> I always thought it must be terribly confusing to live in a large country where the rules vary from one state to the next.
> Here in New Zealand the limits are 50kph (30mph) in residential areas and 100kph (60mph) on the open road, with some intermediate areas of 80kph (50mph).

I can't say for sure, but your comments -- both of the above -- suggest that things work quite differently in New Zealand than they do here. Here there are speed limit signs posted everywhere, and that speed limit could be any number from 5 (though rarely below 20) and up, in increments of 5. It's not confusing from state to state because you'll see a speed limit sign every time the speed limit changes and intermittently in long stretches of road where it doesn't. I can't imagine how you wouldn't have speed limit signs, even if the speed limit is only ever one of three values, but I can't imagine how you'd find varying speed limit laws from state to state confusing otherwise. Enlighten me if you can. Cultural differences in different English-speaking countries always seem to be interesting.