Final answer:
Commercial vehicles such as buses and vehicles carrying hazardous materials have to stop at RR crossings to ensure safety. A traffic signal helps to regulate the speed and flow of traffic to further enhance safety at such intersections.
Step-by-step explanation:
The type of vehicles that are required to stop at railroad (RR) crossings typically include commercial vehicles carrying passengers, such as buses, and vehicles transporting hazardous materials. The specific rules can vary by jurisdiction, but the general idea is that vehicles that would pose a greater risk in the event of a train collision need to take extra precautions. The presence of a traffic signal at intersections, including those near RR crossings, plays a crucial role in ensuring the safety of all road users by regulating vehicle speeds and pedestrian crossing times.