| 1977 – present|
| Castleton, Newport|
Another motorway, the 6-mile (9.7 km) Port Talbot bypass which opened in 1966, was numbered A48(M) before its incorporation into the westward extension of the M4 in the 1970s.
Some maps show the Morriston bypass section of the M4 as also having been originally numbered A48(M), although whether this number was ever used on the ground has been questioned.
A48(M) motorway Wikipedia
The A48(M) is a motorway in Wales between Cardiff and Newport. It is a spur off the M4 into eastern 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.
It opened in 1977, originally as an ending for the M4. The M4 was subsequently extended from junction 29 in 1980 and this section of road was renumbered A48(M). It has been suggested that its continuation westwards was projected to be from St Mellons.
The A48(M) has the unusual feature of having no junctions. At both ends it leads into limited-access junctions. It is also the only motorway entirely in Wales, as the M4 and M48 cross into England.