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And as of today, Californians can legally buy recreational marijuana. But in California and the handful of other states where recreational pot is legal, it is still illegal to drive while under the drug's influence. NPR's Rae Ellen Bichell reported last August that law enforcement and scientists were struggling to find better ways to determine who is impaired. And she started her story in a Denver hotel conference room with 16 State Patrol officers watching a YouTube video.
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BOBBY BLACK: Hi. I'm Bobby Black, senior editor of High Times magazine. And today, we're going to show you how to do a dab.
RAE ELLEN BICHELL, BYLINE: The cops, with their buzz cuts and Mountain Dews, looked curious. Some took notes. After all, they were there for a proper education about weed.
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BLACK: A bowl of water and some tongs if you're seasoning a new nail and, of course, some shatter or wax.
BICHELL: Chris Halsor started this class a few years ago, when Colorado legalized recreational marijuana.
CHRIS HALSOR: The whole point of this class is to get the officers to make correct decisions.
BICHELL: In this state and others, it's legal to be high, but it's illegal to drive while high. Right now, there's no breathalyzer that's proven to work, so it's up to officers like the ones in this room to make the call on whether someone they've pulled over has been driving impaired or not, and therefore, if they should be arrested. But a lot of them haven't been high themselves since some exploratory puffs in high school. They're lacking, Halsor says, in confidence.
HALSOR: Confidence that they're making the right arrest decision and confidence that they're letting people go who really aren't impaired.
BICHELL: The students paged through Dope magazine, chuckled at a photo of an edible called reef jerky and ogled gold-plated blunts at a nearby dispensary. But the real test was sitting in the hotel parking lot in an RV plastered with bumper stickers.
EUGENE BUTLER: Yes. Good music, good company, good weed - it all goes together.
SHARIKA CLARK: That's true.
BICHELL: The four volunteers - John, Christine, Sharika Clark and Eugene Butler - had never met before but they shared a passion for pot, especially the free kind.
CHRISTINE: You got some kief there, too?
BUTLER: But check this out. If you guys ever want to boost your high...
BICHELL: Just to be clear, it's legal to smoke pot on private property in Colorado, so they weren't doing anything wrong. These people, including Sharika Clark, were getting high as a kite for the greater good.
CLARK: We're going to willfully smell like pot around a bunch of cops (laughter).
BICHELL: The volunteers walked into the hotel where the officers were waiting to practice sobriety tests on them. One group started with a volunteer named Christine. She didn't want to share her last name.
AJ TARANTINO: Have you consumed any cannabis today?
CHRISTINE: Oh, yeah.
BICHELL: AJ Tarantino, a trooper with Colorado's State Patrol, took the lead with his colleagues Philip Gurley, Tom Davis and Rich Armstrong observing.
TARANTINO: Would you be willing to do voluntary roadside maneuvers?
CHRISTINE: Of course.
BICHELL: Christine did really well on math, but she didn't do well on other things like balancing, remembering instructions and estimating time.
PHILIP GURLEY: She showed multiple signs of impairment.
TOM DAVIS: Yeah, she'd be going to jail.
RICH ARMSTRONG: Or she'd be arrested.
BICHELL: But then the group moved on to Sharika Clark...
UNIDENTIFIED MAN: Let's get situated here.
CLARK: (Laughter).
BICHELL: ...And things got a little more complicated. Yes, her pupils were huge. And Philip Gurley told her she had a tough time touching her finger to the tip of her nose while her eyes were closed.
GURLEY: But your balance, your counting, your alphabet - all spot on.
BICHELL: So in real life, would they have arrested her for driving impaired?
GURLEY: Yes.
ARMSTRONG: Boy, you're tough. I don't know if I would've or not, to be honest with you.
BICHELL: Right now, these officer's opinions loom large. If they decide you're driving high, you're going to jail, but they are just opinions.
TARA LOVESTEAD: It's too subjective.
BICHELL: Tara Lovestead is a chemical engineer at the National Institute of Standards and Technology in Boulder, Colo. She and her colleagues are looking for the chemical signature of a high so that instead of relying on people to determine cannabis impairment, a standardized test might one day do the trick.
LOVESTEAD: We like to know the human error and the limitations of the human opinion and want to make it as scientific based in fact as we can.
BICHELL: It's actually really hard to do this kind of research because Lovestead works in a federal lab. And federally, cannabis is considered a schedule one substance. So even though she's in Colorado, getting a hold of a sample of THC for research purposes is just as hard as getting a hold of heroin.
LOVESTEAD: Right. We cannot use the stuff down the street (laughter).
BICHELL: It's also a really tricky chemistry problem and that's because of the main chemical in cannabis that gets a person high - THC. In states like Colorado, there is a THC blood test which can show, quote, "presumed impairment," but Lovestead and others maintain that, scientifically speaking, the test isn't reliable.
LOVESTEAD: We just don't know whether or not that means they're still intoxicated or impaired or not.
BICHELL: A leading research group found evidence of THC in the blood of frequent smokers up to a month after they stopped consuming. And in other people, blood samples showed no trace of THC even though the researchers had just watched them smoke joints.
LOVESTEAD: There's no quantitative measure that could stand up in a court of law.
BICHELL: Now, people like Lovestead are setting the standards for a reliable breath test, starting with the fundamental physical properties of THC. But in the meantime, back at the hotel, this course is the best there is. And at least now the officers know what pot strains like OG Kush and Skunk Dog actually smell like.
GURLEY: Yeah, smells like the bottom side of a rock a little bit more.
TARANTINO: I want to smell one that's like - is there one that's fruity?
BICHELL: Rae Ellen Bichell, NPR News.
(SOUNDBITE OF THE MOTET'S "NEMESIS")