DENVER -- With the recreational use of marijuana now legal in Colorado, officers who patrol the state's roads face a new set of challenges. Though smoking or possessing small amounts of cannabis is no longer breaking the law, anyone who drives while impaired is still subject to arrest.
Which raises a knotty question: How many tokes can a driver take before the ability to control a vehicle is compromised to the point of being a danger on the road?
Unlike alcohol, which has an undisputed -- and usually quite apparent -- influence on driving, there is no clear-cut consensus on the amount of marijuana that must be consumed to impair a driver's ability.
This year, as Colorado lawmakers worked out regulatory matters, including taxes on sales, they also passed legislation that set legal limits on marijuana levels in the bloodstream. Under the new law, which took effect on May 28, a driver is assumed to be impaired if a blood test shows a level of tetrahydrocannabinol, or THC, that is five or more nanograms per milliliter. A nanogram is a billionth of a gram.
"What this bill does is create a standard like we have for driving under the influence of alcohol," said State Representative Mark Waller, a Republican from Colorado Springs and the bill's chief sponsor. "Juries have come to expect that standard. So this aids in the prosecution of these cases."
Previous attempts to pass such legislation have met strong opposition. Some legislators felt that the five-nanogram threshold was too low, in part because medical marijuana users always have some THC, the psychoactive ingredient in marijuana, in their blood. Others said that the blood test needed to measure the THC level was too intrusive.
Studies conducted to determine just how much THC it takes to diminish a driver's ability have differed in their conclusions.
Even the National Highway Traffic Safety Administration, which warns against taking the wheel under the influence of any drug that affects the ability to drive safely, acknowledges that detecting impairment caused by marijuana can be trickier than it is for alcohol. The safety agency is studying the effects of cannabis on drivers, and the results are expected late next year.
Some states, like Arizona, have enacted zero-tolerance laws that make driving with any trace of marijuana in the blood unlawful. A handful of other states have set their own thresholds for THC in the bloodstream.
Washington State, where recreational marijuana is also legal, has established the same THC limit as Colorado.
Earlier this year, in a widely viewed broadcast, a Seattle TV station, KIRO, had three volunteers smoke marijuana before driving.
They started out well enough, even after each far exceeded the state's five-nanogram limit. Predictably, the more they smoked, the worse they drove.
Colorado's law tries to strike a balance. Even with its five nanogram standard, the law has some flexibility, letting defendants present evidence that they were not too impaired to drive even when their THC levels exceeded the statutory level.
"I think certainly for the consumer, this is better law than a per se law, because it allows the defendant a chance to offer evidence of their innocence at trial," said Paul Armentano, deputy director of Norml, which advocates for the legalization of marijuana.
"We have this notion that since we have a magic number for alcohol, that we are going to have a similar number for marijuana," he said. "The problem is that marijuana is not metabolized and absorbed by the body in the same way alcohol is."
A spokesman for the Colorado State Patrol, Trooper Josh Lewis, said the law would not have a significant effect on how the police handled stops, though he noted that more officers would be trained in drug recognition.
"We'll still patrol the same way we did before," he said. "We'll look for signs of impairment."
Jeremy Rosenthal, a Denver lawyer who handles cases of driving under the influence, said he thought the law was a good compromise, and he liked that it did not rely solely on a measurement from a blood test.
Mr. Rosenthal said he still thought most juries would end up convicting people who had more than five nanograms in their bloodstream. But allowing a certain measure of leeway in marijuana cases seemed fair to him, given that such cases were different from those involving alcohol.
"I think people in Colorado generally support the right to use marijuana," he said. "But they also don't like people driving around impaired."
This article originally appeared in The New York Times.