The Southern Distributor Road (SDR) is a principal distributor road in Newport that runs from the Coldra roundabout (M4 junction 24) in the east of Newport to Tredegar Park M4 Junction 28) in the west of Newport and includes City Bridge, a bow-string arch bridge spanning the River Usk.

Combined with the M4 in the north, the SDR forms the southern part of a ring road for the city.

The SDR plans were drawn up to make use of this land and to join the two sections together across the river to form a continuous dual carriageway from one side of the city to the other.

On completion it assumed the number A48 replacing the Chepstow Road/Cardiff Road corridor which was downgraded to B4237.

The new road was designed to improve the environment in the city centre, taking traffic away from residential areas, and improving access to industrial areas in the east and south of the city.