Massachusetts General Laws Chapter 90, Section 17 states:
 
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.
 
So, for example, if you were driving in a 'thickly settled' area and the officer clocked you at 40 miles per hour, he or she is within his powers to issue the ticket for speeding.  You can still challenge the ticket, present evidence, and argue that your speed was reasonable for the situation and location you were in, however, the prima facie evidence provision of Section 17 will make it difficult for you to prevail.