Driving While Under the Influence of Weed Proves Hard to Quantify

By May Wilkerson 05/17/16

A new study found that stoned driving laws are not supported by science.

driving while stoned

As marijuana is becoming legal for medical and recreational use in more states, it falls upon lawmakers to figure out the best way to keep the roads safe from high drivers. But whereas with alcohol, legislators have determined a strict legal limit for blood alcohol content (0.08%), marijuana is a little harder to regulate. A new study finds that blood tests are an “unreliable” gauge of a person’s level of pot impairment.

Until now, most laws regulating marijuana-impaired drivers have relied on tests that measure the amount of THC, the active compound in pot, in a person’s blood. But the study, commissioned by the AAA Foundation for Traffic Safety, found that these laws are not supported by science. “There is no concentration of the drug that allows us to reliably predict that someone is impaired behind the wheel in the way that we can with alcohol,” said Jake Nelson, AAA’s director of traffic safety advocacy and research.

The study specifically looked at the laws in six states, where lawmakers have used policies on drunk driving as a basis for how to regulate pot-impaired driving. But the body absorbs alcohol and marijuana differently, said the findings. Whereas the amount of alcohol in the bloodstream correlates directly with how drunk a person is, marijuana impairment only occurs when THC enters the fatty tissues of the brain. People who use weed regularly, including those who take it medicinally, often show no signs of impairment after using the drug, said Jolene Forman, a staff lawyer for the Drug Policy Alliance. She added that pot can remain in the blood for hours, days or even weeks after its effects wear off.

What this means is that the presence of THC in the bloodstream is not a good indicator of whether that person is too impaired to drive. So what’s the solution? The AAA study recommended that states should get rid of laws already in place, which set thresholds for THC content in the blood. Instead, new regulations should require that drivers pass a standard field sobriety test, in addition to a drug assessment which would include a blood test to confirm whether pot (or any other drug) was present in the bloodstream.

But even this kind of more complex testing would pose some problems. Forman said that using blood tests in any capacity could result in unfair punishments. “It would be equivalent to a test that shows that you had a glass of wine three nights prior,” she said. “It tells you nothing about whether the driver is safe now. If we have better tests in the future, then by all means we should use them, but right now those tests are not helpful.”

She also said that this kind of testing would be particularly unfair to medical marijuana patients, who would have high levels of THC in their blood regardless of how sober they are. It would also be unfair to blacks and Latinos, who are more likely to be cited for traffic violations, despite committing them at the same rate as whites.

Over a dozen states are considering legalizing marijuana for recreational or medical use in 2016, but few have coherent proposals for how to prevent people from driving while high. The AAA study, as well as many experts, have called for improved technologies to evaluate drivers’ level of impairment by testing their saliva. But these advancements would require research, and one of the biggest obstacles to this kind of research is that marijuana remains illegal on a federal level.

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May Wilkerson is a writer, comic and Managing Editor @someecards. Co-host of the podcast Crazy; In Bed w/alyssalimp. She is also the top Google result for "insufferable lunatic." Follow this insufferable lunatic on Twitter.