In two months we could have five more states decriminalizing recreational marijuana. The problem is, no one has found out how to punish drivers under the influence of cannabis.

This is not a new issue. Since Colorado and Washington became the first two states to welcome recreational cannabis with more open arms, law enforcement has been struggling to figure out a proper way to categorize THC drivers.

One of the issues is that THC, the active intoxicant in marijuana, works differently in your body than alcohol. Where testing a blood alcohol level can give someone a pretty good indicator of whether someone should be behind a wheel. Sure, there’s some variety in how many drinks it takes a person to get up there, but the standard applies fairly evenly.

Photo Credit: Rusty Clark - On the Air M-F 8am-noon cc
Photo Credit: Rusty Clark – On the Air M-F 8am-noon cc

THC, meanwhile, gets stored in a user’s fat cells, and isn’t water-soluble like alcohol is. Given the lack of a linear relationship between amount of cannabis consumed, THC blood level, and driving performance, it’s tough to simply adapt drunk driving laws to marijuana. Habitual marijuana users can never smoke before operating a vehicle and still get nabbed for THC that’s lingering in their system. They might have five nanograms per millileter of THC (the limit in many places) at any given point.

“You can be positive for THC a week after the last time you used cannabis,” said Mark Kleiman, a professor of public policy at New York University, to NPR. “Not subjectively impaired at all, not impaired at all by any objective measure, but still positive.”

Some people jump off that point even further: The drunk driving laws we have now make for bad policy when adapted to drivers under the influence of marijuana because the risk is far from analogous.  As Canna Law Blog wrote back in 2014:

Our biases — that alcohol and cannabis are equivalent intoxicants, that being stoned must make driving unsafe — run deep. [Eduardo Romano, author of a study in a previously cited New York Times article], despite having authored the study showing that cannabis didn’t increase the risk of crashes versus the sober-baseline, can’t help but succumb to his cultural common-sense, however non-scientific: “Despite our results, I still think that marijuana contributes to crash risk,” he said, “only that its contribution is not as important as it was expected.”

There surely may be legitimate and persuasive reasons why a “stoned driver” shouldn’t be trusted to operate a motor vehicle, perhaps even if such a driver were able to demonstrate otherwise sufficient “field sobriety” before an experienced public-safety officer. Yet the NYT article author, like Mr. Romano, appears to presume that such a driver, merely by virtue of being “stoned”, represents a public safety risk–regardless of the FST results.

Most cannabis-concerned folks are aware that frequent use is likely to build up THC concentrations in the body to such a degree that your blood-THC content might still be five nano-grams per milliliter a day after you last lit up. And cannabis-users and scientists alike understand that such blood-content has little bearing at all on the only question that should matter: is there good reason to believe that the driver was IMPAIRED at the time of driving?

Two years and a handful of states with decriminalization later, and we’re no clearer to finding an answer to that question. Colorado and Washington’s laws are both modeled on alcohol sobriety tests, and in Washington if you test positive for THC you get an automatic DUI-cannabis. And with “drug corridors” popping up in places like Colorado, it’s clear that law enforcement is finding the new laws give them a way in to marijuana drivers.

Of course when the first alcohol blood standard was released it was about twice where we are now, and it took some time for it to be lowered. Testing technology advanced, and the culture’s collective understanding of driving drunk changed. There are some advancements being made for cannabis; phone apps that use memory and sensory tests to more specifically test for THC impairment, new breath tests to detect levels of THC, spit tests that can “tell if you’re driving high in three minutes.”

But until we better understand if such a “potalyzer” for roadside marijuana tests is even the best way to understand impairment, we’re all left with a bit of a buzzkill.