No provision of this title for which traffic signs, signals or devices are expressly required shall be enforced against an alleged violator, if at the time and place of the alleged violation an official traffic control device is not in proper position and sufficiently legible to be seen and understood by a person of ordinary understanding or observation. Whenever a particular section does not expressly state that traffic control devices are required, the absence of a traffic control device shall not constitute a defense of such section. (Ord. 304 § 28, 1963).