NARCOTICS CHECKPOINT 1 MILE AHEAD. NARCOTICS K-9 IN USE. BE PREPARED TO STOP.

Seeing the sign, you slow down. At the roadblock, which is manned by about 30 police officers, one of them approaches you and asks for your license and registration.

The officer stares at you hard, trying to judge whether you're high or drunk. He peers into your car, looking for contraband or other evidence of a crime. Meanwhile, another officer walks a drug-sniffing dog around your car.

After a few minutes, assuming the police do not detect anything that arouses suspicion, you're free to go. Have a nice day.

Thanks to the U.S. Supreme Court, this disconcerting ritual won't be coming to a road near you. But something pretty similar could be.

In a recent 6-to-3 ruling, the Court found that such checkpoints, which Indianapolis used for four months in 1998, violate the Fourth Amendment's prohibition of "unreasonable searches and seizures." Although it should be welcome news to anyone who values his privacy and freedom, the decision is hard to reconcile with earlier cases in which the Court let the demands of law enforcement override Fourth Amendment concerns.

In 1976, for example, the Court approved random traffic stops intended to intercept illegal immigrants at roadblocks up to 90 miles from the border. In 1990 it upheld sobriety checkpoints, at which drivers are stopped to make sure they're not intoxicated without any evidence to suggest they are.

Judging from their track records, these kinds of seizures are, if anything, less reasonable than the roadblocks in Indianapolis, which led to drug-related arrests in 5 percent of stops and arrests on other charges in 4 percent. By comparison, the immigration and sobriety checkpoints considered by the Supreme Court had success rates of 0.12 percent and 1.6 percent, respectively.

In all of these cases, of course, the overwhelming majority of the motorists had done nothing illegal. They were stopped, held by the police, and scrutinized simply because they happened to be traveling on the wrong road at the wrong time.

Despite the practical similarities, the Supreme Court saw a distinction between the roadblocks it had approved and the checkpoints in Indianapolis. Writing for the majority, Justice Sandra Day O'Connor said the immigration and sobriety checkpoints had special justifications: border control in one case, public safety in the other. By contrast, the "primary purpose" of the drug checkpoints was "to detect evidence of ordinary criminal wrongdoing."

If that goal were enough to justify indiscriminate traffic stops, O'Connor observed, "there would be little check on the ability of the authorities to construct roadblocks for almost any conceivable law enforcement purpose....We cannot sanction stops justified only by the generalized and ever-present possibility that interrogation and inspection may reveal that any given motorist has committed some crime."

While O'Connor was understandably anxious "to prevent such intrusions from becoming a routine part of American life," the line she drew is rather fuzzy. Like the drug checkpoints, the other roadblocks were aimed at enforcing criminal laws--against unauthorized immigration and driving while intoxicated.

In any case, drug checkpoints could easily be rejiggered to meet the Court's objection. If police offered rationales the Court has approved, declaring that the primary purpose of a checkpoint was to get drunk and unlicensed drivers off the roads, they could then look for drugs "incidentally."

Given some of the Court's other rulings, the upshot might be the same as allowing drug checkpoints officially identified as such. The Court has said that police are free to peek into a car during an otherwise justified traffic stop. It has also found that the use of specially trained dogs to detect hidden drugs does not constitute a "search" under the Fourth Amendment.

The Court's decision in the Indianapolis case thus invites continued litigation over the "primary purpose" of checkpoint programs. A simpler approach would have been to acknowledge that the earlier roadblock decisions gave police too much power.

Justice Clarence Thomas voted to uphold the drug checkpoints because he thought they were consistent with those precedents, which the Court had not been asked to overrule. But he suggested that the roadblock cases had been wrongly decided.

"I rather doubt," Thomas wrote, "that the Framers of the Fourth Amendment would have considered 'reasonable' a program of indiscriminate stops of individuals not suspected of wrongdoing." Once police are given such authority, they're the ones who need to be stopped.