After running astride the M25 to cross the Thames on a bridge designed by Lutyens, the Runnymede Bridge, the A30 runs parallel to but distant from the M3 until southwest of Basingstoke, bypassing Egham and passing through heathland and Sunningdale, Bagshot bypass, and Camberley where the route almost mirrors the Devil's Highway, a stone (stane) street to Calleva Atrebatum (Silchester Roman town), believed to be older still, then passes close to Hook town centre and in the surrounding country the soil is arable.

[a] From Sutton Scotney village the A30 runs parallel to the latter road as-the-crow-flies 85 miles (137 km) to north-east of Honiton, Devon passing through towns Stockbridge (where it meets its first substantive river since the Thames, the Test) and its trout fishing centres, Shaftesbury, Sherborne, Yeovil, Crewkerne and Chard.

West of Exeter, the A30 is dual carriageway through Devon and into Cornwall, bypassing Whiddon Down, Okehampton and Launceston.

A customary route was long established, even prior to the appointment of Brian Tuke as Master of the King's Posts in 1512, and in William Harrison's Description of England in Holinshed's Chronicles in 1577 this is described as a route from London, through Chard, Honiton, Exeter, Crockernwell, Okehampton, Launceston, Bodmin, and on through Truro.

[7] In 1574, Elizabeth I's Master of Posts Thomas Randolph was given an Order of Council to establish permanent posts on the route between the royal court and Exeter "for the speedy conveyance of all such packets as shall be sent from the Earl of Bedford out of the West Country", with then Earl of Bedford being Francis Russell, later Lord Lieutenant of Devon, based at Tavistock.

[9] The route described by Ogilby started at Hyde Park Corner, and closely mirrored the modern route as far as Exeter, except for three sections from Knightsbridge to Bedfont, Basingstoke to Salisbury via Andover and Exeter to Penzance via Ashburton, Plymouth, and following the Cornish south coast via St Austell.

[13] At the turn of the 19th century, William Hanning created the "New Direct Road", a fast coaching route between London and Exeter.

[17] The running of the Causeway turnpike was overseen by the winner of a public auction and for the year 1880, the winning bid was £591 10s.

In the mid-1960s, numerous councils complained that the Secretary of State for Transport, Barbara Castle, decided that improvements to the A38 from Exeter to Plymouth were of higher priority for funding than any work on the A30.

[25] Following World War II, the Ministry of Transport planned a large-scale upgrade of the A30 across south-west England, with the eventual intention that most of the route would be at least dual-carriageway.

[28] In 1971, the Secretary of State for the Environment, Peter Walker announced many upgrades of the A30 across Devon and Cornwall, identifying the section from Okehampton to Bodmin as a key area of improvement.

The Okehampton bypass, which opened on 19 July 1988, goes to the south of the town, cutting through the northern edge of Dartmoor National Park in Devon.

[35] There were several protests by environmentalists during construction and the particular nature of the DBFO scheme, with a long-lasting occupation of sites on the planned route, focused around Fairmile.

In 2016, President of The Automobile Association, Edmund King, claimed that the action had led to a slowdown in road construction throughout Britain.

[36][37] During 2006 one of the main bottlenecks on the road was removed when the Merrymeet roundabout between Okehampton and Exeter near Whiddon Down was replaced with a grade-separated junction and dual carriageway.

[38] Since the Bodmin to Indian Queens project was completed in late 2007, the new dual carriageway runs to the north of Goss Moor.

[41] On 17 January 2008, British Airways Flight 38 crash-landed near the Great South West Road southeast of Heathrow Airport.

[42] Shortly before the crash landing, the captain of the Boeing 777 involved was able to clear the A30 by raising the flaps, saving the lives of motorists on the ground.

[43] In 2014, the A30 was identified as one of several key routes in the Government's Road Investment Strategy, turning it into a strategic corridor for southwest England.

[44][45] In 2022, the casket of Queen Elizabeth II was driven partially on this road en route to Windsor Castle, her final resting place.

[48] The preferred route was announced in July 2017,[49] and on 6 February 2020, the Secretary of State for Transport approved Highways England's application for a Development Consent Order for the scheme to be constructed.

[53] Arthur Boyt, focus of BBC documentary The Man Who Eats Badgers, described the A30 near Bodmin Moor as a good road for finding roadkill.

[54] In Monty Python's Flying Circus, episode 34: The Cycling Tour, Mr Pither laments "As I lay down to the sound of the Russian gentlemen practising their shooting, I realised I was in a bit of a pickle.