M4 motorway

M4
M4
	M4 highlighted in blue ; Shown with UK motorway network
	Prince of Wales Bridge looking towards Wales, 2009
Route information
	Part of E30
	Maintained by ; National Highways; South Wales Trunk Road Agent
Length	189 mi (304 km)
Existed	1963–present
History	Opened: 1959 (Chiswick Flyover), 1961 (as A4(M)), 1963, 1966 (as A48(M)); Completed: 1996
Major junctions
East end	Chiswick; 51°29′23″N 0°16′41″W / 51.4897°N 0.2781°W
	; J4b → M25 motorway; /; J8/9 → A308(M) motorway/A404(M) motorway; ; J10 → A329(M) motorway; ; J19 → M32 motorway; ; J20 → M5 motorway; ; J21 → M48 motorway; ; J22 → M49 motorway; ; J23 → M48 motorway; ; J29 → A48(M) motorway
West end	Pont Abraham; 51°44′42″N 4°03′54″W / 51.7451°N 4.0651°W
Location
Country	United Kingdom
Counties	Greater London, Buckinghamshire, Berkshire, Wiltshire, South Gloucestershire, Monmouthshire, Newport, Cardiff, Vale of Glamorgan, Rhondda Cynon Taf, Bridgend, Neath Port Talbot, Swansea, Carmarthenshire
Primary; destinations	; London; Heathrow Airport; Slough; Maidenhead; Reading; Newbury; Swindon; Bristol; Newport; Cardiff; Bridgend; Port Talbot; Neath; Swansea
Road network
	Roads in the United Kingdom; Motorways; A and B road zones;

The M4, originally the London-South Wales Motorway, is the third longest 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.