In the first six months of 2013, the patrol's crime lab says, 745 people tested positive for marijuana. Typically, there are about 1,000 positive pot tests on drivers in a full year.
It doesn't necessarily mean there has been a rash of people driving high, said patrol spokesman Bob Calkins. Troopers are looking harder for drivers operating under the influence of pot, and they might be ordering more marijuana blood tests - "We're testing blood we didn't test before," he said.
In addition, the overall number of impaired driving cases handled by the patrol doesn't appear to have risen this year, and should be on track to hit the rough annual average of 20,000 - which could mean that some people are using marijuana instead of alcohol before getting behind the wheel, Calkins said.
"They're still making a very bad decision," he said.
The patrol's crime lab ran the numbers this month and provided the information to the federal government. The Justice Department announced in August it would not sue to block recreational marijuana sales in Washington and Colorado as long as the states satisfy eight federal law enforcement goals, including keeping pot away from children and the black market - and combatting drugged driving.
Washington and Colorado voters legalized the recreational use of marijuana by adults over 21 last year. Both states have set a legal limit of 5 nanograms of active THC per milliliter of blood for drivers; anything above that is a per-se violation of impaired driving laws, similar to blowing 0.08 or above on an alcohol breath test.
The violation is generally a gross misdemeanor punishable by up to a year in jail - and at least one day in custody for a first offense.
Of the 745 people who tested positive for marijuana in the first half of this year, the State Patrol says a slight majority tested above the legal limit. The exact number: 420. It's a curious coincidence, since "420" is an old slang term for marijuana.
In 2011, 506 drivers tested above 5 nanograms. In 2012, it was 609.
The number of positive marijuana blood tests represents a tiny sliver of the roughly 40,000 impaired driving arrests in the state in any given year. The positive pot-test figures reflect cases from the patrol as well as local police agencies.
Kevin Sabet, of Project SAM (Smart Approaches to Marijuana), said it will be interesting to learn the reasons for the increase in drivers testing positive for pot, but the numbers are troubling. His group says it promotes a public-health approach to marijuana.
"It's certainly cause for alarm," he said. "It's very possible people are getting the message that this is OK. Obviously, impaired driving is a major public safety issue."
Alison Holcomb, the American Civil Liberties Union of Washington lawyer who drafted the state's legal pot law, agreed that it's too early to know what was causing the rise, but said she would be tracking the issue - including keeping tabs on whether certain demographics are being disproportionately targeted for marijuana blood tests.
More public education about the dangers of drugged driving, and especially of mixing marijuana and alcohol before getting behind the wheel, is needed, she said. "That's something we'd really like to see the Liquor Control Board and Department of Health collaborate on now," before the state's licensed pot shops open next spring, she said.