I always thought it must be terribly confusing to live in a large country where the rules vary from one state to the next. Australia's pretty much the same size as the US, but I don't think the laws vary (do they Paul?). Of course, it's not as evenly populated.
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). For those of you who haven't been here, which I assume will be most of you, there are only a couple of big cities on each island, with long stretches of road and small settlements in between. It's very difficult to set speed limits for the highways because they vary so much in quality. Driving south from Auckland, the highway is similar to the ones you have in the US, but once you get far enough from the city it becomes pretty much just a two-lane sealed road almost all the way to Wellington.
Also, our terrain over much of the country means that intercity roads have long stretches that are narrow, steep, winding and generally impossible to cover at speeds above maybe 30-40mph, even though they're officially 60. Travelling here is very pleasant and scenic but it takes a lot longer than most visitors think who have just looked at the size of the country on a map. I can't count the number of people I've met who have allowed two weeks to see the whole country, and you couldn't even see the top third of just one island in that time.

Brunnen-G