Efforts to better identify and prosecute impaired drivers are laudable, but the enactment of unscientific and inadvisable ‘per se’ legislation for THC — the primary psychoactive compound in marijuana — and its inert metabolites (byproducts) is not a scientific or advisable approach to addressing traffic safety. Nonetheless, sixteen states now impose either ‘per se’ or zero tolerant ‘per se’ laws for motorists who operate a vehicle with even trace levels of these constituents in their blood or urine, regardless of whether or not they are behaviorally impaired. (Other states require evidence of recent drug use and evidence of behavioral impairment due to said use in order to go forward with a DUI drugs charge.) Additional states, like California, are proposing similar ‘per se’ laws for pot.

Such legal ‘per se’ limits for cannabinoids are arbitrary and problematic. Here is why.

Both THC and carboxy THC (THC-COOH), may be detectable for periods of time extending beyond any reasonable period of impairment, such as days (in the case of THC) or weeks (in the case of carboxy THC). As a result, the US National Highway Traffic Safety Administration affirms, “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 concentrations.”

The United States Department of Transportation Drug Expert Recognition Training materials similarly acknowledge: “Toxicology has some important limitations. One limitation is that, with the exception of alcohol, toxicology cannot produce ‘per se’ proof of drug impairment. That is, the chemist can't analyze the blood or urine and come up with a number that ‘proves’ the person was or wasn't impaired.”

Federal researchers commissioned by the US Department of Transportation to assess whether THC blood levels could be consistently correlated with impaired psychomotor performance determined that they, in fact, could not be. Writing in the report ‘Marijuana and Actual Driving Performance,’ investigators 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. … The answer is very clear: it is not.”

This is because THC possesses unique pharmacokimnetics (absorption patters) that differ dramatically from those of alcohol. For example, peak blood alcohol levels correspond with peak levels of alcohol-induced impaired performance. This is not the case with THC. Because THC blood levels spike almost immediately after inhalation, high THC/blood levels are not necessarily correlated with peak impairment of performance and in some cases they are associated with virtually no changes in performance.

This is not to say cannabis consumption does not possess potential adverse effects of certain pyschomotor skills in specific populations — particularly when consumed by less experienced users or ingested in combination with alcohol — or that marijuana positive drivers overall do not possess a slightly elevated risk of accident. But this risk is far lower than the documented risk associated with drivers who test positive for legal levels of alcohol and/or therapeutic levels of prescribed medications. According to a recently published meta-analysis in the journal Accident Analysis and Prevention of 66 previous studies assessing drug positive drivers and crash risk, marijuana-positive drivers possessed an odds-adjusted risk of traffic injury of 1.10 and an odds-adjusted risk of fatal accident of 1.26. This level was among the lowest estimates documented by the study’s author and it was comparable to the odds ratio associated with penicillin (OR=1.12), anti-histamines (OR=1.12), and antidepressants (OR=1.35), none of which are subject to ‘per se’ criminal penalties. By contrast, a separate study published last month in the journal Injury Prevention reported that drivers with a BAC of 0.01 percent are "46 percent more likely (OR = 1.46) to be officially blamed for a crash than are the sober drivers they collide with." But despite this elevated risk, nobody is calling for ‘per se’ criminal penalties for drivers who operate a motor vehicle after only consuming a single drink of alcohol. Why then would we do so for cannabis?

Proposed ‘per se’ measures for the presence of cannabis’ constituents in blood or urine are an unscientific and disproportionate response to behavior that is already sufficiently addressed by present traffic safety laws which already criminally prohibit driving while impaired by drugs. The imposition and enforcement of such ‘per se’ measures risks inappropriately convicting unimpaired subjects of traffic safety violations, including potentially those persons who have previously consumed in the privacy of their own home some days earlier.

As additional states consider amending their cannabis consumption laws, lawmakers would be advised to consider alternative legislative approaches to address concerns over DUI cannabis behavior that do not rely on solely on the presence of THC or its metabolites in blood or urine as determinants of guilt in a court of law. Otherwise, the imposition of traffic safety laws may inadvertently become a criminal mechanism for law enforcement and prosecutors to punish people who have engaged in legally protected (or depenalized) behavior and who have not posed any actionable traffic safety threat.