The United States Department of Transportation has estimated that in 1970, more than 111 million Americans were licensed drivers and that there were approximately 109 million vehicles registered and regularly traversing America's streets and highways. The Commission-sponsored National Survey (Abelson, et al., 1972) has revealed that approximately 21 million or 15% of all American adults 18 years and over have tried marihuana and that about 6.9 million adults currently use the drug.

At the present time, there is no reliable estimate of the proportion of marihuana users who drive while "high," but America's tragic experience with highway accidents and fatalities involving persons driving under the influence of alcohol raises serious questions about the extent to which marihuana impairs driving skills and performance and thereby constitutes a public safety hazard on this nation's thoroughfares.

In recent years, public safety experts, along with the medical and scientific communities, have devoted increasing attention to the effects of any mind-altering drug on driving, but there is as yet little evidence to inform discussion. As part of its more general concern with the impact of marihuana on public safety, the Commission has reviewed the available research and has concluded that the evidence which presently exists is, at best, inconclusive.

In view of this finding and prior to a rather brief summary of the knowledge we now have, the Commission feels compelled to urge the public to consider these findings as only tentative, to adopt an extremely cautious attitude about the effects of marihuana on driving skill and performance, and perhaps most importantly, to avoid driving at all while under the influence of any mind-altering drug or intoxicant.