There already is a road there. That’s the whole point. It’s one of the busiest in SW England and it is only ~200m away from the monument. The whole area (I.e. the “view”) is a protected historic landscape and it is blighted by the road. The plan is expensive because it is a bored tunnel. It is where the entrances of the tunnel are that is controversial - the middle of the tunnel will be deeper than the archaeology.