Earlier this month two states -- Montana and Oklahoma -- enacted per se legislation prohibiting drivers from operating a motor vehicle if they possessed trace quantities of cannabinoids in their system. Under these new laws, prosecutors no longer have to prove that a defendant is behaviorally impaired by cannabis or that he or she presents a legitimate traffic safety risk. In fact, Oklahoma’s new law even disallows drivers from operating a vehicle with any detectable level of THC or its inert metabolite (byproduct) carboxy THC (aka THC-COOH) -- a nonpsychoactive compound that has been documented to linger in the urine of former cannabis consumers for months after they cease using pot.
 
But Montana and Oklahoma are hardly alone. Oklahoma is now one of nine states to impose criminal sanctions for those who drive with any detectable amount of either THC or its metabolites in their blood or urine (the other eight states are: Arizona, Delaware, Georgia, Illinois, Indiana, Iowa, Rhode Island, and Utah). Two other states, Michigan and Wisconsin, impose similar zero tolerant per se statutes for THC in blood, but exclude marijuana metabolites. In addition, five states -- Montana, Nevada, Ohio, Pennsylvania, and Washington -- impose per se limits for cannabinoids -- meaning that the identifiable presence of THC in blood above a state-specified standards (anywhere between 1ng/ml and 5ng/ml, depending upon the state in question) is proof of a traffic safety violation (under Colorado law, the detection of 5ng/ml or greater of THC in blood gives rise to a "permissive inference" of driver impairment, but this inference may be rebutted in criminal court).
 
While some traffic safety studies have associated the presence of active THC in blood with nominally elevated risk of accident, investigators acknowledge that this risk is far lower than that associated with other illicit and licit drugs, particularly alcohol. Nonetheless, law enforcement officials are increasingly pushing for one-size fits all pot per se standards, despite the fact that scientific experts have consistently declared such standards to be inapplicable to cannabis.
 
For instance, writing in the Journal of the American Medical Association in 1985, an expert panel on drugs and driving concluded: “Except for ethanol, determinations of drug concentrations in body fluids are at present of limited value for establishing driving impairment. … Although psychoactive drugs are those most commonly thought to cause impaired driving, their mere presence in body fluids cannot be construed as evidence of impairment.”
 
In the years since the publication of the JAMA report, little has changed. According to a 1993 US Department of Transportation study, one cannot accurately predict the driving performance of subjects administered cannabis based upon their THC blood levels alone. The authors concluded: “One of the program’s objectives was to determine whether it is possible to predict driving impairment by plasma concentrations of THC and/or its metabolite, THC-COOH, in a single sample. The answer is very clear: it is not. Plasma of drivers showing substantial impairment in these studies contained both high and low THC concentrations; and, drivers with high-plasma concentrations showed substantial, but also no impairment, or even some improvement.”
 
A 2003 US Department of Transportation report, State of Knowledge of Drug-Impaired Driving, reiterated this lack of correlation between illicit drug concentrations and psychomotor impairment, finding, “[F]orensic toxicologists generally have failed to agree on specific plasma concentrations that could be designated as evidence of impairment.”
 
A 2004 National Safety Council report similarly acknowledged, “There is no clear correlation between blood drug concentrations and impairment for many drugs.” Authors added: “In DUI cases involving alcohol, a clear understanding has developed over the past 50 years regarding the relationship between increasing blood concentration and impairment. … The same cannot be said for drugs.”
 
Finally, a recent online fact sheet of the US National Highway Transportation and Safety Administration (NHTSA) once again acknowledges that neither the presence of THC or its metabolites ought to be the basis of determining psychomotor impairment: “It is difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects. Concentrations of parent drug and metabolite are very dependent on pattern of use as well as dose. THC concentrations typically peak during the act of smoking. … 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 concentrations.”
 
That’s what the experts say. So why are cops and politicians claiming otherwise?