'You Can Never Drive': Washington's new standard for drugged driving puts patients in peril.

• Author: Sullum, Jacob

A few weeks after Washington voters approved the marijuana legalization initiative known as I-502, Ronnie Payton was driving south on Highway 167 through a chilly drizzle, heading to his daughter's place in Renton for Christmas dinner. As he passed the Interstate 405 interchange, he collided with an SUV that had merged onto the highway in front of him. Both drivers took the nearest exit and stopped in a parking lot. Although it was a minor accident, the SUV's driver called the police because Payton's insurance card did not list the car he was driving, which belonged to his boss.

That call resulted in Payton's arrest for driving under the influence of a drug (DUID). He was ultimately acquitted, but only because his lawyer managed to dodge a provision of I-502 that in practice makes it illegal for medical marijuana users like Payton to drive.

The state trooper who responded to the accident report, Cliff Roberts, later reported that Payton, a 65-year-old Vietnam veteran who worked for a Seattle organization that arranges housing for homeless people, was shivering and that he had droopy eyelids, bloodshot eyes, and an unsteady gait. Payton mentioned that he used marijuana to treat his glaucoma, as permitted by state law, but said he had not smoked any since 7 a.m., about 12 hours before the accident. Roberts asked Payton to undergo a series of roadside sobriety tests, which included standing on one foot and walking heel to toe. His performance was "extremely poor."

Since a breath test found no alcohol in Payton's system, Roberts arrested him for DUID and read him a statement about Washington's "implied consent" law. Under that law, anyone arrested for DUID had to cooperate with a blood test or lose his driver's license. Payton cooperated, and the test put his THC level at 8.6 nanograms per milliliter, well above the five-nanogram limit established by I-502.

If that test result had been admitted as evidence at Payton's trial, he would have been convicted automatically. But the prosecution agreed not to use the test after Payton's lawyer, Jesse Corkern, argued that it should be suppressed for several reasons, including the fact that the state toxicologist had not yet established standards for measuring THC in blood. Because the jury never heard about the blood test, Corkern was able to win an acquittal, arguing that what Roberts interpreted as signs of intoxication could be explained by Payton's glaucoma, his war injuries (which left him with "a...