Plans for a tunnel near Stonehenge will “enhance and protect” the ancient site but need further work, heritage experts have warned.
Initial designs for the scheme have been unveiled for the first time as a major public consultation starts.
Highways England said the design would “restore the tranquil environment and setting of the monument”.
Opponents have said the tunnel could destroy archaeological treasures and scar the landscape irreparably.
The tunnel, part of the £1.6bn upgrade of the A303 in Wiltshire, aims to “remove sight and sound of the road” as it passes the stone circle and will be dug largely along the existing route.
The location has been chosen to avoid monuments and ancient burial mounds as well as minimise intrusion on views of the winter solstice from Stonehenge.
The National Trust, English Heritage and Historic England welcomed “improvements” to the plans that increase the length of the tunnel with a grass-covered canopy at the western end.
But they urged for the bridge to be widened to form a visual and physical link between monuments in the landscape.
They also raised concerns about a proposal to link two byways, introducing a new route for vehicles close to Stonehenge after the tunnel is built.
A statement by the three bodies said: “This is a once-in-a generation opportunity to reunite this ancient landscape, giving people the opportunity to tread pathways used by our ancestors who built the monuments, to visit and appreciate the monuments and see and hear wildlife without the intrusion of the traffic and noise from the road. “
On Monday experts warned the tunnel could destroy Blick Mead, an archaeological site dating back to the Ice Age.
University of Buckingham archaeologist David Jacques, who is excavating the site, said: “It seems blatantly obvious the Stonehenge landscape is unutterably precious, and if you tamper with it, you are not going to get it back.”
Highways England chief executive Jim O’Sullivan said: “These upgrades in the south west will improve millions of journeys.”
The public’s feedback is sought before the scheme is finalised.
The consultation runs until 6 April.