What is the requirement for flaps on a trailer or semi with 4 tires on the rear axle?

The requirement for flaps on a trailer or semi with four tires on the rear axle is that they must be located on the rear most set. This regulation is designed to reduce the spray from the tires, which can improve visibility for following vehicles, thereby enhancing overall road safety. Flaps help minimize the amount of water, mud, or debris that can be thrown into the air, which is particularly important on wet or muddy roads.

This requirement is generally mandated to ensure that trailers and semis do not contribute to obscuring vision or causing hazardous conditions for other drivers. It is specifically noted for the rear axle since that is where the most substantial amount of spray and debris is generated due to the force and direction of the vehicle's movement. By having flaps at the rear, you effectively mitigate these risks and maintain better safety standards on the road.