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.
- 1 History
- 2 Features
- 3 Current developments
- 4 Proposed developments
- 5 Incidents and accidents
- 6 Junctions
- 7 References
- 8 Outside links
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.
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; it was not originally classed as a motorway. 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.
The 50-mile stretch between Junctions 9 and 15 (Maidenhead and Swindon) was opened to traffic on 22 December 1971. 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.
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. The bus lane was scrapped at the end of 2010 and the third lane was returned to all-traffic use.
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 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. 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.
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.
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. 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. 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. 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.
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.
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.
- 50mph on the Port Talbot elevated section between Junction 40 and Junction 41.
M4 bus lane
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. It was used by 7% of vehicles which carried 21% of the people.
The lane was restored for normal motorway running at the end of 2010 for 18 months using an Experimental Traffic Order and was re-established for the duration of the 2012 Summer Olympics with the intention that it would then be removed permanently.
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 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
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.
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. The section within Wales, some 76 miles, is the responsibility of the South Wales Trunk Road Agent.
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 
J19-J20 managed motorways
Variable speed limit scheme between Junctions 24 and 28
The variable speed limit scheme between Junction 24 and Junction 29 opened in June 2011. 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.
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. 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'.
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.
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. The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469.
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. An extension to the Newport Southern Distributor Road through the old steel works is being considered. 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: "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. 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.
Incidents and accidents
- In June 1984 a crash near Maidenhead resulted in 13 deaths.
- In March 1991 ten people died in a series of crashes in 1991 involving 51 vehicles near Hungerford.
- In May 1995 a coach carrying Royal British Legion members left the road close to the Severn Bridge, resulting in 10 deaths.
- 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. 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.
|miles||Eastbound exits (B carriageway)||Junction||Westbound exits (A carriageway)|
|7.3||Road continues as A4 to Central London||J1
|Start of motorway|
| North Circular A406
South Circular A205
|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)
|J4a|| Heathrow(Terminals 1, 2 & 3) A4|
| Heathrow(Terminals 4, 5 & Cargo), Gatwick Airport, Watford, Oxford,
Stansted Airport (M40, M1, M11, M3, M23) M25
|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
|J6|| Slough (Central) A355|
|Slough (West) A4||J7||Slough (West) A4|
| High Wycombe, Henley A404(M)
|J8/9|| High Wycombe, Henley A404(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
| Newbury, Oxford A34|
|Hungerford, Wantage A338||J14||Hungerford, Wantage A338|
|68.7||Membury services||Services||Membury services|
| Swindon (East) A419
|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
|J17|| Chippenham A350|
|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
| 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, Caldicot A4810 (B4245)|
| City centre A48
Newport (East) B4237
The MIDLANDS (M50)
|J24|| City centre A48|
Newport (East) B4237
|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|
Sarn Park services
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
|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
Penllergaer, Gorseinon A4240
| Swansea A483|
Penllergaer, Gorseinon A4240
|Pontarddulais, Llanelli A4138||J48||Pontarddulais, Llanelli A4138|
|199.2||Start of motorway||J49
| Motorway terminates at a roundabout:|
Pont Abraham services
|Data 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
- 51°29’22"N, 0°16’40"W Eastern end of M1
- 51°29’41"N, 0°29’44"W Intersection of M25 and M4
- 51°27’14"N, 1°18’52"W Intersection of M4 and A34
- 51°33’1"N, 2°33’11"W Almondsbury Interchange – Intersection of M4 and M5
- 51°44’42"N, 4°3’54"W Western end of M4
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- Area 3 Driver Location Signs (map) – Highway Authority, 2009
- Driver Location Signs, M5 J18-11, M4 J22-15 (map) Highway Authority 2009
- "Traffic England Live Traffic Condition Map (selected Popups)". Highways Agency. http://www.trafficengland.com/map.aspx. Retrieved 5 November 2009.
- "Resources". Traffig Cymru/Traffic Wales. http://www.traffic-wales.com/resources. Retrieved 22 August 2011. "Select Telephone & marker post locations."
| ("Wikimedia Commons" has material|
about M4 motorway)
- CBRD Motorway Database – M4
- British Road Database: Motorways – M4 Junction 8/9
- The M4 on SABRE
- The Motorway Archive
|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