This paper reports the initial impact on motor vehicle crashes and casualties resulting from recent efforts to crackdown on drinking drivers in Arizona. Like similar efforts throughout the United States, Arizona lawmakers adopted tough, deterrence-based legislation as the state's chief response to heightened concern with the problem of alcohol-impaired driving. The impact of the new law was measured using interrupted time-series analysis of traffic data. The results indicated that the legal intervention had no deterrent effect. An examination of possible competing events suggested that significant deterrence of drinking and driving was achieved prior to introduction of the new law. Initiation of this effect was attributed to preexisting anti-drunk-driving publicity.