EVERETT — Marijuana increasingly plays a role in fatal crashes on Washington’s roadways, according to a recent state study.

The number of drivers involved in deadly crashes who tested positive for marijuana increased 48 percent from 2013 to 2014, according to the Washington State Traffic Safety Commission report.

“We have seen marijuana involvement in fatal crashes remain steady over the years, and then it just spiked in 2014,” said Staci Hoff, the traffic commission’s data and research director.

A total of 99 drivers involved in fatal accidents tested positive for marijuana in 2014. By contrast, that number was 56 in 2011.

“It certainly seems there has been a real uptick in cases where it was at least partially a factor,” said Tobin Darrow, a Snohomish County deputy prosecutor who handles many vehicular homicide cases.

Statewide, marijuana was found in 28 percent of blood samples taken in impaired driving cases in 2014, according to data provided by the Washington State Patrol. That number has gone up and down over the past five years but was higher last year than in 2012 or 2013.

Washington voters approved a recreational marijuana law in 2012 and state regulators allowed pot shops to open in July 2014.

The new report from the traffic commission breaks down driver data to determine who tested positive for active THC, the impairing substance in marijuana, and those with residual marijuana in their system from days or weeks before.

Among drivers in fatal crashes who tested positive for marijuana, those testing positive for active THC rose from less than half in 2010 to 65 percent in 2013 and 85 percent last year.

Roughly half of the THC-positive drivers exceeded the legal limit of 5 nanograms per milliliter of blood.

“With this data we are finally able to see who was high during the crash versus which drivers had used marijuana in the past few days,” Hoff said. “The answer in 2014 is most of them were high.”

The largest increase in THC-positive drivers involved in fatal accidents was among men ages 21 to 25. There were six in 2013 compared to 19 in 2014.

Many drivers involved in fatal accidents were mixing marijuana and alcohol. In fact, the report said, half of the THC-positive drivers also were considered to be driving drunk.

“Mixing the two accentuates the effects of both,” said Shelly Baldwin, a spokeswoman for the commission. To get the THC information, the traffic commission went back through six years of toxicology reports to create a new category in its database.

Just as the state has seen a reduction in drunken-driving fatalities over the years through a steady public information campaign, it hopes to see the same trend with drugs and marijuana by sharing the data, Baldwin said.

“I think it’s really important to communicate when we are seeing these trends so people can make better informed decisions about what they are going to do,” she said.

Impaired driving — from drinking, drugs or both — typically is involved in about half of the state’s traffic deaths in a given year. It was a factor in 228 of the state’s 462 traffic fatalities in 2014.

Eric Stevick: 425-339-3446; stevick@heraldnet.com.