In a SE Ma small town, there is a rural road that I use frequently and the posted SL is 40 mph (standard state speed limit on such rural roads not posted). The street is about 1 or 2 miles long and has scattered houses along it as past farm land has been sold to people wishing to build homes on it.
For many years this road has had posted speed limit signs of 40 mph and suddenly, some new houses have been built along one part of the street (about a dozen) this year. A few months later there suddenly appeared "30 mph - thickly settled area" at both ends of the house cluster.
A few weeks later the signs were gone. Why? Well, this state has conditions for establishing speed limit areas and (these) residential buildings have to be occupied for a certain distance before qualifying for being a "thickly settled" (30 mph status) area. Turns out the distance of close habitation did not qualify for "thickly settled" and the signs were taken down. Seems these new home owners didn't like traffic "speeding" by their new homes at 40 mph and asked the town to put up thickly settled signs to slow traffic down on this narrow 2 lane paved road. Well, rules are rules and the area, for now, still is posted for 40 mph. Definately an "oops" on the part of the town's officials
So be aware of new posted speed signs in your location and if suddenly they appear, check local and state regulations on posted speed zones before knowing if they are legal!