How Do Cops Know if You’re Driving High?

how do cops know if you're driving high

Though marijuana intoxication doesn’t compromise driving ability nearly as much as alcohol, driving high is never a good idea, and it is just as illegal as drunk driving, even in states with legal recreational cannabis. But weed impairment is tough to measure accurately, as THC (Delta-9 tetrahydrocannabinol, the main psychoactive ingredient in weed) hangs around in your system for up to a month, even if your high dissipated days ago. So, how do cops know if you’re driving high?

Police in the 9 states with legal recreational weed have been using the standard field sobriety test to decide whether a driver is driving high. This is the same tried-and-true test used to test drunk drivers for decades — walking a straight line, focusing the gaze, etc. The trouble with this? A Massachusetts court, after hearing expert testimony, ruled the results of this test were inadmissible for people suspected of high driving.

how do cops know if you're driving high
How do cops know if you’re driving high? Right now, a field sobriety test, just like the one used for alcohol, is the method employed by traffic police.

This has left law enforcement in a bind when it comes to enforcing drugged driving laws. To remedy this problem, the 2016 legalization initiative in California included provisions to allocate a portion of marijuana tax revenue to research a method to test whether people were driving high when they were pulled over.

Massachusetts State Police starting in summer of 2018 are experimenting with a pilot program collecting saliva swabs for testing, working on the theory that saliva concentrations of THC are fluctuate more in correlation with a driver’s level of stoniness.

Civil liberties advocates like the American Civil Liberties Unions have criticized this program, however, as an unreasonable search and seizure, in particular because they say police agencies could use the swabs to maintain an illegal DNA database in addition to simple drug testing. A Massachusetts ACLU official told the Boston Herald the group would likely sue over this practice.

How Do Cops Know If You’re Driving High? Is There A Marijuana Breathalyzer?

So, how do cops know if you’re driving high if a sobriety test is not sufficient and saliva tests are unreliable? Well…if it works as advertised, a solution proposed in spring of 2018 by an Oakland, CA company would make testing marijuana-drugged drivers as easy as it is to test the blood-alcohol level of drunk drivers.

Hound Labs has designed and built a dual Breathalyzer-type device that they say can measure the TCH in your breath vapor to accurately correlate how high you were when a cop administered the test. To develop and test their product, the company enlisted 400 people who had varying marijuana use habits, from occasional to heavy users of both smoked and edible pot preparations.

The Hound studies found “participants had high levels of THC in their breath immediately after smoking, but the level of THC in breath samples dropped to near zero after 2-3 hours.”

how do cops know if you're driving high houndlabs breath
How do cops know if you’re driving high? Well…data from HoundLabs suggests that the level of THC in your breath is associated with recency of use, unlike urine, blood, and hair tests.

Research from Hound Labs also showed that where you smoke determines the THC level in your breath. Specifically, users who consumed cannabis indoors had higher levels of THC in their breath for the duration of their high (4 hours) than those who toked up outdoors.

how do cops know if you're driving high indoors vs. outdoors
How do cops know if you’re driving high? A breathalyzer test from HoundLabs shows that THC is present in your breath after smoking. Strikingly, there is even more THC present if you smoked indoors.

“We offer a solution that not only extends the boundaries of clinical research but also provides better information about recent marijuana use for employers, law enforcement and legislators who are trying to balance public safety with the fair treatment of responsible cannabis users,” said Mike Lynn, CEO and co-founder of Hound Labs in a press release.

Hound Labs, as of August 2018, was readying a pre-production model of its portable dual marijuana/alcohol breathalyzer. If law enforcement agencies start awarding contracts, Hound stands to make a lot of cash: recreational marijuana is legal in 9 states and medical marijuana in 31. Other states allow it for specific medical conditions and several more have legalization ballot measures coming up in the next election.

How Much Does Marijuana Affect Your Driving?

If you smoke weed, and then get in your car, you’ll swerve back and forth in your lane, your reaction and braking time slows and you’re more likely to have your attention diverted away from the road. You’re also more likely to drive below the speed limit and stay farther behind the cars in front of you.

This is according to a detailed review of scientific literature compiled by The National Highway Traffic Safety Administration (NHTSA) prepared a detailed report to Congress, submitted in 2017.

The NHTSA report acknowledged weed-impaired drivers weren’t as dangerous as alcohol impaired ones, but that doesn’t mean we should allow high drivers on the roads.


For the full story, read our article on how cannabis affects your driving.


How do cops know if you’re driving high? (Additional References):

First marijuana ‘breathalyzer’ test created by california company

Jason Lemon


Aug. 5, 2018


“Marijuana Impaired Driving Report to Congress”


July 2017



About Adam Townsend 50 Articles
Adam Maxwell Townsend has been a journalist for 15 years. His reporting has covered science, technology and medicine. Currently, he edits and writes medical and pharmaceutical articles for consumer websites, including WoahStork.

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