Maryland Code, Transportation § 22-413

(a) A person may not drive any bus, truck, trailer, or semitrailer on any highway in this State, unless it is equipped with suitable metal protectors or substantial flexible flaps behind the rear-most wheels of the vehicle or combination of vehicles to prevent the projection of rocks, dirt, water, or other substances to the rear and to minimize side spray.

(b) A person may not cause to be driven any bus, truck, trailer, or semitrailer on any highway in this State, unless it is equipped with suitable metal protectors or substantial flexible flaps behind the rear-most wheels of the vehicle or combination of vehicles to prevent the projection of rocks, dirt, water, or other substances to the rear and to minimize side spray.

(c) A person may not permit to be driven any bus, truck, trailer, or semitrailer on any highway in this State, unless it is equipped with suitable metal protectors or substantial flexible flaps behind the rear-most wheels of the vehicle or combination of vehicles to prevent the projection of rocks, dirt, water, or other substances to the rear and to minimize side spray.

(d) The protectors or flaps shall have a ground clearance of not more than one-third of the horizontal distance from the bottom edge of the protector or flap to the centerline of the axle. However, no protector or flap need be closer to the ground than 6 inches under any condition of loading. The protector or flap shall extend laterally at least the width of the tire or tires being protected.

(e) If any bus, truck, trailer, or semitrailer, is so designed or constructed that the objectives in subsection (a) of the section are accomplished by reason of fender or body construction or other means of enclosure, either permanent or temporary, then the requirements of this section have been satisfied.

(f) This section does not apply to any farm tractor or to any vehicle registered as a farm truck or to uncoupled truck tractors or to pole trailers or other vehicles where the construction is such that complete freedom around the wheel area is necessary to secure the designed use of the vehicle.