36 Pa. Stat. § 670-204
The secretary is hereby authorized to designate by letter, or to name, or number, State highways, or any part or parts thereof, or any combinations of such highways or parts thereof, in such manner as in his discretion will best accommodate and inform the traveling public using the highways of the Commonwealth, and facilitate the keeping of maps and records of the department, and the secretary may change the number, or continue a State highway without it being designated by the route number originally assigned by the General Assembly or assigned by any executive order adopting a connecting road or spur in the manner provided by law prior to the adoption of section 218 of this act.
Whenever the secretary designates any route by common letter, name, or number, for the convenience of the traveling public, he may, in his discretion, include therein any public road, or part thereof, except city, borough, or town streets. Nothing herein contained shall authorize the secretary to construct, maintain, or repair any such road, or part thereof, as a part of the State highway system, nor authorize the secretary to delete from the State highway system any route or portion thereof previously adopted by the General Assembly or by executive order.
36 P.S. § 670-204