Benefits range from reduced journey times and vehicle operating costs (£700 million) to combined CO2e (carbon dioxide equivalent) savings of around £95m.

Commissioned by Dumfries and Galloway, South Ayrshire and Mid and East Antrim Councils and undertaken by independent transport consultancy Sweco, the Strategic and Economic Impacts Report looks at seven options - from bypasses of key towns and rail improvements to full dualling.

The roads - which run from Ayr to Stranraer in Dumfries and Galloway and from Stranraer to Gretna - are mainly single carriageway, with heavy goods vehicles (HGVs) restricted to a 40mph speed limit causing heavy tailbacks.

Congestion is commonplace when the routes go through towns and villages, and on southern stretches of the A77 landslides are frequent.

The A75 and A77 form part of what the Union Connectivity Review terms the North Channel Corridor, citing them as “critical for connectivity for passengers and freight between England, Scotland and Northern Ireland”.

The review recommended the UK Government offer the Scottish Government funding to upgrade the A75, and encouraged the Scottish Government to improve the A77.

And the three councils are calling for urgent action from the Scottish and UK Governments to transform “these vital conduits for communities and commerce”.

The Strategic and Economic Impacts Report is supplemented by detailed appendices and a complementary Economic and Location Impacts Report, which describes what effect A75/77 transport investment would have on jobs and income.

The Economic and Location Impacts Report focusses on south west Scotland, outlining impacts on specific areas and particular groups of society, while also acknowledging the wider strategic impacts investment would bring to Scotland.