They could divert the dual carriageway from the countess roundabout North to the pack horse way past Lark hill camp north of the Stonehenge car park and rejoin the A303 at the roundabout at the western end of the single carriage way.
Oh no they couldn't. The traffic in that area has increased significantly with Army Re Basing and the housing developments to the east of Larkhill. The Garrison has also been extended from its core to the West with new entrances and exits.