The M48 crosses the River Severn alongside the M4. It's an oxbow lake of motorway, being the old route of the M4 across the original Severn Bridge. When the Second Severn Crossing was built, the M4 moved across there, and the M48 took its place.
Despite being effectively bypassed, this little motorway does have a life of its own. It has two of its own junctions, one in England for the A403, making it an easy way into Wales from the industrial areas of Avonmouth, and one in Wales serving Chepstow and the A466. As well as carrying local and commuter traffic, it's also an important diversion route in the event that the Second Severn Crossing is closed.
The Severn Bridge itself makes a journey on the M48 more than worthwhile. The bridge is spectacular. It's not as big as the Second Severn Crossing, whose epic scale makes it a wonder in its own right, but it is particularly handsome and was always far more famous than its newer cousin. The M48 also crosses the Wye Bridge immediately to the west of the Severn, which would - anywhere else - be a famous landmark in its own right. It's a fine cable-stayed bridge that crosses the River Wye and, with it, the border between England and Wales.
Nonetheless, now that it's not the main road any more, it's not a priority for funding. When the motorway's number changed, money was saved on new road signs by just blanking out the first digit of the junction number - the former junction 21 becoming 1 and 22 becoming 2.
The M48 may not be correctly numbered. Despite connecting to the M4 at each end, it lies north of the M4 and west of the M5, so according to the rules established for motorway numbers in the late 1950s, it should really have a number beginning with a 5. It may be that the number M48 was chosen because it connects to the M4 at each end and runs parallel to part of the A48, so while its number may be technically in error, it looks perfectly at home. This numbering oddity is explored in considerable depth in CBRD's pages on Road Numbers.