When approaching a railroad crossing without any signals or gates, drivers should:

Answer: C — Slow down and prepare to stop if necessary.

Approaching a railroad crossing without any signals or gates can be dangerous as it could mean that a train is about to pass through. Therefore, it is important to slow down and prepare to stop if necessary. Option A is incorrect because 15 feet may not be a safe distance to stop, especially if the train is coming at a high speed. Option B is incorrect because it is never advisable to speed up when crossing railroad tracks as it increases the risk of an accident. Option D is incorrect because not changing your speed can be dangerous if a train is about to pass through, and it is always better to be prepared to stop if needed.