Upgrades to some of England's busiest roads, including a controversial tunnel past Stonehenge, could be scrapped because they are not value for money.
The National Audit Office (NAO), which scrutinises government spending, said plans for 16 routes were on hold.
The schemes being "reconsidered" to make sure they are "deliverable and affordable" are part of an £11.4bn investment announced in 2014.
Neither the NAO or Department for Transport have said which are at risk.
Highways England, which is responsible for major roads, has identified the 16 schemes that could be cancelled, delayed or redesigned, the NAO said.
The NAO concluded that several problems were created by the plans being drawn up in just 17 months in order to publish them before the May 2015 general election.
The equivalent rail planning takes about 30 months.
The spending watchdog also claimed the Department for Transport (DfT) chose projects without knowing enough about whether they represented the best use of taxpayers' money.
The Five-year Road Investment Strategy featured 112 major projects to begin before 2020, such as smart motorway upgrades and converting single-carriageway A-roads to dual carriageways.
The overall plan includes:
- the tunnel at Stonehenge
- upgrading the M1 to a smart motorway around Sheffield and Rotherham
- improving electronic signage, emergency roadside telephones and CCTV cameras on the M11 between Stansted Airport and Cambridge
Edmund King, president of the AA, said motorists would be frustrated about delayed upgrades and that the public should be told which of the plans have stalled "so that they know if they are stuck on a road to nowhere".
The director of motoring research charity the RAC Foundation, Steve Gooding said: "Highways England has had a mountain to climb in gearing up for the scale of work involved, so it shouldn't surprise us that in some respects it is still in the foothills."
Among the major plans are:
- North West: Improving links to the Port of Liverpool
- North West and Yorkshire: Completing the "smart" lane on the M62 from Manchester to Leeds
- North East: £290m to make the A1 dual carriageway all the way from London to Ellingham, 25 miles from the Scottish border
- Midlands: Improving the M42 to the east of Birmingham
- East: £300m to upgrade the east-west connection to Norfolk, by making sections of the A47 dual carriageway and improving its connections to the A1 and A11
- South West: £2bn to make the entire A303 and A358 to the region dual carriageway, including a tunnel at Stonehenge
- London and South East: Improving one-third of the junctions on the M25, £350m of improvements to the A27 along the south coast
A further 54 schemes are scheduled to start in 2019-20, which would cause significant disruption to motorists, increase prices and put pressure on resources at Highways England, the NAO said.
Its report stated that by August last year the amount by which forecast costs exceeded available funding had reached £841m.
But Highways England chief executive Jim O'Sullivan said he was confident the organisation would complete the work without overspending, and a "delivery plan" would be published in June.
The DfT said it would consider the findings of the report.