Professor Stephen Glaister of Imperial College advocated road pricing to fund projects.
"We accept tunnels for rail, but not for roads. Is it worth doing, and who will pay? Road pricing solves both problems," he said.
Former transport minister Stephen Ladyman had ordered an investigation into alternatives to the tunnel when costs soared .
The 2.1km bored tunnel would have taken the A303 under its current alignment, freeing Stonehenge from the pressures of passing traffic.
There were strong environmental arguments both for and against the scheme.
"Due to significant environmental constraints across the whole of the World Heritage Site, there are no acceptable alternatives to the 2.1km bored tunnel scheme," said Harris.