M4 motorway

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M4 sign

The M4 is a motorway which runs between London and Carmarthenshire. Major towns and cities along the route include Reading, Swindon, Bristol, Newport, Cardiff and Swansea. Originally referred to as the London-South Wales Motorway, the majority, including a suspension bridge over the River Severn, was constructed between 1965 and 1971; the missing link in Port Talbot was completed in 1993. A new Severn bridge was opened in 1996 with the M4 rerouted to use it.

The M4 runs close to the A4 from London to Bristol. After crossing the River Severn it follows the A48 through South Wales, using the Brynglas Tunnels at Junction 25a, Newport and terminates just north of Pontarddulais. The area of land along the M4, with its towns and cities, is known as the M4 corridor.


A new road from London to South Wales was first proposed in the 1930s, and the Ministry of Transport announced plans for the M4 as one of the first major post-war trunk road improvement projects in 1956.[1]

The Chiswick flyover, a short section of elevated dual-carriageway built to reduce the impact of traffic travelling between central London and the west, opened in 1959;[2] it was not originally classed as a motorway.[3] 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 it was only following the creation of the Welsh Office that the Government became committed to a high-standard dual carriageway to Pont Abraham in Carmarthenshire.[4]

The 50-mile stretch between Junctions 9 and 15 (Maidenhead and Swindon) was opened to traffic on 22 December 1971.[5] The missing link in Port Talbot 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.[6]

The Second Severn Crossing

In June 1999 the section of the third lane (the lane nearest the central reservation) between Junctions 2 and 3 was converted to a bus lane and opened as a pilot scheme. The scheme was made permanent in 2001. A lower speed limit was introduced along the bus lane section at the same time.[7] The bus lane was scrapped at the end of 2010 and the third lane was returned to all-traffic use.[8]

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.[9]

Between 2007 and January 2010 the section from Castleton (Junction 29) to Coryton (Junction 32) was widened to six lanes.[10] The scheme was formally opened in 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 stations provided along the route, with an aspiration for further stations to be provided along the M4 into South West England over time.[11] A similar claim was made for a 30-mile section of road in Scotland close to Aberdeen in September 2009 with refuelling points at Bridge of Don, Ellon and Peterhead.[12]

The original (A48(M)) bridge over River Neath, is to the right, the new M4 bridge is to the left

In October 2010 the new Secretary of State for Transport, Philip Hammond, announced that the bus lane in the London section would be suspended for 18 months from 24 December 2010 but brought back for the 2012 Summer Olympics, after which it was likely to be scrapped permanently.[13]

Between 2008 and 2010, Junction 11, near Reading, was extensively remodelled with a new four-lane motorway junction, two new road bridges and other works.[14] The £65m 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.[15] It also involved the movement of the local Highways Agency and Fire Service offices, and the construction of a long footbridge network, a new bus-lane and a new gyratory. Sound barriers for nearby residential areas were also installed.[16] In April 2008, the decision to preserve a rare Vickers machine gun pillbox and turn it into a bat roost was announced by the developers.[17]

The table below shows the timeline for the construction of the motorway on a section by section basis.[18][19][20][21][22][23]

Year opened 1961 1963 1965 1966 1967 1971 1972 1977 1980 1994
Section(s) J7–9 J5–7 J1–5 J18–23
J22–28 J9–18 J44–46 J28–29


Toll bridge

The westbound carriageway tolls (left) and the three-lane eastbound carriageway

The M4 crosses the River Severn via a toll bridge, the second of only two on the UK motorway network – the first was the original Severn Bridge, now part of the M48. Tolls are charged in the westbound direction only; to prevent queueing onto the bridge itself the toll plaza is situate over two miles from the end of the western abutment.

Speed limits

For the majority of its length, the national speed limit applies. Exceptions include the following:

  • 40mph on the Chiswick Flyover within London in both directions.
  • 60mph between Junction 4 and the Chiswick Flyover eastbound only.
  • 50mph when approaching the toll plaza after leaving the Severn Crossing, to protect plaza staff moving between the toll booths.[24]
  • 50mph on the Port Talbot elevated section between Junction 40 and Junction 41.

M4 bus lane

M4 bus lane near Norwood Green, Ealing

Between 1999 and 2010 there was a controversial 3.5-mile bus lane on the eastbound (London-bound) carriageway from Junction 3 (A312) to the western end of the Chiswick Flyover near Brentford, covering part of the 15-mile journey between Heathrow Airport 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.[25] It was used by 7% of vehicles which carried 21% of the people.[7]

The lane was restored for normal motorway running at the end of 2010 for 18 months[8] using an Experimental Traffic Order[26] and was re-established for the duration of the 2012 Summer Olympics with the intention that it would then be removed permanently.[8]

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.[27]

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 Brentford's Golden Mile. This section was designed to have a heated road surface to reduce icing in winter, however, due to the high costs in preventing icing by this method the heating is no longer used.

Four level stack interchanges

M4 Junction 4b / M25 Junction 15 near Heathrow Airport

The M4 has two of only three four-level stack interchanges in the UK, including the first UK example at the junction with the M5 (Junction 20/"Almondsbury Interchange") and the other at the junction with the M25 (J4b). Junction 4b also has to make provision for a railway line passing beneath the M4. Due to the nature of such junctions, it is impossible to make a u-turn at Junction 20 or Junction 4b.


The M4 passes through the Brynglas Tunnels at Junction 25a, Newport. These are the only twin–bored tunnels on the United Kingdom motorway network.

Notable junctions

Junction 8/9 at Maidenhead, Berkshire, is the only one in the UK with dual numbers. This a relic from when the M4 used to turn north after junction 8, where it met the A308, and head for the original Junction 9, where it ended on a roundabout 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.

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 military installation mainly used for storing munitions. The M4 entrance allows easier access for the large vehicles used to carry the munitions.

The section of the M4 in South Wales has to thread its way through mountainous terrain and built-up areas, so there are some unusual junction layouts:

Junction 27 (High Cross, Newport) is a normal grade-separated roundabout junction, but subject to 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.
Junctions 30–31 (East Cardiff) were set aside for intermediate additional interchanges at the time of construction. 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 traffic to and from local destinations to the west and one for places to the east. The former leads to and from a spur leading to the roundabout in Briton Ferry, formerly known as Junction 41a, and the original bridge over the River Neath, 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 Port Talbot. As a result, it is possible in both directions to travel almost two miles on the motorway, 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 a creating a "right-turn" on a roundabout. Similarly, slip roads pass under or over the main motorway at Junctions 41 and 42.


Maintenance of the section of the motorway within England, some 123 miles, is the responsibility of the Highways Agency.[28] The section within Wales, some 76 miles, is the responsibility of the South Wales Trunk Road Agent.[29]

Current developments

Boston Manor viaduct repairs

In July 2012 the London to Heathrow section (between Junctions 1 and 3) was closed due to structural repairs being required at the Boston Manor viaduct [30]

J19-J20 managed motorways

Work is currently taking place to install[31] a stretch of managed motorway on the M4 between Junctions 19 (M32) and 20 (M5), and is estimated for completion in Spring 2014.[32]

Variable speed limit scheme between Junctions 24 and 28

During construction of the electronic indicator signs for the variable speed limit scheme at Junction 27 and a new concrete reservation

The variable speed limit scheme between Junction 24 and Junction 29 opened in June 2011.[33] This section of the motorway was originally built in 1967 and has many non-standard gradients and a number of tight bends. It is hoped the flow of traffic will be smoothed and motorway capacity increased for the 8 miles between the two junctions. The scheme should also save money through fewer accidents in the long term. Widening this section of the motorway is not possible because of the two-lane Brynglas Tunnels and existing housing close to the motorway.[34]

Proposed developments

Hard shoulder running, Junction 3 to 12

In 2010 it was announced that hard shoulder running would be introduced from Junction 3 to Junction 12.[35] As of January 2012 no further information is available on the Highways Agency Website other than mention of the scheme on the main map showing the network and proposed works where it is proposed that work will start 'post 2015'.[36]

New junctions: 15a, 18a

A new Junction 15a, being considered by Wiltshire Council, this would give access to South and Central Swindon as well as to Wroughton, Marlborough and Devizes via the existing A4361, as well as a possible Junction 18a which would connect with the A432 and A4174 giving better access to Bristol, via Mangotsfield, and also a direct link with Yate.[37]

M4 Junction 31

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), have been rekindled after proposals for a new business park on a 125-acre site north of the M4 were submitted to Cardiff Council.[38] The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469.[39]

M4 Relief Road around Newport

Plans for an 'M4 Relief Road' around Newport were first announced by the Welsh Office in 1991, but made little progress. The Welsh Assembly Government revived the scheme as the 'New M4' tolled bypass in 2007 but later abandoned it for financial reasons.[40] An extension to the Newport Southern Distributor Road through the old steel works is being considered.[41] This road is already a dual carriageway. A public consultation exercise on options for improving the capacity of the M4 corridor around Newport opened on 5 March 2012. Its website states that:[42] "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."

Port Talbot, Junctions 40 and 41

There have been calls to close the slip roads at Junction 40 and 41 (at Port Talbot) to improve traffic flow. The motorway has only two lanes on this stretch and is a major traffic congestion blackspot. Junctions 40 and 41 (at Port Talbot) have very short slip roads which have not been modernised.[43] The Port Talbot peripheral distributor road is under development, which should divert local traffic away from the M4. In future, it is hoped to extend the M4 to Carmarthen, but this depends on finance.[44]

Incidents and accidents

  • In June 1984 a crash near Maidenhead resulted in 13 deaths.[45]
  • In March 1991 ten people died in a series of crashes in 1991 involving 51 vehicles near Hungerford.[45]
  • In May 1995 a coach carrying Royal British Legion members left the road close to the Severn Bridge, resulting in 10 deaths.[45]
  • In July 2002, Gus Dudgeon 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 July 2011 a lorry fire in the Brynglas Tunnels closed the motorway.[46] Although there were no injuries, one tunnel remained closed and a contraflow system was in place in the remaining tunnel for about one month, causing major travel delays.[47][48]


M4 motorway
miles Eastbound exits (B carriageway) Junction Westbound exits (A carriageway)
7.3 Road continues as A4 to Central London J1
[coord 1]
Start of motorway
North Circular A406
South Circular A205
Chiswick A315
J2 Staines, Hounslow, Brentford A4
Heston services Services Heston services
Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow A312 J3 Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow A312
Heathrow(Terminals 1, 2 & 3) (A4)
Uxbridge (A408)
J4a Heathrow(Terminals 1, 2 & 3) A4
Uxbridge (A408)
Heathrow(Terminals 4, 5 & Cargo), Gatwick Airport, Watford, Oxford,
Stansted Airport (M40, M1, M11, M3, M23) M25
[coord 2]
Heathrow(Terminals 4, 5 & Cargo), Gatwick Airport, Maidstone, Watford, Oxford, Stansted Airport (M40, M1, M11, M3, M23, M20) M25
Colnbrook, Langley A4
Eton, Datchet B470
J5 Colnbrook, Langley A4, Datchet B470
Slough (Central) A355
Windsor A322
J6 Slough (Central) A355
Windsor A322
Slough (West) A4 J7 Slough (West) A4
High Wycombe, Henley A404(M)
Maidenhead A308(M)
J8/9 High Wycombe, Henley A404(M)
Maidenhead A308(M)
Reading (East), Wokingham, Bracknell A329(M) J10 Reading (East), Wokingham, Bracknell A329(M)
Basingstoke, Reading (Central) A33 J11 Basingstoke, Reading (C & S) A33
Reading services Services Reading services
Reading (West), Theale A4 J12 Reading (West), Theale A4
Newbury, Oxford A34
Chieveley services
[coord 3]
Newbury, Oxford A34
Chieveley services
Hungerford, Wantage A338 J14 Hungerford, Wantage A338
68.7 Membury services Services Membury services
Swindon (East) A419
Marlborough A346
Oxford (A420)
J15 Swindon A419, Marlborough A346
Swindon (West), Royal Wootton Bassett, RAF Lyneham, Calne A3102 J16 Swindon (West), Royal Wootton Bassett, RAF Lyneham, Calne A3102
Chippenham A350
Cirencester A429
J17 Chippenham A350
Cirencester A429
96.7 Leigh Delamere services Services Leigh Delamere services
Bath, Stroud A46 J18 Bath, Stroud A46
Bristol M32 J19 Bristol M32
The SOUTH WEST, Bristol (West), The MIDLANDS, Gloucester M5
Almondsbury Interchange
[coord 4]
The SOUTH WEST, Bristol (West) M5(S)
The MIDLANDS, Gloucester M5(N)
No access J21 Chepstow M48
Avonmouth M49 J22 Avonmouth M49
127.0 No tolls Tolls Toll booth
129.2 Chepstow M48 J23 No access
Magor, Caldicot A4810 (B4245)
Magor services
Magor, Caldicot A4810 (B4245)
Magor services
City centre A48
Newport (East) B4237
Monmouth A449
J24 City centre A48
Newport (East) B4237
Monmouth A449
137.9 No access J25 Caerleon B4596
138.3 No access J25a Newport, Cwmbran A4042
Brynglas Tunnels Tunnel Brynglas Tunnels
Newport, Cwmbran, Caerleon A4051 J26 Newport A4051
High Cross B4591 J27 High Cross B4591
Newport A48
Risca, Brynmawr A467
J28 Newport A48
Risca, Brynmawr A467
143.6 No access J29 Cardiff East and South A48(M)
Cardiff East A4232
Cardiff Gate services
Cardiff East A4232
Cardiff Gate services
Cardiff North, Merthyr Tydfil A470 J32 Cardiff North, Merthyr Tydfil A470
Cardiff West, Cardiff Airport, Barry, Penarth A4232
Cardiff West services
Cardiff West, Cardiff Airport, Barry, Penarth A4232
Cardiff West services
Llantrisant, Rhondda A4119 J34 Llantrisant, Rhondda A4119
Pen-coed A473 J35 Pen-coed, Bridgend A473
Maesteg A4063
Sarn Park services
Maesteg A4063
Sarn Park services
Porthcawl, Pyle A4229 J37 Porthcawl, Pyle A4229
Port Talbot A48 J38 Port Talbot A48
178.3 No access (on-ramp only) J39 No access
Port Talbot A4107 J40 Port Talbot A4107
Port Talbot A48 J41 Port Talbot, Baglan A48
Swansea A483
Briton Ferry A48
J42 Swansea A483
Neath, Merthyr Tydfil A465 J43 Neath, Merthyr Tydfil A465
Swansea A48 J44 Swansea A48
Swansea, Pontardawe, Morriston, Clydach A4067 J45 Swansea, Pontardawe, Morriston, Clydach A4067
Swansea, Llangyfelach B4489 J46 Swansea, Llangyfelach B4489
Swansea A483
Pontarddulais A48
Penllergaer, Gorseinon A4240
Swansea services
Swansea A483
Pontarddulais A48
Penllergaer, Gorseinon A4240
Swansea services
Pontarddulais, Llanelli A4138 J48 Pontarddulais, Llanelli A4138
199.2 Start of motorway J49
[coord 5]
Motorway terminates at a roundabout:
Carmarthen A48
Ammanford A483
Pontarddulais A48
Pont Abraham services
Data[49][50][51][52] from driver location signs and location marker posts are used to provide distance and carriageway identification information. Where a junction spans several hundred yards and the data is available, both the start and finish values for the junction are shown.
Coordinate list
  1. 51°29’22"N, 0°16’40"W Eastern end of M1
  2. 51°29’41"N, 0°29’44"W Intersection of M25 and M4
  3. 51°27’14"N, 1°18’52"W Intersection of M4 and A34
  4. 51°33’1"N, 2°33’11"W Almondsbury Interchange – Intersection of M4 and M5
  5. 51°44’42"N, 4°3’54"W Western end of M4


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  39. "Business park plan ‘would hit green land’". icwales. 19 December 2007. http://icwales.icnetwork.co.uk/business-in-wales/business-news/2007/12/19/business-park-plan-would-hit-green-land-91466-20266885/. 
  40. "Plans for the M4 Toll (Newport)". BBC News. 20 September 2007. http://news.bbc.co.uk/2/hi/uk_news/wales/7003690.stm. Retrieved 12 May 2010. 
  41. "The New M4 Project – Magor to Castleton". Welsh Assembly Government. 13 July 2009. http://wales.gov.uk/publications/accessinfo/drnewhomepage/transportdrs2/transportdrs2009/m4projmagor-castleton/?lang=en. 
  42. M4 corridor enhancement measures: Overview. Accessed 5 March 2012
  43. "Are M4 plans on the right track?". South Wales Evening Post. 12 December 2007. http://www.thisissouthwales.co.uk/displayNode.jsp?nodeId=161818&command=displayContent&sourceNode=161644&contentPK=19049034&folderPk=88499&pNodeId=161375. 
  44. "Unitary Development Plan". Carmarthenshire Council. c. 2010. http://www.cartoplus.co.uk/carmarthenshire/english/text/07_t_transport.htm. 
  45. 45.0 45.1 45.2 "Death toll on British roads". Daily Mail (London). http://www.dailymail.co.uk/news/article-187995/Death-toll-British-roads.html. "13 died in a crash on the M4 near Maidenhead, Berks, in June 1984." 
  46. "Traffic chaos after M4 Brynglas tunnel lorry blaze". BBC News. 26 July 2011. http://www.bbc.co.uk/news/uk-wales-south-east-wales-14288312. Retrieved 9 April 2013. 
  47. "M4 tunnel fire: Brynglas tunnel reopens". BBC News. 29 July 2011. http://www.bbc.co.uk/news/uk-wales-south-east-wales-14344483. Retrieved 9 April 2013. 
  48. "M4 tunnel fire sparks relief road debate". BBC News. 27 July 2011. http://www.bbc.co.uk/news/uk-wales-14318891. Retrieved 9 April 2013. 
  49. Area 3 Driver Location Signs (map) – Highway Authority, 2009
  50. Driver Location Signs, M5 J18-11, M4 J22-15 (map) Highway Authority 2009
  51. "Traffic England Live Traffic Condition Map (selected Popups)". Highways Agency. http://www.trafficengland.com/map.aspx. Retrieved 5 November 2009. 
  52. "Resources". Traffig Cymru/Traffic Wales. http://www.traffic-wales.com/resources. Retrieved 22 August 2011. "Select Telephone & marker post locations." 

Outside links

("Wikimedia Commons" has material
about M4 motorway)
Motorways in the United Kingdom

Great Britain: M1  • M2  • M3  • M4  • M5  • M6  • M6 Toll  • M8  • M9  • M10  • M11  • M18  • M20  • M23  • M25  • M26  • M27  • M32  • M40  • M42  • M45  • M48  • M49  • M50  • M53  • M54  • M55  • M56  • M57  • M58  • M60  • M61  • M62  • M63  • M65  • M66  • M67  • M69  • M73  • M74  • M77  • M80  • M85  • M90  • M180  • M181  • M271  • M275  • M602  • M606  • M621  • M876  • M898  • Sections of A road: A1(M)

Former motorways marked in italics

Northern Ireland: M1  • M2  • M3  • M5  • M12  • M22