Section 17. No person operating a motor vehicle on any way shall run it at a rate of speed greater than is reasonable and proper, having regard to traffic and the use of the way and the safety of the public. Unless a way is otherwise posted in accordance with the provisions of section eighteen, it shall be prima facie evidence of a rate of speed greater than is reasonable and proper as aforesaid (1) if a motor vehicle is operated on a divided highway outside a thickly settled or business district at a rate of speed exceeding fifty miles per hour for a distance of a quarter of a mile, or (2) on any other way outside a thickly settled or business district at a rate of speed exceeding forty miles per hour for a distance of a quarter of a mile, or (3) inside a thickly settled or business district at a rate of speed exceeding thirty miles per hour for a distance of one-eighth of a mile, or (4) within a school zone which may be established by a city or town as provided in section two of chapter eighty-five at a rate of speed exceeding twenty miles per hour. Operation of a motor vehicle at a speed in excess of fifteen miles per hour within one-tenth of a mile of a vehicle used in hawking or peddling merchandise and which displays flashing amber lights shall likewise be prima facie evidence of a rate of speed greater than is reasonable and proper. If a speed limit has been duly established upon any way, in accordance with the provisions of said section, 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; but, notwithstanding such establishment of a speed limit, every person operating a motor vehicle shall decrease the speed of the same when a special hazard exists with respect to pedestrians or other traffic, or by reason of weather or highway conditions. Any person in violation of this section, while operating a motor vehicle through the parameters of a marked construction zone or construction area, at a speed which exceeds the posted limit, or at a speed that is greater than is reasonable and proper, shall be subject to a fine of 2 times the amount currently in effect for the violation issued. Except on a limited access highway, no person shall operate a school bus at a rate of speed exceeding forty miles per hour, while actually engaged in carrying school children.