It is against the law to drive while under the influence of marijuana. It has always been assumed that cannabis, like alcohol, impairs the perception, coordination, reflexes and judgment necessary for the safe operation of a motor vehicle. And, of course, there have been governmental studies addressing the question: Does marijuana impair driving?
Interestingly, however, the findings do not necessarily support popular opinion…. On the one hand, the California Department of Justice has found that marijuana undoubtedly impairs psychomotor abilities that are functionally related to driving and that driving skills may be impaired, particularly at high-dose levels or among inexperienced users. "Marijuana and Alcohol: A Driver Performance Study", California Office of Traffic Safety Project No. 087902 (Sept. 1986).
Contradicting these conclusions, however, are two federal studies. The U.S. Department of Transportation conducted research with a fully interactive simulator on the effects of alcohol and marijuana, alone and in combination, on driver-controlled behavior and performance. Although alcohol was found consistently and significantly to cause impairment, marijuana had only an occasional effect. Also, there was little evidence of interaction between alcohol and marijuana. Accidents and speeding tickets reliably increased with alcohol, but no marijuana or combined alcohol-marijuana influence was noted. "The Effects of Alcohol on Driver-Controlled Behavior in a Driving Simulator, Phase I", DOT-HS-806-414.
A more recent report entitled "Marijuana and Actual Performance", DOT-HS-808-078, noted that "THC is not a profoundly impairing drug….It apparently affects controlled information processing in a variety of laboratory tests, but not to the extent which is beyond the individual’s ability to control when he is motivated and permitted to do so in driving". The study concluded that:
An important practical objective of this study was to determine whether degrees of driving impairment can be actually predicted from either measured concentration of THC in plasma or performance measured in potential roadside "sobriety" tests of tracking ability or hand and posture stability. The results, like many reported before, indicated that none of these measures accurately predicts changes in actual performance under the influence of THC…
The researchers found that it "appears not possible to conclude anything about a driver’s impairment on the basis of his/her plasma concentrations of THC and THC-COOH determined in a single sample".
Note: "THC" stands for Delta-9-tetrahydrocannabinol, which is the intoxicating ingredient in marijuana. THC is fairly quickly converted by the body into inert metabolites, which can stay in the body for hours or even days. It is these metabolites that police blood tests in DUI arrests detect and measure. In other words, (1) marijuna may not impair driving ability at all, and (2) the blood "evidence" only measures an inactive substance which may have been there for days.