No provision of this chapter for which official traffic control devices are required shall be enforced against an alleged violator if, at the time and place of the alleged violation, an official device is not in proper position and sufficiently legible to be seen by an ordinarily observant person. If a particular section does not state that official traffic control devices are required, such section shall be effective even though no devices are erected or in place. (2014 Code § 15-604)