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 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.

Start

Castleton

End

St Mellons

Passes

None

Connects to

M4

Length

2 miles

Open Junctions Section
Oct 1977 Entire motorway

Exit list

Symbols and conventions are explained in the key to exit lists. You can click any junction to see its full details.

Junction   Eastbound               Westbound  
M4 J29 Casnewydd
Newport
M4 Link

M4
EAST


M4
N/A
LanesLanesLanesLanes LanesLanesLanesLanes
2 miles, 2 lanes 2 miles, 2 lanes
Llaneirwg
St Mellons
A48

A48
WEST
A48



Caerdydd
Cardiff
A48
LanesLanesLanesLanes Signs LanesLanesLanesLanes Signs
Routes

Picture credits

With thanks to Maurice Frank for information in this section.

In this section

What's new

Imperfectly Odd: Batheaston Bypass

It has viaducts, a tunnel and plenty of controversy, but the amazing Batheaston Bypass doesn’t really work. What went wrong?

South London's lost motorways

Completing the story of London's epic Ringways, we've just published the Southern Radials, five more motorways that never saw the light of day.

To the north east!

Two new additions to our collection of Opening Booklets take us to Darlington and Middlesbrough.

Share this page

Have you seen...

From War to Worboys

The tale of how British traffic signing developed between the Second World War and the mid-1960s, bringing us from a system designed at the turn of the century to the signs we still use today.

About this page

Published22 April 2017

Last updated6 June 2020