Massachusetts has an unique unposted limit law. If a limit is not posted outside of a thickly settled area, you are not speeding unless you exceed 50 (divided highway) or 40 (undivided) for a quarter mile. In a thickly settled area other than a school zone, exceeding 30 for an eighth of a mile is speeding.
MGL 90-1 defines: "Thickly settled or business district", the territory contiguous to any way which is built up with structures devoted to business, or the territory contiguous to any way where the dwelling houses are situated at such distances as will average less than two hundred feet between them for a distance of a quarter of a mile or over.
The Massachusetts Turnpike, which for legal purposes includes the Boston Harbor tunnels, is exempt from the state prima facie speed limits. The signs you see are not speed limits for the purposes of MGL 90-17. These signs are informing you of Turnpike regulations: 730 CMR 7.08(6)(c) sets absolute limits on the Turnpike. The fine and insurance surcharge are the same as for speeding.
The Metropolitan District Commission has a regulation prohibiting speeds greater than posted (350 CMR 4.01(2)).
Here are some frequently asked questions about tickets and traffic court in Massachusetts.
See also historic speed laws of Massachusetts