Colorado's measure doesn't make any changes to the state's driving-under-the-influence laws, leaving lawmakers and police to worry about its effect on road safety.
"We're going to have more impaired drivers," warned John Jackson, police chief in the Denver suburb of Greenwood Village.
Washington's law does change DUI provisions by setting a new blood-test limit for marijuana -- a limit police are training to enforce, and which some lawyers are already gearing up to challenge.
"We've had decades of studies and experience with alcohol," said Washington State Patrol spokesman Dan Coon. "Marijuana is new, so it's going to take some time to figure out how the courts and prosecutors are going to handle it. But the key is impairment: We will arrest drivers who drive impaired, whether it be drugs or alcohol."
Drugged driving is illegal, and nothing in the measures that Washington and Colorado voters passed this month to tax and regulate the sale of pot for recreational use by adults over 21 changes that. But law enforcement officials wonder about whether the ability to buy or possess marijuana legally will bring about an increase of marijuana users on the roads.
Statistics gathered for the National Highway Traffic Safety Administration showed that in 2009, a third of fatally injured drivers with known drug test results were positive for drugs other than alcohol. Among randomly stopped weekend nighttime drivers in 2007, more than 16 percent were positive for drugs.
Marijuana can cause dizziness and slowed reaction time, and drivers are more likely to drift and swerve while they're high.
Marijuana legalization activists agree people shouldn't smoke and drive. But setting a standard comparable to blood-alcohol limits has sparked intense disagreement, said Betty Aldworth, outreach director for Colorado's Campaign to Regulate Marijuana Like Alcohol.
Most convictions for drugged driving currently are based on police observations, followed later by a blood test.
"There is not yet a consensus about the standard rate for THC impairment," Aldworth said, referring to the psychoactive chemical in marijuana.
Unlike portable breath tests for alcohol, there's no easily available way to determine whether someone is impaired from recent pot use.
There are different types of tests for marijuana. Many workplaces test for an inactive THC metabolite that can be stored in body fat and remain detectable weeks after use. But tests for current impairment measure for active THC in the blood, and those levels typically drop within hours.
According to the National Highway Traffic Safety Administration, peak THC concentrations are reached during the act of smoking, and within three hours, they generally fall to less than 5 nanograms per milliliter of blood -- the same standard in Washington's law, one supporters describe as roughly equivalent to the .08 limit for alcohol.
Two other states -- Ohio and the medical marijuana state of Nevada -- have a limit of 2 nanograms of THC per milliliter. Pennsylvania's health department has a 5-nanogram guideline that can be introduced in driving violation cases, and a dozen states, including Illinois, Arizona, and Rhode Island, have zero-tolerance policies.
In Washington, police still have to observe signs of impaired driving before pulling someone over, Coon said. The blood would be drawn by a medical professional, and tests above 5 nanograms would automatically subject the driver to a DUI conviction.
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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