The A30 is an old trunk road (main road) which runs from central London right down to Land's End, the westernmost point of the mainland England (though not Britain), and is sometimes called the Great South West Road. At 284 miles (457 km) long it is the third longest 'A' road in the United Kingdom, behind the A1 and A38. The A30 makes up part of the main freight and tourism route to South West England, and in many parts it has been dualled and also now bypasses many of the old towns through which it originally ran.

In Hampshire, Wiltshire and Dorset the road runs through many towns, such as Salisbury, Shaftesbury, Sherborne and Yeovil, so the more rural A303 road - which runs parallel to the A30 a few miles to the north, and is the trunk road between Basingstoke and Honiton - takes the bulk of traffic. The A303 is being upgraded to make it continuous dual carriageway East of Ilminster. West of Ilminster the A358 will be upgraded to the M5 motorway at Taunton. Between Basingstoke and Honiton the A30 remains, for most of the route, a single carriageway carrying only local traffic.

A major new portion (from the 1980s onwards) bypasses Okehampton and skirts the northern edge of Dartmoor in Devon, thence into Cornwall via the gateway town of Launceston and then on into Cornwall via Bodmin Moor.

Opened in August 1999 is a new dual carriageway link between Honiton and Exeter in East Devon, giving ready access to Exeter Airport. There were many protests by environmentalists against the damage caused by the building of this road, with a long-lasting occupation of sites on the planned route, focused around the village of Fairmile .