Why do US routes in the southeast have concurrent state routes?
Prior to the US-numbered routes, many states had already established state route systems. With the advent of US routes, most following existing state routes, most states removed the original state route numbers at some time, but AL, FL, GA, and TN kept them in their records for maintenance purposes. Most are unposted outside GA, although a few errors get out. GA, however, posts them consistently. AL, FL, and GA also give maintenance numbers to the Interstates (GA’s are in the 400s). None of these are signed, although GA 400 is part of the system and several FL routes continue on past the end of the Interstate (e.g. FL 400 in Daytona Beach) or take a short but distinctly different path near it (e.g. FL 9 in Miami). (FL 10 is the secret number of US 90 west of Jacksonville, so the “exposed” portion of FL 10 being near I-10 is coincidental – I-10’s hidden designation is FL 8). OR maintains its roads with a similar concept, with “highways” being the original system and mostly now unposted,
