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Here’s the problem.

Anyone who drives a vehicle on our roads wants to feel safe from impaired drivers. It’s dangerous enough out there without worrying about intoxicated motorists. Nobody would argue with that.

We know driving under the influence of alcohol can be dangerous and deadly. Roadside tests have been devised that can determine alcohol in blood fairly accurately and that can be generally correlated to one’s level of intoxication. It’s not perfect, but it works well enough to stop many otherwise dangerous drivers, and it makes the rest of us feel safer.

Cannabis just doesn’t work that way. While tests can detect delta-9 THC levels in blood, urine and saliva, no one has been able to correlate those numbers to a level of intoxication. Though two people might have the same amount of THC in their blood, one smoking a joint for the first time and getting behind the wheel will have a completely different experience than a longtime daily user or medical-marijuana patient.

This is particularly frustrating for law enforcement, especially in states that have legalized adult-use. A test makes people feel more comfortable about drivers and cannabis use, especially those who don’t partake at all. But whether that’s a reliable way to measure intoxication is still in question.

In Colorado, legislators came up with a compromise of sorts. Any driver found to have a delta-9 THC level of 0.5 percent per milliliter of blood can be arrested as impaired, but the state also allows drivers to challenge that in court, especially medical patients.

The state recently surveyed 18,000 residents to gauge how it could better educate people about cannabis and driving, and found that more than 70 percent of users admitted they had driven after using it in the last year. Many felt they were better drivers after using cannabis, and others wanted “credible, nuanced, and empirical (evidence) that could prove the dangers of driving after using cannabis.” 

That’s the rub. Researchers, though plenty are working on it, continue to find no way to accurately measure cannabis intoxication, and many feel there might not be one. “More and more are concluding there is no number that can measure impairment,” says lawyer Chris Halsor, a Denver lawyer who teaches classes to police departments around the country about cannabis and impairment. A blood sample, he notes, just doesn’t capture the delta-9 THC in a person’s brain. 

Democratic leadership in a US House of Representatives’ committee has introduced a bill that includes a provision that would require legal marijuana states to consider ways to promote education about the dangers of cannabis-impaired driving while curbing such behavior. The language is tucked in a larger bill about completing long-term infrastructure projects.

But if we can’t actually measure impairment, what good is any of this? The main reason breathalyzer technology works for alcohol impairment is because of what is called the partition ratio, a math formula that basically converts alcohol in someone’s breath to a blood-alcohol level. But, Halsor says, “There’s nothing that suggests that there’s a partition ratio for marijuana.”

I participated in one of Halsor’s “Marijuana DUI Investigations” classes last year. I don’t want impaired drivers on the road creating danger to others, but I also feel like I’m in control when I drive after using cannabis, so I wanted to learn more.

The class consisted of four of us getting as stoned as we possibly could and then interacting with the officers, lawyers and prosecutors taking the class. I think everybody learned something. I realized that the roadside tests used to test for alcohol impairment aren’t useful for cannabis—I can’t walk a straight line when I’m sober, for instance. And while some officers consider cannabis users as petty criminals or stupid stoners and are probably out to harass them, I found that most are more interested in people getting home safely.

Now a University of Colorado study casts even more doubt on our chances of coming up with a blood test for cannabis impairment. CU researchers looked at 121 cannabis users, half using legal concentrates made with oils that have higher THC percentages and others who used flower. Those who inhaled the concentrates showed significantly higher levels of THC in their blood before, directly after ingestion and one hour later, but all users reported about the same feeling of intoxication and the same ability to perform balance and cognitive tests at the same times.

That hasn’t stopped other states from trying. Oklahoma, which recently voted to allow medical cannabis, has decided to spend several hundred thousand dollars on a breathalyzer pilot program to help police determine whether a person is using cannabis and, more importantly, whether they are impaired.

Halsor’s classes focus on watching a person’s behavior rather than relying on blood levels or roadside tests because you can learn more by observation than numbers and the simple fact that a reliable test, if even possible, is a long way off. “There is no silver bullet roadside device at this moment in time or in the near future,” he says, “that we will be able to rely on.”