(a) Notwithstanding section 17 or any other general or special law to the contrary, the city council, the transportation commissioner of the city of Boston, the board of selectmen, park commissioners, a traffic commission or traffic director of a city or town that accepts this section in the manner provided in section 4 of chapter 4 may, in the interests of public safety and without further authority, establish a speed limit of 25 miles per hour on any roadway inside a thickly settled or business district in the city or town on any way that is not a state highway.
(b) Upon establishing a speed limit under this section, the city or town shall notify the department. The operation of a motor vehicle at a speed in excess of a speed limit established under this section shall be a violation of section 17.