There are only 2 possible bypass routes, both start at Berwick Bassett. One aims to join up with the A4 at the sanctuary the other goes behind Windmill hill passed Julians house and joins the A4 on the Calne road.
Neither of these will ever happen because of the huge amount of archaeology they would destroy.
Traffic damaging the stones, Na.
I've seen cars hit the Swindon stone at 60 miles and harldy even scratch the stone but totaled the car.
PeteG

Nah, sod the tunnel (not literally - that would be hideous!). How about a flyover? That'd have the added advantage of some spectacular aerial views of the stones, which'd be a real boon for us amateur photographers!

A tunnel! Now why didn't I think of that? Maybe we could run it under Silbury itself thereby providing an in-depth (no pun intended) archaeological examination of the Hill while at the same time reinforcing its foundations and sucking out the polystyrene in-fill from below (simultaneously of course pouring in a chalk compound from above). The Avebury Bypass Tunnel could even be enlarged BELOW Silbury to provide an underground car park for Avebury visitors or, perhaps, even to emerge a little further west at West Kennet Long Barrow! Perhaps WKLB could even be set at the centre of a super roundabout - visible to all but accessible only to the foolhardy.

The tunnel possiblties are endless! Thank you (and I haven't even begun to consider the flyover option yet) though I still kinda think a simple bypass is the answer.