(a) Safety guards or flaps are required on all trucks, trailers, or semitrailers (in combination with a towing vehicle), if the rearmost axle of the vehicle (or combination) has four tires or more, or at least two super single tires. They are not required on buses, pole trailers, motor homes, or truck tractors. In this section, "super single tire" means a wide-base, single tire that may be used in place of two standard tires on the same axle.
(b) Safety guards or flaps shall be located and suspended behind the rearmost wheels of such vehicle or if in combination behind the rearmost wheels of such combination to within eight inches of the surface of the roadway.
(c) A tolerance of four inches will be allowed.
(d) Safety guards or flaps shall be at least as wide as the tires they are protecting.
(e) When trailers and semitrailers are operated in combination with a towing vehicle, safety guards or flaps will be required on the rearmost axle of such combination.
(f) Safety guards or flaps shall be of metal, rubber, rubberized material, or other substantial material, capable of remaining in place back of rear wheels by their own weight or structure while the said vehicle is being operated.
(g) The construction of safety guards or flaps will be such that they will remain in proper place back of rear wheels and will be rigid enough to prevent slush, mud, gravel, and other roadway material or debris being transmitted from the vehicle's rear wheels to the windshield of the following vehicle.
(h) Safety guards or flaps should be securely mounted, as wide as the tire that it is protecting, not split or torn to the extent that it is ineffective and the bottom edge of the safety guard or flap shall be no more than 12 inches from the surface of the roadway while the vehicle is standing still, or otherwise not in motion.
(i) A flexible safety guard or flap may swing upwards and backwards while the vehicle is moving, so long as the safety guard or flap continues to block the trajectory of slush, mud, gravel, and other roadway material or debris which would otherwise be transmitted to the windshield of a following vehicle while both vehicles are in motion.

Source Note: The provisions of this §21.4 adopted to be effective April 12, 2012, 37 TexReg 2430