Driving While Stoned

The New York Times reported earlier this week that driving under the influence of marijuana is significantly less risky than driving with a blood-alcohol concentration of 0.08.  That’s a good thing, since the Times also reported that impairment from marijuana is difficult to detect using the current battery of standardized field sobriety tests and difficult to confirm through subsequent laboratory tests.  The article summarized several recent studies making these findings and noted the conclusion of some experts that public resources would be better spent combating alcohol-impaired driving, including perhaps lowering the per se threshold for alcohol concentrations to 0.05,  than in establishing a per se limit for blood-THC content or devising roadside tests to detect for marijuana impairment.

Why is driving while stoned less risky? The simple answer is that marijuana and alcohol have different physiology. Drivers impaired by alcohol tend to overestimate their skills and drive faster.  Drivers impaired by marijuana do the opposite. A professor of public policy interviewed for the Times article noted the old joke about “‘Cheech and Chong being arrested for doing 20 on the freeway.’”  (The Times ran a picture of the famous comedic duo to accompany its story, which, if you ask me, it should have saved for Throwback Thursday (#tbt)). Studies estimate that drivers who are stoned are twice as likely to crash.  A 20-year-old driver who has a blood-alcohol concentration of 0.08 percent, in contrast, is almost 20 times more likely to be in a fatal accident than a sober driver.

Why is marijuana impairment hard to detect? THC is the chemical that gives marijuana its psychoactive punch. It can take as long as four hours for THC metabolites to appear in urine after smoking, and urine can test positive for those metabolites days or weeks after a person last smoked.  As the National Highway Transportation Administration’s fact sheet on marijuana puts it, this is “well past the window of intoxication and impairment.” And because concentrations of THC in a person’s blood depend in part upon the pattern of marijuana use, it is difficult to establish a relationship between a specified blood concentration and performance impairing effects.  The Times reports that regular marijuana smokers could have a blood-THC content that meets the limits set in Colorado and Washington for THC concentration a full day after they last smoked.

Driving while stoned is illegal in North Carolina as is driving under the influence of any “impairing substance.” Yet, given the scientific limitations discussed above, stoned driving can be a tough case for the State to prove. While the State may establish per se impairment by proving that a person had any amount of a Schedule I controlled substance in his or her blood or urine, to prove impairment from marijuana, a Schedule VI controlled substance, the State must prove that the person’s physical or mental faculties were appreciably impaired by that substance at the time he or she drove.

Governor Pat McCrory recently appointed members to a Statewide Impaired Driving Task Force that is charged with developing a statewide plan for preventing and reducing impaired driving. Should the prevention of stoned driving be a priority for this group?  Or should it instead focus on alcohol-impaired driving?  What should its strategy be?  Adoption or amendment of per se limits?  Expanding options for treatment?  Something else?

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