The Holy Grail for Law Enforcement: Accurate Roadside Testing for Driving While Stoned

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Nearly every state has to weather the argument from law enforcement about how to find and get stoned drivers off the road. But developing roadside testing devices to use for finding reliable, standardized levels of THC intoxication continues to be a slippery, moving problem with no solution in sight.

One of the more hotly contested issues of the cannabis industry is how to determine if a driver is too high to drive.

Every person who argues against legalizing cannabis points to the lack of a roadside testing device for law enforcement to measure THC intoxication and impaired driving as one of the reasons they won’t support legalization.

But blood or saliva testing for THC intoxication has proven to be iffy at best. Peak levels of THC inside the human body quickly lower after consumption. And THC consumers can also test positive when they haven’t consumed for weeks.

States have had to comply with some sort of legal action just to satisfy naysayers. So far, six states have adopted “per se” (inherently illegal) limits on THC found during a blood test, from 1 nanogram per milliliter in Oregon to 5 nanograms per milliliter in Montana and Washington. Other states have a zero tolerance for THC, including Illinois (which just legalized recreational consumption), Arizona, Indiana, Oklahoma and Rhode Island (1).

While all the details about “driving-while-stoned” (also being called “buzzed driving”) are being worked out, jumping into the breach of mis- or no-information come the usual scare tactics.

For example, a recent blog published by the National Conference of State Legislatures claimed that the number of drivers killed in crashes who tested positive for marijuana doubled from 2007 to 2015, conceding that testing for cannabis use is “problematic,” and that drivers are often tested for high blood concentration of alcohol “and rarely tested for additional substances." (2)

Testing dead drivers for cannabis? Hardly possible. “Many responsibility analyses used THC from coroner reports, but interpretation of those levels is complicated by postmortem redistribution of THC,” according to a study published in Addiction magazine in May, 2019 (3). THC broadly disperses away from blood vessels inside the human body postmortem, meaning different concentrations of THC can be collected from the body but it depends on where from the body they are collected, according to the study.

The National Highway Traffic Safety Administration (NHTSA), in a 2017 report to Congress about marijuana-impaired driving, spent most of the study discussing alcohol impairment, but included information from a study that found that a driver who was under the influence of both alcohol and THC would be a better driver because “marijuana mitigated some of the effects found with alcohol by reducing the time spent above the speed limit.” (4)

In another study the report cited (5), “subjects dosed on marijuana showed reduced mean speeds, increased time driving below the speed limit and increased following distance during a car following task.” NHTSA concluded with the somewhat ironic complaint that it couldn’t do any further studies about cannabis and driving due to “Congressional prohibition” on cannabis.


The American Automobile Association is stumped as well. In a 2016 evaluation of over 5,000 drivers, they found that there was no evidence “that any objective threshold exists that established impairment, based on THC concentrations measured in specimens collected from cannabis-positive subjects placed under arrest for impaired driving. Among both samples of drivers who came into contact with law enforcement and were subsequently placed under arrest for DUI, only 30-49 percent would have been considered impaired under a per se standard set at 5 ng/mL, depending on whether alcohol or other drugs are detected and taken into consideration. Based on this analysis, a quantitative threshold for per se laws for THC following cannabis use cannot be scientifically supported.” (6)

The study in Addiction magazine concluded about the same thing: No increase in crash risk, after adjustment for age, sex, and use of other impairing substances, in drivers with THC less than 5 nanograms per milliliter. “There was no statistically significant association between THC level and risk of responsibility. There was significantly increased risk in drivers who had used alcohol, sedating medications, or recreational drugs other than cannabis.” (3) 

Someone, somewhere will be a zillionaire tomorrow if they can make a field testing kit for reliable and actionable THC intoxication. So far, there have only been a few takers.

There is a German-made device, the Drager DrugTest 5000, that has been used since 2015 by police in Norway to do an analysis of THC levels during suspected drug-impaired driver stops. In a study about the device published in the Journal of Analytical Toxicology, while officers said that the device was somewhat helpful, it was plagued with false positives, issues with oral swabs analysis versus blood analysis, issues with drivers using other substances in addition to THC like cocaine and alcohol, and even THC being absorbed within the unit’s collection device (7).

Cannabix Technologies, based in Vancouver, British Columbia, has been working on a THC breathalyzer since 2014. The handheld unit analyzes breath instead of saliva for THC content. Development has been slow but steady. The unit is currently going into a pilot testing stage through a partnership with the University of Florida (8).

Another handheld device that tests for both alcohol and cannabis intoxication is being developed by Hound Labs in Oakland, California. The device is touted as being “1 billion times more sensitive” than today’s crop of alcohol breathalyzers (9). A recent clinical trial at the University of California-San Francisco demonstrated that the unit can measure molecules in breath to one trillionth of a gram per liter of breath (10).

Other researchers are reportedly working on a sensor-studded cap to be worn on a subject’s head to check for THC impairment (11).

Chances are that a device that can accurately measure THC intoxication to the satisfaction of both science and law enforcement is still years away. So for now, law enforcement has to do it old-school, pulling over drivers who appear to weave or change lanes or smell of cannabis or other subjective observations that put a driver in a category of possible THC intoxication. But without proof of actual THC intoxication to any level of reasonable standardized impairment, a good attorney can get the case tossed. And it's back to square one.




4. impaired-driving-report-to-congress.pdf