Which of the following describes a "clear" signal?

A "clear" signal is specifically described as a state of a controlled signal when the proceed/lunar aspect is displayed. This indicates that the track is clear and the train is authorized to proceed without restrictions. The clear signal is crucial for ensuring the safe operation of trains, as it allows crews to be informed about the status of the track ahead, enabling them to maintain proper speed and prevent accidents.

In contrast, the other options describe different types of signals. For example, a signal indicating a train is approaching does not indicate that the track is clear, but rather that caution is needed. A signal that indicates a track is closed implies that no train movement is allowed in that area, and a cautionary signal denotes that the movement must be slow due to potential hazards. Therefore, these alternatives do not define what is meant by a "clear" signal in the context of roadway worker protection.