Driving While High: What Does ‘Impairment’ Really Mean?

By: Chris Nesi

Driving under the influence of any motor skill-inhibiting substance, whether it comes from a flask, a bong, or a prescription bottle, is both illegal and dangerous. 

While the scientific and law enforcement communities generally agree about what constitutes “impairment” under the influence of alcohol, a similar consensus on what determines a driver’s sobriety under the influence of marijuana remains elusive.

As the national tide shifts ever closer to legalizing recreational marijuana use, law enforcement officials are grappling with applying existing driving under high drivers’ influence laws. However, some states’ imprecise approach in implementing these “drugged driving” laws has created an environment that is potentially subject to abuse by law enforcement and with questionable benefit to public safety.

The lack of consensus on identifying marijuana-impaired drivers will be an ongoing concern as the nation’s marijuana laws and consumption practices evolve.

In a 2013 report in the Humboldt Journal of Science Relations, NORML Deputy Director Paul Armentano warned that enforcement of so-called “per se” drugged driving laws, under which drivers can be convicted of a traffic safety offense for operating a motor vehicle with even trace amounts of THC or metabolites found in their bodily fluids, “may inadvertently become a criminal mechanism for law enforcement and prosecutors to punish those who have engaged in legally protected behavior and who have not posed any actionable traffic safety threat.”

Eighteen states have such per se laws, which means if a driver is pulled over and ultimately arrested on suspicion of DUI, they could be subject to additional criminal charges if their bodily fluid tests positive for a pre-determined amount of THC or its metabolites.

This effectively eliminates the state’s requirement to prove a driver is actually impaired, as failing a field sobriety test would, relying instead on the presence of a substance in an individual’s blood or urine, a questionable yardstick for measuring a driver’s ability to operate their motor vehicle safely.

“It is far from established that the identification of either THC or the carboxy THC metabolite may be consistently correlated with behavioral impairment,” Armentano testified before the Nevada Subcommittee on the Medical Use of Marijuana in August.

While most drugs completely leave your system within a few days, marijuana lingers in the fat cells before slowly being passed through the bloodstream and eliminated from the body. The speed of that process varies depending on the frequency of use, especially if use is chronic, as in the case of a medical marijuana patient, who may even be immune to the psychoactive effects of cannabis.

“Experienced cannabis consumers … become tolerant to the substance’s behavioral effects. These subjects also retain trace concentrations of THC and/or carboxy THC for extended periods of time well beyond the duration of impairment,” Armentano testified.

Some states impose a specific “THC limit,” which measures the amount of THC and its metabolites in someone’s blood, similar to a blood-alcohol test. Washington and Colorado, where recreational cannabis use is legal for adults over age 21, use 5 ng/ml (five billionths of a gram per milliliter of blood) as the legal limit, with zero tolerance for drivers under 21 the states’ recreational cannabis laws. Other states have different thresholds. Armentano said these numbers are arbitrary, as trace amounts of THC and its metabolites in the blood have not been unimpeachably linked to motor function impairment. 

“These levels were not chosen because of any scientific evidence indicating that psychomotor impairment or greater likelihood of an accident is likely to occur at or above these levels,” Armentano wrote in an email.

And yet, twelve states have “zero tolerance” per se laws, where any trace of THC can result in criminal charges, regardless of when consumption of the substance took place.

Jonathan Adkins is the executive director of the Governors Highway Safety Administration, which deals with several traffic safety issues at the state level, including impaired driving. Adkins said the organization is not opposed to alcohol or legal marijuana consumption, but it supports per se laws and their expansion to other states. He stopped short, however, of saying unequivocally that the laws prevent traffic fatalities.

“We don’t have the research to show [per se laws]prevent deaths or accidents, but they’re one of the many tools law enforcement can use, and we support them.”

When asked whether drivers with trace amounts of THC in their system could be considered legally impaired, Adkins conceded, “Different people react very differently to marijuana. And that’s part of the controversy. Right now, it’s more about what we don’t know than what we do. We’re having trouble keeping up with all changes to the legislation.”

Legal precedent has been set, which calls into question the logic of charging and convicting a driver for trace amounts of THC and metabolites in the blood or urine, whether the driver in question is a legal cannabis consumer or not.

In 1997, The Supreme Court of Georgia reversed a lower court’s conviction of a non-medical cannabis user who was pulled over for speeding and arrested for having drugs in his system while driving. The high court ruled that his conviction violated the Constitution’s equal protection laws by “arbitrarily” distinguishing legal and illegal cannabis users.

Last month, the Arizona Supreme Court held that drivers in the state could not be convicted “based merely on the presence of a non-impairing metabolite that may reflect the prior usage of marijuana.”

Despite more nuanced rulings at the state level, the Federal Government has not yet budged from a stricter view of the dangers of marijuana consumption by drivers. President Obama’s 2012 National Drug Control Strategy report encourages even more states to adopt zero-tolerance per se drugged driving laws.

The NDCS report does mention that it’s working on creating a unified standard for drug testing, including researching the development of a “reliable, widely-available roadside test.” It also says it’s working on learning more about what constitutes intoxication when it comes to marijuana.

A policy of “Let’s find out more and enforce accordingly” is a slight but meaningful step up from previous decades’ apparent policy of “Let’s not find out more and enforce haphazardly.” However, until a peer-reviewed scientific determination can show the correlation between the amount of THC in bodily fluids and the inability to drive, arresting and charging sober motorists with DUI for trace particles of THC in their blood is nothing short of draconian.  

10 states in which you (for now) absolutely do not want to drive with cannabis in your system.

  • ARIZONA (Zero Tolerance for THC/Metabolites, but with legal precedent against it)
  • DELAWARE (Zero Tolerance for THC/Metabolites)
  • ILLINOIS (Zero Tolerance for THC/Metabolites)
  • INDIANA (Zero Tolerance for THC/Metabolites with no exception for MMJ patients)
  • IOWA (Zero Tolerance for THC/Metabolites with no exception for MMJ patients)
  • GEORGIA (Zero Tolerance for THC/Metabolites, but with legal precedent against using bodily fluid testing to determine intoxication)
  • MICHIGAN (Zero Tolerance for THC)
  • OKLAHOMA (Zero Tolerance for THC/Metabolites)
  • RHODE ISLAND (Zero Tolerance for THC/Metabolites)
  • UTAH (Zero Tolerance for THC/Metabolites)
  • WISCONSIN (Zero Tolerance for THC with no exception for MMJ patients)

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