Final answer:
Mud flaps on trucks or trailers must reach within 8 inches of the ground to minimize flying debris and prevent hazards on the road.
Step-by-step explanation:
In the case of trucks or trailers, mud flaps are required to reach within 8 inches of the ground. This is to ensure that the flying debris, such as rocks or loose gravel, is minimized and doesn't pose a hazard to other vehicles on the road. The mud flaps help prevent the debris from being thrown up into the air by the tires and causing damage to nearby vehicles or pedestrians.