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

(b) No provision of this act 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 in position approximately conforming to the requirements of this act, such devices shall be presumed to have been so placed 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 pursuant to the provisions of this act and purporting to conform to the lawful requirements pertaining to such devices shall be presumed to comply with the requirements of this act, unless the contrary shall be established by competent evidence.