On most interstates, there shouldn't even be a speed limit (and if so, it should be around 80-85), save for the "safe and prudent" approach that Montana used before the feds blackmailed them into shutting that down.
One of our more dangerous roads, frequented by soldiers, pineapple trucks, and tourists headed to the north shore driving 5 miles under the ridiculously low-set speed limit, has a speed limit sign of 35 posted not far from Schofield Barracks. Someone has quite neatly tagged the sign to close the 3 to an 8... and I do believe some take the "85" seriously.