What does the hand signal for 'reduce speed' entail?

The hand signal for 'reduce speed' involves making a slight horizontal movement at arm's length. This signal is designed to be clear and easily visible to those who need to receive the instruction, such as train operators or maintenance crews. The motion is subtle yet distinct enough to convey the message without causing confusion.

This method allows for effective communication in situations where verbal commands may not be easily heard or understood due to noise or distance. Using horizontal movements is a common practice in signaling, as it can be seen from a distance, ensuring that the instruction to reduce speed can be promptly acted upon.

The other options involve different movements or gestures that do not represent the 'reduce speed' command, making them unsuitable for this specific instruction.