A new report claims that there is uncertainty over plans to dual the A303 and build a tunnel under Stonehenge.
The Amesbury and Berwick Down road improvement project, which involves building a tunnel beneath the World Heritage Site, is still at a very early stage.
However a report by the National Audit Office claims that there are risks and uncertainty around it being delivered on time and achieving the desired benefits.
The project forms part of the A303 and A358 road corridor which links the South East and South West of England.
The Department for Transport plans to upgrade the entire A303/A358 to a dual carriageway by 2029.
It is made up of eight individual projects and allowing cars to travel 60mph along the corridor.
The scheme also involves building a 3.3km tunnel beneath Stonehenge.
The Department for Transport and Highways England hope the tunnel will reduce congestion, support economic growth and improve the setting of the World Heritage Site.
It is expected to cost between £1.5 billion and £2.4 billion and could open to traffic by December 2026.
The project is only estimated to deliver £1.15 in benefits for every £1 spent, in part due to the high cost of building a tunnel.
Almost three quarters of these benefits will come from removing the road around the World Heritage Site.
However the National Audit Office says these benefits are based on asking the public how much they would pay to have the road removed making any total uncertain.
Highways England says there are significant additional benefits if all eight projects are complete. Adding it hoped that this would maximise its return on investment on the Amesbury to Berwick Down project.
The Department for Transport says it will approve each proposal on its own merits and has committed to start two other projects by March 2020.
The National Audit Office says that if it does not upgrade the whole corridor, it will not be able to help unlock the full growth potential in the South West.
The Amesbury to Berwick Down project has been delayed because of decisions about how it will be funded.
Previous attempts to construct a tunnel have been cancelled due to escalating costs and disagreements between stakeholders.
The National Trust and Historic England have agreed to the idea in principal but other bodies, including the UNESCO World Heritage Committee, have raised concerns.
The National Audit Office recommends that Highways England and the Department for Transport stick to the opening date of December 2026 to ensure it remains realistic.
- £1.5 to £2.4bn - estimated cost range (2016 prices) to build Amesbury to Berwick Down project (including a tunnel under Stonehenge)
- £524m - estimated maintenance costs (2016 prices) of Amesbury to Berwick Down project over 60 years
- 3.3km - current length of proposed tunnel under Stonehenge
- 73% - the cultural heritage benefits as a percentage of total monetised project benefits
- 1.15:1 - latest benefit-cost ratio for Amesbury to Berwick Down project
- December 2026 - estimated date for the Amesbury to Berwick down project to be open to traffic
- 8 - number of projects needed to complete the A303/A358 road corridor works
- 2029 - intended completion date for all eight projects along the A303/A358 road corridor