Where the boundary line of any city, borough, or incorporated town traverses longitudinally the legal limits of a highway, and a portion only of such highway, situate either outside of or within the municipality, has been or may hereafter be established as a State highway, the portion of the highway which the act fails to include because of the presence of the boundary line, whether such portion may be outside of or within the corporate limits of the city, borough, or incorporated town, is hereby established as, or shall hereafter automatically become, a State highway, and shall be constructed and maintained by the department in accordance with the provisions of this act governing State highways in cities or boroughs or in townships, as the case may be.