US Supreme Court: Automobile Searches
The Supreme Court recognizes three distinct doctrines permitting the police to search automobiles without warrants. First, since Carroll v. United States (1925), a warrant has not been required so long as there is probable cause to believe that the vehicle contains contraband or evidence of a crime. This “automobile exception” has been greatly expanded since Carroll. For example, the Court held in California v. Carney (1985) that a mobile home capable of traveling on a highway was included within the exception, and in Wyoming v. Houghton (1999), the Court held that the police may use the exception to search the personal belongings of passengers (but not the passengers themselves).
Second, the Court held in New York v. Belton (1981) that the police may automatically search an automobile's passenger compartment without a warrant after arresting an occupant of the vehicle because a criminal may hide contraband or weapons in the vehicle before the arrest. Since the Court held in Atwater v. City of Lago Vista (2001) that the police may arrest motorists for even petty traffic violations, the police now have an incentive to arrest minor traffic violators in order to perform “Belton searches” of their automobiles.
“Inventory searches” are the third doctrine permitting the police to perform warrantless automobile searches. In Colorado v. Bertine (1987), the Court held that the police may thoroughly search vehicles that have been lawfully impounded for any reason. The Court explained that such inventory searches are justified to protect the owner from misappropriation, to protect the police from false claims of theft, and to prevent dangerous items from being stored on police property.
Taken together, the expansion of the automobile exception, Belton searches, and inventory searches have largely eliminated the expectation of privacy that American motorists may once have had in their automobiles.
http://www.answers.com/topic/automobile-searches