75 Pa. C. S. § 3111.  Obedience to traffic-control devices

  (a) GENERAL RULE.-- Unless otherwise directed by a uniformed police officer or any appropriately attired person authorized to direct, control or regulate traffic, the driver of any vehicle shall obey the instructions of any applicable official traffic-control device placed or held in accordance with the provisions of this title, subject to the privileges granted the driver of an emergency vehicle in this title.
 
   (b) PROPER POSITION AND LEGIBILITY OF DEVICE.-- No provision of this title 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, the section shall be effective even though no devices are erected or in place.
 
   (c) PRESUMPTION OF AUTHORIZED PLACEMENT.-- Whenever official traffic-control devices are placed or held in position approximately conforming to the requirements of this title, the 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) PRESUMPTION OF PROPER DEVICES.-- Any official traffic-control device placed or held pursuant to the provisions of this title and purporting to conform to the lawful requirements pertaining to such devices shall be presumed to comply with the requirements of this title, unless the contrary shall be established by competent evidence.