If no crossing sign is visible, when should the whistle signal be sounded?

The whistle signal should be sounded 15 to 20 seconds before entering a crossing when no crossing sign is visible because this timing ensures that the warning is given in advance, allowing anyone in the vicinity, such as pedestrians or vehicles, to be alerted to the incoming train. This advance notice is crucial for safety, as it provides adequate time for individuals to react and clear the area, thus reducing the risk of accidents.

In many railway regulations, the specified timeframe allows the sound of the whistle to travel and be heard, providing an effective means of communication about the approaching train. Since crossings without signage can be particularly hazardous due to the lack of visual warning, the requirement to signal well in advance enhances public safety by compensating for the absence of markings that usually indicate a crossing.