The state chapter of the American Civil Liberties Union found that such low-level charges were filed in just 120 cases in 2013, down from 5,531 cases the year before.
"The data strongly suggest that I-502 has achieved one of its primary goals - to free up limited police and prosecutorial resources," Mark Cooke, criminal justice policy counsel with the state ACLU, said in a news release.
Ian Goodhew, deputy chief of staff at the King County prosecutor's office, said that hasn't been the case in his office. He said prosecutors only handled a few misdemeanor pot cases a day before the law went into effect.
"There's no great relief of workload," Goodhew said. "All this has meant is maybe our calendar in District Court in the Seattle division is maybe, instead of 46 cases in a day, 44 or 43 or 42. We're no longer filing misdemeanor marijuana cases, but we were not expending any significant resources on those cases at the time I-502 passed."
Cooke conceded the law hasn't fundamentally changed what prosecutors do every day, but said when considered more broadly, I-502 has saved resources, from basic investigation and filing of paperwork to court time. He noted King County's adult misdemeanor pot cases fell from 1,435 in 2009 to 14 last year.
"I can't fault their logic," said Mitch Barker, executive director of the Washington Association of Sheriffs and Police Chiefs. "If we took speeding off the books, that would free up time. If we took robbery off the books, that would free up time.
"The question we all have to look at is, is it good public policy? My sole concern is that when you expand access to marijuana for adults, you expand access for underage people."
The pot cases that were filed in the state last year likely involved people caught with more than the ounce of weed, or 28 grams, they're allowed to have under Washington's Initiative 502, but less than the 40 grams that can trigger felony possession charges.
The data, which came from Washington's Administrative Office of the Courts, also suggest racial disparities remain a concern in marijuana charges, Cooke said.
Prior to I-502's passage in 2012, blacks were nearly three times as likely as whites to face misdemeanor marijuana possession charges in Washington, and that remained true among the 120 cases filed last year, he said. Of the 120, white defendants accounted for 82 cases and blacks for 11. That equated for whites to 2 cases per 100,000 residents; for blacks, to 5.6 per 100,000.
The number of misdemeanor filings for those over 21 had been dropping for several years, the group said, from 7,964 in 2009 to 5,531 in 2012. Court filings for all drug felonies, including marijuana growing and selling, has remained fairly constant since 2009, at about or slightly under 20,000.
Among people under 21, misdemeanor marijuana possession charges have also fallen in the past two years from 4,127 in 2011 to 3,469 in 2012 and 1,963 last year. People under 21 aren't allowed to have pot under the state law, so reasons for that decline weren't immediately apparent.