Congestion on the A303 isn't a question of rural public transport requirements; it's the major highway down to the West Country (at least for those who prefer a more scenic or romantic route to the dull M4/M5 alternative) and its shrinkage from dual carriageway to single-lane at that point is what causes the bottleneck around Stonehenge. The only alternative to a tunnel would be to widen the existing road but that would be even more destructive/disruptive. No wonder this decision's been debated and postponed almost as long as the Heathrow expansion.