The Fourth Amendment to the U.S. Constitution prohibits most warrantless searches by police, requiring them first to obtain a warrant from a judge or magistrate. Various exceptions to this rule apply during traffic stops, when police can act on anything they see, hear, or smell that gives them a reasonable suspicion of illegal activity. This could result in a traffic stop for suspected driving while intoxicated (DWI) leading to more serious charges, or a stop for a lesser traffic violation leading to a suspicion of DWI. A defendant in a New Jersey DWI case, State v. Mercado, challenged the search of his vehicle, which police claimed was justified under the “protective sweep” exception to the Fourth Amendment’s warrant requirement.
The protective sweep exception is largely based on another Fourth Amendment exception known as the plain-view doctrine, which holds that police do not violate a person’s Fourth Amendment rights if they investigate something that they can easily see from a reasonable vantage point. If an officer stops a car because of something other than DWI, for example, the officer may be able to investigate possible DWI if an open alcohol container is visible inside the car. This also applies to something an officer can smell, such as the odor of alcohol or marijuana.
A 1983 U.S. Supreme Court decision, Michigan v. Long, addressed the plain-view doctrine in a traffic stop for suspected DWI. The officers searched the defendant’s vehicle because they “had reason to believe that the vehicle contained weapons potentially dangerous to the officers.” Instead, they found marijuana. The Supreme Court identified the protective sweep exception more specifically in 1990 in Maryland v. Buie. It held that an officer may search the immediate area when they have a specific reason to believe that another person is present who could pose a threat to themselves or others.