CBS News Correspondent Barry Bagnato reports the ruling means that except in special circumstances, police must have some specific suspicion of wrongdoing before they can pull over a car to try to catch a potential lawbreaker.
American Civil Liberties Union legal director Steven Shapiro says the ruling gives a clear message.
"Today's decision sends a clear message that even a conservative court is not willing to countenance the serious erosion of our basic constitutional rights in the name of the war on drugs," Shapiro said Tuesday.
The 6-3 ruling weighed privacy rights against the interests of law enforcement. The majority found that Indianapolis' use of drug-sniffing dogs to check all cars pulled over at the roadblocks was an unreasonable search under the Constitution.
The majority, in an opinion written by Justice Sandra Day O'Connor, said the ruling does not affect other kinds of police roadblocks such as border checks and drunken-driving checkpoints. Those have already been found constitutional.
But the reasoning behind those kinds of roadblocks - chiefly that the benefit to the public outweighs the inconvenience - cannot be applied broadly, O'Connor wrote.
"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," the opinion said.
Lawyers for Indianapolis conceded that the roadblocks erected there in 1998 detained far more innocent motorists than criminals.
The city said its primary aim was to catch drug criminals. Civil liberties advocates called the practice heavy-handed and risky, and asked the Supreme Court to ban it.
Law enforcement in and of itself is not a good enough reason to stop innocent motorists, the majority ruling concluded.
The court was not swayed by the argument that the severity of the drug problem in some city neighborhoods justified the searches.
"While we do not limit the purposes that may justify a checkpoint program to any rigid set of categories, we decline to approve a program whose primary purpose is ultimately indistinguishable from the general interest in crime control," the majority opinion said.
Cars were pulled over at random in high-crime neighborhoods in Indianapolis, motorists questioned, and a drug-sniffing dog led around the cars. Most motorists were detained for about three minutes.
The city conducted six roadblocks over four months in 1998 before the practice was challenged in federal court.
Police stopped 1,161 cars and trucks and made 104 arrests. Fifty-five of the arrests were on drug charges.
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