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How High is Too High to Drive?

As legalized pot spreads, states are dazed and confused about what to do with stoned drivers.



On the evening of February 23, 2010,  Rodney Koon was pulled over for doing 83 in a 55 zone outside Traverse City, Michigan. The deputy who stopped Koon noticed that the inside of  his Toyota RAV4 smelled funny, and the middle-aged carpenter admitted that he'd taken a few hits of marijuana six hours earlier.

As a pipe-carrying medical-marijuana user (for a hernia and rheumatoid arthritis), Koon thought that the law was on his side. The cop thought otherwise and took him in for a blood test, which revealed traces of THC, or tetrahydrocannabinol, pot's psychoactive ingredient. Koon was charged with violating the state's "zero tolerance" drugged-driving law. He's still fighting the charges; if convicted, he faces a suspended license and, since he has a previous DUI, up to a year in prison.

Koon's case centers on a contentious practical issue that has emerged with the spread of pot legalization: how to define—and prevent—stoned driving. Between 2009 and 2011, the number of THC-positive  blood samples obtained from drivers by Colorado police more than doubled. A study found that about 7 percent of California drivers surveyed on two weekend nights last summer  tested positive for THC. Nearly the same percentage had alcohol in their blood; 1 percent had both pot and booze. (Drivers who agreed to participate in the study were given legal immunity, handed $20, and hooked up with a ride home if necessary.) But just how stoned is too stoned to drive?

Figuring that out isn't easy. In all 50 states, if you're pulled over on suspicion of driving under the influence, you must submit to a Breathalyzer test or face arrest and possibly a blood test. Yet cops lack anything like a Breathalyzer for THC, and studies have shown that the field sobriety test widely used by police departments correctly  fingers stoned drivers only about 30 to 50 percent of the time; drunks are detected 80 percent of the time. Some police departments are trying to improve those odds: The Colorado State Patrol employs specialized drug recognition experts armed with  a 12-step protocol that includes one-leg stands, finger-to-nose tests, and checking for "a lack of ocular convergence." Although Colorado does not have a legal limit on blood THC levels, it  wins convictions on 90 percent of its drugged-driving cases.

Ten states, many of which have legalized medical marijuana, simply make it illegal to drive with any trace of marijuana in your blood. Other states essentially regulate the drug like alcohol, requiring drivers to stay below a set limit of cannabinoids in their blood. When Washington voters legalized pot last November, they  also outlawed driving with a blood THC level over 5 nanograms per milliliter—about half the level detected in Koon. Ohio and Nevada's limit is even stricter: 2 ng/ml. These rules appeal to a public accustomed to drunk-driving standards, and they give police a simple benchmark for making arrests.

But these approaches don't account for what scientists know about marijuana's effects on drivers. "The reality is that alcohol and cannabis are two very different drugs that affect people in very different ways," says  Jan Ramaekers, a psychology and neuroscience professor at Maastricht University in the Netherlands. A 2009 study funded by the National Institute on Drug Abuse found that  THC can persist in chronic pot smokers' blood for a week after they stop smoking, sometimes at levels in excess of 3 ng/ml. Other research shows that those residual blood levels (and sometimes  even much higher levelsdon't impair most heavy users' psychomotor skills. If the goal is to arrest only people who are driving dangerously, Ramaekers says, then laws like Washington's could lead to a rash of false convictions.

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