Should the Government Be Worried About Stoned Drivers?

Congress is looking into the effects of intoxicated driving, but the jury's still out on how to best police it.

Marijuana plants grow at Perennial Holistic Wellness Center, a not-for-profit medical marijuana dispensary in operation since 2006, on September 7, 2012 in Los Angeles, California.  (National Journal)

The House Oversight and Government Reform Committee has mostly concerned itself of late with the IRS scandal, the problematic rollout of the Obamacare website, and the terrorist attack on the U.S. Embassy in Benghazi, Libya. But on Thursday, the panel turned its attention to marijuana use by drivers, including pilots, subway train operators, mariners, and school bus drivers.

At the hearing, called "Planes, Trains and Automobiles: Operating While Stoned," members heard testimony about the impact of driving under the influence of marijuana. Rep. John Mica, who once held up a fake joint at another hearing, lambasted the federal government for its "chaotic and inconsistent policies on marijuana."

There's no federal benchmark for THC — the psychoactive ingredient in marijuana — like there is for blood alcohol content, because under federal law any consumption of marijuana is illegal. Some states that have decriminalized medical or recreational marijuana use have implemented their own benchmarks. Colorado, for example, has set a limit of 5 nanograms of THC per milliliter of blood.

Since THC is fat-soluble, it can remain in your system long after its intoxicating effects have worn off. In states such as Illinois, where marijuana is still illegal, having any trace of THC in your system — even if you smoked weed days before getting pulled over — can get you a DUI.

Colorado traffic cops have also been trained to recognize the behavior of drivers who may be under the influence of marijuana, as part of its "Drive high, get a DUI" campaign.

But the science behind marijuana intoxication and driving is still murky. There's no such thing as a "weed breathalyzer." And unlike alcohol, marijuana (and other drugs) can have vastly different effects, depending on who's using it. THC does not metabolize as predictably as alcohol, so it's more difficult to set a concrete level at which someone can be considered intoxicated — and impaired.

And unlike alcohol DUIs, which are tracked at the state level, marijuana intoxication is not monitored as closely, making it more difficult to ascertain how much of a problem it is. One study found that, between 1999 and 2010, the proportion of people killed while driving with marijuana in their system tripled, from 4 percent to 12 percent. But that data was inconclusive, because it's impossible to know whether those drivers consumed marijuana that day, or a week before their accident.

Among the research into intoxicated driving, one data point remains true: Alcohol use is the No. 1 cause of death on American roadways. In 2012, alcohol accounted for 31 percent of driving fatalities — killing more than 10,000 people nationwide.

A 1993 report from the National Highway Traffic Safety Administration found that drivers under the influence of marijuana are generally slower, clumsier, and have difficulty staying in their lane. "Drivers under the influence of marijuana retain insight in their performance and will compensate when they can, for example, by slowing down or increasing effort," the report's authors found at the time. "As a consequence, THC's adverse effects on driving performance appear relatively small."

Some users can still drive competently with a high dosage of THC. One driver in Washington — where marijuana is now legal — was able to pass a road test with seven times the legal limit of THC in her system. Still, with any intoxicating substance, it's better to be safe than sorry. But while marijuana is an increasing concern on the road, the more dangerous problem continues to be our relationship to a substance that's already legal.