Mich. Comp. Laws § 252.316: Illuminated signs.

Sec. 16.

(1) A sign that is subject to this act may be illuminated so as to allow the sign to be seen and read but the illumination shall be employed in a manner that prevents beams or rays of light from being directed at any portion of the main-traveled way of the highway in a manner that interferes with safe driving.

(2) A sign containing changing illumination shall not be erected in any area except in an incorporated city or village over 35,000 in population where the department determines it is consistent with customary usage in the area. A sign permitted under section 18(f) is not a sign containing changing illumination.

(3) A sign shall not be so illuminated that it obscures or interferes with the effectiveness of an official traffic sign, device, or signal.

(4) All lighting shall be subject to any other provisions relating to lighting of signs presently applicable to all highways under the jurisdiction of the state.