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Colorado's marijuana DUI law is modeled on the one for alcohol, which sets a number to determine when someone is too intoxicated to drive. For pot, that number is five nanograms of THC per milliliter of blood. Anything above that and the law says you shouldn't be driving.

More states may come up with their own marijuana DUI guidelines. Voters in five states from California to Maine are deciding this November whether to legalize recreational marijuana. They're weighing the good, the bad and the still unknown. Issues like driving while stoned are still in the "unknown" category.

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, says 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," he explains.

That's why adapting drunk driving laws to marijuana makes for bad policy, says 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 says. "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 cops really need is a simple roadside sobriety test. Scientists at UCSD are among researchers working on several apps that could measure how impaired one is behind the wheel. 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, NPR reports.