A public inquiry into the planned redevelopment of roads around Stonehenge is due to open on Tuesday.
Before (top) and after, as envisaged in the plan
Under the proposals, the A303 would be widened and re-routed under the stones in a 2.1km bored tunnel, a scheme supported by the government.
The National Trust says it supports the tunnel idea, but wants it to be longer.
It is concerned about plans to site exits on "archaeologically sensitive ridgelines" and says the landscape would be impacted by traffic.
Martyn Heighton, from the Trust, said: "The inquiry needs to consider whether the proposed tunnel is long enough to conserve and enhance Stonehenge's spirit of place, landscape and archaeological interest."
Under the proposals, a Bronze Age earthwork which formed the ceremonial approach to the stones would be severed by the A303, claims the Trust.
Work will start by 2005 if the scheme is approved.