Colorado Pot Debate Continues: When Are Stoners Too Stoned To Drive?

June 11, 2013

Last fall, voters in Colorado and Washington state gave a big thumbs-up to recreational marijuana use. Rick Steves and Doritos retailers rejoiced, but lawmakers have been far less excited -- largely because there's little agreement on when the THC found in pot impairs driving skills.

As the New York Times reports, both states have modeled their marijuana laws on the well-known blood-alcohol content standard that determines whether a motorist is too drunk to drive. Like the U.S. blood-alcohol limit that sits at .08, Colorado and Washington set a threshold of five nanograms of THC per milliliter of blood: anything at that level or higher indicates impairment.

Colorado state representative Mark Waller was the chief sponsor of the Colorado bill that set THC driving limits. He liked the way that his bill was consistent with other laws and its easy application in court: "What this bill does is create a standard like we have for driving under the influence of alcohol.... Juries have come to expect that standard. So this aids in the prosecution of these cases."

The only problem is, marijuana and alcohol affect humans very differently. Countless tests have been conducted on blood-alcohol levels and motor skills, and there appears to be a direct correlation between high blood-alcohol content and impairment. The effects are felt by drivers regardless of height, weight, or sex: a 140-pound woman with a .08 level will be just as impaired as a 110-pound man registering .08.

THC is different, though. Researchers have yet to agree on the magic number at which THC significantly impairs driving ability. In test situations witnessed earlier this year, subjects had to get themselves far, far above the five nanogram threshold before their driving skills were affected. And the participants clearly understood that they shouldn't have been driving when they reached that point. How many drunk drivers would've said the same?

It's also good to remember that determining THC levels requires taking blood samples, which seems more invasive than administering a breath test. In fact, we'd call it far more invasive. That could have some impact on the constitutionality of these laws.

Despite all this confusion -- or perhaps because of it -- Colorado officials say that THC levels alone won't be the bar for driving impairment. Driving behavior appears to be the #1 criteria, and if motorists can prove that they were driving responsibly at the time of their arrest, they may have an out in court. 


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