Significantly more drivers pulled over by police in Washington state are testing positive for marijuana since legalization of the drug's recreational use took effect in January, according to figures released this week by the Washington State Patrol.

In the first six months with pot legal in the state, 745 drivers stopped by police tested positive for the drug's psychoactive ingredient, THC, in their blood, the data show.

Over half of those were over the state's new legal limit of 5 nanograms of THC per milliliter of blood.

By contrast, in each of the last two full years, about 1,000 drivers who were pulled over tested positive for THC.

The increase comes despite the fact that recreational-use pot stores will not open in Washington state until next year.

Washington State Patrol spokesman Bob Calkins said the findings, while preliminary, indicate more people may be driving impaired than was the case before Washington and Colorado in January became the first states to legalize recreational use of the drug.

Some 20 states and the District of Columbia allow medical marijuana use.

Calkins said that, in the first half of the year, the overall number of people pulled over by the State Patrol on suspicion of driving under the influence, whether of alcohol or drugs, remained roughly on par with figures from the last two years.

The State Patrol arrests about 20,000 people a year on suspicion of impaired driving. A similar number of arrests are made by other police agencies around the state.

Whether people are driving under the influence of pot, alcohol or prescription drugs, Calkins said, "It all comes back to a bad decision to drive while impaired."

Kevin Sabet, co-founder of Project Smart Approaches to Marijuana, a group that opposes its widespread use, said the results are troubling but not surprising.

"People are getting the impression that marijuana use is okay," he said. "Even before one recreational store opens in Washington, we are already seeing the effects."

Douglas Hiatt, a Seattle-based criminal defense lawyer and marijuana advocate, said the state's new "stoned driving" standard wrongly encourages police to pull over more drivers on suspicion of marijuana use.

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Hiatt, who opposed Washington state's pot legalization law in part over concerns about its driving provision, pointed to the zero-tolerance standard for drivers under age 21, which he said makes teens vulnerable to police officers looking for easy arrests.

"It's like shooting fish in a barrel," he said. "It hits the kids of color the hardest." (Editing by Dan Whitcomb and Gunna Dickson)