Motorways in UK: M4 (London-South Wales)

M4 is one of the major motorways in United Kingdom, London to Bristol and on to South Wales. 

This southern east-west motorway passes through fourteen counties: Greater London, Berkshire, Buckinghamshire, Wiltshire, Gloucestershire, Monmouthshire, Newport, Cardiff, Vale of Glamorgan, Rhondda Cynon Taff, Bridgend, Neath Port Talbot, Swansea and Carmarthenshire.

The section of the motorway within England has 123 miles (198 kilometers) and the section within Wales has 76 miles (122 kilometers). The maintenance of the English section is the responsibility of the Highways Agency, while the Welsh section is the responsibility of the Welsh Government. 

The English section was constructed between 1965 and 1971, while the Welsh section was completed in 1993. A new Severn bridge, known as the Second Severn Crossing, was opened in 1996 with the M4 was rerouted to use it.

After a suspension bridge over the River Severn was constructed in 1966, a new Severn bridge was opened thirty years later, in 1996, with the motorway rerouted to use it. 

You can find here a list of M4 motorway junctions and service areas. http://www.12go.co.uk/motorways.cfm?motorway=M4 

M4 is one of the three motorways that cross the River Thames, going upstream, the other being the M25 (the London orbital motorway) and the M2 (London-Southampton).