'Tis the season for shopping and celebrating ... and a spike in drunk driving.

It happens the weekend after Thanksgiving all the way through the New Year's, said Washington State Patrol Sgt. Mark Crandall.

With the legalization of pot, police expect to see more drivers impaired -- not by alcohol, but marijuana.

You go from zero legality to full legality, people are going to make some bad judgments. Regrettably they're gonna put themselves behind the wheel and regrettably we've got to arrest them, Crandall said.

Crandall is one of 217 Drug Recognition Experts (DREs) in Washington state. These experts get called to the scene when a driver seems impaired but manages to blow below the legal limit for alcohol on the roadside breathalyzer test. The DRE officer puts a suspect through a round of additional tests looking for evidence of drug impairment.

And through that evaluation we do such things as blood pressure and pulse because certain drugs will elevate and certain drugs will decrease blood pressures and pulses. We look at their eyes, we take them into a dark room and look at their eyes, because eyes will do certain things with certain drugs, Crandall said.

The evaluation takes 40 minutes to an hour. Afterwards, the DRE officer typically obtains a blood sample from the suspect either voluntarily or by court order. In some states police do the blood draws themselves, but in Washington draws are done at a hospital or by a medic at an accident scene.

The blood samples are then analyzed at the State Toxicology Lab in Seattle. The state's new marijuana law says a person is legally impaired if he or she has 5 nanograms of THC per milliliter of blood. THC is the active ingredient in pot. But even toxicologists can t say how much marijuana it takes to reach that point.

With alcohol it s basically math: How much a person drank, how quickly the person drank it, how much the person ate, and how much the person weighs. With pot there s a long list of what ifs.

You don't know the dose of marijuana that they used. You don't know how much they used. You don't know the potency of the marijuana that was used, said Brian Capron, Acting Lab Manager at the Washington State Toxicology Laboratory.

That s what worries Amy Freedheim. She prosecutes the worst cases in King County DUIs that kill or maim. Freedheim is bracing for an increase in marijuana DUIs.

I think the concern is people who never would have broken the law, would never have gone that extra step to obtain marijuana illegally, who may now say, 'Let me try it!' said Freedheim.

And don t think Freedheim can t come after you if your blood THC level is under the 5 nanograms set by the new law.

These are people who've been killed by impaired drivers ... or in vehicular homicide crashes, said Freedheim, pointing to a wall of photos in her office -- faces of people who died in DUI accidents and vehicular homicide crashes.

Among the faces on Freedheim s wall of victims is Lindsey Austin from Seattle just 19 years old when she died. She was as killed on October 16, 2006; she was a passenger in a man's car who had been smoking marijuana, said Freedheim.

The driver, Edward James Hills, had a THC level of just 1.6 Nanograms/Ml. That s less than a third of what s allowed under Washington's new law.

But that just shows you there is no safe amount to drive on, said Freedheim.

A Drug Recognition Expert examined Hills and deemed him impaired from the marijuana, and Freedheim got a conviction for DUI homicide. That was six years ago; Hills is still in prison, serving a 16-year sentence.

According to the judge who handed down the sentence, Hills was an infrequent user of marijuana and would have felt the effects of the drug more pronounced than a more frequent user. Marijuana impairs driving by distorting time and spaced and delayed reaction time and decreased vigilance.

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