A Michigan traffic safety law that prohibits the operation of a motor vehicle by persons who possess any presence of THC in their blood, regardless of whether or not they are behaviorally impaired by the substance, may not be strictly applied to state-qualified medical cannabis patients. So decided the Michigan Supreme Court on Tuesday in the case People v Koon.

In a unanimous opinion, the Court determined that legal protections extended to state-qualified patients under the Michigan Medical Marihuana Act, enacted by voters in 2008, supersede the state’s zero tolerance, internal possession law. As a result, the Court determined that state prosecutors must establish that authorized patients charged under the statute are actually impaired by their cannabis use in order to gain a DUI criminal conviction.

According to the syllabus of the Opinion:

“The MMMA [Michigan Medical Marihuana Act] does not define what it means to be ‘under the influence,’ but the phrase clearly contemplates something more than having any amount of marijuana in one’s system and requires some effect on the person. Thus, the MMMA’s protections extend to a registered patient who internally possesses marijuana while operating a vehicle unless the patient is under the influence of marijuana. The immunity from prosecution provided under the MMMA to a registered patient who drives with indications of marijuana in his or her system but is not otherwise under the influence of marijuana inescapably conflicts with MCL 257.625(8) [the state's zero tolerance per se DUI law], which prohibits a person from driving with any amount of marijuana in her or system.”

The state’s zero tolerance per se drug law remains applicable to non-patients. Under such laws, motorists are guilty per se (in fact) of a criminal traffic safety violation if they engage in the act of driving while detectable levels of certain controlled substances or, in some cases, their inert metabolites (byproducts) are present in the defendants’ blood or urine. Proof of actual impairment is not a requirement for a conviction under the law.

To date, ten states — Arizona, Delaware, Georgia, Illinois, Indiana, Iowa, Michigan, Rhode Island, Utah, and Wisconsin — have enacted legislation imposing zero tolerance per se thresholds for the presence of cannabinoids and/or their metabolites. (State-authorized medical cannabis patients in Arizona and Rhode Island are exempt from prosecution under these per se statutes unless the state can provide additional evidence of psychomotor impairment.)

Five additional states impose non-zero-tolerant per se thresholds for cannabinoids in blood: Montana (5ng/ml — law takes effect on October 1, 2013), Pennsylvania (1ng/ml), Ohio (2ng/ml), Nevada (2ng/ml) and Washington (5ng/ml). Most recently, Colorado lawmakers approved legislation stating that the presence of THC/blood levels above 5ng/ml “gives rise to permissible inference that the defendant was under the influence.” State-qualified patients in Colorado, Montana, and Nevada are not provided legal exemptions from these statutes, although legislation is presently pending in Nevada to do so.

NORML believes that it is inadvisable to infer behavioral impairment based on the presence of blood/cannabinoid levels alone — a position that we outline here, here, and in public testimony here.

Such caution is similarly expressed by the United States National Highway Transportation and Safety Administration, which acknowledges: “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.”

A 2013 review of per se drugged driving laws and their impact on road safety found “no evidence that per se drugged driving laws reduce traffic fatalities.”