Arizona v. Johnson, 555 U.S. 323 (Jan. 26, 2009)

Summarizing existing law, the Court noted that a “stop and frisk” is constitutionally permissible if: (1) the stop is lawful; and (2) the officer reasonably suspects that the person stopped is armed and dangerous. It noted that that in an on-the-street encounter, the first requirement—a lawful stop—is met when the officer reasonably suspects that the person is committing or has committed a criminal offense. The Court held that in a traffic stop setting, the first requirement—a lawful stop—is met whenever it is lawful for the police to detain an automobile and its occupants pending inquiry into a vehicular violation. The police do not need to have cause to believe that any occupant of the vehicle is involved in criminal activity. Also, an officer may ask about matters unrelated to the stop provided that those questions do not measurably extend the duration of the stop. The Court further held that to justify a frisk of the driver or a passenger during a lawful stop, the police must believe that the person is armed and dangerous.