But I can't remember if something is 60 or 70!
Try and remember that it's down to whether or not it's a divided highway. Two-way traffic = 60 mph, divided with some sort of barrier or median strip = 70 mph. I'm sure it's confusing that you don't actually see a sign which shows "60" or "70" explicitly, just the "National Speed Limit" sign (white disk with diagonal black bar).
The reason for that sign is historical. Before the mid 1960s there was no absolute maximum speed limit on open roads, just the same sort of rule as existed in a number of American states in the past to drive at a speed reasonable and prudent for the conditions. So originally that sign actually meant "End of speed limit," hence its design. When the upper limit was introduced in the 1960s, they kept the sign and just changed its meaning (although at one time there were "real" 70 signs to be seen on motorways; I remember them still being around in the 1970s).
The posted limits often seem somewhat illogical and inconsistent, especially in rural areas such as this where you can be driving along a fairly fast, main road which has had its speed limit lowered to 50 mph under one of the accident-prevention schemes, yet the moment you turn onto a narrow, twisty little backlane the first thing you see is the NSL sign putting the limit back to 60 mph! It's basically because a lower speed limit can be posted only on specific stretches of road when an order has been so made, so anything else is 30 or 60/70 by default.
It's confusing I know, but at least if you know the history it might help to make a little more sense as to why things are the way they are.
