Supporters of Washington's measure said they included the standard to allay fears that legalization could prompt a drugged-driving epidemic, but critics call it arbitrarily strict. They insist that medical patients who regularly use cannabis would likely fail even if they weren't impaired.
They also worry about the law's zero-tolerance policy for those under 21. College students who wind up convicted even if they weren't impaired could lose college loans, they argue.
Jon Fox, a Seattle-area DUI attorney, said he's interested in challenging Washington's new standard as unconstitutional. Under due process principles, he said, people are entitled to know what activity is prohibited. If scientists can't tell someone how much marijuana it will take for him or her to test over the threshold, how is the average pot user supposed to know?
By contrast, he noted, the science on alcohol is well established. Some states publish charts estimating how many drinks it will take a person of a certain weight over a certain time to reach .08.
But such a challenge to Nevada's marijuana DUI limit failed in 2002, when the state Supreme Court ruled that the Legislature has broad authority to set driving standards. The U.S. Supreme Court declined to review that case, said Las Vegas DUI attorney Michael Becker.
"Marijuana affects everyone differently," Becker said. "The prevailing opinion of forensic toxicologists is that a 2-nanograms standard, such as exists in Nevada, absolutely results in convictions where individuals are not actually under the influence. But the 5-nanograms standard more closely approaches the mean threshold of prevailing opinion."
Colorado's legalization measure didn't set a driving standard — an intentional omission by the activists who wrote it because the issue has proven divisive. Lawmakers in Colorado, which has an established medical marijuana industry, have tried but failed three times to set a THC driving limit.
Drugged driving cases in Colorado were up even before the legalization vote. In 2009, the state toxicology lab obtained 791 THC-positive samples from suspected impaired drivers. Last year, it had 2,030 THC-positive samples.
Colorado lawmakers are preparing to take up driving standards yet again when they convene next year.
"I believe a 5-nanogram limit will save lives," said Colorado Republican state Sen. Steve King, sponsor of previous driving-high bills.
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