Final answer:
Typical vehicles like bicycles, motorcycles, and cars are required to stop at railroad crossings only when indicated, while commercial vehicles carrying hazardous materials must always stop.
Step-by-step explanation:
The vehicles that are always required to stop at RR crossings are certain types of commercial vehicles, such as buses and trucks that carry hazardous materials, not the options listed in the question. It is a common misconception that typical passenger vehicles like bicycles, motorcycles, or cars are always required to stop at railroad crossings. However, these vehicles are generally required to stop only when there is an indication of an approaching train, such as flashing lights, a lowering gate, or a visible train. As for trains, by their very nature, they travel on the rails and do not stop at their own crossings.