Following traffic signs and signals

(a) The driver of any vehicle shall obey the instructions of any official traffic-control device applicable thereto placed in accordance with law, unless otherwise directed by a traffic or 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 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 proper position and sufficiently legible to be seen by an ordinarily observant person. Whenever a particular section does not state that signs are required, the section shall be effective even though no signs are erected or in place. (c) Whenever official traffic-control devices are placed in position approximately conforming to law, 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 law and purporting to conform to the lawful requirements pertaining to such devices shall be presumed to comply with the requirements of this chapter, unless the contrary is established by competent evidence.