DENVER (AP) — Angeline Chilton says she can’t drive unless she smokes pot.

The suburban Denver woman uses medical marijuana to ease multiple sclerosis symptoms and says she never would get behind the wheel right after smoking. But her case underscores a problem that no one’s sure how to solve: How do you tell if someone is too stoned to drive?

States that allow medical marijuana have grappled with determining impairment levels for years. And voters in Colorado and Washington state will decide this fall whether to legalize the drug for recreational use, bringing a new urgency to the issue.

A Denver marijuana advocate says officials are scrambling for limits in part because more drivers acknowledge using the drug.

“The explosion of medical marijuana patients has led to a lot of drivers sticking the (marijuana) card in law enforcement’s face, saying, ‘You can’t do anything to me — I’m legal,”’ said Sean McAllister, a lawyer who defends people charged with driving under the influence of marijuana.

It’s not that simple. Driving while impaired by any drug is illegal in all states.

But it highlights the challenges law enforcement officers face using old tools to try to fix a new problem. Most convictions for drugged driving now are based on police observations, followed later by a blood test.

Authorities envision a legal threshold for pot that would be comparable to the blood-alcohol standard used to determine drunken driving.

But unlike alcohol, marijuana stays in the blood long after the high wears off a few hours after use, and there is no quick test to determine someone’s level of impairment — not that scientists haven’t been working on it.

Dr. Marilyn Huestis of the National Institute on Drug Abuse, a government research lab, said that soon there will be a saliva test to detect recent marijuana use.

But government officials say that doesn’t address the question of impairment.

“I’ll be dead — and so will lots of other people — from old age before we know the impairment levels” for marijuana and other drugs, said White House drug czar Gil Kerlikowske.

Authorities recognize the need for a solution. Marijuana causes dizziness and slowed reaction time, and drivers are more likely to drift and swerve while they’re high.

Dr. Bob DuPont, president of the Institute for Behavior and Health, a non-government institute that works to reduce drug abuse, says research proves “the terrible carnage out there on the roads caused by marijuana.”

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