M4 motorway

M4 shield

Route information
Part of
Maintained by
Highways England
South Wales Trunk Road Agent
Length189 mi[1] (304 km)
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°29′23″N 0°16′41″W / M4 motorway (eastern end))
 Junction 4b.svg UK-Motorway-M25.svg
J4b → M25 motorway
Junction 8-9.svg UK-Motorway-A308 (M).svg/UK-Motorway-A404 (M).svg
J8/9 → A308(M) motorway/A404(M) motorway
Junction 10.svg UK-Motorway-A329 (M).svg
J10 → A329(M) motorway
Junction 19.svg UK-Motorway-M32.svg
J19 → M32 motorway
Junction 20.svg UK-Motorway-M5.svg
J20 → M5 motorway
Junction 21.svg UK-Motorway-M48.svg
J21 → M48 motorway
Junction 22.svg UK-Motorway-M49.svg
J22 → M49 motorway
Junction 23.svg UK-Motorway-M48.svg
J23 → M48 motorway
Junction 29.svg UK-Motorway-A48 (M).svg
J29 → A48(M) motorway
ToPont Abraham
51°44′42″N 4°03′54″W / 51°44′42″N 4°03′54″W / M4 motorway (western end))
CountiesGreater London, Berkshire, Wiltshire, Gloucestershire, Monmouthshire, Newport, Cardiff, Vale of Glamorgan, Rhondda Cynon Taf, Bridgend, Neath Port Talbot, Swansea, Carmarthenshire

London Heathrow Airport
Port Talbot
Road network

The M4, a motorway in the United Kingdom running from west London to southwest Wales, was originally referred to as the London-South Wales Motorway. The English section to the Severn Bridge was constructed between 1961 and 1971; the Welsh element was completed in 1993. The Second Severn Crossing officially renamed the Prince of Wales Bridge, was inaugurated on 5 June 1996 by HRH The Prince of Wales and the M4 was rerouted. Apart from its two spurs—the A48(M) and the M48—the M4 is the only motorway in Wales.

From London to Bristol, the M4 runs close to the A4. After crossing the River Severn, toll-free, the motorway follows the A48, to terminate at the Pont Abraham services in Carmarthenshire. The major towns and cities along the route include Slough, Reading, Swindon, Bristol, Newport, Cardiff, Port Talbot and Swansea.


A new road from London to South Wales was first proposed in the 1930s. In 1956, the Ministry of Transport announced plans for the first major post-war road improvement projects.[2]

The Chiswick flyover, a short section of elevated dual-carriageway, not originally classed as a motorway, opened in 1959 to reduce the impact of traffic travelling between central London and the west.[3][4]

M4 bridge over the River Neath (left)
original (A48(M)) bridge (right)

The Maidenhead bypass opened in 1961 whilst J1-J5 opened in 1965. The stretch from J18 to the west of Newport was opened in 1966, including the Severn Bridge (now part of the M48). The Port Talbot by-pass, also built in the 1960s and now part of the M4, was originally the A48(M) motorway, a number now allocated to a short section of motorway near Cardiff. The Ministry of Transport originally intended that the M4 would terminate at Tredegar Park west of Newport, and following the creation of the Welsh Office that the Government became committed to a high-standard dual carriageway to Carmarthenshire.[5]

The English section of the motorway was completed on 22 December 1971 when the 50-mile (80 km) stretch between junctions 9 and 15 (Maidenhead and Swindon) was opened to traffic.[6] The Welsh section was completed in 1993, when the Briton Ferry motorway bridge opened. The Second Severn Crossing opened in 1996, together with new link motorways on either side of the estuary to divert the M4 over the new crossing. The existing route over the Severn Bridge was redesignated the M48, and the new M49 was opened to connect the new crossing to the M5.[7]

Other Languages
العربية: خط إم4 السريع
Cymraeg: M4
Deutsch: M4 motorway
español: Autopista M4
italiano: Motorway M4
עברית: כביש M4
Simple English: M4 motorway
srpskohrvatski / српскохрватски: M4 (Engleska)