U.S. 39
	The main purpose of US 39 is to eliminate the split routes of US 31 between Louisville, KY and Nashville, TN.  Seeing as how odd-numbered routes increase to the west, I'd replace US 31W with US 39 and US 31E with plain old US 31.  US 39 would also take over Alt US 31 from Nashville to Pulaski, TN.