In five states this fall — California, Arizona, Nevada, Maine and Massachusetts — voters will be deciding whether marijuana should be legal for recreational use. And any of those states that do legalize marijuana will have to wrestle with the question of how to enforce laws against stoned drivers.
It has been legal to smoke pot for fun in Colorado since January 2014, and the state modeled its marijuana driving-under-the-influence law on the one for alcohol. If a blood test shows a certain level of THC, the mind-altering compound in marijuana, the law says you shouldn’t be driving.
It sounds straightforward, but consider the case of Abby McLean, a stay-at-home-mom from the Denver suburbs.
McLean, 30, was driving home from a late dinner with a friend two years ago when she came upon a DUI roadside checkpoint.
“I hadn’t drank or smoked anything, so I was like, ‘Let’s go through the checkpoint,'” she recalled.
McLean is a regular marijuana user but she insists she never drives while high.
Still, the officer at the checkpoint told her he smelled marijuana and that her eyes were bloodshot. Eventually he whipped out handcuffs, and McLean said she started to panic: “Like, massive panic attack. And, ‘Oh, my God, I have babies at home. I need to get home. I can’t go to jail!’ ”
She didn’t go to jail that night, but she got home hours late. A blood test later revealed McLean had five times the legal limit of THC allowed in Colorado, which is five nanograms of THC per milliliter of blood.
It may sound like an open and shut case that could have resulted in any number of penalties. But McLean’s attorney, Nadav Aschner, had a field day in court with Colorado’s marijuana intoxication limit.
“Even the state’s experts will say that number alone is something, but generally not enough, and we really hammered that home,” he said. Aschner got a hung jury and McLean pleaded to a lesser offense.
Still, McLean’s trip through the criminal justice system is emblematic of numbers that suggest a sharp increase in marijuana DUI arrests in Colorado. So far this year, State Patrol data show that total DUI citations this year rose to 398 through early July, compared with 316 in for the same period 2015.
It turns out, measuring a person’s THC is actually a poor indicator of intoxication. Unlike alcohol, THC gets stored in your fat cells, and isn’t water-soluble like alcohol, said Thomas Marcotte, co-director of the Center for Medicinal Cannabis Research at the University of California, San Diego.
“Unlike alcohol, which has a generally linear relationship between the amount of alcohol you consume, your breath alcohol content and driving performance, the THC route of metabolism is very different,” Marcotte said.
That’s why adapting drunk driving laws to marijuana makes for bad policy, said Mark Kleiman, a professor of public policy at New York University. “You can be positive for THC a week after the last time you used cannabis,” he said. “Not subjectively impaired at all, not impaired at all by any objective measure, but still positive.”
Still, Colorado and five other states have such laws on the books because pretty much everyone agrees that driving stoned can be dangerous, especially when combined with alcohol.
What police say they really need is a simple roadside sobriety test. Scientists at UCSD are among researchers working on several apps that could measure how impaired a driver is. One has a person follow a square moving around a tablet screen with a finger, which measures something called “critical tracking.” Another app measures time distortion, because things can slow way down when a person is high.
The THC route of metabolism is very different.
Those tests are still experimental.
Denver District Attorney Mitch Morrissey said the uncertainty doesn’t mean Colorado should throw out its THC blood test. He said it may not be perfect, but it gives juries another piece of evidence to consider at trial.
“I think that putting in a nanogram level makes sense,” said Morrissey. “I can’t tell you what level it should be. I don’t think Colorado’s is right. I don’t think it should be as high as it is. I think it should be lower.”
Morrissey remembers trying alcohol DUI cases as a young prosecutor. The science wasn’t settled then either, the blood alcohol standard was about twice as high as it is now, and it took years for it to be lowered.
“I think that has to do with better testing better technology,” which Morrissey said will improve eventually for marijuana too.
In the meantime, some regular marijuana users, like Abby McLean, are scared to drive for fear of failed blood tests.
“I haven’t gone out really since then, because I’m paranoid to run into the same surprise, ‘Oh oh, there’s a DUI checkpoint.'”
This story is part of a partnership that includes Colorado Public Radio, KPCC, NPR and Kaiser Health News.