The A48 is a major trunk road in Great Britain. It runs from the A40 at Highnam west of Gloucester to the A40 at Carmarthen. Before the construction of the M4 motorway and the first Severn Bridge in the mid 1960s it was the principal route into south Wales. For most of its journey through 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. Continuing west along the South Wales coast, it connects Chepstow (also bypassed), Newport, Cardiff, Cowbridge, Bridgend, Pyle, Port Talbot, Neath, Swansea before terminating at Carmarthen.
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).