LONDON — Stonehenge, the prehistoric monument in England that draws hundreds of thousands of visitors each year, is closer to getting a traffic tunnel nearby to “enhance and protect” the tranquil environment of the ancient landscape.

Initial designs for the project at Stonehenge, a World Heritage Site, were unveiled on Thursday. They are part of a project set to cost 1.6 billion pounds, or about $2.2 billion, to upgrade a highway near the monument in the county of Wiltshire, in southwest England.

The designs address concerns that the tunnel portals and deep cuttings would scar the landscape and cause irreparable damage. Heritage groups had also been concerned that the proposed tunnel exit would be too close to the Normanton Down Barrows, a collection of tombs that is a part of the Stonehenge landscape.

In the proposal, the latest and most significant one put forward by Highways England, the British government organization overseeing the project, the tunnel would be longer than previously envisaged: 1.8 miles.