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Route of the A48(M)

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 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, remains unresolved.

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.


Start Castleton (M4)
Finish Cardiff (A48)
Passes None
Length 2 miles
Connects to M4


Views of the A48(M) from on and off the road. If you have a photo to contribute, contact me.


The A48(M) is... a very boring two lane motorway. Sorry.
Photo by Steven Jukes


Flying over the M4, the bridge is very wide. Unlike the ghost carriageway, there's no mystery here - the most likely reason is to provide adequate forward visibility, not for any additional lanes.
Photo by Steven Jukes

Construction Timeline

When the various parts of the A48(M) were built, listed in chronological order.

Open Jct Section
Oct1977 Entire motorway

Exit List

All the junctions and destinations along the route.

Junction   Eastbound               Westbound
(M4 J29) Casnewydd
M4 Link


N/A More information about this junction
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2 miles, 2 lanes Up   Down 2 miles, 2 lanes
St Mellons

More information about this junction
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With thanks to Maurice Frank for information in this section.