Mich. Comp. Laws § 247.313

Sec. 3. Whenever such barricading device shall be constructed, installed, operated or maintained, the public authorities having jurisdiction and control over the highway at any such place shall install and maintain at the side of such highway, immediately adjacent to such device, reflectorized warning signs with the words "automatic barrier" in letters not less than 3 inches high. Whenever such device is located at a railroad intersection, additional reflectorized warning signs, of a design to be prescribed by the state highway commissioner, shall be installed and maintained on both sides thereof at a distance not less than 400 feet therefrom when such intersection is located on a highway where vehicular traffic is permitted to travel at speeds in excess of 30 miles per hour, and not less than 200 feet therefrom when such intersection is located on a highway where vehicular traffic is permitted to travel at speeds not in excess of 30 miles per hour, and such device and warning signs shall be in lieu of all other protection at such intersection. The advance warning signs required by any other law of this state shall also be installed and maintained.