36 Pa. Stat. § 670-220
When any State highway route or combination of State highway routes, or portions thereof, in the opinion of the secretary, is inadequate for present or anticipated traffic, and the cost of securing the right of way necessary for a highway of adequate design on the existing location would be unwarranted in the judgment of the secretary, or the topography adjacent to the existing location is such that widening is impractical, and the establishment and construction of a parallel highway, together with the retention of the existing highway, will provide ample width of highway for public use, the secretary may establish, construct and maintain a parallel or approximately parallel highway as a State highway in any township, borough, incorporated town or city, or combination of such political subdivisions, and retain the present highway as a part of the system of State highways. A plan for the establishment and construction of such parallel or approximately parallel highway shall be prepared by the department, signed by the secretary, and approved by the Governor and filed as a public record in the department.
The secretary may control the direction of the flow of traffic over parallel or approximately parallel highways established under the provisions of this section by the erection of approved one-way traffic signs at the beginning and ending of each parallel or approximately parallel highway, and at the entrance of all roads, highways, alleys or streets connecting therewith.
Damages occasioned by the taking of private property for the construction of parallel or approximately parallel State highways established under the provisions of this section, shall be ascertained and paid as provided by Article III of this act.
36 P.S. § 670-220