The main issue isn't the rubbernecking. It's that a largely dual carriageway A303 is reduced to single carriageway at this point. Similar jams occur frequently at other points along the road where this happens.
True, but the other direction is simply the rubber necking as it’s single lane on the approach. Plus in the direction you are stating, there’s a clear backlog from directly next to the henge - 20mph traffic until the instant people are past where it heads straight back to 60mph. A hedge can only but help that situation.