The following various sections of public roads shall respectively be adopted by the Commonwealth as State highways, to be constructed and maintained at the sole expense of the Commonwealth, under the provisions of present or future laws governing State highways: Provided, however, That any county, township, borough, or incorporated town, or any two or more of such municipalities acting jointly, may pay the whole or any part of the cost of the construction of said highways, that is to say:
36 P.S. § 1350