Final answer:
Local laws vary, but drivers are typically required to slow down or stop at railroad crossings without gates if they cannot see 400ft in both directions, to ensure they can stop safely for oncoming trains.
Step-by-step explanation:
The question pertains to driving regulations and speed limits at a railroad crossing without gates or controls. When you are within 100ft of the crossing and cannot see the tracks 400ft in both directions, different regions have varying speed limit laws. However, the typically recommended speed in such a situation is slow or stop if you don't have a clear view of the tracks because a train might be approaching. The question does not provide an explicit answer, but drivers should always exercise caution and adhere to their local traffic laws, which may require slowing down to a speed where it's safe to stop if a train approaches (sometimes as low as 15-20 miles per hour or the speed which allows for a safe stop).