Vehicle Searches. In general, the Fourth Amendment does not require that police obtain a warrant to search an automobile when they have probable cause to believe it contains contraband or evidence of criminal activity. This exception to the warrant requirement--the “automobile exception”--stems from both the inherent mobility of vehicles and the reduced expectation of privacy that results from their pervasive regulation. Even if a vehicle's mobility is temporarily restricted or potential drivers have been secured, many circuits have held that warrantless searches are still valid. The automobile exception has been applied to motor homes and portable campers, as well as trains, planes, and water-going vessels.
If police have probable cause to search an entire vehicle, they may search all compartments, containers, and packages within the vehicle, including those belonging to passengers. In California v. Acevedo, the Supreme Court confronted a variant of this scenario: police had probable cause to believe that a particular container--a brown sack--held contraband, and they watched the defendant place the container in the trunk of his car. Although a warrant is generally required to search containers, the Court held that, when placed in a vehicle, a container may be searched on probable cause alone. In addition, the part of the vehicle for which there is probable cause to believe the container will be found may be searched, but a search of the entire vehicle is unlawful absent probable cause to search the entire vehicle.
Vehicle searches performed after exigent circumstances lapse are valid as long as the police legitimately could have searched the automobile at some point. In Chambers v. Maroney, the Supreme Court upheld the warrantless seizure and subsequent search of a car at a police station because there was probable cause at the time of the stop to justify an immediate search. Moreover, in Florida v. Meyers, the Court upheld the warrantless search of an impounded car that had already been subject to a legitimate inventory search.
An officer with probable cause to believe that a motorist has violated a traffic law may temporarily detain the motorist--regardless of the officer's motivations or suspicions may conduct a limited search of the motorist and vehicle for weapons upon reasonable belief that the motorist is potentially dangerous. Absent such potential danger, an officer may not search the passenger compartment of an arrested driver's vehicle unless the officer reasonably believes that it may contain evidence of the offense for which the driver is being arrested, or the arrestee has physical access to the passenger compartment during the search. Officers may conduct a drug-detection dog sniff test on a car during a lawful traffic stop, but they may not prolong an otherwise completed traffic stop.
Legal challenges to the validity of a search under this exception are typically considered by the courts only if brought by the owner or by an operator with legitimate possession of the vehicle.