Mud flap requirements

Written by lynn rademacher
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Mud flap requirements
Mud flaps are designed to protect other drivers from flying road debris. (Thinkstock Images/Comstock/Getty Images)

Mud flaps are pieces of heavy rubber material that are mounted behind the tires of large trucks and other large equipment. The purpose of mud flaps is to protect the driver behind the truck from being hit with flying road debris. Mud flaps also help to reduce the amount of water spray that is thrown from truck tires in wet driving conditions. Several states have specific mud flap regulations; however, there are some basics that are universal to all mud flap applications regardless of the location.

Reflective Stripping

Reflective stripping on large commercial vehicles was implemented as a requirement in 1992 according to the FMCSA (Federal Motor Carrier Safety Administration) branch of the U.S. Department of Transportation. As a part of this regulation, it became required for mud flaps to be outfitted with reflective stripping across the top of the mud flap. The only exception to this rule is if there is a step bar at the back of the trailer that is closer to the ground than the top of the mud flaps. In this situation the reflective stripping may be applied to the lower bar.


A properly mounted mud flap will prevent debris from leaving the wheel of the truck or trailer at an angle greater than 22 degrees. The Department of Transportation (DOT) has created a mathematical formula to help truck drivers and transportation companies to determine the proper location of where to mount the mud flaps. Mud flaps should not be mounted so close to the tires that they are able to rub on the tire and create additional friction. Nor should they be mounted so far back from the last trailer tire that the mud flap becomes ineffective at deflecting road debris. DOT officials check a truck's mud flap locations while performing routine roadside weight checks.

Distance from Ground

The U.S. Department of Transportation requires the bottom edge of the mud flap to never be more than 10 inches from the ground. Some states have set even stricter standards and required mud flaps to extend even closer to the ground. A distance greater than 10 inches may allow road debris to fly past the mud flap and hit the following car at an angle greater than 22 degrees.

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