The A48(M) is a little spur from junction 29 of the M4 to let traffic flow more smoothly between Cardiff and the Severn Bridges. Once upon a time, for about three years, the M4 ended here, but this little bit of road was only ever going to be a spur and its life as part of a major motorway was always meant to be short-lived. The road was laid out from the outset to allow construction of the M4 Cardiff Bypass - it was built complete with the overbridge for junction 29 in place.
One of the mysteries of this road, much discussed at Pathetic Motorways, is the A48(M)'s ghost carriageway. About half this motorway's brief mileage is spent with the two carriageways separated as it approaches the merge with the M4. But alongside the westbound carriageway, there appears to be another roadway running parallel - exactly where the eastbound side would be if it wasn't busy flying over the M4. This appeared to be evidence that the A48(M) had been originally built without provision for this junction, as it looked like an old abandoned version of the eastbound side. But that's not the case.
The official, somewhat incredible story is that this was a temporary carriageway built while the westbound side was being reconstructed. Quite why the highway authorities went to such lengths to rebuild one half of a short spur motorway, or why they would need to when the road must have been less than 20 years old at the time, isn't very clear.
The A48(M) now has an uncertain future, with plans for the new M4 alignment at Newport threatening to relegate this to a non-motorway route as part of the widespread changes the new motorway will bring.