Study: Evidence Fails to Support DUI Impairment Levels for Cannabis

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Study: Evidence Fails to Support DUI Impairment Levels for Cannabis

By Paul Armentano, NORML

WASHINGTON, DC — Available science fails to support the imposition of driving under thedriving influence (DUI) impairment thresholds for cannabis in a manner that is analogous to the per se limits already in place for alcohol, according to the conclusions of a November 2014 publication published by the United States Department of Transportation, National Highway Traffic Safety Administration (NHTSA). Per se traffic safety laws criminalize those who operate a vehicle with trace or specific levels of a controlled substance in their bodily fluids, even in the absence of any further evidence indicating that the subject was behaviorally impaired.

States the paper’s authors, “Every state has enacted a law defining drivers who are at or above .08 grams per deciliter BAC as ‘legally impaired,’ but there are no similar, commonly accepted impairment levels for other drugs.” Nonetheless, despite this lack of consensus, authors acknowledge that “some state laws have established levels for some drugs at which it is illegal to operate a motor vehicle” – a position which they concede is not evidence based.

“The alcohol laws are based on evidence concerning the decreased ability of drivers across the population to function safely at these BACs,” they write. “Such evidence is not currently available for concentrations of other drugs.”

Delta Extrax

Eleven states presently impose a zero tolerant per se DUI limit for THC and, in some cases, the presence of its inert metabolite carboxy THC. Five additional states impose per se limits for the presence of either THC or carboxy THC.

NHTSA’s recent acknowledgement is far from the first time that the federal traffic safety agency has opined against the imposition of per se limits for THC. In an online factsheet, entitled Drug and Human Performance: Cannabis/Marijuana, the agency concludes: “It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects. … It is inadvisable to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH [carboxy THC metabolite] concentrations.”

Full text of the NHTSA report, “Understanding the Limitations of Drug Test Information, Reporting, and Testing Practices in Fatal Crashes,” appears online at:


  • Tom Rector
    December 15, 2014

    I sent you information on the NTSB study published in September 2014. The study was performed because of the data I sent them. The NTSB called on the FAA to clarify it’s policy on cannabis and to study drug use by pilots because toxicology studies show cannabis is only found in 2 to 2 1/2% in non-drug tested pilots. Sedating antihistamines (sold otc) is found 4 times as often as cannabis and 3 times more often than all drugs we test for on a standard drug screen.we have to end drug testing if we’re going to end prohibition.

  • Harry Om
    December 17, 2014

    Why do people use cannabis, in the first place? To make a change in their normal life. They either use cannabis for pain abatement or they use it recreationally like alcohol, to relax and get a buzz. It’s impairment no matter how one looks at it, i.e., and altered state of the mind and of the body.

    Just because that impairment has not been narrowed down to the equivalency of .08 DUI, doesn’t mean there is no impairment.

  • michele
    July 28, 2015

    Is there any new information of how individual states are treating dui for thc ? In regards to levels of impairment at the time of stop?

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