What classifies a road as a rural highway?

today, while driving home from traffic court(ironic), im drivin via back roads, througl a small town with a 30 mph speed limit. Well, after passing through the town, the road clears out to nothing more than a few farms on the sides, so i assume im on a rural highway now, as im no longer really in town and its been around a mile since the last speed limit sign. I accelerate a bit, and get nailed for 46 in a 30. about 1/2 mile later i see a sign "end 30 mph zone" how close do speed limit signs have to be, and what classifies a rural highway (55mph in illinois, if no posted speed limit)?