The Fourth Amendment restricts when a police officer may stop a person. Generally, an officer must have probable cause to believe that the person has committed a crime in order to stop them. When police officers conduct a traffic stop, there is no question that they are allowed detain the driver. But what happens if one of the passengers of the vehicle wants to leave in the middle of the stop?
Ordinarily, when a police officer seeks to enter a person's vehicle without their permission, they need to have some reasonable suspicion that its occupants have engaged in criminal activity. One exception to this rule is the "community care-taker" exception. This doctrine dates back to a 1973 Supreme Court Case called Cady v. Dombrowski. In that case, a police officer took custody of the defendant's vehicle following a traffic accident so that he could move the vehicle out of oncoming traffic. Upon learning that there was a revolver in the trunk of the vehicle, the officer opened the trunk and removed the gun.
It is not unusual for law enforcement officers to turn a traffic stop into a full blown search and seizure of the vehicle that has been stopped. How is this accomplished? First, many times the officers, who have made what seems to be a routine traffic stop, know they will find contraband in the vehicle. The traffic stop is used as an excuse to get inside the vehicle, without the police having to reveal the source of their information that the car is carrying. After issuing a traffic citation, a "casual" request to search the vehicle is made. If consent is declined, the officers call back-up to bring a drug sniffing dog. If the dog alerts to the presence of contraband, the vehicle is searched based upon that alert and no one, including the driver ever knows the actual reason for the search.