The Amesbury and Berwick Down project – which involves building a tunnel beneath the Stonehenge World Heritage Site – is at an early stage but there are risks and uncertainty around it being delivered on time and achieving the benefits government hopes it will bring, according to today’s report by the National Audit Office.

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 (the Department) aims to upgrade the entire A303/A358 to dual carriageway over the 14 years to 2029 through eight individual projects and allow mile a minute journey speeds along the corridor.

The Amesbury and Berwick Down project involves building a tunnel of 3.3km beneath the Stonehenge World Heritage Site. The Department and Highways England expect the tunnel to reduce congestion, support economic growth and improve the setting of the World Heritage Site. It is forecast to cost between £1.5 billion and £2.4 billion, subject to funding approval from HM Treasury and the outcome of commercial negotiations with contractors, with a likely cost of around £1.9 billion (including VAT), and to 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. 73% of total benefits are cultural heritage benefits from removing much of the surface road from the World Heritage Site. However, these benefits are based on asking the public how much they would pay to have the road removed from the World Heritage Site and as such are inherently uncertain1.

Highways England believes there are significant additional benefits if all eight projects, covering the entire road corridor, are completed and that this will maximise its return on investment on the Amesbury to Berwick Down project. The Department intends to approve each project on its own business case and has committed to start two other projects alongside the Amesbury to Berwick Down project by March 2020. However, it considers the remaining five projects to be low to poor value for money. Highways England may therefore struggle to justify future investment if they are assessed on an individual basis. 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. It was initially going to be publicly financed, but in October 2016 HM Treasury instructed the Department to use private finance, delaying the planned start of construction from March 2020 to December 2021. In October 2018, the Chancellor of the Exchequer cancelled future private finance deals, which included the Amesbury to Berwick Down project. At February 2019, Highways England had spent £53 million on the project. HM Treasury has granted it a further £21.5 million of funding for pre-construction works. The government says it remains committed to the project, but it is not clear how the project will be funded.

There are risks that Highways England and the Department will need to manage to ensure the project has a realistic chance of delivering value to taxpayers. Highways England is still working to an open date of December 2026 despite delays to the project, resulting in a very tight construction timetable. There are also geological and archaeological risks2. While Highways England is working to lessen these risks, it also needs to make sure it can support the project throughout its life; the operation, maintenance and renewal costs are expected to be £524 million (2016 prices) over 60 years.

Previous attempts to construct a tunnel have been cancelled due to escalating costs and disagreements between stakeholders. Highways England has managed to gain agreement in principle from key stakeholders such as the National Trust and Historic England but other bodies, including the UNESCO World Heritage Committee, have raised concerns.

The NAO recommends that Highways England and the Department keep in view the open to traffic date of December 2026 to ensure it remains realistic. They should also engage effectively with other government departments and stakeholders to ensure all the expected benefits of the project are delivered.

"The tunnel at Stonehenge is currently only just value for money by the Department’s own business case. Based on experience, project costs tend to grow rather than fall, at least in the early years. It will take a very special effort by the Department to protect public value up to completion."

Amyas Morse, the head of the NAO

Read the full report

Improving the A303 between Amesbury and Berwick Down

Notes for editors

Key facts

£1.5 to £2.4bn
Estimated cost range (2016 prices) to build Amesbury to Berwick Down project (which includes the tunnel under Stonehenge)

estimated maintenance costs (2016 prices) of Amesbury to Berwick Down project over 60 years 3.3km current length of proposed tunnel under Stonehenge

the cultural heritage benefits as a percentage of total monetised project benefits

latest benefit-cost ratio for Amesbury to Berwick Down project, excluding wider economic impacts

December 2026
estimated date for the Amesbury to Berwick down project to be open to traffic

number of projects needed to complete the A303/A358 road corridor works

intended completion date for all eight projects along the A303/A358 road corridor


Notes for Editors

  1. Highways England estimated the value of heritage benefits by asking respondents in a survey how much they would be willing to pay to remove the road from the World Heritage Site. While it followed HM Treasury guidance in arriving at this estimate, these benefits are uncertain because they are based on a hypothetical situation and are difficult to measure. The Department questioned whether people might have responded with a higher value than they would have done in a real-life decision-making scenario and advised decision makers to treat them cautiously.
  2. Highways England has undertaken archaeological surveys to avoid disturbing archaeological sites. However, there remains a risk that unidentified sites may be discovered during construction, which may cause delays and increase costs. In 2007, the government cancelled the planned tunnel project at Stonehenge because of unforeseen difficulties in the construction work. These included the presence of phosphatic chalk, which can be difficult to drill through and dispose of, which increased expected costs. Highways England has undertaken preliminary ground investigations, but it remains possible that there are further phosphatic chalk deposits or unstable cavities, which are unlikely to be identified before construction begins. See paragraph 3.11
  3. Press notices and reports are available from the date of publication on the. Hard copies can be obtained by using the relevant links on our website.
  4. The National Audit Office scrutinises public spending for Parliament and is independent of government. The Comptroller and Auditor General (C&AG), Sir Amyas Morse KCB, is an Officer of the House of Commons and leads the NAO, which employs some 785 people. The C&AG certifies the accounts of all government departments and many other public sector bodies. He has statutory authority to examine and report to Parliament on whether departments and the bodies they fund have used their resources efficiently, effectively, and with economy. Our studies evaluate the value for money of public spending, nationally and locally. Our recommendations and reports on good practice help government improve public services. Our work led to audited savings of £741 million in 2017.

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