How far must you stop from a railroad crossing when lights are flashing?

Answer: B — At least 15 feet

When approaching a railroad crossing where the lights are flashing, it's crucial to stop at least 15 feet away. This distance allows for adequate safety as it gives you a clear line of sight to the tracks and sufficient distance to avoid any potential hazards from an approaching train. The flashing lights are a warning that a train is coming, and stopping at this specified distance helps to ensure that you are not in danger from the train or obstructing its path. Additionally, this regulation is designed to protect both drivers and pedestrians, ensuring that everyone has time to react to the warning signals.