No provision of this chapter for which a traffic control device is required shall be enforced against an alleged violator if, at the time and place of the alleged violation, the required traffic control device is not in position and sufficiently legible or audible, as the case may be, to be seen or heard by an ordinarily observant or attentive person; but whenever a particular section does not state that a traffic control device is required, such section shall be effective even though no such device is in place.
('73 Code, § 15-17)