36 Pa. Stat. § 670-703
If the department relocates a part or section of a State highway route included within which is a county bridge, the responsibility for the construction and maintenance of which is placed upon the Commonwealth by section seven hundred one of this act, upon the opening of the new highway to vehicular traffic and the abandonment of the section of highway supplanted by the relocation as a State highway route, the bridge located on the abandoned section of highway shall revert to the county, and the subsequent maintenance and construction thereof shall become the obligation of the county if the section of highway is to remain for the accommodation of vehicular traffic. If in the opinion of the secretary, it is not necessary that such highway be continued as a public road, he may remove or dispose of such county bridge or authorize it to be removed and disposed of by the county commissioners.
36 P.S. § 670-703