No provision of this subchapter, for which signs are required, shall be enforced against an alleged violator if, at the time and place of the alleged violation, an official sign is not in place and sufficiently legible to be seen by an ordinarily observant person. Whenever a particular provision does not state that signs are required, the provision shall be effective, even though no signs are erected or in place.
('88 Code, § 18.234.2)