The A48 is a major trunk road in Great Britain. It runs from the A40 at Highnam  west of Gloucester to the A40 at Carmarthen. Before the construction of the first Severn Bridge in 1966 it was the principal route between South Wales and South West England. For most of its journey through South Wales, it runs almost parallel to its successor. Before the construction of the Second Severn Crossing, during times of high winds at the Severn Bridge the A48 was used as part of the diversion route, and is still marked as a Holiday Route.