Provisions of this code that require signs shall not be enforced against an alleged violator if, at the time and place of the alleged violation, an official sign was not in proper position and was not sufficiently legible so as to be seen by an ordinarily observant person. When a particular rule does not state that signs are required, the rule shall be effective even though signs are not erected or in place.
Mich. Admin. Code R. 28.1305