The A48 is a major trunk road in Great Britain. It runs from the A40 at Highnam, 3 miles (4.8 km) west of Gloucester, to the A40 at Carmarthen. Before the construction of the first Severn Bridge, in 1966, it was the principal route between South Wales and South West England. For most of its journey through South Wales, it runs almost parallel to its successor. Before the construction of the Second Severn Crossing, during times of high winds at the Severn Bridge the A48 was used as part of the diversion route, and is still marked as a Holiday Route.
From Gloucester, the A48 runs through the villages of Minsterworth, Westbury-on-Severn, connects to a link road to Cinderford in the Forest of Dean then through Newnham, Blakeney and bypassing the town of Lydney (the bypass was built in the 1990s) on the west bank of the River Severn. Crossing the Wales-England border at Chepstow and continuing west close to the South Wales coast, it connects Newport, Cardiff, Cowbridge, Bridgend, Pyle, Port Talbot, Neath, Swansea before terminating at the junction with the A40 near the centre of Carmarthen.
There is a motorway section (the A48(M)) which is a spur from the M4 running from junction 29 on the west side of Newport. The A48(M) has the unusual feature of having no junction options at both ends – it leads into limited-access junctions. Near the east of Cardiff, at St Mellons, it ends by flowing onto the A48 (Eastern Avenue) and through Cardiff. It is 2 miles (3.2 km) long and is a 2-lane motorway throughout its length. At St Mellons it runs continuously into a further 6 miles (9.7 km) of the dual-carriageway A48, which also features (albeit narrow) hard shoulders. The original A48 continues to link Newport and Cardiff.
Just before Junction 44 of the M4 motorway, there is an abandoned dual carriageway trunk road, just to the left side of the motorway. This road used to have two service stations, which have been demolished.
The A48 from Highnam to Newport runs adjacent to the River Severn. It runs through a series of villages, until it reaches Chepstow where it crosses the Wales-England border. From the M4 Motorway at J15 near Swindon, traffic is directed for Wales if it is over the Severn Bridge weight limit of 44 tonnes. Traffic is directed onto the A419, then onto the A417 after Cirencester, and at Gloucester, onto the A40. This road has some speed cameras, as there have previously been incidents here, and have been erected to prevent them happening again. During busy periods, such as the Severn Bore, this road may become busy with parked cars near the river's edge. Apart from morning and evening rush hours the road is generally quite empty and free running, with no heavy traffic reports. The road also runs next to the Forest of Dean. The woods may be viewed from the roadside, as may the hills of the Dean. There is a level crossing in Lydney. Until Chepstow, there is a height limit under the low railway bridges. Tall vehicles are directed to Newport on the A40. For some parts of this route, short distance dual carriageways occur, especially on steep hills.
At Chepstow, the road links Gloucestershire with Monmouthshire. The road runs through Chepstow. There is access to the Forest of Dean in Chepstow. At the end of the road in Chepstow (at the roundabout), the Primary Route ends here, and it meets the A466, a road that provides access to the Wye Valley and to the M48 motorway, originally the M4. Access to the M4 is available on this road. The A48 becomes a secondary route here, and continues bypassing Caldicot, Caerwent and Langstone.
The A48 then continues to M4 junction 24, where vehicles exceeding the height limit may rejoin the A48. The A449 provides access to Usk and the A40 near Raglan. When the Severn Bridge is closed in bad weather conditions, the traffic is directed onto the A449. After this roundabout, the road follows through to Newport. There are some minor routes that take you to Newport Town Centre, but the main route is the A4042, leading directly to Newport town centre and Caerleon. This is after the Newport International Sports Village , a sports village with facilities including a Swimming Pool, Tennis Courts, Football Stadium, Cricket Pitch, Velodrome and many more facilities. Passing through Newport, there are views of the industrial town, with views of historic features such as the Transporter Bridge. The road then reaches the M4 again, at J28.
The original (1923) route of the A48 was Worcester to Carmarthen via Malvern, Ledbury, Ross-on-Wye, Monmouth, Newport, Cardiff, Bridgend, Neath and Llanelli. In 1935 it was rerouted east of Newport, replacing the A437 between Newport and Gloucester. The road from Worcester to Newport became part of the A449, apart from the section between Ross and Monmouth (which became part of the A40).
In June 2008, a 27-mile (43 km) stretch of the A48 between Chepstow - Gloucester was named as the most dangerous road in the South West of England. This single carriageway stretch had 45 fatal and serious injury car accident collisions between 2004 and 2006, and was rated as medium risk in the EuroRAP report published by the Road Safety Foundation.