threat of legal punishment, albeit a relatively small fine, reduced by one-half the number of customary offenders.