Campaigners lost a High Court battle to block the road last year
Archaeology work has started ahead of the building of a relief road through protected countryside in Dorset.
An area of the Ridgeway was fenced off on Wednesday in preparation for the 50,000 sq-m excavation of the area.
The road to Weymouth and Portland will provide extra access before the area hosts the 2012 Olympic sailing events.
Environmental groups lost a High Court legal challenge last year to stop the route, which goes through an Area of Outstanding Natural Beauty (AONB).
Campaigners, including Campaign to Protect Rural England (CPRE) and Transport 2000, claimed the route will cut through the legally protected countryside and affect the ancient woodland and a nature reserve.
But the judge refused permission for a judicial review of Dorset County Council's decision to grant itself permission for the £84.5m project.
Archaeologists will recover and record anything found which is of historical importance.
Miles Butler, Dorset County Council director for environment, said: "An investigation of this size hasn't been done for many years.
"We've been told that finds of interest could include evidence of a prehistoric ritual landscape, ancient burial sites and remains related to farming from the Roman and medieval periods."
Oxford Archaeology will be carrying out the advance investigations.
David Score, Oxford Archaeology senior project manager, said: "This excavation presents a superb opportunity for archaeologists to investigate a small part of this historic area in detail.
"It has the potential to uncover evidence that will contribute to our understanding of how people used the Dorset landscape in the past."