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A small group of volunteers spent much of the last year getting drunk and stoned on marijuana furnished by the federal government before getting behind the wheel.

The volunteers were part of what federal scientists say was the most comprehensive study ever conducted on how marijuana, and pot combined with alcohol, affect drivers. The data now being analyzed ultimately will help regulators decide how stoned is too stoned to drive. It's similar to the studies conducted to develop levels for drunken driving. Volunteers were recruited from around Iowa City, home to the University of Iowa's National Advanced Driving Simulator.

"They were happy to participate," says Marilyn Huestis, chief of chemistry and drug metabolism at the Intramural Research Program at the National Institute on Drug Abuse.

RELATED: Roadside tests check for impairment

The participants never got on real roads. Instead, they drove for about 40 minutes behind the wheel of NADS, a federally funded simulator that can mimic the look and feel of everything from urban parking lots to darkened gravel roads. Deer jump out unexpectedly. Passing cars swerve.

Before getting behind the NADS wheel, each volunteer was required to consume specific combinations of marijuana and alcohol, or a placebo. Because the university has a smoke-free campus, the volunteers had to use a vaporizer to consume their marijuana, which was furnished under strict rules by a federal garden at the University of Mississippi.

Each of the 19 drivers who completed the six combinations of pot and alcohol gave blood and saliva tests during their drives to check intoxication levels, Huestis says. She says the entire experiment took three years to design and administer.

The testing finished this spring, and now scientists are studying the 250 variables checked by the tests. They hope to have initial data available by October.

"In this country, there's a huge controversy over whether there should be zero tolerance or there should be some level that's acceptable. It's a terribly difficult problem," Huestis says. "We will be looking at what are the kinds of functions that are affected, and whether they are significantly different … whether alcohol is on board or not."

Colorado State Trooper J.J. Wolff has made a career of tracking down drunken and drugged drivers. As one of the state's leading experts in identifying impaired drivers, Wolff knows many Americans are watching what's happening on Colorado roads. He says he's not yet seen a major increase in stoned drivers, but state troopers are definitely looking.

"I have personally not seen more stoned drivers, not arrested more stoned drivers," says Wolff, who also trains new troopers on how to recognize impaired drivers. "From my point of view, that's good."

To check whether drivers might be impaired by alcohol, marijuana or prescription drugs, Wolff puts them through a series of voluntary roadside tests. The tests, which include standing on one leg while counting silently, and walking in a straight line, check someone's sense of time and motor skills. Marijuana, like alcohol, is a central nervous system depressant. That means it affects how someone perceives time, Wolff says.

"My first objective is to make sure you're OK to drive," he says.

One of the most useful tools police officers have is the portable breath test, which instantly checks a driver's intoxication level through the amount of alcohol they exhale. In the past few weeks, Wolff has been testing a new kind of rapid screening system for marijuana use. The test takes about 10 minutes, using a drop of saliva from a driver. Most of the commonly used marijuana tests require a blood draw and take days to yield results. Those tests can really tell only whether someone has used marijuana in the past week or so, not whether they are impaired now.

Wolff says the federal research will help by leading to specific guidelines on what's considered impaired. For alcohol, police generally say that someone with a blood-alcohol level of .08% is too drunk to drive. Wolff and Huestis say decades of federal research on alcohol impairment has given lawmakers and police sound science upon which to base drunken-driving laws. Marijuana hasn't been studied as carefully.

Colorado has significantly boosted the number of "drug-recognition expert" officers patrolling its roads, and other states are following suit. In Cheyenne, Wyo., just north of the Colorado state line, all officers there have received special training to better recognize marijuana-impaired drivers.

"When it comes to cannabis, it's a lot trickier than when it comes to alcohol," Wolff says. "I can't tell you if one joint is going to make you high to the point where you can't drive. That's a really hard question to answer at this point. ... "The safest thing to do right now: If you are going to drink any amount, don't drive. And if you are going to consume any amount of cannabis, don't drive."

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