What must be obeyed when a signal indicates an approach speed limit of 30mph?

When a signal indicates an approach speed limit of 30 mph, the requirement is to approach at a speed not exceeding the posted speed. This means that the train must be operated at speeds that are at or below 30 mph, ensuring safe approach conditions to the signal.

Obeying the posted speed limit is critical for safety, as it allows for appropriate reaction time in case the signal indicates a stop or caution, and it helps maintain control of the train as it approaches potentially critical areas like junctions or crossings.

Other options suggest actions that would either not comply with the speed limit or could compromise safety. Proceeding at any speed below 30 mph does not necessarily guarantee adherence to the maximum allowable speed, which could lead to disqualification of this option. Accelerating to approach the signal disregards the necessity to respect the posted limit, potentially creating unsafe conditions. Stopping until the signal changes would be overly cautious and is not typically required unless directed by the signal itself, which may not be the case when just approaching with a speed limit in place.

Thus, the correct answer emphasizes the importance of adhering to the safely established speed limit of 30 mph while approaching the signal, reinforcing the significance of operating within prescribed limits in rail operations.