(a) The driver of any vehicle shall obey the instructions of any official traffic-control device, applicable thereto placed or held in accordance with the provisions of this chapter, unless otherwise directed by a police officer, subject to the exceptions granted the driver of an authorized emergency vehicle in this chapter.

(b) 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. Whenever 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.

(c) Whenever official traffic-control devices are placed or held in position approximately conforming to the requirements of this chapter, such devices shall be presumed to have been so placed or held by the official act or direction of lawful authority unless the contrary shall be established by competent evidence.

(d) Any official traffic-control device placed or held pursuant to the provisions of this chapter and purporting to conform to the lawful requirements pertaining to such devices shall be presumed to comply with the requirement of this chapter, unless the contrary shall be established by competent evidence.