Any person who has been pulled over by an officer for a traffic violation may have their car sniffed by a drug dog during the traffic stop. The U.S. Supreme Court, in Illinois v. Caballes, stated that a drug dog could perform a sniff of a stopped car regardless of the reason for the stop. According to the Supreme Court, a sniff is not a search and therefore, the odor of a car is not protected by the Fourth Amendment.
This does not mean that an officer can keep you stopped longer than necessary though to allow a K9 unit to arrive. In fact, the Supreme Court held, in Rodriguez v. U.S., that a traffic stop could not be prolonged longer than necessary to wait for a K9 unit to arrive and perform a dog sniff. Though this now prevents officers from extending the time of the search to allow a K9 unit to arrive, the standard traffic stop time is not fixed. This means that an officer must only show that the length of the traffic stop was ‘reasonable'. Depending on the circumstances, a traffic stop lasting nearly a half hour could still be considered reasonable. Additionally, K9 unites may arrive on scene early during the course of a traffic stop and not extend the stop at all.