The M4, originally the London-South Wales Motorway, is a motorway in the United Kingdom running from west London to southwest Wales. The English section to the Severn Bridge was constructed between 1961 and 1971; the Welsh element was largely complete by 1980, though a non-motorway section around Briton Ferry bridge remained until 1993. On the opening of the Second Severn Crossing in 1996, the M4 was rerouted over it. The line of the motorway from London to Bristol runs closely in parallel with the A4. After crossing the River Severn, toll-free since 17 December 2018, the motorway follows the A48, to terminate at the Pont Abraham services in Carmarthenshire. The M4 is the only motorway in Wales apart from its two spurs: the A48(M) and the M48. The major towns and cities along the route—a distance of approximately 189 miles (304 km)—include Slough, Reading, Swindon, Bristol, Newport, Cardiff, Bridgend, Port Talbot and Swansea.
