A new paper issued by the United States National Highway Traffic Safety Administration (NHTSA) advises against the imposition of per se impairment levels for THC.
Per se DUI laws, which exist nationwide for alcohol and are imposed for THC and/or the carboxy THC metabolite in more than a dozen states, criminally prohibit the operation of a motor vehicle if the driver possesses specific quantities of a controlled substance in their bodily fluids. Under these strict liability standards, drivers who test positive for even residual quantities of the drug or its inert metabolite in their blood or urine are guilty of a traffic safety violation, even in the absence of any further evidence indicating that the subject was behaviorally impaired.
However, according to the November 2014 NHTSA paper
, entitled ‘Understanding the Limitations of Drug Test Information, Reporting, and Testing Practices in Fatal Crashes,’ per se DUI limits are inappropriate for substances other than alcohol.
Authors conclude: “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. Some state laws have established levels for some drugs at which it is illegal to operate a motor vehicle. The alcohol laws are based on evidence concerning the decreased ability of drivers across the population to function safely at these BACs. Such evidence is not currently available for concentrations of other drugs.”
The authors add, “Current knowledge about the effects of drugs other than alcohol on driving performance is insufficient to make judgments about connections between drug use, driving performance, and crash risk.”
NHTSA’s latest paper is consistent with previous acknowledgements made by the federal transportation agency opposing the enactment of per se DUI thresholds for cannabis. Speaking before the US House of Representatives, Committee on Oversight and Government Reform in July, NHTSA’s Dr. Jeffrey Michael acknowledged
,“The available evidence does not support the development of an impairment threshold for THC (in blood) which would be analogous to that (of) alcohol.”
Dr. Michael’s statements mimic those provided by NHTSA’s online cannabis fact sheet
, which states: “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 [metabolite] concentrations.”
Nonetheless, to date, 11 states impose zero tolerant
per se DUI limits for THC and five additional states impose per se limits for the presence of the cannabinoid. In 2012, the White House National Drug Control Strategy Report called for the imposition of zero tolerant per se standards for cannabis in every state, including in those states that allow for the plant’s legal consumption.