OPINION:  The laws today typically define "drunk driving" as "driving under the influence of alcohol and/or drugs", with marijuana included in the meaning of "drugs".  In most states, the very presence of marijuana in a driver's blood is either illegal in itself or presumptive of impaired driving.  The prevailing view for years has been that cannabis, like alcohol, impairs the coordination, reflexes, perception and judgment necessary for the safe operation of a vehicle.

But does marijuana, in fact, adversely effect the ability to drive safely?  On this issue, governmental studies are in conflict.

The federal government'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. "The Effects of Alcohol on Driver-Controlled Behavior in a Driving Simulator, Phase I", DOT-HS-806-414. The study found that alcohol consistently and significantly caused impairment -- but that marijuana had only an occasional effect. Further, there was little evidence of interaction between alcohol and marijuana. Finally, speeding tickets and accidents went up with the use of alcohol, but no marijuana or combined alcohol-marijuana influence was noted. 

Coming to a different conclusion, however, the California Department of Justice found that marijuana does impair impair driving skills, particularly at high-dose levels or among inexperienced users. "Marijuana and Alcohol: A Driver Performance Study", California Office of Traffic Safety Project No. 087902.

Yet, a more recent federal study has found that "THC [the intoxicating ingredient in marijuana] 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".  "Marijuana and Actual Performance", DOT-HS-808-078

The study concluded 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".  

Should evidence of marijuana in a driver's body be sufficient for a drunk driving conviction?  It should be noted that the "THC" mentioned is fairly quickly converted by the body into inert metabolites.  These metabolites can stay in the body for hours or even days. And it is these metabolites that are detected and measured by blood tests from DUI arrests.

In other words, (1) marijuana may not impair driving ability at all, and (2) the blood "evidence" usually measures only an inactive substance which may have been present for days.

About the author:  Lawrence Taylor is an Orange County DUI attorney, a former Fulbright Professor of Law (Japan), and the author of the standard legal textbook in the field, Drunk Driving Defense, 6th edition (cited as authority by the California Supreme Court).