Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 90, § 18B

(a) Notwithstanding section 18 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 designated safety zones on, at or near any way in the city or town which is not a state highway, and with the approval of the department if the same is a state highway. Such safety zones shall be posted as having a speed limit of 20 miles per hour.

(b) The operation of a motor vehicle in such zone at a speed exceeding the speed limit established under this section shall be a violation of section 17.