(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 traffic or 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 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,
such section shall be effective even though no signs are erected or in