Marijuana use appears to have increased as a factor in deadly crashes last year in Washington.

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Marijuana use appears to have increased as a factor in deadly crashes last year in Washington.

New data from the Washington Traffic Safety Commission shows the number of drivers involved in fatal crashes with THC in their body increased from 38 in 2013 to 75 this past year. About half those 75 drivers had active THC — the main psychoactive chemical in pot — above the level that legally determines intoxication.

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

Commission staff aren’t sure why.

One obvious reason is that state-regulated pot stores opened in 2014, providing access to legal weed. But the first few stores didn’t open until July, and their supply was scarce. Seattle, allotted 21 stores by state officials, saw only one shop selling pot until late September.

What’s more, there were more marijuana-involved fatal crashes in the first half of 2014, before stores opened, than in the second half of the year.

There are other confounding factors. Half the drivers with active THC in their blood also were under the influence of alcohol, and the majority of those were legally intoxicated.

The newly released data also doesn’t account for prescription drugs in the marijuana-positive drivers in fatal crashes.

“We’re not saying A equals B,” said commission spokeswoman Shelly Baldwin about the link between fatal crashes and marijuana use. “We’re saying this is a factor. We look for trends and this is a trend we’re seeing.”

A follow-up report in September should provide more detail, she said.

THC is commonly detected in two forms. It can be the active kind, delta-9-tetrahydrocannabinol, which has an intoxicating effect and dissipates quickly. Or it can be the inactive kind, carboxy-THC, which remains in your system for days, even weeks, but isn’t intoxicating at that point.

If active THC exceeds 5 nanograms per milliliter of blood, that’s enough to convict a driver of DUI notwithstanding other evidence.

Previously, the commission knew that 2014 saw an increase in the number of marijuana-involved fatal crashes. But it didn’t know if the drivers had active or inactive THC in their blood.

Researchers went back into the individual cases to determine the kind and level of THC in the fatal-crash drivers.

The new data seems to indicate that more Washingtonians feel comfortable driving after consuming pot, Baldwin said. The commission’s preliminary analysis suggests it is not safe to drive while high, she said, as some consumers believe.