A303 improvements part of £15bn 'roads revolution'
Improvements to the A303 form part of a £15bn "roads revolution" announced by the prime minister.
David Cameron said the investment would see "hundreds of extra lane miles on our motorways and trunk roads".
Mr Cameron said years of stalling improvements to arteries like the A303 had "held parts of our country back".
The A303 is the shortest route from London and the South East to the South West, but it is known for its "notorious" congestion black spots.
It is 92 miles (148km) long and runs through five counties - Hampshire, Wiltshire, Dorset, Somerset and Devon.
"This will be nothing less than a roads revolution - one which will lead to quicker journey times, more jobs, and businesses boosted right across the country," said Mr Cameron at the Confederation of British Industry annual conference in London.
He told business leaders updating the UK's infrastructure was a key part of the government's long-term economic plan and would see investment in more than 100 projects by the end of the decade.
Mr Cameron said the Autumn Statement - due in three weeks' time - would have "at its heart the biggest, boldest and most far-reaching road improvement programme in four decades".
The programme of investment in the road network will include improvements to:
• The A303 to the South-West
• The A1 north of Newcastle
• The A1 Newcastle-Gateshead western bypass
• Trans-Pennine roads in the north of England,
• The A47 in the east of England
• The A27 on the south coast