WASHINGTON -- The U.S. Supreme Court yesterday agreed to decide whether police who stop a motorist for speeding must have "reasonable, articulable" suspicion that there are illegal drugs in the car before allowing a drug-detecting dog to sniff the vehicle.

The justices agreed to a request by the state of Illinois that it review a ruling by the Illinois Supreme Court that set aside the marijuana trafficking conviction of Roy I. Caballes, who was pulled over by a state trooper for traveling 71 miles per hour in a 65-mph zone. A second trooper arrived with a drug-sniffing K-9 dog, which indicated that there were drugs in Caballes' trunk. The troopers opened the trunk and found the marijuana.

Ruling in Caballes' favor, the Illinois Supreme Court said troopers violated the Fourth Amendment ban on "unreasonable searches and seizures" by prolonging what should have been a routine traffic stop.

The court added that the troopers lacked "sufficient justification for implementing a canine sniff. The police did not detect the odor of marijuana in the car or note any other evidence suggesting the presence of illegal drugs."

A background check performed while Caballes was pulled over did turn up information that he had twice been arrested on marijuana charges.

Ralph E. Meczyk, Caballes' lawyer, said the case had national significance because police everywhere have been using traffic stops to make more generalized searches. Although some state supreme courts have recognized greater privacy rights for motorists under state constitutions, Meczyk said, a U.S. Supreme Court decision in Caballes' favor would curb police discretion nationwide.

(In Pennsylvania, according to Trooper Linette Quinn, a state police spokeswoman, troopers in a traffic stop who have reason to suspect "drug activity" can call for a drug-sniffing dog and, if the dog picks up a scent, can seek a search warrant to open a car trunk.)

The outcome of the Illinois case in the U.S. Supreme Court could depend on whether the court views use of a drug-sniffing dog as a "search," which requires probable cause of wrongdoing, or a non-intrusive examination comparable with police observing something in "plain sight."

A dissenting Illinois Supreme Court justice in the Caballes case noted that in a 2000 U.S. Supreme Court ruling dealing with police roadblocks, the majority justices wrote, "[T]he fact that officers walk a narcotics-detention dog around the exterior of each car ... does not transform the seizure into a search."