Under the Fourth Amendment to the United States Constitution all unreasonable searches and seizures are prohibited. A search conducted without a valid search warrant is per se unreasonable unless the circumstances of the search fall within one of the exceptions to the search warrant requirement. These exceptions include:

(1) consent, (2) search incident to a lawful arrest, (3) stop and frisk, (4) probable cause to search with exigent circumstances, (5) the emergency doctrine, (6) an inventory search, (7) plain view, and (8) an administrative search of a closely regulated business.

This issue arises frequently following a traffic stop wherein the arresting officer suspects the vehicle may contain contraband.  The officer first requests consent to search the vehicle based on an alleged odor of marijuana.  If the driver consents, then the search is valid.  If the driver refuses, then the officer must create reasonable suspicion through a canine unit (drug dog) to alert the officer to marijuana.  The canine's alert provides this necessary element to the search.  If the officer sees, in plain view, a roach or paraphernalia within the vehicle, then he has probable cause to arrest the driver and may search the vehicle "incident to arrest."  If  this search turns up more contraband or illegal items, then the seized items are generally admissible at trial.

My experience has been that most drivers consent to the search out of fear or subtle intimidation.  The reality is that every driver may refuse the officer's request but then is subject to further detention and questioning.  Most will take the path of least resistance.  But once consent is granted, then the search is valid.  I'm not suggesting drivers should automatically decline a search request, but under many circumstances the officer will not be able to procure the canine unit without an unlawful or lengthy detention of the driver.  The courts examine the duration of the detention, among other factors, when determining the legality of the stop.  The judicial review will always consider the "totality of the circumstances" and whether any of the eight exceptions listed apply to the facts.