36 Pa. Stat. § 971
From and after the adoption of this act, all those certain existing public roads, highways, turnpikes, and toll-roads, or any parts or portions thereof, subject to the provisions hereinafter made in the case of turnpikes and toll-roads, forming and being main traveled roads or routes between the county-seats of the several counties of the Commonwealth, and main traveled roads or routes leading to the State line, and between principal cities, boroughs, and towns, shall be known, marked, built, rebuilt, constructed, repaired, and maintained by and at the sole expense of the Commonwealth; and shall be under the exclusive authority and jurisdiction of the State Highway Department, and shall constitute a system of State Highways, the same being more particularly described and defined as follows:
36 P.S. § 971