The reason for the construction of the tunnel is to preserve the actual look of Stonehenge. I consider the aerial photographs taken of Stonehenge prove the damage to the historical content of Stonehenge has already been done by the service road put in, which has completely altered the outlook of the stones. The tunnel is being done to preserve the natural habitat, there is no need to build a tunnel to preserve what has already been damaged.
Regarding the time delay on passing Stonehenge, it is caused, to a degree, by rubber-necking. If a line of trees was planted between the current road and the stones, restricting the view, that would speed up the traffic movement and they would only need to put a dual carriage way by the current road, without the massive expense of digging a tunnel.
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