Why not! Because a ring road would do massive, irreparable damage to the Wiltshire landscape and the surrounding archaeology - the Avebury World Heritage site is quite big after all. I agree its not ideal to have the road going through the stone circle though in its historical context it is only during the past 50/60 years that the lorries and untenable amounts of traffic have thundered through. There is also the A4 running between Silbury and WKLB with frequent accidents on that stretch of road because people drive too fast. At least they (mostly) slow down through Avebury.

I don't have an alternative solution, sorry.