One of the exceptions to the Fourth Amendment's warrant requirement is the so-called ''automobile'' exception. If a vehicle is stopped by police, a warrantless search of the vehicle is usually allowable under this exception if the police have probable cause to believe that it contains evidence of a crime. This is based on the principle of exigency, or the need for immediate action arising out of the likely disappearance of the vehicle and the possibility of destruction of the evidence.
Generally, a speeding or traffic violation by itself does not justify the search of a vehicle, but this rule is not without exceptions. If the officer has a reasonable belief that they might be assaulted by the driver, or if the driver fails to produce a valid license or produces a forged one, a search will normally be valid.
In most instances, a valid search of a vehicle can include any containers within the car, such as packages or luggage. While there must be a connection between a crime and the search, there is no rule that the search be limited only to items relating to that crime; i.e., the police can search the driver and the car if they have a reasonable belief, or probable cause to believe, that there is a gun in the driver's possession or in the car.
When there is probable cause to search a vehicle, it is not necessary that the search take place immediately. A valid search can occur long after the initial stop and even after the car has been moved to the police station or impound lot.
When a car is impounded, the police can search it for the purposes of taking an inventory of its contents. It is not necessary that the search be conducted for the purpose of seeking evidence of a crime. While the police can search the entire car and catalog its contents, they are limited in what they can do with the contents; for an inventory search to be valid, it must be made pursuant to standardized police procedures.
If a driver gives a police officer permission to search their car, the warrant requirement is, of course, not necessary. Consent can be either ''express'' or ''implied.''
Implied consent usually becomes a factor when a driver is suspected of driving under the influence of alcohol (''DUI'' or ''DWI''); many states now have statutes that specify that when an officer has a reasonable suspicion that a driver is impaired, the driver's use of the public streets is considered implied consent to search of the driver's person and vehicle.
Express consent arises when a police officer asks for and receives permission to search the driver's car. When such consent is given, the officer usually is permitted to open a closed container in the car if it might reasonably hold the object of the search.