The A48 is a major road extending from a junction with the London to Fishguard A40 trunk road (nprn 419867) at Highnam in Gloucestershire to Carmarthen. Historically it was the most important route into and across south Wales and follows closely the original Roman trade route.

On designation in 1922-3 the A48 ran between a junction with the A44 in Worcester via Great Malvern, Ledbury, Ross on Wye, where it crossed the A40, Monmouth, Usk, Newport, Cardiff, Cowbridge, Bridgend, Port Talbot, Neath, Morriston, Loughor, Llanelli, Kidwelly and Carmarthen where it joined the A40. The A48 crossed the Wales - England border two miles north of Monmouth.

Following re-routing in about 1935, the northern end of the A48 was moved east to a junction with the A40 at Highnam, Gloucester (1) from where it took over the route of the A437 via Lydney, Chepstow, crossing into Wales at Chepstow Bridge (nprn 43154); it now crosses via a modern bridge at ST 53950 94100 and continues via Newport, Cardiff, Cowbridge, Bridgend, Port Talbot, Neath, Morriston, Pontarddulais and Carmarthen where it rejoins the A40. The western end was re-routed between Morriston and Carmarthen via Pontarddulais around 1935. There are several re-alignments and improvements,  by-passes (e.g at Baglan and Cowbridge) and new dual carriageway sections such as that to the east of Carmarthen and round Newport. The road is now largely superceded by the M4 Motorway (nprn 417517) as a main route across south Wales.

(1) The A40 was re-routed at the same time to take over the former A48 between Ross on Wye, Monmouth and Raglan. (see nprn 419867)

C.Parry & B.A.Malaws, RCAHMW, 18 January 2011 and 26 December 2022.