Police cannot use random roadblocks to search out drug law violators, the Supreme Court ruled on Monday. The ruling, on a case in which Indianapolis police put up checkpoints for precisely that purpose in inner city neighborhoods, came on a 6-3 vote, with Chief Justice William Rehnquist and conservative Justices Antonin Scalia and Clarence Thomas dissenting.

The majority opinion, written by Justice O'Connor, found that the city's use of drug-sniffing dogs to check all vehicles was an unreasonable and therefore unconstitutional search barred by the Fourth Amendment.

The ruling maintained a distinction between random stops or searches designed to catch criminals, such as the present case, and those whose primary reason is to benefit the public good or public safety, such as sobriety checkpoints and border checks.

O'Connor wrote that the constitutional protections requiring police to have reasonable suspicion before stopping and searching a car would not allow that reasoning to be applied to cases in which law enforcement ends are paramount.

"If this case were to rest on such a high level of generality, there would be little check on the authorities' ability to construct roadblocks for almost any conceivable law enforcement purpose," she wrote.

"We have never approved a checkpoint program whose primary purpose was to detect evidence of ordinary criminal wrongdoing," O'Connor wrote. "Rather, our checkpoint cases have recognized only limited exceptions to the general rule that a seizure must be accompanied by some measure of individualized suspicion."

And, O'Connor continued, if the Court allowed such random searches, "the Fourth Amendment would do little to prevent such intrusions from becoming a routine part of American life."

In his dissent, joined by Scalia and Thomas, Chief Justice Rehnquist wrote that the test for random stops was "whether they serve a significant state interest with minimal intrusion on motorists." Rehnquist, who earlier also supported the sobriety checks and border checkpoints, found that random stops and searches by drug-sniffing dogs "are executed in a regularized and neutral manner. And they only minimally intrude upon the privacy of motorists. They should therefore be constitutional."

And Clarence Thomas gave new evidence that his legal mind is a strange universe, indeed, when he signed onto Rehnquist's dissent, but also questioned the legality of any "indiscriminate stops of individuals not suspected of wrongdoing," which was not directly at issue in the case.

Although the Rehnquist court has long been considered pro-police, this ruling, along with a handful of others suggests a trend on the Supreme Court toward reining in some of the excesses of law enforcement.

Earlier this year the court ruled unanimously that police may not stop and search someone based solely on an anonymous tip that the person is carrying a weapon. Also this year, the court ruled that Border Patrol agents could not search passengers' bags as part of a routine immigration search.

Brooklyn Law School professor Susan Herman told the Washington Post the decisions suggest that "the court wants to hold the line and to recognize that there are rules."