A road tunnel under Stonehenge is one of nine proposals with an estimated cost of £1.2 billion to be scrutinised by Whitehall officials to ease the traffic nightmare on the A303.
The Department for Transport has published details of a study, to be completed this summer, laying bare for the first time the scale of potential improvements on the second main road connecting London and the South West.
Officials will be looking at the entire 110-mile "A303-A30-A358 corridor" that starts near Exeter in Devon and finishes close to Andover in Hampshire.
The A358, linking the A303 at Ilminster with the M5 near Taunton, is also under consideration for an upgrade.
Nine blackspots on the so-called "Highway to the Sun" will be given special attention, notably where the dual carriageway narrows to a single lane.
The proposed schemes are based on previous studies, and have been championed by a consortium of South West authorities, which is where the estimated costs have been drawn from. They are:
12 km on A303 Amesbury to Berwick Down, Stonehenge - £536m.
3.9 km on A303 Wylye to Stockton Down - £32m.
12km on A303 Chicklade Bottom to Mere - £146m.
5.5km on A303 Sparkford to Ilchester - £47m.
A30 Podimore Roundabout - £24m.
A303 Cartgate Roundabout - £28m.
10km on A303 South Petherton to Southfields, also known as the Ilminster bypass - £67m.
14km on A358 Southfields to M5 junction 25 - £275m.
23km A303 Southfields to Honiton - £50m.
With ministers anxious to make a decision on which of six proposed road-building schemes across the country to go ahead with at George Osborne's autumn statement this year, the report acknowledges it "may not be possible to develop full strategic outline business cases for each of the prioritised infrastructure proposals".
The comment may raise alarm that a once-in-a-generation opportunity risks being squandered.
Plans for a road tunnel beneath Stonehenge, a world heritage site, and a bypass have been proposed before but were dropped in 2007 due to the estimated cost.
The "scoping" report also warns "great weight should be given to conserving landscape and scenic beauty in nationally designated areas", underlining concern over the impact on Stonehenge and the Blackdown Hills in Somerset.
A DfT spokesman said six studies would "develop solutions to some of the most notorious and long-standing hotspots on our road network", adding: "Following discussion and input from stakeholders, the scope of work for each study which includes the A303, has been finalised and work is now progressing with analysing the evidence for each route."