Another motorway was numbered A48 ( M ) before its incorporation into the westward extension of the M4 in the 1970s .
Another motorway is the 6 - mile ( 9.7 km ) Port Talbot bypass which opened in 1966 .
Another motorway , the 6 - mile ( 9.7 km ) Port Talbot bypass which opened in 1966 , was numbered A48 ( M ) before its incorporation into the westward extension of the M4 in the 1970s .