M4 motorway

M4 shield
M4
MapM4 highlighted in blue

Shown with UK motorway network
New Severn Bridge - panoramio.jpg
Route information
Part of E30
Maintained by
National Highways
South Wales Trunk Road Agent
Length189 mi (304 km)
Existed1963–present
HistoryOpened: 1959 (Chiswick Flyover), 1961 (as A4(M)), 1963, 1966 (as A48(M))
Completed: 1996
Major junctions
East endChiswick
51°29′23″N 0°16′41″W / 51.4897°N 0.2781°W / 51.4897; -0.2781 (M4 motorway (eastern end))
Major intersections
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 endPont Abraham
51°44′42″N 4°03′54″W / 51.7451°N 4.0651°W / 51.7451; -4.0651 (M4 motorway (western end))
Location
CountryUnited Kingdom
CountiesGreater 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
M3 M5

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.