In response to Caballes, the Court clarified in Rodriguez v. United States (2015) that an officer may not unreasonably prolong the duration of a traffic stop to conduct a dog sniff.

[1] The Fourth Amendment to the United States Constitution protects against unreasonable searches and seizures by the federal government and its officials, such as police officers.

In United States v. Place (1983), the Court ruled that a canine sniff of a person's property by a trained detection dog does not constitute a "search" within the meaning of the Fourth Amendment.

[4] On November 12, 1998, Illinois state trooper Daniel Gillette stopped Roy Caballes for speeding on Interstate 80 as it passed through LaSalle County.

[6] Another trooper, Craig Graham of the State Drug Interdiction Team, overheard the transmissions and informed the dispatcher he would meet up with Gillette to conduct a canine sniff on Caballes's car.

[14] After unsuccessfully moving to suppress the marijuana evidence before trial, Caballes was convicted of narcotics trafficking and sentenced to 12 years in prison and a $256,136 fine.

[16] The Supreme Court granted certiorari to the case in order to answer the question of whether the Fourth Amendment requires reasonable suspicion to justify using a drug-sniffing dog during a routine and otherwise legitimate traffic stop.

In United States v. Place (1983), the Court had held that a dog sniff is sui generis because it discloses only the presence or absence of narcotics.

Caballes argued that it was wrong to assume that the alerts of drug-sniffing dogs reveal only information regarding the presence or absence of narcotics.

Souter pointed to a study relied on by the State of Illinois in its reply brief, indicating that "dogs in artificial testing situations return false positives anywhere from 12.5% to 60% of the time, depending on the length of the search".

The fact that a dog sniff is sui generis only matters if the sole determinant of what is "reasonable" is the length of time a traffic stop lasts.

Motorists [would not] have constitutional grounds for complaint should police with dogs, stationed at long traffic lights, circle cars waiting for the red signal to turn green."