Well, over here in Oz, all roads have speed limits which Tesla knows about and displays. Will have to try myself and will report. Although I'm fairly sure you're confusing engaging AP when under the speed limit at which point it will set it to the speed limit, minus or plus the amount you configure in the settings. So if you configured it to be +5 mph of the speed limit, that's what it gets set to. But if you were already driving at 30mph over the speed limit and engage AP, it will keep your speed. And it makes sense, in case it has incorrectly determined the speed limit, it won't cause safety issue by driving too slowly.