G.L. c. 90, § 17, as amended through St. 1986, c. 689, § 7, governs “Speed limits” in the Commonwealth. “Shortly stated the statute forbids the running of an automobile at a rate of speed greater than is reasonable and proper, and declares what rates of speed shall be prima facie evidence of the rate forbidden.” Commonwealth v. Cassidy.
“If a speed limit has been duly established upon any way, in accordance with the provisions of said section [e.g., G.L. c. 90, § 18], operation of a motor vehicle at a rate of speed in excess of such limit shall be prima facie evidence that such speed is greater than is reasonable and proper …”. G.L. c. 90, § 17.
Contrariwise, it follows that the Selectmen’s use of phony speed limits as prima facie evidence is a violation of G.L. c. 90, § 17, and is thus ultra vires.