The eastern section from Denham, Buckinghamshire to Wheatley, Oxfordshire is better served by the M40 and its former function of linking London with Cheltenham and Gloucester has been taken by the M4, A419 and A417 via Swindon.

At Marble Arch it joins the A5 Edgware Road as far as the Marylebone Flyover to become Westway (classified A40(M) as an elevated motorway) then meets Western Avenue.

Becoming a dual carriageway, it passes GCHQ in Cheltenham and the three-level stacked roundabout junction with the M5 motorway.

There it connects with the M50 motorway, and forms part of the high quality dual carriageway between South Wales and the English Midlands.

The A40 is dualled for over 1.5 miles (2.4 km) as it approaches a junction east of Brecon with the A470 north, which is the main north–south road through mid-Wales.

Continuing as a dual carriageway, the A40 and A470 concurrent bypass Brecon to the south, crossing the River Usk here.

Beyond this point the A40 continues as a single carriageway, now south of the River Usk, and roughly follows the northern edge of the Brecon Beacons National Park.

The A40 continues west along the Tywi valley to Carmarthen where as a dual carriageway it forms the eastern bypass, meeting the terminus of the A48 at Pensarn.

[8] St Clears to Haverfordwest dualling There were plans in 2002 for a major improvement of the 23-mile stretch between St Clears and Haverfordwest which included upgrading to a dual carriageway; described as an extension of the national motorway network to the West Wales coast by virtue of the route from the M4 motorway being entirely dual carriageway.

Within a couple of years, the project appeared to be dying a very quiet death, causing local newspapers to report it being an election stunt for the two marginal constituencies that would best benefit from the improvements.

The political party at the centre of the row instead directed the project deferment to damning environmental statements by Friends of the Earth Cymru.

Instead, it recommended upgrades to the existing route including bypasses around Robeston Wathen and Llanddewi Velfrey using a three-lane option.

Constructed in 1994, the road started east of Black Bridge on the original A40, then running north of the town before to a new roundabout just west of Llain Cottage.

The design capacity of a single carriageway two-lane highway is more than adequate to cater with the anticipated traffic flows for the foreseeable future.

The scheme would improve the A40 between Llanddewi Velfrey and Penblewin, to the west of St Clears and meet the aim of the targeted investment in infrastructure along the east–west road corridor in south Wales.

[14] From the east, it is proposed that a roundabout would be created just north of Glenfield Farm, where the straight section from St Clears ends at Bethel Chapel, and then take a new route north-west of the town of Llanddewi Velfrey, south-west over Pentroydin Fawr and Penttroydin Fech farms with cattle underpasses, a new underpass beneath the existing Llanddewi Velfrey to Llanfallteg road, before going over the original A40 at Ffynnon Wood.

To the west of Ffynnon Wood, the road would then cross back over the original A40 to the east of Henllan Lodge in a way to maintain the tree lined avenue to Henllan, then run parallel to the A40 on the north side of the existing A40 to a new roundabout at Penblewin and the junction with the A478.

54% felt the section from Ffynnon Wood and Penblewin Roundabout needed improvement, with both orange and purple routes preferred by 42% of the respondents.

The Welsh Assembly Government explains this increase as being the result of inflation and land costs, saying the original budget did not include either, as well as additional statutory procedures, additional works following some design standards, and the increase in VAT.