In the ordinary case, a search of private property must be both reasonable and conducted pursuant to a properly issued search warrant. However, law enforcement officers are empowered to search an automobile without a warrant, so long as it can be demonstrated that exigent circumstances rendered the obtaining of a warrant an impossible or impractical alternative and that probable cause existed for the search. The doctrine was initially premised on the notion that there was a constitutional difference between houses and cars, which are inherently mobile. However, mobility is no longer the prime justification for the automobile exception; rather, it is the diminished expectation of privacy which surrounds the automobile.
Although an automobile passenger possesses a reduced expectation of privacy, the passenger does not lose his privacy interest just because he happens to be in the vehicle. Courts utilize a totality of the circumstances principle to determine if a passenger should be frisked and his belonging searched. In determining whether a passenger will be subjected to a frisk, companionship to a driver suspected of criminal activity is only one factor necessary to make the determination. The circumstances must also suggest that the passenger is armed and dangerous and poses a danger to the officer. Notwithstanding the justifications necessary for a frisk, if an officer has probable cause to stop the driver of the vehicle, the officer may, as a practical matter, order the passenger out of the vehicle for the duration of the stop.
Similarly, the United States Supreme Court has ruled that before a passenger in the vehicle can be arrested, there must be "probable cause" based upon the "totality of the circumstances" that the passenger was engaged in a common criminal enterprise with the arrested driver.
With respect to searches, when a police officer makes a lawful custodial arrest of the occupant of an automobile, the officer may, as a contemporaneous incident of that arrest, search the automobile's passenger compartment, even though the occupants have exited the automobile, and the vehicle has been secured. While the search of a vehicle does not automatically extend to an item that is clearly associated with the nonarrested passenger, courts have allowed police, on the basis of the automobile exception, to search a passenger's personal belongings inside an automobile that the police had probable cause to believe contained contraband or dangerous weapons.
In view of the unique treatment of an automobiles under the Fourth Amendment, passengers in an automobile lack standing to challenge the constitutionality of automobile searches and seizures.