The roads do change more than you think, I update maps regularly and I can still find myself coming up to islands that aren't on the sat-nav, and also finding that the new road I am on shows me driving across a field and the sat-nav keeps trying to re-calculate my route.
On the other hand, I don't think that sat-navs that show the speed limit have any chance of keeping up with the ever-lowering speed limits on our roads, I am always amazed that the speed limit on a main 'A' road can drop from 60 to 30 for a mile or two just because there are a couple of houses. It used to be a legal requirement that to introduce a 30mph speed limit the street lighting had to be really good, but nowadays councils seem to introduce 30 limits as and when they please, even with no street lights.