Mass. Gen. Laws ch. 166, § 21D: Warning signs; size; posting

Section 21D. Warning signs shall be placed: (1) Within the equipment readily visible to operator of cranes and other equipment when at the controls of such equipment;

(2) On the outside of equipment in such number and locations as to be readily visible to mechanics or other persons engaged in the work operations.

Warning signs shall be not less than five inches in height, nor less than seven inches in width.