I agree that this is probably how the road came into being - a sheep run, which became a track then a road. We have to remember that, apart from enlightened antiquaries, Avebury was largely ignored during the two millennia when Christianity was dominant. The stones were just a backdrop to the farming activities of the area. When the road was being metalled (would it have been the early part of the 20th century - VBB will know) I don't suppose much thought was given to the Avenue. We have Alexander Keiller to thank for that.