Frontage roads and state-maintained bridges. Mostly unsigned, except for far western Kentucky. Only other place I ever saw one signed was in Letcher County, near Whitesburg off KY 15, but that signage was removed and I think the state may have abandoned that route.
What's scary is that the state numbering series jumps from 3xxx to 6xxx, which means they may anticipate the four-digit route number going into the 4xxx or 5xxx series. New state routes are getting signed in the 36xx range now.