In the USA, legal requirements for mud flaps on vehicles, especially commercial trucks and trailers, are established to enhance road safety by minimizing debris and spray thrown by the tires.
It’s important to note that actual laws regarding mud flaps are enforced on a state level and specific guidelines can vary. Any info contained in this post are overall general guidelines.
The Federal Motor Carrier Safety Regulations (FMCSRs) require commercial vehicles to be equipped with mud flaps if their rear tires are not fully covered by the vehicle’s body or fenders. These regulations, outlined in 49 CFR Part 393.86, specify that mud flaps must be of sufficient width to cover the full width of the tire and should be mounted to prevent debris from being thrown onto the road, thereby reducing the risk of accidents caused by flying debris. Additionally, many states have their own specific regulations regarding the installation, size, and material of mud flaps, often requiring them to extend to within a certain distance from the ground to ensure effective coverage. Compliance with both federal and state regulations is essential for legal operation and to avoid fines. The bottom of the mud flaps should be no more than 1/3 of the distance of distance of the from the center of the rear tire to where the mud flap is located. For example, if the mud flap is 42 inches behind the center of the rear tire, it can’t be more than 14 inches off the ground. Some states have specific height requirements, such as 8 inches in Texas, Arizona, Delaware, and Missouri, or 14 inches in Alaska. Generally speaking about 6” above the road surface is sufficient. If mud flaps are too close to the ground they can get damaged more easily. Although not specifically required it’s a good thing to have anti-spray mud flaps with vertical grooving on the back sides. This helps to reduce road spray and channels water back down towards the ground.