IN the campaign over legalizing marijuana, the loudest voices for “no” came from medical users who feared they would unfairly lose their right to drive. They can relax. Initiative 502’s standard for driving under the influence has not led to the crackdown they predicted.

There has been no jump in “green DUIs,” said the Washington State Patrol’s toxicologist, Dr. Fiona Couper, at the hearing in Olympia Wednesday of the House Public Safety Committee. Seattle DUI attorney Patricia Fulton reported “absolutely no effect” in her defense practice.

The amount of the psychoactive chemical THC is not as closely related to the amount of impairment as is the case with alcohol, so there is argument about the standard. But THC does cause impairment.

Police and prosecutors say their focus is on the impairment. If a person drives erratically and fails the field sobriety test, a DUI conviction does not require 5 nanograms of THC per milliliter of blood. The finding makes a conviction easier, but the impairment comes first — as it should.

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“We aren’t going to arrest somebody unless there’s impairment,” said Lt. Rob Sharpe of the State Patrol’s Impaired Driving Section.

Blood testing is not new and not done casually. It’s done at a medical center, and takes money and time. Couper said 1,000 to 1,100 drivers were tested last year statewide, with the median result slightly below 5 nanograms. About one-third also tested positive for alcohol.

Amy Freedheim, King County senior deputy prosecuting attorney, said, “We look at the driving, the crash and the driver at the time of the arrest.” If the blood test is at least 5 nanograms, she will use it to instruct the jury. Then it matters — but impairment comes first.

If the standard becomes a problem, it can be changed. Meanwhile, the law should be enforced as usual. The DUI laws are about keeping the roads safe, and the need for that did not change with Initiative 502.