The State Department of Transportation, hereinafter referred to as the department, shall, as early as practicable after July 26, 1949, designate such streets or roads, including viaducts and bridges, as are municipal connecting link roads in each city or town of this state under the then existing state highway system and, thereafter, shall designate such municipal connecting link roads for additional state roads in the state highway system at the time such roads are opened for travel. Such designation shall also be made promptly of any street or road in any city or town not constituting a part or portion of the state highway system wherever necessary to care for arterial or main highway traffic, and the provisions of this article shall apply to such streets or roads so designated. The department shall keep a record of such municipal connecting link roads so designated and shall promptly, after designation in each case, furnish to the city or town involved a statement of the municipal connecting link roads so designated therein.