Now that recreational marijuana is legal in Colorado, the state has increased education about the dangers of drugged driving and stepped up enforcement, according to NPR. The problem, experts say, is that there is no widespread agreement about how much marijuana impairs a person’s ability to drive.

Colorado’s new state limit for marijuana use while driving is 5 nanograms per milliliter of blood of THC, the drug’s psychoactive chemical.

John Lacey, a traffic safety expert, says marijuana doesn’t metabolize predictably like alcohol. “It makes setting an absolute level where everyone is impaired, like we have for alcohol, much more difficult for marijuana and for other drugs,” he told NPR. “They just behave differently than alcohol does.”

He noted drivers who use marijuana tend to drive more slowly, have trouble staying in their lane, and don’t respond as quickly as drivers who don’t use the drug. He advises people to stay off the road after they’ve used marijuana.

Colorado has added dozens of drug recognition experts to its ranks of law enforcement. While the state has started to keep track of marijuana DUI citations, most local police departments do not.

A study published earlier this year concluded that fatal car crashes that involved marijuana tripled in the past decade. One in nine drivers involved in a fatal crash tests positive for marijuana, according to the Columbia University researchers.