This bridge used to connect the local farmer to pasture either side of the motorway so is wide enough to accommodate farm vehicles and livestock. But today it is a public footpath that crosses the fields to the North to / from Mathern Mill, and skirts the golf club driving range and practice green to the South in the direction of St. Pierre and Ifton Hill.
If you are approaching from the south then follow the line of the public footpath from the designated parking (use waypoint as guide). Do not attempt to take any short-cuts across the corner of the driving range and practice areas!
The M4 construction in the 1960’s and 1970’s was considered the greatest feat of road building in South Wales since the Romans left! Making Cardiff and Newport far more accessible to England and with the addition of the original Severn Bridge (just a mile or so to the East) it cut the road distance between London and Cardiff by 18 miles and between Bristol and Cardiff by 55 miles.
In fact, it apparently stated in the Severn Bridge souvenir brochure that the M4 Severn Bridge was due to end at a roundabout at Crick on the A48 (about a mile or so to the West of this footbridge), and it was only after construction started that the decision was made to extend the road to The Coldra (now Junction 24).
This stretch beneath you was renamed the M48 in 1996 when the second Severn Crossing was opened and carried the M4.