The A30 begins at Henlys Roundabout, a junction with the A4 near Hounslow. It runs along the south side of Heathrow Airport, then past Ashford and Staines-upon-Thames, before reaching the M25 motorway. This first section is entirely dual carriageway.
After crossing the M25, the A30 runs parallel to the M3 all the way to Basingstoke, bypassing Egham and passing through Bagshot, Camberley, Hartley Wintney and Hook.
Just west of Basingstoke, at junction 8 of the M3, the A303 begins. The A30 runs parallel to this road all the way to just north-east of Honiton, passing through Stockbridge, Salisbury, Shaftesbury, Sherborne, Yeovil, Crewkerne and Chard. (From M3 Junction 8 to Sutton Scotney the designated A30 is subsumed into the A303 to Bullington Cross, and then follows the old A34. The old A30 persists as five miles of unclassified road between Popham Airstrip and Sutton Scotney).
Between the M25 and Honiton, the A30 is mostly single carriageway, carrying local traffic. However, there are short stretches of dual carriageway from Camberley to Basingstoke, which has a dualled inner ring road, two between Stockbridge and Salisbury (also with dualled inner ring road shared with the A36), and between Sherborne and Yeovil.
This section is a trunk road as far as Penzance. It is mostly dual carriageway, but there are several short sections of single carriageway.
To pass Exeter, through traffic can join the M5 motorway for three miles. West of Exeter, the A30 is dual carriageway through Devon and into Cornwall, bypassing Whiddon Down, Okehampton and Launceston. The dual carriageway continues through Cornwall to Bodmin Moor, where there is a two-mile stretch of single carriageway. The dual carriageway resumes until Carland Cross, after which there is a single carriageway stretch to Chiverton Cross.
From Chiverton Cross, the dual carriageway bypasses Redruth and Camborne. The A30 returns to single carriageway west of Camborne, and a mid-1980s bypass takes the road around Hayle. Between Hayle and Penzance, the A30 returns to the original route and it passes through several villages. Approaching Penzance, the A30 briefly becomes a dual carriageway once again. Once west of Penzance, the A30 becomes a more rural road running through or past several villages, before terminating at Land's End.
A large section of the A30 follows the course of the historic London – Land's End coaching road. The road appeared on John Ogilby's map of Britain in 1675, and was covered by Ogilby's later strip-maps showing "The Road from London to The Land's End in Cornwall". The coaching route started at Hyde Park Corner, closer to the centre of London than the modern A30, but broadly followed the modern route as far as Exeter, except for running from Basingstoke to Salisbury via Andover. Beyond Exeter, the route went via Plymouth and then followed the Cornish south coast via St Austell down to Penzance, some distance away from the modern A30. Ogilby described it as "The Post-Office making this one of their Principal Roads" and thought the section through Surrey and Hampshire was "in general a very good Road with suitable Entertainment". It is described as "the Great Road to Land's End" in the Magna Britannia, published in the early 19th century.
As the coaching road to Land's End was a major route, it was a popular place for highwaymen. William Davies, also known as the Golden Farmer, robbed several coaches travelling across Bagshot Heath. He was hanged in 1689 at a gallows just off what is now the A30 between Bagshot and Camberley. The Jolly Farmer pub was reportedly built near the site of the gallows, and is now a road junction.
At the turn of the 19th century, William Hanning created the "New Direct Road", a fast coaching route between London and Exeter. The road deviated from Ogilby's route running via Amesbury and Ilminster, rejoining the older road at Honiton. It became popular with postal services such as The Subscription. In 1831, a race was held between London and Exeter via the New Direct Road, which resulted in a dead heat. 170 miles (270 km) were covered in 13 hours, compared to a typical early 18th century time of four days. In response to the competition of routes, a new turnpike road was built west of Chard, avoiding the historic route to Honiton via Stockland, with several steep hills. This road met the New Direct Road near Upottery.
Historically, the route between London and Land's End was also called the "Great South-West Road". In the 21st century, the name only refers to a small section of the road near Heathrow.
The A30 was one of the first roads to be classified by the Ministry of Transport for funding in 1921. It followed Ogilby's route up to Exeter, then the basic route of the modern A30 through Okehampton, Launceston and Bodmin to the Greenmarket in Penzance, where it ended. It was extended to Land's End in 1925.
The Great South West Road section of the A30 around Heathrow had been planned as the western end of the Great West Road project, one of the first bypasses built for motor traffic. Construction began in 1914 but was quickly halted because of World War I. It resumed construction in 1919. The full route from Chiswick to Ashford was opened by King George V on 30 May 1925.
Following the construction of a bypass around Basingstoke, the route of the A30 was changed on 1st April 1933 to run by Sutton Scotney and Stockbridge, rejoining the original route at Lopcombe Corner east of Salisbury. An alternative route, the A303 was created out of existing roads at the same time between Micheldever Station and the Blackdown Hills, that followed the basic course of Hanning's New Direct Road. The A30 remained the principal route between London and Exeter, until the A303 became a trunk road in 1958, receiving central Government funding and relegating the parallel A30 to a local road.
By the mid-20th century, large sections of the A30 were struggling to cope with the increasing demands of road traffic. 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. Cornwall County Council complained that the A30 through the county was narrow and twisted, and known as the "stage coach trail".
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. The M3 motorway was planned as a replacement for the A30 between London and Popham. Following a public enquiry in 1966, the line was fixed the following year. The work was completed as far as Bagshot in 1971, then to Sunbury-on-Thames in 1974. 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 2.2-mile (3.5 km) Honiton dual-carriageway bypass opened in early December 1966 at a cost of £984,000. Between Honiton and Exeter, the A30 is dualled and at Exeter it merges briefly with the M5 motorway. The Hayle bypass was first proposed in the late 1970s. It was controversial, and Dora Russell protested against its construction. It was completed in 1985.
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. In the 1980s, the route of the bypass was the subject of a prolonged campaign from conservationists, including Sylvia Sayer, who preferred a route to the north of the town through agricultural land.
The section between Honiton and Exeter in East Devon was upgraded in 1999 to dual carriageway, giving quicker access to Exeter International Airport. This road was built under the Design Build Finance Operate (DBFO) scheme by the private consortium Connect A30, who receive a shadow toll from the Government for each vehicle travelling along the road. Archaeological investigations during the work found a Roman cavalry garrison and later settlement at Pomeroy Wood. 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. Swampy received press attention for his part in this protest. Along with other controversial road plans, including the M3 completion over Twyford Down and the Newbury Bypass, the action led to a slowdown in road construction throughout Britain.
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.
Since the Bodmin to Indian Queens project was completed in late 2007, the new dual carriageway runs to the north of Goss Moor. The previous road has been converted to a cycle lane. In December 2012 it was announced that 2.8 miles (4.5 km) from Temple to Higher Carblake would be upgraded to a dual carriageway. Building started in early 2015, and is expected to be complete by spring 2017. This work will make the A30 continuous dual carriageway between the M5 at Exeter and Carland Cross in Cornwall.
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. This includes further dual carriageway improvements east of Honiton towards the Blackdown Hills.
The stretch between Carland Cross and Chiverton Cross would have meant a continuous dual carriageway from Exeter right through to Camborne; however this was shelved in 2006 as it not considered a regional priority.