The driver of a vehicle has been detained when he or she submits to a police stop. (U.S. v. Cortez) Such detention requires reasonable suspicion of criminal activity, or a traffic violation or equipment defect. (U.S. v. Sharpe) The question of whether a passenger has been detained sometimes depends on the kind of vehicle the passenger is riding in.
In two cases arising from Florida drug interdiction inspections, the U.S. Supreme Court said that when officers boarded buses during scheduled stops and asked passengers for consent to search, the passengers had not necessarily been detained, because the officers had done nothing that would have communicated to a reasonable innocent person that he or she was not at liberty to ignore the police and terminate the encounter. (Florida v. Bostick; U.S. v. Drayton) The court has said that this same rule applies to passengers of trains, planes, and taxicabs. (Bostick; Brendlin)
"Two officers walked up to Bostick on the bus, asked him a few questions, and asked if they could search his bags [where they found cocaine]. No seizure occurs when police ask questions of an individual, ask to examine the individual's identification, and request consent to search his or her luggage—as long as the officers do not convey a message that compliance with their requests is required." (Florida v. Bostick)
By contrast, the court recently ruled that everyone riding in a private passenger vehicle is necessarily detained whenever that vehicle is stopped by police. Bruce Edward Brendlin was a front-seat passenger in a car that was stopped by police for an investigation that prosecutors later conceded was not supported by reasonable suspicion. As a result of the stop, Brendlin was found to be in possession of drugs and manufacturing paraphernalia. His motion to suppress this evidence as the "fruit" of an unlawful stop was denied in state court, on the ground that passengers have no "standing" to contest the stop of a vehicle, since they have not personally been detained.
The U.S. Supreme Court unanimously reversed this holding and ruled that neither drivers nor passengers would feel free to leave and so are all detained, and all have a right to challenge the legality of the stop in a suppression motion. (This means they would also be able to sue for violation of their Fourth Amendment rights based on an unjustifiable stop.)