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‘Not backed by science’: Washington still struggles with marijuana DUI limit

File photo (Joe Raedle/Getty Images)

SEATTLE — Four years after I-502 made recreational marijuana legal in Washington state, it’s not any easier to scientifically determine if someone behind the wheel is too stoned to drive.

When the law was enacted, Washington decided on a “per se” DUI threshold of five nanograms of THC per milliliter of blood drawn.  Drivers over that number are presumed impaired, and could be charged with a DUI.

The problem, says Washington Traffic Safety Commission research director Staci Hoff, is the number is completely arbitrary.

RELATED: Blood test reveals hit-and-run suspect was at 54 times the legal limit for marijuana

“(The per se blood limit) wasn’t backed by scientific theories then, and it’s not backed by scientific theories now,” Hoff said.

Though the per se limit of alcohol is not perfect, .08 is a better predictor of intoxication than the limit for THC, Hoff says. That's because THC is a fat soluble drug. Chronic users who haven't smoked recently could still exhibit a level of 10 or 20 nanograms of THC per milliliter of blood.

And someone who smokes for the first time might register zero THC in the blood, while still being too impaired to drive.

"More and more research is coming out debunking this mythical link between THC level in the blood and level of impairment," Hoff said.

It's not so much the level of THC in your blood, but the level of impairment that law enforcement agencies are concerned with. People should move away from thinking 'oh, if I'm under 5 nanograms of THC, I'm good to drive.'

That just isn't the case, says Washington State Patrol Lt. Rob Sharp.

"Regardless of the concentration of the substance, if you exhibit the effects and your driving is impaired, you can be arrested," Sharp said.

Troopers use a combination of impairment gauges, including roadside sobriety tests.

But roadside sobriety tests are not always perfect at detecting marijuana impairment either, Hoff says.

Marijuana greatly impacts cognitive abilities. Many of the steps in the standard field sobriety test, such as walking a straight line or touching your nose, are physical tests. Someone high on marijuana might pass physical tests just fine.

Law enforcement agencies have developed more cognitive tests, but those combined with physical tests take quite a long time roadside, Hoff says.

Hoff says a combination of THC tests, cognitive tests, and physical tests is ideal. It might be best for the state to get rid of the 5 nanograms limit Hoff says, and use a roadside exam that tests for any presence of THC. Those roadside exams are in the works; Washington State University is developing a saliva test to determine immediately if THC is present.

Hoff said that regardless of what blood tests show, the ultimate goal is to get impaired drivers off the road.

"We are getting back to the root of traffic safety, which is really impairment," Hoff says.