The Commissioner of Highways may classify, designate, and mark state highways and provide a uniform system of traffic control devices for such highways under the jurisdiction of the Commonwealth. Such system of traffic control devices shall correlate with and, so far as possible, conform to the system adopted in other states. All drivers of vehicles shall obey lawfully erected traffic control devices. No provision of this section relating to the prohibition of disobeying traffic control devices or violating local traffic control devices shall be enforced against an alleged violator if, at the time and place of the alleged violation, any such traffic control device is not in proper position and sufficiently legible to be seen by an ordinarily observant person.