Like many medical marijuana patients, Greg Duran says he drives in fear, knowing he could be busted at any moment for driving under the influence.
As he merges onto Interstate 70 north of Denver, Duran explains that he's probably over the state's new marijuana limit: 5 nanograms per milliliter of blood of THC, the psychoactive chemical in pot.
"It would be devastating if I lost my car. It would change everything," Duran says.
He needs marijuana to treat nausea from his vertigo and he tries to keep a constant amount in his body in order to keep his condition at bay, he says. Because the drug can stick around in the bloodstream long after a person has smoked, Duran says the THC limit will surely net innocent drivers.
With more than 100 recreational and 500 medical marijuana stores now open for business in Colorado, stoned drivers are a growing concern. As a result, the state is stepping up education and enforcement but that leaves people like Duran worried.
Sean McAllister, a Denver criminal defense attorney, says he's getting five calls a week from clients with pot DUI citations.
"If you were talking about this concept with alcohol and told people, 'We got a test that can say if you drank in the last 24 hours and if you fail it we're going to arrest you for DUI,' we would be occupying the Capitol right now," McAllister says.
Users don't have guideposts for marijuana impairment, he says. Is half a joint too much? What about two or five bong rips? No one is sure.
It doesn't help that marijuana doesn't metabolize predictably like alcohol, says John Lacey, a traffic safety expert based in Maryland.
"It makes setting an absolute level where everyone is impaired, like we have for alcohol, much more difficult for marijuana and for other drugs," Lacey says. "They just behave differently than alcohol does."
And drivers behave differently on marijuana than after drinking. They drive slower, but they also have trouble staying in their lane and lack a quick response time.
Lacey says it's best to stay off the road. Some studies indicate that stoned drivers are 33 percent more likely than sober drivers to be involved in a fatal crash. That's enough of a risk to prompt a new state educational campaign.
In one TV ad a man is trying to light a gas grill that happens to be missing a propane tank. Words come on the screen saying, "Grilling high is now legal. Driving to get the propane you forgot isn't."
If drivers don't heed the warning, State Trooper Nicholas Hazlett will be waiting.
"It is the toughest school that an officer will go through in law enforcement," says Hazlett, who is certified as a drug recognition expert.
The state has added dozens of these specialists to law enforcement ranks. They're needed, since there is no easy way to test for marijuana impairment. "I wish we did, but no, we don't have a marijuana Breathalyzer," Hazlett says.
The state patrol has just started keeping track of marijuana DUI citations but most local police departments don't specifically track marijuana DUIs in their own districts. That makes it hard to know the true scope of the problem.
Hazlett has a particular interest in the issues of driving while intoxicated, since he lost an uncle to a drunk driver many years ago. So far, since pot was legalized he hasn't seen a big increase in stoned drivers. But even one is too many.
The danger isn't worth it, Hazlett says. If you've smoked pot, it's best to stay off the road altogether.