No provision of this traffic code for which signs are required shall be enforceable against an alleged violator if at the time and place of the alleged violation the sign herein required is not in proper position and sufficiently legible to be seen by an ordinarily observant person. However, when any particular section of this traffic code does not state that signs are required, such section shall be effective without signs being placed to give notice thereof.
('84 Code, § 70.20)