RENEE MONTAGNE, HOST:
Five states hold ballot initiatives this November on whether to legalize recreational marijuana. And many voters and policymakers are studying Colorado's experience where pot has been legal for nearly three years. Still, one issue remains hazy there - how to enforce laws on driving while stoned. Ben Markus from Colorado Public Radio and Stephanie O'Neill of member station KPCC in Los Angeles report.
BEN MARKUS, BYLINE: This story starts with a stay-at-home mom from the Denver suburbs.
STEPHANIE O'NEILL, BYLINE: Her name is Abby McLean. And she was driving home from a late dinner with a friend two years ago when she came upon a DUI roadside checkpoint.
ABBY MCLEAN: I hadn't drank or smoked anything, so I was like, let's go through the checkpoint.
O'NEILL: McLean is a regular marijuana user, like, five times a day. But she insists she never drives while high.
MARKUS: Still, the cop at the checkpoint tells her he smells marijuana, that her eyes are bloodshot. Eventually, he whips out handcuffs and McLean freaks.
MCLEAN: Like, massive panic attack. And, oh, my God, I have babies at home. I need to get home. I can't go to jail.
MARKUS: A blood test later revealed McLean had five times the legal limit of THC. That's the mind-altering compound in marijuana - so an open-and-shut case, right?
O'NEILL: Actually, no. McLean's attorney, Nadav Aschner, had a field day in court with Colorado's marijuana intoxication limit.
NADAV ASCHNER: Even the state's experts will say that number alone is something but generally not enough. And we really hammered that home.
MARKUS: Aschner got a hung jury, and McLean pleaded to a lesser offense. But hers is just one of a number of Colorado cases where this is now happening.
O'NEILL: Turns out, measuring a person's THC level is actually a poor indicator of intoxication because, unlike alcohol, THC stores in your fat. Tom Marcotte is co-director of the Center for Medicinal Cannabis Research at the University of California at San Diego.
TOM MARCOTTE: 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.
MARKUS: And that's why adapting drunk driving laws to marijuana makes for bad policy, says Mark Kleiman. He's a professor of public policy at New York University.
MARK KLEIMAN: You can be positive for THC a week after the last time you used cannabis, not subjectively impaired at all, not impaired at all by any objective measure but still positive.
O'NEILL: Still, Colorado and five other states have such laws on the books because pretty much everyone agrees that driving while stoned can be dangerous, especially when combined with alcohol.
MARKUS: What cops really need is a simple roadside sobriety test, and a common iPad may offer that solution.
O'NEILL: The scientists at UC San Diego are now working on several iPad apps that could eventually be used to measure how impaired you are behind the wheel, like one where you have to follow a square moving around a screen with your finger which measures something called critical tracking. And then there's this other one that measures time distortion because things slow way down when you're high.
MARKUS: But that's all still experimental. In the meantime, some regular marijuana users like Abby McLean are scared to drive for fear of failed blood tests.
MCLEAN: I haven't gone out really since then because I'm paranoid to run into the same surprise. Uh oh, there's a DUI checkpoint.
O'NEILL: A checkpoint that for McLean could mean potentially thousands more dollars in attorney's fees to defend herself. For NPR News, I'm Stephanie O'Neill in Los Angeles.
MARKUS: And I'm Ben Markus in Denver. Transcript provided by NPR, Copyright NPR.