Michigan Compiled Laws, Chapter 480. Motor Carrier Safety § 480.25

Sec. 15.  A truck, truck tractor, trailer, semitrailer, or any combination of these, when used on a highway, shall be constructed, equipped, or operated to prevent water or other road surface substances from being thrown from the rear wheels of the vehicle or combination at tangents exceeding 22- 1/2 degrees measured from the road surface.  If a flap type device is used, it shall not have attached any type of lamp, breakable reflective material, or reflecting buttons nor may the device extend beyond the maximum width of the vehicle or combination.