What does a sign marked 'W' indicate?

A sign marked 'W' typically serves as a signal to indicate a specific action that drivers should take, which, in this case, is to sound the horn as per a specific rule. This type of sign is often related to situations where the driver must alert others, often in environments such as rail crossings or areas where visibility is limited, ensuring safety by making their presence known.

While the other possible answers may pertain to traffic regulations in different contexts, they do not align with the specific function of the 'W' sign. For instance, a speed limit sign would typically be marked with a 'S' or a numeric value, and a stop sign would be distinctly marked with an eight-sided shape and clear wording. Thus, understanding the function of the 'W' sign is crucial for safety and compliance with traffic rules.