In a 6-3 opinion, the court stressed that the Fourth Amendment forbids police from searching persons without some specific reason to believe that they did something wrong.
While police have broad authority to stop motorists for traffic violations, they do not have a general authority to stop cars "to detect evidence of ordinary criminal wrongdoing," wrote Justice Sandra Day O'Connor for the court.
Tuesday's ruling is the third this year that breathes new life into the Fourth Amendment. It comes as a mild surprise because until recently, the justices had sided regularly with law enforcement in the war on drugs.
Earlier this year, the court ruled that police may not stop and search a pedestrian based entirely on a vague and anonymous tip phoned to police headquarters. The justices said the Fourth Amendment requires more specific evidence of wrongdoing.
The justices also ruled that police may not squeeze or feel a traveler's bags in a random search for illegal drugs. In that ruling in the case of 4th Bond vs. United States, the justices threw out drug evidence against a bus passenger who was arrested after an officer felt a brick of meth-amphetamine in the passenger's satchel. The court said a traveler's hand bags are private and off limits to searches, except when an officer has a specific reason to look for drugs.
Narcotics roadblocks are rare, but the Indianapolis case tested whether they could be used nationwide. In August 1998, city police there set up six checkpoints to stop cars. Their intention was to cut the flow of illegal drugs in and out of the city.
When a motorist was stopped, an officer asked to see his or her driver's license. At the same time, a second officer with a drug-sniffing dog circled the vehicle. If the first officer or the dog detected anything suspicious, the vehicle was pulled aside and searched.
In a four-month period, police said they stopped 1,161 motorists and made 104 arrests. Fifty-five of the arrests were for drug offenses and 49 were for other reasons. When several detained motorists complained about the stops, the American Civil Liberties Union sued the city, contending that the stops were unconstitutional.