Local authorities in their respective jurisdictions may erect and maintain signs designating residence and business districts, highways and steam or interurban railway grade crossings and such other signs as may be deemed necessary to carry out this title and such additional signs as may be appropriate to give notice of local parking and other special regulations. Local parking and other special regulations shall not be enforceable against an alleged violation if, at the time and place of the alleged violation, an appropriate sign giving notice thereof is not in proper position and sufficiently legible to be seen by an ordinarily observant person.