How close to the surface must mud flaps on trucks or trailers extend?

Mud flaps on trucks or trailers are required to extend close to the surface of the road to effectively minimize the amount of debris and water that can be kicked up by the vehicle's tires. This helps to enhance safety by reducing the risk of such materials striking other vehicles or pedestrians, as well as improving visibility for other drivers by limiting the splashing effects during wet conditions.

The requirement for mud flaps to be within 8 inches of the road surface serves as a standard in ensuring that they function properly while accommodating a variety of driving conditions. This specification allows for optimal safety and performance, thus making it the correct answer to the question.