A police officer pulls you over at a checkpoint and asks, "Have you been drinking?" Assuming he wants to know whether you have consumed alcohol in the last few hours, such that it might be affecting your ability to drive, you say no. "Not at all?" he asks. Well, you admit, you did have a beer the night before, whereupon he arrests you for driving under the influence.

If that scenario makes sense to you, you should have no problem with Michigan's new policy regarding driving and drug use. As recently interpreted by the state Supreme Court, Michigan law prohibits marijuana smokers from driving long after the drug's psychoactive effects have disappeared. A dozen states have similar policies, and federal drug officials think all of them should, which would in effect revoke or periodically suspend the driver's licenses of more than 25 million Americans.

Michigan law bars someone from driving "if the person has in his or her body any amount of a controlled substance listed in schedule 1," which includes marijuana, THC (marijuana's main active ingredient), and their "derivatives." So even before last week's decision by the Michigan Supreme Court, unimpaired drivers could be arrested with tiny, inconsequential traces of THC in their blood. In contrast with this "zero tolerance" rule, the legal cutoff for drinkers is a blood alcohol concentration of 0.08 percent.

The Michigan Supreme Court made the double standard worse by declaring that 11-carboxy-THC, a nonpsychoactive marijuana metabolite that can remain in a person's blood or urine for days or weeks, counts as a forbidden THC "derivative." The upshot is that many regular marijuana smokers can never legally drive in Michigan, whether they're intoxicated or not, while occasional smokers are barred from driving for days after each dose.

"It is irrelevant that an 'ordinary' marijuana smoker allegedly does not know that 11-carboxy-THC could last in his or her body for weeks," the court said. "It is also irrelevant that a person might not be able to drive long after any possible impairment from ingesting marijuana has worn off."

The four judges in the majority bent over backward to reach this bizarre conclusion. They cited several definitions of "derivative" that could be read to include 11-carboxy-THC, most of which also would render ubiquitous chemicals such as carbon dioxide "controlled substances," meaning that no one would be allowed to drive. They chose the one definition of "derivative" that avoided this absurd result while still allowing 11-carboxy-THC to be counted as a disqualifying blood contaminant.

The three dissenters noted that such a conclusion is contrary to the law's intent (to protect the public from impaired drivers) and inconsistent with state and federal criteria for Schedule I substances (which are supposed to be psychoactive chemicals or precursors to them). They also argued that the ruling results in an unconstitutionally vague law that invites arbitrary and discriminatory enforcement.

Given variations in metabolism and laboratory standards, marijuana smokers can never be sure whether they're legally permitted to drive in Michigan. The statute as interpreted by the Michigan Supreme Court therefore does not give people enough information to know when they are violating it -- a basic requirement of due process and the rule of law.

Treating unimpaired drivers as if they were intoxicated is fundamentally unfair, and treating a drug metabolite with no pharmacological action like the drug itself makes no sense if the goal is preventing accidents. But the drug warriors who see Michigan as a model for the nation have other goals in mind.

Proponents of "zero tolerance" laws such as drug testing consultant J. Michael Walsh and former federal drug czar Robert DuPont see them as a way of deterring drug use and forcing users into "treatment." If the point is to make the penalties for smoking marijuana more severe, let's have a debate about that, instead of pretending the issue is traffic safety.