"Ignorance of the law is no excuse"

[Hyena Dandy]

and a second's worth of though should be more than it takes to realize that you can't go highway speeds through a town, so I really have no sympathy for this particular issue.

It depends, honestly. Because where the town limits are, you might not (particularly if not familiar of the area) actually know you're IN a town. The place my family vacations, the main road is also Route 172, (which turns into Blue Hill Road) and it goes from "This is the highway, whatever you want" to "Now you're in town" Well before you actually see any buildings. And if they chose to put a speed limit at the very outskirts of town, which they don't, you very well could not know you're in town.

I agree on the "You can probably slow down fast enough, if you see the sign" thing. But as for the "Common sense" part... Not sure I actually agree that your common sense would kick in without the sign.