M4 near Birchgrove A470 in Cardiff A48(M) in St Mellons M4 / A449 near Langstone The A48 is a trunk road in Great Britain running from the A40 at Highnam, 3 miles (4.8 km) west of Gloucester, England, to the A40 at Carmarthen, Wales.

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 since 1995, bypassing Lydney on the west bank of the River Severn.

It crosses the England–Wales border at Chepstow and continues westwards close to the South Wales coast passing Newport, Cardiff, Cowbridge, Bridgend, Pyle, Port Talbot, Neath and Swansea, before terminating at the junction with the A40 near the centre of Carmarthen.

Apart from morning and evening rush hours, the road is generally quite empty and free running, with no heavy traffic reports.

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 June 2008, the 27-mile (43 km) Gloucester – Chepstow stretch of the A48 was named as the most dangerous road in South West England.

This single carriageway had 45 fatal and serious injury collisions between 2004 and 2006, and was rated as medium risk in the EuroRAP report published by the Road Safety Foundation.