The state traffic laws regulating the speed of vehicles shall be applicable upon all streets within the city, except as this traffic code, as authorized by state law, declares and determines upon the basis of engineering and traffic investigation that certain speed regulations shall be applicable upon specified streets or in certain areas, in which event it shall be prima facie unlawful for any person to drive a vehicle at a speed in excess of any speed so declared in this traffic code when signs are in place giving notice thereof.
('71 Code, § 72.11) Penalty, see § 70.99