What informs police that a "protective frisk" may be necessary during a stop?

The necessity for a "protective frisk," often referred to as a stop-and-frisk, is primarily informed by the officer's reasonable belief that the individual might be armed or dangerous. This standard is established under the Fourth Amendment, which protects against unreasonable searches and seizures.

When an officer makes a Terry stop, which is a brief, investigatory detention, they must have a reasonable suspicion based on specific, articulable facts that the person being stopped is involved in criminal activity. If, during this encounter, the officer develops a reasonable belief that the individual poses a threat to their safety, particularly if there’s reason to suspect that the person is armed, they are justified in conducting a protective frisk for their own safety and the safety of others in the vicinity.

This judicially-created exception allows officers to ensure their safety while carrying out their duties without needing a warrant or probable cause to arrest. The key is the officer's perception of a threat based on the totality of the circumstances, which is a foundational aspect in evaluating whether a protective frisk is warranted in any given situation.