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

motorway A controlled-access highway is a type of highway that has been designed for high-speed vehicular traffic, with all traffic flow—ingress and egress—regulated. Common English terms are freeway, motorway, and expressway. Other similar terms ...

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 Briton Ferry () is a town and Community (Wales), community in the county borough of Neath Port Talbot, Wales. The Welsh name may indicate that the church, ''llan'', is protected from the wind, ''awel''. Alternatively, ''Sawel'' may be a deri ...

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 London is the Capital city, capital and List of urban areas in the United Kingdom, largest city of both England and the United Kingdom, with a population of in . London metropolitan area, Its wider metropolitan area is the largest in Wester ...

Bristol Bristol () is a City status in the United Kingdom, cathedral city, unitary authority area and ceremonial county in South West England, the most populous city in the region. Built around the River Avon, Bristol, River Avon, it is bordered by t ...

runs closely in parallel with the A4. After crossing the

River Severn The River Severn (, ), at long, is the longest river in Great Britain. It is also the river with the most voluminous flow of water by far in all of England and Wales, with an average flow rate of at Apperley, Gloucestershire. It rises in t ...

, toll-free since 17 December 2018, the motorway follows the

A48 A48 may refer to: * A48 motorway (France), a road connecting the A43 and Grenoble * A48 road (Great Britain), a road connecting Gloucester, England and Carmarthen, Wales * Autovía A-48, a motorway under construction connecting Cadiz and Algecira ...

, to terminate at the Pont Abraham services in

Carmarthenshire Carmarthenshire (; or informally ') is a Principal areas of Wales, county in the South West Wales, south-west of Wales. The three largest towns are Llanelli, Carmarthen and Ammanford. Carmarthen is the county town and administrative centre. ...

. The M4 is the only motorway in

Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...

apart from its two spurs: the A48(M) and the M48. The major towns and cities along the routea distance of approximately include

Slough Slough () is a town in Berkshire, England, in the Thames Valley, west of central London and north-east of Reading, at the intersection of the M4, M40 and M25 motorways. It is part of the historic county of Buckinghamshire. In 2021, the ...

Reading Reading is the process of taking in the sense or meaning of symbols, often specifically those of a written language, by means of Visual perception, sight or Somatosensory system, touch. For educators and researchers, reading is a multifacete ...

Swindon Swindon () is a town in Wiltshire, England. At the time of the 2021 Census the population of the built-up area was 183,638, making it the largest settlement in the county. Located at the northeastern edge of the South West England region, Swi ...

, Bristol, Newport,

Cardiff Cardiff (; ) is the capital city, capital and List of urban areas in the United Kingdom, largest city of Wales. Cardiff had a population of in and forms a Principal areas of Wales, principal area officially known as the City and County of Ca ...

Bridgend Bridgend (; or just , meaning "the end of the bridge on the Ogmore") is a town in the Bridgend County Borough of Wales, west of Cardiff and east of Swansea. The town is named after the Old Bridge, Bridgend, medieval bridge over the River Og ...

Port Talbot Port Talbot (, ) is a town and community (Wales), community in the county borough of Neath Port Talbot, Wales, situated on the east side of Swansea Bay, approximately from Swansea. The Port Talbot Steelworks covers a large area of land which d ...

and

Swansea Swansea ( ; ) is a coastal City status in the United Kingdom, city and the List of urban areas in the United Kingdom, second-largest city of Wales. It forms a Principal areas of Wales, principal area, officially known as the City and County of ...

History

A new road from London to South Wales was first proposed in the 1930s. In 1956 the

Ministry of Transport A ministry of transport or transportation is a ministry responsible for transportation within a country. It usually is administered by the ''minister for transport''. The term is also sometimes applied to the departments or other government a ...

announced the plans for the first major post-war road improvement projects. 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. M4 River Neath Crossing

The

Maidenhead Maidenhead is a market town in the Royal Borough of Windsor and Maidenhead in the county of Berkshire, England. It lies on the southwestern bank of the River Thames, which at this point forms the border with Buckinghamshire. In the 2021 Census, ...

bypass (J7-J9) opened in 1961, the section from Slough to Maidenhead (J5-J7) opened in 1963 and J1-J5 opened on 24 March 1965 incorporating the Chiswick Flyover. The stretch from J18 to the west of Newport was opened in 1966, including the Severn Bridge. The

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 Tredegar Park () is a country park situated in the Coedkernew area of Newport, Wales. It lies close to junction 28 of the M4 motorway The M4, originally the London-South Wales Motorway, is the third longest motorway in the United Kingd ...

west of Newport, and following the creation of the Welsh Office that the Government became committed to a high-standard dual carriageway to

The English section of the motorway was completed on 22 December 1971 when the stretch between junctions 9 and 15 (Maidenhead and

) was opened to traffic. The Welsh section was largely completed between 1970 and 1980, though a non-motorway section remained across the

bridge until 1993, when a second motorway-only bridge opened. In 1996, the Second Severn Crossing opened with new link motorways on either side of the estuary to divert the M4 over the new crossing. At the same time, the original route over the Severn Bridge was redesignated the M48, and the M49 was opened to link the new crossing with the M5 at Avonmouth. The new M49 shortened the route between South Wales and the South West, and reduced traffic at the busy M4/M5 junction at Almondsbury.

After completion

In April 2005, speed checks carried out by police camera vans between junction 14 and junction 18 led to a public protest, involving a "go-slow" of several hundred vehicles along the affected sections of the motorway. Between 2007 and January 2010, the section from Castleton (junction 29) to Coryton (junction 32) was widened to six lanes. The scheme was formally opened on 25 January 2010 by Ieuan Wyn Jones the Deputy First Minister for Wales. During 2009, the Newport section of the motorway between junctions 23a and 29 was upgraded with a new concrete central barrier. In February 2010, it was proposed that the M4 in South Wales would become the first hydrogen highway with

hydrogen station A hydrogen infrastructure is the infrastructure of points of hydrogen production, truck and pipeline transport, and hydrogen stations for the distribution and sale of hydrogen fuel, and thus a crucial prerequisite before a successful commerciali ...

s provided along the route, with an aspiration for further stations to be provided along the M4 into

South West England South West England, or the South West of England, is one of the nine official regions of England, regions of England in the United Kingdom. Additionally, it is one of four regions that altogether make up Southern England. South West England con ...

over time. Between 2008 and 2010, junction 11 was extensively remodelled with a new four-lane junction, two new road bridges and other works. The £65million scheme included work on the Mereoak roundabout and part of the A33 Swallowfield Bypass near Shinfield, and also the conversion of the two existing bridges, one of which is available only to pedestrians and cyclists and the other to buses. It also involved the movement of the local

Highways Agency National Highways (NH), formerly Highways England and before that the Highways Agency, is a government-owned company charged with operating, maintaining and improving motorways and major A roads in England. It also sets highways standards u ...

and

Fire Service A fire department (North American English) or fire brigade (English in the Commonwealth of Nations, Commonwealth English), also known as a fire company, fire authority, fire district, fire and rescue, or fire service in some areas, is an organi ...

offices, and the construction of a long footbridge network, a new

bus lane A bus lane or bus-only lane is a lane restricted to buses, generally to speed up public transport that would be otherwise held up by traffic congestion. The related term busway describes a roadway completely dedicated for use by buses, whilst ...

and a new gyratory. Sound barriers for nearby residential areas were also installed. In April 2008, the decision to preserve a rare

Vickers machine gun The Vickers machine gun or Vickers gun is a Water cooling, water-cooled .303 British (7.7 mm) machine gun produced by Vickers Limited, originally for the British Army. The gun was operated by a three-man crew but typically required more me ...

pillbox and turn it into a bat roost was announced by the developers.

Former bridge tolls

Tolls were charged on the Severn Bridge(s) from opening until 2018. In 1966 the toll was 2s 6d (post-

decimalisation Decimalisation or decimalization (see American and British English spelling differences, spelling differences) is the conversion of a system of currency or of weights and measures to units related by Power of 10, powers of 10. Most countries have ...

equivalent £0.125) for cars, rising to £1 in the late 1980s. Around 1991 the toll was doubled but charged in the westbound direction only, to reduce queuing. After 1996, the tolls were equal westbound-only on both bridges, and rose steeply after 2000 to a peak of £6.70 for cars in 2017, leading to protests from Welsh businesses. Tolls on both bridges over the

were eliminated on 17 December 2018, and the former toll booths were removed in 2019.

Timeline of construction

Maintenance

Maintenance of the Second Severn Crossing and the of motorway in England is the responsibility of

National Highways National Highways (NH), formerly Highways England and before that the Highways Agency, is a State-owned enterprise, government-owned company charged with operating, maintaining and improving Roads in England, motorways and major A roads in Eng ...

. The in Wales is the responsibility of the South Wales Trunk Road Agent.

Features

Speed limits

For the majority of its length, the national speed limit applies. Exceptions include the following: * on the Chiswick Flyover within London in both directions. * between junction 4 and the Chiswick Flyover, eastbound only. * on the

elevated section between junction 40 and junction 41. The fixed speed camera was removed in 2006 as it was believed to be causing tailbacks. In July 2014, an average speed camera system was installed; it generated around £500,000 in fines in the first six months. * between junctions 24 and 28 at Newport.

Smart motorway

The first section of smart motorway on the M4, between junctions 19 (M32) and 20 (M5) north of Bristol, has variable speed limits and includes a part-time hard-shoulder. Completion was in summer 2014. Another section between junctions 24 and 29 in Newport had variable speed limits until 2021, when it was changed to a permanent limit with average speed cameras. In 2010, it was announced that the motorway would be changed to a smart motorway between junctions 3 and 12. With a length of , on completion it became the longest smart motorway scheme in the United Kingdom. Work started in autumn 2018 and was completed ahead of schedule in December 2021 at a cost of £848 million. Further conversions to smart motorways have been halted indefinitely.

Brynglas tunnels

The Brynglas Tunnels carry the M4 under Brynglas Hill in Newport. The tunnels are the first and only twin–bored tunnels in the UK motorway network (the Dartford Tunnel is not classified as part of the

M25 motorway The M25 or London Orbital Motorway is a major ring road encircling most of Greater London. The motorway is one of the most important roads in the UK and one of the busiest. Margaret Thatcher opened the final section in 1986, making the M25 th ...

). In July 2011, a lorry fire in one tunnel closed the motorway. Although there were no injuries and no deaths, the tunnel remained closed and a contraflow system was in place in the remaining tunnel for about one month, causing major travel delays.

Bus lane

In June 1999, the M4 bus lane was created on the third lane between junctions 2 and 3, initially as a pilot scheme and then a permanent arrangement from 2001. A lower speed limit was introduced along this section at the same time. The bus lane was on the eastbound carriageway; from the western end of the Chiswick Flyover near

Brentford Brentford is a suburban town in West (London sub region), West London, England and part of the London Borough of Hounslow. It lies at the confluence of the River Brent and the River Thames, Thames, west of Charing Cross. Its economy has dive ...

to junction 3 (A312), covering part of the journey between

Heathrow Airport Heathrow Airport , also colloquially known as London Heathrow Airport and named ''London Airport'' until 1966, is the primary and largest international airport serving London, the capital and most populous city of England and the United Kingdo ...

and central London. The lane which had no intermediate exits was for use by buses, coaches, motorcycles, emergency vehicles and licensed taxis but not mini-cabs. In December 2010, the bus lane was suspended for 18 months. It was reinstated temporarily for the

2012 Summer Olympics The 2012 Summer Olympics, officially the Games of the XXX Olympiad and also known as London 2012, were an international multi-sport event held from 27 July to 12 August 2012 in London, England, United Kingdom. The first event, the ...

and then permanently decommissioned.

Porous road surface

Near junction 35 of the M4, there is a stretch of the motorway that has a surfacing of porous asphalt that improves drainage and reduces noise. When driving in heavy rain drivers notice a reduction in road spray from other vehicles and improved visibility. This special surface was publicised in an episode of the BBC's '' Tomorrow's World'' programme. This was the site of the first trial of the new road surface when it was laid down in 1993.

Elevated and heated section

The elevated section of the M4 in West London, built in the 1960s, is mostly directly above the A4 and extends over parts of

's Golden Mile. This section was designed to have a heated road surface to reduce icing in winter.

Four-level stack interchanges

The M4 has two of the three four-level stack interchanges in the UK, including the first UK example at junction 20, the " Almondsbury Interchange" with the M5. The other is at junction 4b, the " Thorney Interchange" with the M25; this interchange has to make provision for a railway line passing beneath the M4. Due to the nature of these junctions, one cannot make a U-turn at either of them.

Notable junctions

Junction 8/9 near

Berkshire Berkshire ( ; abbreviated ), officially the Royal County of Berkshire, is a Ceremonial counties of England, ceremonial county in South East England. It is bordered by Oxfordshire to the north, Buckinghamshire to the north-east, Greater London ...

, and

High Wycombe High Wycombe, often referred to as Wycombe ( ), is a market town in Buckinghamshire, England. Lying in the valley of the River Wye, Buckinghamshire, River Wye surrounded by the Chiltern Hills, it is west-northwest of Charing Cross in London, ...

Buckinghamshire Buckinghamshire (, abbreviated ''Bucks'') is a Ceremonial counties of England, ceremonial county in South East England and one of the home counties. It is bordered by Northamptonshire to the north, Bedfordshire to the north-east, Hertfordshir ...

is the only one in the UK with dual numbers. This arose at the time when the M4 turned north near junction 8, where it met the A308, and headed for the original junction 9, where the motorway ended at a

roundabout A roundabout, a rotary and a traffic circle are types of circular intersection or junction in which road traffic is permitted to flow in one direction around a central island, and priority is typically given to traffic already in the junct ...

interchange with the A4. When the westward extension was opened, junction 8 was closed and a new junction built a little to the west, taking both numbers. The road to the A4 became A423(M) and later A404(M), and the junction with the A4 became 9B. Junction 9A is the exit for Cox Green and White Waltham. To the west of junction 13 on the eastbound carriageway there are a set of sliproads signposted "Works Unit Only". The signs have red borders, implying a military exit. It is a back entrance to RAF Welford, a Second World War airfield and now an RAF/

USAF The United States Air Force (USAF) is the Air force, air service branch of the United States Department of Defense. It is one of the six United States Armed Forces and one of the eight uniformed services of the United States. Tracing its ori ...

military installation mainly used for storing munitions. The M4 entrance allows easier access for the large vehicles used to carry the munitions. Plans for the "missing" Junction 31, also known as the Thornhill interchange, for which planning permission was originally granted in September 1991 (but subsequently expired), were rekindled after proposals for a new business park on a site north of the M4 were submitted in 2007 to Cardiff Council. The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469. A freedom of information request in 2010 to Cardiff Council shows that whilst the land that would enable this junction should continue to be strategically protected, the decision to formally abandon the proposed Junction 31 Thornhill was made in October 2007 and there had been no subsequent mention of it in Cardiff Council Strategic or Planning meetings since. In South Wales, the M4 has to thread its way through mountainous terrain and built-up areas, so there are some unusual junction layouts. Junction 27 (

High Cross A high cross or standing cross (, , ) is a free-standing Christian cross made of stone and often richly decorated. There was a unique Early Medieval tradition in Ireland and Britain of raising large sculpted stone crosses, usually outdoors. Th ...

) is a normal grade-separated roundabout junction, but has severe space constraints: traffic joining the motorway must initially travel in the opposite direction to the intended direction of travel, before making a sharp left-hand turn from the slip road onto the motorway. At the time of construction, junction numbers 30 and 31 were reserved for future intermediate interchanges. Junction 30 ( Cardiff Gate) has since been added, but there are no current plans to construct Junction 31 ( A469 road). Junction 39 can only be used to access the motorway from a single slip road onto the westbound carriageway from the A48 at junction 38. There is no exit from the motorway at this junction. Junction 41 comprises two different junctions; one for local traffic to and from the west and one from the east. The former leads to and from a spur leading to the roundabout in

, formerly known as junction 41a, and the original bridge over the

River Neath River Neath () is a river in south Wales running south west from the point at which its headwaters arising in the Brecon Beacons National Park converge to its mouth at Baglan Bay below Briton Ferry on the east side of Swansea Bay. Course U ...

, which would allow access onto the stretch of the M4 from junction 43 westward. The second, eastern junction leads to and from the A48 towards

. As a result, one can travel for almost on the motorway in either direction, both joining and then leaving the motorway at junction 41. Junction 44 is unusual in that the eastbound entrance dives under the inside of the junction, effectively creating a "right-turn" on a roundabout. Similarly, slip roads pass under or over the main motorway at junctions 41 and 42. There have been calls to close the

slip road In the field of road transport, an interchange (American English) or a grade-separated junction (British English) is a road junction that uses grade separations to allow for the movement of traffic between two or more roadways or highways, usi ...

s at junctions 40 and 41 to improve traffic flow. The motorway has only two lanes on this stretch and is a major traffic congestion blackspot. The short slip roads have not been modernised. A small-scale trial of closing the westbound on-slip of junction 41 between 7 am and 9 am and from 4 pm to 6 pm on weekdays started on 4 August 2014 but following heavy criticism from local businesses and residents, was stopped on 29 May 2015.

Relief road

On 5 June 2019, the

Welsh Government The Welsh Government ( ) is the Executive (government), executive arm of the Welsh devolution, devolved government of Wales. The government consists of Cabinet secretary, cabinet secretaries and Minister of State, ministers. It is led by the F ...

scrapped the proposal for a proposed motorway south of Newport. The Welsh Assembly Government had revived the scheme as a tolled bypass in 2007 and later abandoned it for financial reasons. An extension to the Newport Southern Distributor Road through the old Corus steel works was considered. This road is already a dual carriageway. A public consultation exercise on options for improving the capacity of the

M4 corridor The M4 corridor is an area in the United Kingdom adjacent to the M4 motorway, which runs from London to South Wales. It is a major hi-tech hub. Important cities and towns linked by the M4 include (from east to west) London, Slough, Bracknell, M ...

around Newport opened on 5 March 2012. Its website states that: "the motorway around Newport does not conform to today's motorway standards. It lacks continuous hard shoulders, has closely spaced junctions with sub-standard slip road visibility and narrows to a restricted two lane section through the Brynglas Tunnels. Heavy congestion occurs along this stretch and either side of it at peak hours."

Junctions

{, class="plainrowheaders wikitable" , - !scope=col, County !scope=col, Location !scope=col, mile !scope=col, !scope=col, !scope=col, Destinations (EB) !scope=col, Destinations (WB) !scope=col, Notes , - , rowspan=6,

Greater London Greater London is an administrative area in England, coterminous with the London region, containing most of the continuous urban area of London. It contains 33 local government districts: the 32 London boroughs, which form a Ceremonial count ...

, rowspan=6, London Borough of Ealing/

London Borough of Hounslow The London Borough of Hounslow ( ) is a London borough in west London, England, forming part of Outer London. It is governed by Hounslow London Borough Council. The borough stretches from near Central London in the east (Chiswick) to the b ...

London Borough of Hillingdon The London Borough of Hillingdon () is a London borough in Greater London, England. It forms part of outer London and West London, being the westernmost London borough. It was formed in 1965 from the districts of Hayes and Harlington Urban Distr ...

, style="background:#eaecf0;text-align:right;", 7.3 , style="background:#eaecf0;text-align:right;", 11.8 , style="text-align:center;", 1 , ''Road continues as '' , ''Start of motorway'' , - style="text-align:center;" , style="background:#eaecf0;text-align:right;", 8.3
8.5 , style="background:#eaecf0;text-align:right;", 13.4
13.7 , style="text-align:center;", 2 ,
, , - style="text-align:center;" , style="background:#eaecf0;text-align:right;", 12.5
12.8 , style="background:#eaecf0;text-align:right;", 20.1
20.6 , style="text-align:center" colspan=3, '' Heston services'' , - style="text-align:center;" , style="background:#eaecf0;text-align:right;", 13.4
13.7 , style="background:#eaecf0;text-align:right;", 21.5
22.1 , style="text-align:center;", 3 , style="text-align:center" colspan=2, Heathrow 20 airtransportation

(Terminals 4, 5 & Cargo), Hayes, Harrow,

Hounslow Hounslow ( ) is a large suburban district of West London, England, west-southwest of Charing Cross. It is the administrative centre of the London Borough of Hounslow, and is identified in the London Plan as one of the 14 metropolitan cen ...

A312 , - style="text-align:center;" , style="background:#eaecf0;text-align:right;", 15.2
15.7 , style="background:#eaecf0;text-align:right;", 24.5
25.2 , style="text-align:center;", 4 , style="text-align:center" colspan=2, Heathrow 20 airtransportation

(Terminals 2 & 3) (A4)

Uxbridge Uxbridge () is a suburban town in west London, England, and the administrative headquarters of the London Borough of Hillingdon, northwest of Charing Cross. Uxbridge formed part of the parish of Hillingdon in the county of Middlesex. As part ...

A408 , - style="text-align:center;" , style="background:#eaecf0;text-align:right;", 16.8
17.7 , style="background:#eaecf0;text-align:right;", 27.0
28.5 , style="text-align:center;", 4b
'' Thorney Interchange''
Thorney Interchange – Intersection of M4 and M25 , style="text-align:center" colspan=2,
, - style="text-align:center;" , rowspan=4,

, rowspan=4,

– Windsor–

boundary and

, style="background:#eaecf0;text-align:right;", 19.1
19.5 , style="background:#eaecf0;text-align:right;", 30.7
31.4 , style="text-align:center;", 5 , style="text-align:center" colspan=2,

Colnbrook Colnbrook is a village in the Borough of Slough, Slough district in Berkshire, England. It lies within the historic counties of England, historic boundaries of Buckinghamshire, and straddles two distributaries of the River Colne, Hertfordshire, ...

, Langley, Slough (East) A4
Eton, Datchet B470 , - style="text-align:center;" , style="background:#eaecf0;text-align:right;", 22.9
23.2 , style="background:#eaecf0;text-align:right;", 36.8
37.4 , style="text-align:center;", 6 , style="text-align:center" colspan=2, Slough (Central) A355
Windsor A322 , - style="text-align:center;" , style="background:#eaecf0;text-align:right;", 24.7
25.0 , style="background:#eaecf0;text-align:right;", 39.8
40.3 , style="text-align:center;", 7
''

Trumpet interchange In the field of road transport, an interchange (American English) or a grade-separated junction (British English) is a road junction that uses grade separations to allow for the movement of traffic between two or more roadways or highways, usi ...

'' , style="text-align:center" colspan=2, Slough (West) A4 , - style="text-align:center;" , style="background:#eaecf0;text-align:right;", 27.8
28.2 , style="background:#eaecf0;text-align:right;", 44.7
45.4 , style="text-align:center;", 8/9 , style="text-align:center" colspan=2,

, Henley A404(M)

A308(M) , - style="text-align:center;" , 35.0
35.8 , 56.4
57.6 , Reading (East),

Wokingham Wokingham ( ) is a market town and civil parish in Berkshire, England. It is the main administrative centre of the wider Borough of Wokingham. At the 2021 census the parish had a population of 38,284 and the wider built-up area had a populati ...

Bracknell Bracknell () is a town and civil parish in Berkshire, England, the westernmost area within the Greater London Built-up Area, Greater London Urban Area and the administrative centre of the borough of Bracknell Forest. It lies to the east of Re ...

A329(M) , J10
'' Partial cloverleaf interchange'' , Reading (East),

A329(M) , - style="text-align:center;" , 40.6
41.2 , 65.4
66.3 ,

Basingstoke Basingstoke ( ) is a town in Hampshire, situated in south-central England across a valley at the source of the River Loddon on the western edge of the North Downs. It is the largest settlement in Hampshire without city status in the United King ...

, Reading (Central & South) A33 , J11 ,

, Reading (Central & South) A33 , - style="text-align:center;" , 43.5
43.8 , 70.0
70.5 , '' Reading services'' , ''Services'' , '' Reading services'' , - style="text-align:center;" , 45.3
45.7 , 72.9
73.5 , Reading (West), Theale A4 , J12 , Reading (West), Theale A4 , - style="text-align:center;" , 56.9
57.2 , 91.5
92.1 , Newbury,

Oxford Oxford () is a City status in the United Kingdom, cathedral city and non-metropolitan district in Oxfordshire, England, of which it is the county town. The city is home to the University of Oxford, the List of oldest universities in continuou ...

A34
'' Chieveley services ( Moto)'' , J13
''Services''
Intersection of M4 and A34 (E05) , Newbury,

A34
'' Chieveley services ( Moto) '' , - style="text-align:center;" , 64.4
64.8 , 103.7
104.3 , Hungerford,

Wantage Wantage () is a historic market town and Civil parishes in England, civil parish in the Vale of White Horse, Oxfordshire, England. Although within the boundaries of the Historic counties of England, historic county of Berkshire, it has been a ...

A338 , J14 , Hungerford,

A338 , - style="text-align:center;" , 68.7 , 110.5 , '' Membury services'' , ''Services'' , '' Membury services'' , - style="text-align:center;" , , , ''Entering

'' , , ''Entering

Wiltshire Wiltshire (; abbreviated to Wilts) is a ceremonial county in South West England. It borders Gloucestershire to the north, Oxfordshire to the north-east, Berkshire to the east, Hampshire to the south-east, Dorset to the south, and Somerset to ...

'' , - style="text-align:center;" , 76.4
77.1 , 122.9
124.0 ,

(Central & East) A419
Marlborough A346

( A420) , J15 , Swindon (Central & East) A419, Marlborough A346 , - style="text-align:center;" , 82.4
82.8 , 132.6
133.2 ,

(West), Royal Wootton Bassett, Wroughton, MoD Lyneham,

Calne Calne () is a town and Civil parishes in England, civil parish in Wiltshire, southwestern England,OS Explorer Map 156, Chippenham and Bradford-on-Avon Scale: 1:25 000.Publisher: Ordnance Survey A2 edition (2007). at the northwestern extremity ...

A3102 , J16 , Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham,

A3102 , - style="text-align:center;" , 94.8
95.3 , 152.5
153.4 ,

Chippenham Chippenham is a market town in north-west Wiltshire, England. It lies north-east of Bath, Somerset, Bath, west of London and is near the Cotswolds Area of Outstanding Natural Beauty. The town was established on a crossing of the River Avon, ...

A350

Cirencester Cirencester ( , ; see #Pronunciation, below for more variations) is a market town and civil parish in the Cotswold District of Gloucestershire, England. Cirencester lies on the River Churn, a tributary of the River Thames. It is the List of ...

A429 , J17 ,

A350

A429 , - style="text-align:center;" , 96.7 , 155.7 , '' Leigh Delamere services'' , ''Services'' , '' Leigh Delamere services'' , - style="text-align:center;" , , , ''Entering

'' , , ''Entering

South Gloucestershire South Gloucestershire is a unitary authority area in the ceremonial county of Gloucestershire, South West England. Towns in the area include Yate, Chipping Sodbury, Kingswood, Thornbury, Filton, Patchway and Bradley Stoke. The southern p ...

'' , - style="text-align:center;" , 104.9
105.6 , 168.8
170.0 ,

Bath Bath may refer to: * Bathing, immersion in a fluid ** Bathtub, a large open container for water, in which a person may wash their body ** Public bathing, a public place where people bathe * Thermae, ancient Roman public bathing facilities Plac ...

Stroud Stroud is a market town and civil parish in Gloucestershire, England. It is the main town in Stroud District. The town's population was 13,500 in 2021. Sited below the western escarpment of the Cotswold Hills, at the meeting point of the ...

A46 , J18 ,

A46 , - style="text-align:center;" , 112.8
113.2 , 181.5
182.1 , Bristol M32 , J19 ,

M32 , - style="text-align:center;" , 115.8
116.3 , 186.3
187.2 , The South West,

(West), The Midlands,

Gloucester Gloucester ( ) is a cathedral city, non-metropolitan district and the county town of Gloucestershire in the South West England, South West of England. Gloucester lies on the River Severn, between the Cotswolds to the east and the Forest of Dean ...

, Bristol Airport 20 airtransportation

, M5 , J20
'' Almondsbury Interchange''
Almondsbury Interchange – Intersection of M4 and M5 , The South West, Bristol (West) M5(S)
The Midlands, Gloucester M5(N) , - style="text-align:center;" , 117.7
189.5 , 189.5
190.0 , No access , J21 ,

Chepstow Chepstow () is a town and community in Monmouthshire, Wales, adjoining the border with Gloucestershire, England. It is located on the tidal River Wye, about above its confluence with the River Severn, and adjoining the western end of the ...

M48 , - style="text-align:center;" , 121.3
121.6 , 195.2
195.7 , Avonmouth, Bristol Airport 20 airtransportation

Weston-super-Mare Weston-super-Mare ( ) is a seaside town and civil parish in the North Somerset unitary district, in the county of Somerset, England. It lies by the Bristol Channel south-west of Bristol between Worlebury Hill and Bleadon Hill. Its population ...

M49 , J22 , Avonmouth M49 , - style="text-align:center;" , rowspan=2 , 121.7
124.1 , rowspan=2 , 195.9
199.8 , ''Entering

'' , rowspan=2 , '' Second Severn Crossing''
Second Severn Crossing , ''Entry into Wales'' , - style="text-align:center;" , ''Entry into England'' , ''Entering

Monmouthshire Monmouthshire ( ; ) is a Principal areas of Wales, county in the South East Wales, south east of Wales. It borders Powys to the north; the English counties of Herefordshire and Gloucestershire to the north and east; the Severn Estuary to the s ...

'' , - style="text-align:center;" , 127.0 , 204.4 , ''No toll plaza'' , ''Bridge Toll'' , style="background:#dcdcfe;", ''Former toll plaza (closed Dec 2018)'' , - style="text-align:center;" , 129.2 , 208.0 ,

M48 , J23 , No access , - style="text-align:center;" , 131.4
132.2 , 211.4
212.8 , Magor, Caldicot A4810
'' Magor services'' , J23A
''Services'' , Magor, Caldicot A4810
'' Magor services'' , - style="text-align:center;" , , , ''Entering

'' , , ''Entering the City and County Borough of Newport'' , - style="text-align:center;" , 134.8
135.6 , 217.0
218.3 ,

City centre A city centre, also known as an urban core, is the Commerce, commercial, Culture, cultural and often the historical, Politics, political, and geographic heart of a city. The term "city centre" is primarily used in British English, and closely e ...

Newport (East) B4237
Monmouth A449
The Midlands ( M50) , J24 ,

Newport (East) B4237
Monmouth A449 , - style="text-align:center;" , 137.9 , 222.0 , No access , J25 , Caerleon B4596 , - style="text-align:center;" , 138.3 , 222.6 , No access , J25A , Newport (East),

Cwmbran Cwmbran ( ; , also in use as an alternative spelling in English) is a town in the county borough of Torfaen in South Wales. Lying within the Historic counties of Wales, historic boundaries of Monmouthshire (historic), Monmouthshire, Cwmbran was ...

A4042 , - style="text-align:center;" , 138.8
139.1 , 223.4
223.8 , '' Brynglas Tunnels'' , ''Tunnel'' , '' Brynglas Tunnels'' , - style="text-align:center;" , 139.1
139.5 , 223.8
224.5 , Newport (North & South),

, Caerleon A4051 , J26 , Newport (North & South) A4051 , - style="text-align:center;" , 140.8
141,4 , 226.6
227.6 ,

B4591 , J27 ,

B4591 , - style="text-align:center;" , 142.0
142.9 , 228.5
230.0 , Newport (West)

Risca Risca () is a town in the Caerphilly County Borough and within the Historic counties of Wales, historic boundaries of Monmouthshire (historic), Monmouthshire in south-east Wales. It is split into two Community (Wales), communities; Risca East an ...

, Brynmawr A467 , J28 , Newport (West)

, Brynmawr A467 , - style="text-align:center;" , 143.6 , 231.1 , No access , J29 ,

(South & East) A48(M) , - style="text-align:center;" , , , ''Entering the City and County Borough of Newport'' , , ''Entering the City and County of Cardiff'' , - style="text-align:center;" , 146.6
147.3 , 236.0
237.0 , Cardiff (East) A4232
'' Cardiff Gate services'' , J30
''Services'' , Cardiff (East) A4232
'' Cardiff Gate services'' , - style="text-align:center;" , , , ''Non- existent'' , J31 , ''Non-existent'' , - style="text-align:center;" , 151.9
244.5 , 244.5
246.1 ,

Merthyr Tydfil Merthyr Tydfil () is the main town in Merthyr Tydfil County Borough, Wales, administered by Merthyr Tydfil County Borough Council. It is about north of Cardiff. Often called just Merthyr, it is said to be named after Tydfil, daughter of K ...

(North & Central) A470 , J32 , Cardiff (North & Central),

A470 , - style="text-align:center;" , 155.4
156.0 , 250.1
251.1 , Cardiff (West), Barry, Penarth A4232

Cardiff Airport Cardiff Airport () is an airport in Rhoose, Vale of Glamorgan. It is the only airport offering commercial passenger services and cargo services in Wales. The airport is owned by the Welsh Government, operating it at arm's length as a commercia ...

'' Cardiff West services'' , J33
''Services'' , Cardiff (West), Barry, Penarth A4232

'' Cardiff West services'' , - style="text-align:center;" , 157.5
158.1 , 253.5
254.4 , Llantrisant, Rhondda A4119
Royal Glamorgan Hospital , J34 , Llantrisant, Rhondda A4119
Royal Glamorgan Hospital , - style="text-align:center;" , , , ''Entering the City and County of Cardiff'' , , ''Entering the

Vale of Glamorgan The Vale of Glamorgan ( ), locally referred to as ''The Vale'', is a Principal areas of Wales, county borough in the South East Wales, south-east of Wales. It borders Bridgend County Borough to the west, Cardiff to the east, Rhondda Cynon Taf t ...

, the

Rhondda Cynon Taf Rhondda Cynon Taf (; RCT; also spelt as Rhondda Cynon Taff) is a Principal areas of Wales, county borough in the South East Wales, south-east of Wales. It consists of five valleys: the Rhondda Fawr, Rhondda Fach, Cynon, Taff () and Ely valleys, ...

, Bridgend County Borough and

Neath Port Talbot Neath Port Talbot () is a Principal areas of Wales, county borough in the South West Wales, south-west of Wales. Its principal towns are Neath, Port Talbot, Briton Ferry and Pontardawe. The county borough borders Bridgend County Borough and Rhon ...

'' , - style="text-align:center;" , 163.9
164.4 , 263.7
264.5 , Pen-coed A473 , J35 ,

, Pen-coed A473 , - style="text-align:center;" , 167.5
168.3 , 269.6
270.8 ,

A4061
Maesteg A4063
Princess of Wales Hospital
'' Sarn Park services'' , J36
''Services'' ,

A4061
Maesteg A4063
Princess of Wales Hospital
'' Sarn Park services'' , - style="text-align:center;" , 172.8
173.5 , 278.1
279.3 , Pyle,

Porthcawl Porthcawl () is a town and community in the Bridgend County Borough of Wales. It is located on the south coast, west of Cardiff and south-east of Swansea. Historically part of Glamorgan and situated on a low limestone headland on the South ...

A4229 , J37 ,

, Pyle A4229 , - style="text-align:center;" , , , ''Entering the

, the

, Bridgend County Borough and

'' , , ''Entering the City and County of Swansea'' , - style="text-align:center;" , 177.0
177.3 , 284.9
285.3 ,

, J38 ,

, - style="text-align:center;" , 178.3 , 286.9 , No access (on-ramp only) , J39 , No access , - style="text-align:center;" , 179.7
179.9 , 289.2
289.6 ,

A4107 , J40 ,

A4107 , - style="text-align:center;" , 180.9
181.3 , 291.1
291.8 ,

, J41 ,

, - style="text-align:center;" , 183.9
184.9 , 295.9
297.3 ,

Swansea (South)

A483 The A483, officially described as the Swansea to Manchester Trunk Road, although now ending in Chester, is a major road in the United Kingdom. It runs from Swansea in Wales to Chester in England via Llandovery, Llandrindod Wells, Oswestry and W ...

, J42 , Swansea (South)

, - style="text-align:center;" , 184.9
185.7 , 297.5
298.8 ,

Neath Neath (; ) is a market town and Community (Wales), community situated in the Neath Port Talbot, Neath Port Talbot County Borough, Wales. The town had a population of 50,658 in 2011. The community of the parish of Neath had a population of 19,2 ...

A465 , J43 ,

A465 , - style="text-align:center;" , 187.0
187.4 , 300.9
301.6 , Swansea (East)

, J44 , Swansea (East)

, - style="text-align:center;" , 188.8
189.3 , 303.9
304.7 , Pontardawe, Swansea (North & Central) A4067 , J45 , Pontardawe, Swansea (North & Central) A4067 , - style="text-align:center;" , 190.8
191.4 , 307.0
308.0 , Llangyfelach B4489
Felindre
Morriston Hospital , J46 , Llangyfelach B4489
Felindre
Morriston Hospital , - style="text-align:center;" , 192.4
193.1 , 309.6
310.7 , Swansea (West)

Gorseinon

'' Swansea services'' , J47
''Services'' , Swansea (West)

Gorseinon

'' Swansea services'' , - style="text-align:center;" , , , ''Entering the City and County of Swansea'' , , ''Entering

'' , - style="text-align:center;" , 196.4
197.1 , 316.0
317.2 , Pontarddulais,

Llanelli ; ) is a market town and community (Wales), community in Carmarthenshire and the Preserved counties of Wales, preserved county of Dyfed, Wales. It is on the estuary of the River Loughor and is the largest town in the Principal areas of Wales, ...

A4138 , J48 , Pontarddulais,

A4138 , - style="text-align:center;" , 199.2 , 320.6 , ''Start of motorway'' , J49
Terminus
''Services''
Western end of M4 , Motorway terminates at a roundabout:

Carmarthen Carmarthen (, ; , 'Merlin's fort' or possibly 'Sea-town fort') is the county town of Carmarthenshire and a community (Wales), community in Wales, lying on the River Towy north of its estuary in Carmarthen Bay. At the 2021 United Kingdom cen ...

Ammanford Ammanford ( ) is a town and community (Wales), community in Carmarthenshire, Wales. At the 2021 United Kingdom census, 2021 census the community had a population of 5,445, and the wider built up area had a population of 8,285. Ammanford is se ...

Llandeilo Llandeilo () is a town and Community (Wales), community in Carmarthenshire, Wales, situated where the River Towy is crossed by the A483 road, A483 on a 19th-century stone bridge. At the 2021 United Kingdom census, 2021 census the community had ...

Pontarddulais A48
'' Pont Abraham services'' , - ;Coordinate list Data from driver location signs and location marker posts are used to provide distance and carriageway identification information. Where a junction spans several hundred metres and the data is available, both the start and finish values for the junction are shown.

E30

Although not signed, European route E30 includes most of the M4. The entire route runs between Cork in Ireland and

Omsk Omsk (; , ) is the administrative center and largest types of inhabited localities in Russia, city of Omsk Oblast, Russia. It is situated in southwestern Siberia and has a population of over one million. Omsk is the third List of cities and tow ...

in Russia.

Major incidents and accidents

* In June 1984 a crash near

resulted in 13 deaths. * In March 1991, ten people died in the 1991 M4 motorway crash near Hungerford. The crash involved 51 vehicles in foggy conditions. * In May 1995 a coach carrying Royal British Legion members left the M4 close to the Severn Bridge, resulting in 10 deaths. * In April 1998, drummer Cozy Powell died following a car accident on the M4 near Bristol. * In July 2002,

Gus Dudgeon Angus Boyd "Gus" Dudgeon (30 September 1942 – 21 July 2002) was an English record producer, who oversaw many of Elton John's most acclaimed recordings, including his commercial breakthrough, " Your Song". Their collaboration led to seven US N ...

, a music producer known for his work with

Elton John Sir Elton Hercules John (born Reginald Kenneth Dwight; 25 March 1947) is a British singer, songwriter and pianist. His music and showmanship have had a significant, lasting impact on the music industry, and his songwriting partnership with l ...

, and his wife died when the car he was driving veered off the M4 between Reading and Maidenhead. The inquest recorded a verdict of accidental death. * In May 2025 a white Ford Transit minibus overturned on the link road at junction 10 of the M4 to the A329(M) near Reading, Berkshire, at about 14:25 BST on 11 May. A six-year-old boy was also killed in the crash. Five other children injured in the incident have since been discharged from hospital.

References

External links

CBRD Motorway Database – M4

British Road Database: Motorways – M4 Junction 8/9
* The Motorway Archive *

*

*

*

*

*

*
Map of the preferred route around Newport (relief road)
{{DEFAULTSORT:4-0004m Constituent roads of European route E30 Motorways in England Motorways in London Motorways in Wales Roads in Berkshire Roads in Bristol Transport in Bridgend County Borough Transport in Cardiff Transport in Carmarthenshire Transport in the London Borough of Hillingdon Transport in the London Borough of Hounslow Transport in Monmouthshire Transport in Neath Port Talbot Transport in Newport, Wales Transport in Rhondda Cynon Taf Transport in South Gloucestershire District Transport in Swansea Transport in Wiltshire