Attorney: Marijuana May Not Impair Driving Ability At All

Drunk driving laws today typically define "driving under the influence" as covering both alcohol and drugs, with marijuana included as "drugs." In most states, the very presence of marijuana in a driver's blood is either illegal in itself, or is considered proof of impairment.

"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," said DUI attorney Lawrence Taylor.

But none of us has the pot equivalent of those ubiquitous "I was so drunk I totaled my car when I was a teenager" type of stories. And the highway carnage that would accompany marijuana's popularity -- like that which has accompanied alcohol's -- seems never to have happened.

The blood or urine tests typically used to determine the presence of marijuana metabolites don't measure impairment or intoxication; in fact, such tests can detect marijuana days or even weeks after it was last ingested, long after any "impairment" is in the distant past.

The federal government's Department of Transportation (DOT) did research with a fully interactive simulator on the effects of alcohol and marijuana, alone and in combination, on driver 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 only had an occasional effect.

Further, contrary to drug warrior mythology, there was little evidence of interaction between alcohol and marijuana.

Speeding tickets and accidents went up with the use of alcohol, but no marijuana influence on speeding and accidents was noted. Additionally, alcohol-impaired drivers who also smoked marijuana showed no additional impairment from the pot.

The California Department of Justice came to a different conclusion, claiming that marijuana does 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).

But a more recent federal study found that "THC [the active 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 federal study says that "It appears not possible to conclude anything about a driver's impairment on the basis of his/her plasma concentrations of THC... determined in a single sample."

"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," said Taylor.

"Prosecutors readily acknowledge that a person who smoked a week earlier couldn't possibly be impaired by marijuana, and yet they still prosecute, and often win, these cases," noted a commenter on Taylor's webpage.