For marijuana users -- and there are tens of thousands in Washington, lawful and otherwise -- the question about Initiative 502 is its threshold standard of intoxication for driving on public roads. This is an issue, but it should not be the deciding one. Under I-502, a driver who has 5 nanograms of active THC per milliliter of blood is legally under the influence of drugs. That's like the standard for alcohol. A driver whose blood is at least 0.08 percent alcohol is legally drunk. A driver can be arrested and convicted of driving under the influence with less than 0.08 percent if his driving is bad enough, but with 0.08 percent, there is no argument about it. Patrol officers do not have a blood test for marijuana. If a driver's alcohol test exonerates drink, the officer can call in a drug-recognition expert who can decide to do a blood draw. If the draw picks up an intoxicant, that's evidence against the driver, but Robert Calkins, spokesman for the Washington State Patrol, says convictions are based mostly on testimony about the driving. Under Initiative 502 a blood level of 5 nanograms of THC would mean guilty. Clearly such guilty. Clearly, such a standard would make the work of the police and courts much easier. If the standard is scientifically valid, it should be in the law. About that, the pro- and anti-502 sides offer studies, none conclusive. Most states have no standard or a zero standard, because most states consider