Mass. Gen. Laws ch. 89, § 4A: Driving vehicles in a single lane; motorcycles, riding and passing

Section 4A. When any way has been divided into lanes, the driver of a vehicle shall so drive that the vehicle shall be entirely within a single lane, and he shall not move from the lane in which he is driving until he has first ascertained if such movement can be made with safety. The operators of motorcycles shall not ride abreast of more than one other motorcycle, shall ride single file when passing, and shall not pass any other motor vehicle within the same lane, except another motorcycle.