Based in California, Cannabiz Confidante is a blog exploring the legal cannabis industry: from cannabis business ("cannabiz") to legal issues, lifestyle and social justice.

Driving High: Detecting Impaired Drivers

Driving High: Detecting Impaired Drivers

Is there a reliable roadside test for detecting marijuana consumption? Do DUI laws rely on tests that measure actual impairment from marijuana?



Drugged driving is a legitimate concern, especially in the states where marijuana is legal. We’re all familiar with the dangers of drinking alcohol and driving, but we know very little about marijuana and driving. While there have been a fair number of studies over the years, there is still little consensus about whether (and how much) consuming marijuana impairs driving or how best to detect marijuana exposure. How can we test for marijuana consumption?

Blood Tests

THC blood tests are currently considered to be the most reliable way to detect marijuana consumption. They are conducted under the same circumstances as suspected drunk driving. In Colorado, law enforcement may request blood tests to detect marijuana impairment after arresting the driver under suspicion of a DUI. Roadside blood testing is invasive and impractical and a blood draw is only taken at a hospital with the driver's expressed consent. 

Cannabis DUI impairment detection

Blood tests for alcohol are considered to be highly accurate because BAC correlates with actual observed psychomotor impairment. However, unlike alcohol, the amount of THC in the blood doesn't necessarily correlate to actual psychomotor impairment. Peak THC blood levels do not consistently correspond with levels of peak impairment.  One study showed that people who inhale THC typically reach their highest THC blood level within minutes, well before the drug's impairing effects have reached their peak. Furthermore, THC can be detected in the bloodstream for days or even weeks after the impairment has dissipated.

When it comes to determining legal impairment, the poor correlation between THC levels in the blood and actual impairment is problematic and concerning. There are currently 18 states (not including California!) that have some variation of a zero tolerance laws for marijuana while driving. Zero tolerance makes it illegal to drive with the presence of THC (or its metabolites) in your bloodstream.

These laws presume that a driver whose THC levels exceed a threshold level is impaired, even though research shows THC and its metabolites can remain in the driver's bloodstream for days or weeks after impairment has dissipated.  As Mark A. R. Kleiman, a professor specializing in issues involving drugs and criminal policy at NYU, so aptly puts it: “A law against driving with THC in your bloodstream is not a law you can know you are obeying except by never smoking marijuana or never driving.”

Urine Tests

Without much further discussion needed, urine tests present the same problems as blood tests. THC metabolites can be detected in a driver's urine for weeks after impairment has dissipated. This is the same reason why people agonize so much over employment drug tests.

Saliva/Breath Tests

Saliva and breathalyzer tests for marijuana are the newest testing methods and have not been proven to be consistently accurate. However, they have the potential to be much more sensitive in testing recent marijuana exposure - between 4 and 24 hours after inhalation. Another benefit: oral swabs and breathalyzer tests are less invasive and more convenient to perform in a roadside sobriety test.

There have been many iterations of tests but none have been found to be reliable enough for widespread use by law enforcement in the U.S. Research is still ongoing, though, and we may be very close to developing a sufficiently sensitive and accurate device. The Colorado State Patrol is participating in a 3-year pilot program testing five different kinds of oral detection devices. 

In California, group of Stanford engineers has developed a mobile device called a ‘potalyzer’ that detects THC molecules in the range of 0 to 50 nanograms per milliliter of saliva. The minimally invasive test would allow police officers to collect a saliva sample from a driver’s mouth using a cotton swab and use the device to analyze and report the results in as little as three minutes. Awesome, right?

But before oral swabs or 'potalyzers' can become the roadside test for marijuana impairment, a few critically important questions remain. Does THC concentration in saliva reliably correlate with psychomotor impairment? What is the time range for detection of THC in saliva? (i.e. what's the likelihood it'll detect THC after impairment has dissipated?)

Smoking Marijuana While Driving, Driving High

Bottom Line

We need a reliable test to detect legal impairment due to marijuana consumption. Under a majority of state laws, impairment is what makes driving under the influence dangerous and illegal. A test that criminalizes the mere presence of THC in the blood is not a reliable measure of impairment. Nobody wants to be arrested for a DUI days or even weeks after impairment has dissipated.

A test of legal impairment should measure a factor (e.g. biological matter, physical symptoms or behavioral cues) that actually correlates with psychomotor impairment.  It must also be sensitive enough to detect recent use - within hours, not days or weeks. Ideally, the test should also be minimally invasive and convenient for law enforcement officers to use in field sobriety tests.

Perhaps an oral swab or breathalyzer test will prove to the be the most reliable option. Perhaps no biological test for THC will be as reliable as the alcohol breathalyzer, and we'll have to rely the standard physical test (e.g. one leg stand, heel-to-toe steps, etc). The bottom line is, whatever detection method we use, marijuana DUI laws must penalize actual THC impairment rather than mere presence in the body.

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