If police lawfully stop a car, are they allowed to search passengers and their belongings? As with many legal questions, the answer depends.
The Fourth Amendment protects against unreasonable searches and seizure. But the U.S. Supreme Court has said that in certain circumstances it's reasonable—and therefore legal—for police to search passengers and their things.
As a general rule, police are supposed to have a warrant before they conduct any search. But there are lots of exceptions to this warrant rule.
A search of a vehicle is one such exception—often called the "automobile exception." To inspect the interior of a car, the police usually need probable cause to believe they'll find incriminating evidence inside—not a warrant.
("Inventory searches" are another exception to the warrant rule that could apply to passenger searches.)
Once the police have probable cause, everything inside the car that could conceal the item they're looking for is fair game. For example, imagine police have probable cause to look for drugs and find a backpack in a car. It doesn't matter whom the backpack belongs to (passenger or driver)—the officers may look inside the bag because it could contain drugs.
Example: Officer Colombo pulls a car over for making an illegal left turn. Inside the car are four teenagers. Colombo notices a hypodermic syringe—that, upon further inspection, he observes contains traces of drugs—in the driver's shirt pocket. Colombo orders everyone out of the car, frisks them, and begins to search the car for drugs. The officer picks up a purse from the back seat; one of the passengers identifies it as hers. Officer Colombo opens the purse, finds drugs inside, and places the purse's owner under arrest. The arrest and drug seizure were valid. Because Officer Colombo had the right to search the car for drugs, he also had the right to search items belonging to passengers that could reasonably contain drugs. (Wyoming v. Houghton, 526 U.S. 295 (1999).)
Example: Officer Colombo pulls a car over for making an illegal left turn. Inside the car are four teenagers. The officer notices an illegal automatic weapon sticking out under the front passenger seat. Colombo orders all the occupants out of the car, frisks them, and begins to search the car for other weapons. He picks up a wallet from the back seat; one of the passengers identifies it as his. Officer Colombo carefully searches the wallet and finds drugs inside. He places the wallet's owner under arrest. This seizure and arrest for drug possession are probably not valid. Because Officer Colombo had the right to search the car for weapons-related evidence, he also had the right to search property belonging to passengers that could reasonably contain weapons. Because no weapon could be concealed in the wallet, the search of the wallet was arguably illegal and the arrest based on it invalid.
Having probable cause to search a vehicle doesn't automatically give police the right to search a passenger's person or clothing. But some circumstances do allow police to search passengers. For instance, an officer who legally arrests a passenger can then search the person "incident to arrest." Police are also allowed to pat down (or frisk) a passenger they reasonably suspect is armed and dangerous. (U.S. v. Di Re, 332 U.S. 581 (1948); U.S. v. Robinson, 414 U.S. 218 (1973); Arizona v. Johnson, 555 U.S. 323 (2009).)
Search-and-seizure law is complicated and frequently changes. It's also possible that your state's law provides greater protection than the federal law discussed in this article. So, if you've been arrested, get in touch with an experienced criminal defense attorney.