Roads strategy comes as RAC Foundation predicts there could be 7m more road users in England and Wales within 20 years

More than 80 new road schemes, including a long-awaited plan to tunnel under Stonehenge, will be announced on Monday in what ministers describe as the most far-reaching roads programme in decades.

The £15bn initiative, which covers investment lasting up to 2021, will be set out in the government’s first road investment strategy, which also includes improvements to junctions on the M25, to the A27 in Sussex, to approaches to Liverpool, and to the A1 in the north-east of England. It comes as a report from the RAC Foundation predicts there could be an additional 7 million road users in England and Wales – taking the total to 43 million – within 20 years.

The most eye-catching proposal is to spend £2bn turning the A303 into a strategic corridor to the south-west, partly by building a 1.8-mile dual-carriageway tunnel at Stonehenge. The road, which links the M3 to Devon and is known as the “holiday trail”, is notorious for its tailbacks, where frustrated families queue for hours en route to the English Riviera and beyond. The improvements are designed to enable road users to drive on dual carriageway from London to within 15 miles of Land’s End.

Around £1.5bn will also be spent adding an extra lane to certain motorways, turning them into “smart motorways”, where the extra lane can be used to manage traffic, improving links between London, Birmingham, Manchester and Yorkshire.

The £15bn was first allocated in the 2013 Whitehall spending round, but Monday’s announcement sets out details about how the money will be allocated for the first time. It contains more than 100 projects, of which 84 are new.

Patrick McLoughlin, the transport secretary, said this was “the biggest, boldest and most far-reaching roads programme for decades”. Officials said there had been nothing comparable since the motorway building programme came to an end in the late 1970s.

But there is disagreement among many groups over the Stonehenge site, including archaeologists, wildlife enthusiasts, druids and drivers.

English Heritage and the National Trust have joined forces for the first time – a move greeted with deep suspicion by the Stonehenge Alliance, which includes the druids – to assess the proposals.

English Heritage is the custodian of the stone circle, and the National Trust owns thousands of acres around it; their joint aim is a pastoral setting where the relationship between hundreds of monuments, some older than Stonehenge, can be understood and enjoyed.

The Stonehenge Alliance, however, had been campaigning for a longer tunnel. Kate Fielden, an archaeologist and member of the alliance, said English Heritage and the trust should not be considering affordability, only what was best for the site. “The bottom line is that the tunnel and portals are going to be an irreversible change in the landscape. We need to think really big on this.”

John Cridland, director-general of the Confederation of British Industry, recently called the A303 “a glorified car park” and urged the government to “just get on with” widening the road.

But there are other groups that would rather the government thought again. The UK branch of Icomos, the international council which advises Unesco on world heritage sites (Stonehenge joined the list in 1986, when sorting out the road was identified as a priority) has told the Department for Transport: “Associated portals and dual carriageways could have a highly adverse impact on other parts of the world heritage landscape that cannot be set aside, however great the benefits of a tunnel.”

In a joint statement, English Heritage and the National Trust said: “The existing A303 is highly detrimental … we will continue to work with the Department for Transport to identify a solution that both improves the world heritage site and is achievable, including a tunnel option.”

Phil McMahon and Nick Snashall, archaeologists from English Heritage and the National Trust respectively, have spent weeks walking all over the site studying every hump and hollow, trying to work out where the tunnel and its four-lane-wide entrance portals and inevitable lighting would do the least damage.

A 1.3-mile tunnel would get traffic out of sight of the stones, but bring it roaring back out of the earth close to important monuments including the bronze age Sun Barrow, so called because it lines up with the stone circle at the winter solstice. Moving the tunnel exit downhill and further west would reduce the light and noise impact, but affect a cemetery of well-preserved burial mounds – which would be spared by the pig-field option, south of the current road line.

McMahon freely admits that in 2007, when the government scrapped the proposed tunnel on cost grounds, English Heritage had got it wrong: “We were mainly concerned with the view looking out from the stone circle. We now understand and take on board the immense importance of the landscape.”

Danny Alexander, chief secretary to the Treasury and chair of the cabinet committee on infrastructure, said: “The dramatic upgrade of the A303 near Stonehenge will deliver the best for the future, whilst preserving the best from the past.”

The Department for Transport said the Stonehenge scheme was “at a very early stage” because it would have to go to consultation.

The announcements include:

South-west £2bn to convert to dual carriageway the entire A303 and A358, including a tunnel at Stonehenge. This will allow road users to drive on a dual carriageway from London to within 15 miles of Land’s End.

North-east £290m to complete the conversion to dual carriageway of the A1 all the way from London to Ellingham, 25 miles from Scottish border.

North-west and Yorkshire Completing the “smart” (lane-increasing) motorway along the entire length of the M62 from Manchester to Leeds. Improvements to trans-Pennine capacity.

North-west Improving links to the Port of Liverpool.

South £350m of improvements to the A27 along the south coast.

East of England £300m to upgrade the east-west connection to Norfolk by making sections of the A47 dual-carriageway.

London and south-east Improving one-third of the junctions on the M25, Britain’s busiest motorway.

Midlands improving the M42 to the east of Birmingham, improving the connections to Birmingham airport, and paving the way for the new High Speed 2 interchange station.