Researchers Work to Answer Important Questions About Driving While Stoned

Researchers at the University of California, San Diego’s Center for Medicinal Cannabis Research are more than halfway through a study that may help give scientists, policymakers and the public a better understanding about what constitutes marijuana-impaired driving.

“We have a lot of people waiting with bated breath,” said Tom Marcotte, Ph.D., a neuropsychologist leading the study at the Center for Medicinal Cannabis Research (CMCR).

As recreational and medical cannabis are legalized throughout the world, law enforcement agencies and legislators have been challenged to define impaired driving. Unlike scientific tests that measure alcohol levels in the blood and breath, there’s a lack of scientific consensus about how much THC constitutes impairment.

Existing data are unclear. A Centers for Disease Control and Prevention 2017 marijuana and driving fact sheet warns the public that driving while under the influence of cannabis can slow reaction time, distort perception and impair coordination. Yet the CDC acknowledges that current testing methods and police procedures can’t definitively determine if marijuana use actually increases the risk of car crashes.

Not only is there no accurate roadside test for drug levels in the body, according to the report, but marijuana can also linger in the body for days or weeks, making it impossible to draw direct conclusions.

A recent survey by the Colorado Department of Transportation  found that 69 percent of cannabis users have driven while under the influence of cannabis at least once in the past year, and 27 percent admitted that they drive high almost daily. More significantly, preliminary data showed that 40 percent of recreational users and 34 percent of medical users said they don’t think being under the influence of cannabis impacts their ability to drive safely.

The CMCR study, which began in early 2017 and is funded by California’s AB 266, the Medical Marijuana Regulation and Safety Act, doesn’t aim to develop an accurate marijuana breathalyzer, but it does intend to answer some key questions about driving stoned.

The researchers are focusing on the following:

  • The impact of various THC-dosing levels on driving performance
  • How many hours after initial use that driving impairment lasts
  • If saliva or breath can serve as a useful substitute for blood samples in courtroom proceedings
  • If using a specially-designed iPad application can help in identifying acute impairment during standardized field sobriety tests

“The relationship between THC levels and impairment is not linear like alcohol,” Marcotte said. “You can be impaired with high levels, but you can also be impaired with low levels.  … Frequent users can also have chronic low levels and be unimpaired. At least when it comes to blood, it’s not particularly useful right now.”

So far, more than 100 participants ages 21 to 55 have gone through testing for the study. Researchers plan to have a total of 180 when the study concludes, potentially by the spring of 2019.

The research subjects must be in good health, current cannabis users and have valid driver’s licenses. He said researchers haven’t had much trouble finding participants.

“I think they find it novel and interesting,” Marcotte said. “We’ve had no problem recruiting.”

“Hopefully, we’ll give a lot of information to law enforcement, policy[makers]and users,” Marcotte said.

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