What does an illuminated "S" signal signify?

An illuminated "S" signal signifies "Stop." This signal is critical for maintaining safety and order in operational environments, particularly in situations where immediate halting of movement is required, such as at a train crossing or an intersection. The "S" signal is a clear and universally recognized indication that all operators must cease their activities, allowing time for any necessary precautions to be taken or for other signals to determine the next course of action.

In contrast to the other options, which may imply diminished urgency or alternative actions—like slowing down or proceeding—the "S" signal provides a definitive directive to halt activity entirely, prioritizing safety above all. Understanding the implications of this signal is essential for effective tower operation, as it helps prevent potential accidents and ensures the smooth flow of transportation systems.