Section 5. If the department determines, after public notice and a hearing of all parties interested, that public necessity and convenience require that a way should be laid out or be taken charge of by the commonwealth, it shall file in the office of the county commissioners for the county where the way is situated a certified copy of a plan thereof and a certified copy of a certificate that it has laid out and taken charge of said way in accordance with said plan, and shall file in the office of the clerk of each town where the way is situated a copy of the plan showing the location of the portion lying therein and a copy of the certificate that it has laid out and taken charge of said highway in accordance with said plan, and thereafter said way shall be a state highway, and shall be constructed by the department at the expense of the commonwealth; but any state highway so laid out and constructed may be abandoned or discontinued as provided in section twelve. The width of a state highway shall be such as the department deems necessary. If the width of a state highway be less than that of the way previously existing, that portion of the way which lies between the boundary or location lines of the state highway and the boundary lines of the way previously existing shall remain a public way unless the department determines that it should be abandoned, or the county commissioners of the county, or the city or town in which the way is situated, having jurisdiction of the way, abandon at any time said portion in the manner provided by law for the alteration, relocation or discontinuance of public ways.