What is the primary function of a traffic signal's red light?

The primary function of a traffic signal's red light is to indicate stop. This signaling is crucial for ensuring the safety of all road users, including drivers, pedestrians, and cyclists. When a red light activates, it commands vehicles to halt at the intersection, preventing potential collisions and allowing for the safe flow of traffic in the opposite direction or for pedestrians crossing the street. Stopping at a red light is a fundamental rule of traffic safety, enabling orderly movement and minimizing accidents.

The other options refer to different scenarios: preparing for a turn typically involves a green light or a turn signal, while alerting emergency vehicles or signaling a school zone may involve different types of signals or signage. However, the red light's unmistakable purpose is to stop vehicles, which maintains traffic order and safety.