Agreed - Assuming you get in on time (of course!) and it takes you approx 30 min to get your rental car and on the road... it will take you about an hour to get to Greensboro, but shouldn't expect too much traffic there until closer to "rush" hour. The only places in NC that get "rush" hour are GSO, RDU and CLT. Even then, its nothing like what you might expect in a larger city.

In case you're interested, the best way to get from RDU to Roanoke would be I-40W, then take the airport exit in GSO (HWY 68 N). Hwy 68 will merge with Hwy 220 N just above Greensboro, which will take you right into Roanoke.

Have a safe trip!