I81Expect big backups and long delays if you are heading from Roanoke to Christiansburg this afternoon or evening, the result of an unscheduled detour off Interstate 81. VDOT officials say crews discovered unstable rock slope today along the area where they are building five miles of new truck climbing lanes. As a result, all southbound traffic on 81 is being taken off the highway at the Dixie Caverns exit and detoured onto US 11/460 up the mountain to Christiansburg. VDOT says the detour will last at least into tomorrow morning as crews determine how to deal with the unstable rock.