Final answer:
Before backing under a truck trailer, always ensure the area is clear of obstacles for safety. While other factors like tire inflation and lights are important, the immediate concern is to avoid accidents during the backing maneuver.
Step-by-step explanation:
Before backing under a truck trailer, it's crucial to ensure that the area is clear of obstacles. This is vital for safety to prevent accidents or collisions with objects, people, or other vehicles. While making sure tires are inflated is essential for general operation, and having lights on is necessary for visibility, especially in low-light conditions, the most immediate concern before backing up is to check the surroundings. Ensuring the engine is off is not a prerequisite for backing under a trailer since the engine typically needs to be running to maneuver the vehicle. When passing a freight truck with a trailer that's bouncing up and down slowly on the highway, it suggests that the trailer is likely nearly empty. A heavily loaded trailer would have less bounce due to the weight damping the suspension's movement.