It is responsible for managing motorways and trunk roads in South Wales on behalf of the Welsh Government.

The agent manages the motorways and trunk roads in the fourteen principal areas of the south of Wales, from the Severn Bridge in the east to Milford Haven in the west.

Prior to the South Wales Trunk Road Agency being established, motorways and trunk roads in Wales were managed by the Ministry of Transport, later being taken on by the Welsh Office.

The National Assembly for Wales took responsibility for devolved powers on 1 July 1999,[2] as part of this process, transport was transferred from the Parliament of the United Kingdom to the National Assembly for Wales and with it responsibility for the trunk road network, including motorways.

[4] The South Wales Trunk Road Agency (SWTRA) acts as Agent Highway Authority for the Welsh Government Neath Port Talbot County Borough Council manage and maintains the trunk road network on behalf of the Transport and Strategic Regeneration division of the Welsh Government for SWTRA.