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More Washington Drivers Use Pot And Drive; Effect On Safety Disputed

When Washington state legalized recreational marijuana, people wondered if it would mean more stoned drivers on the roads. Two and a half years later, one trend is clear: Police are arresting more drivers with pot in their systems — but what's not clear yet is what that means for traffic safety.

Since legalization, the Washington state toxicology lab — the group that tests blood samples from DUI cases — says a lot more of samples are positive for marijuana. Three years ago, about 19 percent of the samples contained THC, the key ingredient in pot. This year, that percentage is up to 33 percent. (See that data here and here.)

That worries Kevin Sabet, president of Smart Approaches to Marijuana, a group critical of the new legal pot industry.

"I think really what it says is that people are getting the message from legalization that marijuana is safe," Sabet says. "They're making money off of heavy, habitual, regular users, and my worry is that a lot of those users think it's perfectly OK to get behind the wheel after they smoke."

Still, that 33 percent figure comes with a big caveat. It's not a percentage of all drivers on the road. It's just a measure of the drivers who've had their blood taken under suspicion of DUI. It could be that Washington state patrol and the police are just getting better at recognizing stoned drivers and pulling them over.

But a grimmer of statistic has added to the concern.

"What we see is that in 2014 we had a fairly good spike in marijuana involvement in crashes," says Shelly Baldwin of the Washington Traffic Safety Commission. They've done an interesting new analysis of blood tests of drivers in fatal accidents.

Last year, they saw an increase in the percentage of blood samples showing THC — but they weren't distinguishing between active THC and what's known as "carboxy THC." That's the metabolized THC that hangs around your system long after you've sobered up.

"Carboxy certainly sticks around in the system for a long time. So it didn't tell us what we needed to know about behaviors out there," Baldwin says.

So what they did is go back to those blood samples and find the ones with active THC — indicating more recent marijuana use. And they found that those cases are becoming more common. Before legalization, about half of the fatality blood samples containing marijuana had active THC. Now that's up to 85 percent, and Baldwin says that does point to changing behaviors.

"It's telling me that people are using and driving," Baldwin says.

Paul Armentano of the pro-marijuana group NORML cautions, "We must not conflate the detection of these compounds as evidence of impairment."

Armentano says you actually can't really tell how impaired someone is just by looking at THC levels in the blood, and other experts agree with him on this. He also points to new research showing that even active THC can linger in the body long after the high — especially in people who use a lot of pot.

"Is this greater prevalence associated with any tangible decrease in overall traffic safety? That's the ultimate question. And there's really nothing in this data that I've seen so far to indicate that it is," Armentano says.

And in fact, the total number of traffic fatalities has remained about the same since legalization. Still, traffic safety officials in the state call the growing percentages of active THC in blood samples "alarming," and it's a trend they say they're keeping an eye on.

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Martin Kaste is a correspondent on NPR's National Desk. He covers law enforcement and privacy. He has been focused on police and use of force since before the 2014 protests in Ferguson, and that coverage led to the creation of NPR's Criminal Justice Collaborative.