This pretty much covers everything. I would think a sign on public property or a right-of-way could be considered "foreign substance" if it is placed there without permission. ===================================
Michigan NATURAL RESOURCES AND ENVIRONMENTAL PROTECTION ACT (EXCERPT) Act 451 of 1994
324.8901 Definitions. Sec. 8901.
As used in this part: (a) "Litter" means all rubbish, refuse, waste material, garbage, offal, paper, glass, cans, bottles, trash, debris, or other foreign substances. (b) "Public or private property or water" includes, but is not limited to, any of the following: (i) The right-of-way of a road or highway....... (ii) A park, playground, building, refuge, or conservation or recreation area. (iii) Residential or farm properties or timberlands.
324.8902 Littering property or water prohibited. Sec. 8902. (1) A person shall not knowingly, without the consent of the public authority having supervision of public property or the owner of private property, dump, deposit, place, throw, or leave, or cause or permit the dumping, depositing, placing, throwing, or leaving of, litter on public or private property or water other than property designated and set aside for such purposes.