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Riding High: Arizona's Zero-Tolerance Stance on Pot and Driving

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According to a widely reported study published in the British Medical Journal last year, "acute cannabis consumption nearly doubles the risk of a collision resulting in serious injury or death."

Sounds serious. But if it's true that pot is a major risk for drivers, and if it's also true that — as surveys show — as many as 15 million Americans may be regular marijuana users, why are there so few pot-caused accidents in the Phoenix area?

Police and other experts offer theories.

People often go out for drinks, which involves driving, points out Scottsdale police Sergeant Mark Clark. But with no marijuana bars (Phoenix's Vapor Lounge for medical-pot patients is an exception), people under the influence of pot may have less reason to get behind the wheel.

But there's also the simple fact that marijuana doesn't get users as wasted as alcohol or pills.

A chart seen by college students across the country shows a continuum of alcohol effects based on BAC: At three times the legal limit of alcohol, most people struggle to stay conscious. At five times the legal limit, death is a very real possibility for alcohol users.

No such continuum exists for pot, which is less toxic than aspirin or Tylenol.

Clearly, marijuana and alcohol are two very different beasts in terms of how they act on the body and affect the brain. This was shown in dramatic fashion during a 1993 on-road study of marijuana conducted by the National Highway Traffic Safety Administration, which concluded that it wasn't possible to predict driving impairment by the level of THC in the blood.

The blood of drivers "showing substantial impairment in these studies contained both high and low THC concentrations; and drivers with high plasma concentrations showed substantial, but also no impairment, and even some improvement."

Later studies lean toward the idea that concentrations in the neighborhood of 2 to 5 nanograms per milliliter may be predictors of impairment, though not always. Still, that some drivers with high THC concentrations showed no impairment or even "improvement" in driving is remarkable. A similar result for any study involving alcohol and driving would be unimaginable.

At the same time, the revelation that some drivers with low THC concentrations can show "substantial impairment" should raise concern among any marijuana user who doesn't want to hurt someone in a crash.

Novice users may be at particular risk of that.

For example, a 16-year-old crashed into one car, fled, then hit a parked car last year. He admitted to Phoenix police that he felt "light-headed" after smoking pot and knew he shouldn't have been driving. A blood test showed he had 2 nanograms per milliliter of active THC in his body. If he'd been stopped in Nevada, he would've been at the legal limit for that state, but the teen wouldn't have been considered too high to drive in Washington state.

Compare that to the findings of a 2009 study of heavy marijuana users funded by the National Institute on Drug Abuse. The study took 25 long-term heavy users and put them on the wagon for a week in a secure clinic where they could be examined (and where they couldn't toke up). NIDA researcher Marilyn Huestis, who participated in the study, tells New Times that some of these heavy users reported smoking up to 60 joints a day before they abstained for science.

By day seven, six of the 25 still had measurable amounts of active THC in their systems. A user who checked in with 7 ng/ml in his blood still had 3 ng/ml in his system on the last day. The residual THC still affects the brain and, based on other studies, could cause impairment for driving, Huestis says.

When ingested, the active ingredient in pot finds common receptors in the brain that are involved in "critical functions . . . control of movement, divided attention, critical tracking, staying in the lane," she says.

One thing the study didn't research, though, was users' actual driving histories. Asked whether her research made her comfortable enough to predict the percentage of suspected cannabis-caused crashes in Phoenix last year, Huestis declined to throw out numbers.

Daniel Rees, an economics professor at the University of Colorado-Denver, says his research shows that traffic fatalities have declined in states that have legalized marijuana for medical use and that the decline appears to be related to the new marijuana laws. Rees says his paper on the study will be published this month in the Journal of Law and Economics.

His theory is that enough people in the pro-pot states have substituted marijuana for alcohol use to make a difference in the crash trends. Even if the study from the British Medical Journal is correct about the crash risk for drivers doubling after pot use, Rees pointed to a 2004 study that showed drivers with a .08 BAC were 10 times more likely not just to be in a crash, but to cause one.

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Ray Stern has worked as a newspaper reporter in Arizona for more than two decades. He's won numerous awards for his reporting, including the Arizona Press Club's Don Bolles Award for Investigative Journalism.