With Pot Legal, Police Worry About Road Safety It's settled. Pot, at least certain amounts of it, will soon be legal under state laws in Washington and Colorado. Now, officials in both states are trying to figure out how to keep stoned drivers off the road. 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.