State: Number of drivers high on pot on the increase

SEATTLE -- New statistics released by the Washington State toxicologist indicate a rise in the number of cases of drivers exceeding the state's new blood THC level for marijuana.

The mandatory DUI limit -- also known as a per se limit -- is 5 milligrams/liter of delta-9 THC on a person's blood stream. That's the active ingredient in marijuana that can cause impairment.

In 2011, the state reported 1,036 cases of drivers testing positive for delta-9 THC in their blood. In 2012, the number dropped to 988. But in the first six months of 2013, there was 745 cases, which would project to nearly 1500 for the year.

Of those cases, 506 exceeded the legal limit of 5 milligrams/liter of delta-9 THC in 2011. That number increased to 609 in 2012. In first six months of 2013, it was 420 which projects to 820 for the year.

But the Washington State Patrol says its not just a numbers game. Troopers are looking for signs of impaired driving and do not necessarily rely on the state's new per se marijuana limit.

"You don't have to hit the target to be considered impaired," said Lt. Rob Sharpe, the Impaired Driving Section Commander for the Washington State Patrol.

"You can be impaired below those numbers as well, because we are looking for impairment when we encounter someone on the road way," Sharpe said.

There is a another factor that could skew future numbers. Because there is no field sobriety unit like a breathylzer to test a driver's blood alcohol levels, a blood draw is required to test for delta-9 THC .

A U.S. Supreme Court ruling led Washington State legislators to the elimination of the implied consent warning for blood.

"That means we don't read a warning like we do for alcohol in a breath test, we need a search warrant," Sharpe said.

The change took effect September 28. Now law enforcement needs to contact a judge to sign off on a search warrant before blood is drawn from a suspected impaired driver.

Unlike alcohol, which can linger in the blood for hours, experts say THC in a person's blood can disappear quickly within minutes after the last exposure to marijuana.

If a driver appears impaired when they are pulled over, experts say their blood THC levels drop significantly by the time a typical blood draw is taken after a driver is taken into custody.