The city's top marijuana defense lawyer says that if voters pass an initiative next year to legalize marijuana, pot arrests won't necessarily go down—in fact, they could go up. "I thought I was about to be put out of business," says Jeffrey Steinborn, who has defended pot cases in Washington State for decades. But after looking at the measure more closely, he says, "It means I'm still in business."

If passed by voters, the initiative filed on June 22 would allow adults to possess an ounce of marijuana and require the state to license marijuana stores and distributors. However, it would also create a cutoff level in the bloodstream for THC, the psychoactive compound in marijuana, for drivers. Backers of the initiative—who have the support of national funders—say they wrote the provision to prevent people from driving while high on marijuana. The THC limit would be five nanograms of THC per milliliter of blood, which they argue is analogous to the .08 cutoff for drivers who have consumed alcohol.

"The problem is that marijuana is not alcohol; it is processed very differently," says Paul Armentano, a certified expert witness on marijuana DUIs and deputy director of the National Organization for the Reform of Marijuana Laws. He points to research that finds marijuana can stay in the bloodstream longer than alcohol without necessarily impairing all users, and the levels can spike long after someone feels high. The strict cutoff could eliminate defenses in court, Armentano warns, though he acknowledges that the cutoff point may serve as a metric of impairment for most people.

Douglas Hiatt, another marijuana defense attorney in Seattle, says that for authorized medial marijuana patients, the initiative would "essentially take away their right to drive."

Meanwhile, Alison Holcomb, campaign director for New Approach Washington, which is sponsoring the measure, concedes the science on the subject is imperfect. But a cutoff for THC was a necessary part of the initiative, she says, "to identify a place where the vast majority of drivers can safely operate a motor vehicle on the road." She points out that officers would still need probable cause to stop a driver and administer a test.

Steinborn insists, "This could increase marijuana arrests as the police go apeshit. They will go after DUIs, and it will be like shooting fish in a barrel."