Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 90, § 17C

(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.