Whenever by this or any other chapter of this City Code any parking time limit is imposed or parking is prohibited on designated streets or portions of streets it shall be the duty of the Police Chief to erect appropriate signs giving notice thereof and the regulations shall not be effective unless signs are erected and in place at the time of any alleged offense. When signs are erected giving notice thereof, no person shall disobey the restrictions stated on such signs.
(Code of Iowa, Sec. 321.236)