.Driving While High

Determining whether a person is too ‘impaired’ to drive

Blood-alcohol tests often aren’t really conclusive when it comes to determining whether a person is too impaired to drive. Many people who are over the legal limit are probably perfectly fine to drive, even if doing so might get them into legal trouble. But many aren’t, and the line has to be drawn somewhere. Even if you think the common blood-alcohol limit of 0.08% is set too low, at least it’s a meaningful number that everyone can agree is the standard even if they disagree that it should be. And most sane people would agree that a person with a blood-alcohol level of three or four times the legal limit shouldn’t be behind the wheel.

But there is nothing like that for cannabis, though courts and law-enforcement agencies have been pretending otherwise for years. The equation began to change when states started legalizing weed for adult use about a dozen years ago. As with alcohol, weed is now legal to possess and use, but that use might lead to trouble when it comes to driving. 

But unlike with alcohol, there’s no reliable measure of  “impairment.”

THC, the cannabinoid that is most associated with the high cannabis provides, stays in the body long after the effects wear off. Testing a driver’s blood for the presence of THC is therefore meaningless. This has been widely known all along, but that didn’t stop cops, prosecutors, and courts from leveling criminal charges against people “caught” driving days, weeks, or even months after the last time they got high.

The federal government, at least, seems to be waking up to this fact, albeit with frustrating (and typical) slowness. Most recently, as reported last week by Marijuana Moment, a Justice Department researcher declared that law enforcement and the courts should accept that measuring concentrations of THC in the bloodstream is not a reliable measure of impairment.

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“Maybe what we need to do is kind of get away from that idea that we can sort of have a number when it comes to marijuana and have that mean that you’re impaired,” said DOJ researcher Frances Scott during an episode of the “Justice Today” podcast published last month. “And it may get into some different types of measures than we’re used to doing. So maybe it’s not a blood measure or a breath measure.”

Scott, a physical scientist with the National Institute of Justice’s Office of Forensic Sciences, explicitly stated what most of us have known all along: “If you have chronic users versus infrequent users, they have very different concentrations correlated to different effects. So the same effect level, if you will, will be correlated with a very different concentration of THC in the blood of a chronic user versus an infrequent user.”

Leaving aside her use of the phrase “chronic users” (which is both funny and problematic, for different reasons) that’s just the start of what makes blood tests useless for cannabis impairment, especially as compared to blood-alcohol tests. The same amount of the same weed will affect even the same person (whether a “chronic” user or not) differently at different times. That’s true of booze, too, but much less so than with cannabis.

What’s more, it’s not even clear that being high is particularly problematic when it comes to driving, as the federal government has previously stated.

While being high certainly can negatively affect things like response times and motor skills, some research has shown that such effects aren’t particularly troublesome in the aggregate. One major study, published in 2019 by the journal Addiction, “found no increase in crash risk, after adjustment for age, sex, and use of other impairing substances” in drivers with less than 5 nanograms of THC per milliliter of blood.”

Not that any of this should make anyone comfortable with the idea of, say, their pilot getting stoned before takeoff, their surgeon being high, or even of their cabdriver being wasted. If a reliable, simple test of impairment could be devised, it could be useful. But so far, there simply is no such thing.


  1. There is a new class of technology called “Impairment Detection Technology” that seeks to measure impairment. My company, Gaize, is one example. We use a series of ocular tests to detect current impairment and report that instantly.

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  2. “If a reliable, simple test of impairment could be devised, it could be useful. But so far, there simply is no such thing.” Actually, there IS a scientifically validated measure of impairment from cannabis–an app called Druid from Impairment Science, Inc. Druid measures impairment in 1 minute from any source, e.g., cannabis, alcohol, fatigue, concussion. Grounded in cognitive neuroscience (e.g., reaction time, balance), Druid is sensitive enough to distinguish different levels of impairment from different amounts of THC consumed (Johns Hopkins Medical School study). In the App Store and Google Play, free for two weeks. Used by companies and scientists around the world.

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