What must be true for police to lawfully stop a car?

For police to lawfully stop a car, it is essential that they have probable cause to believe that a traffic law has been violated. This standard allows law enforcement to make an immediate assessment based on the observable behavior of the vehicle and its driver. Probable cause can originate from witnessing a traffic infraction, such as speeding or running a stop sign, which provides a reasonable basis for initiating a traffic stop.

The requirement for probable cause distinguishes a lawful stop from one that may violate constitutional protections against unreasonable searches and seizures. A traffic infraction, even a minor one, serves as sufficient justification for police intervention while the vehicle is in motion. This principle is rooted in the Fourth Amendment of the U.S. Constitution, which governs searches and seizures and ensures citizens' rights against arbitrary law enforcement actions.

In contrast, a warrant is generally not necessary for police to stop a vehicle for a traffic violation, as the presence of probable cause at the moment of the stop is adequate. Consent from the driver is also not a prerequisite; officers can stop a vehicle based solely on their observation of an infraction. Lastly, while an eyewitness complaint can be valuable, it is not a requirement for conducting a lawful stop, as police can independently verify violations themselves.