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 construction of the Second Severn Crossing, officially renamed the Prince of Wales Bridge and inaugurated on 5 June 1996 by the Prince of Wales, caused the M4 to be rerouted.
|Part of |
|Maintained by |
South Wales Trunk Road Agent
|Length||189 mi (304 km)|
|History||Opened: 1959 (Chiswick Flyover), 1961 (as A4(M)), 1963, 1966 (as A48(M))|
J4b → M25 motorway
J8/9 → A308(M) motorway/A404(M) motorway
J10 → A329(M) motorway
J19 → M32 motorway
J20 → M5 motorway
J21 → M48 motorway
J22 → M49 motorway
J23 → M48 motorway
J29 → A48(M) motorway
|Counties||Greater London, Berkshire, Wiltshire, Gloucestershire, Monmouthshire, Newport, Cardiff, Vale of Glamorgan, Rhondda Cynon Taf, Bridgend, Neath Port Talbot, Swansea, Carmarthenshire|
The line of the motorway from London to Bristol runs closely in parallel with the A4. After crossing the River Severn, toll-free since 17 December 2018, the motorway follows the A48, to terminate at the Pont Abraham services in Carmarthenshire. The M4 is the only motorway in Wales apart from its two spurs: the A48(M) and the M48. The major towns and cities along the route—a distance of approximately 189 miles (304 km)—include Slough, Reading, Swindon, Bristol, Newport, Cardiff, Bridgend, Port Talbot and Swansea.
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.
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.
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. 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.
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. The Welsh section was completed in 1993, when the Briton Ferry motorway 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.
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 stations provided along the route, with an aspiration for further stations to be provided along the M4 into South West England 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 £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.
Timeline of construction
For the majority of its length, the national speed limit applies. Exceptions include the following:
- 40 miles per hour (64 km/h) on the Chiswick Flyover within London in both directions.
- 60 miles per hour (97 km/h) between junction 4 and the Chiswick Flyover eastbound only.
- 50 miles per hour (80 km/h) on the Port Talbot 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 generates around £500,000 in fines each six months.
The M4 has two sections of smart motorway. The one between junctions 19 (M32) and 20 (M5) north of Bristol has variable speed limits and a part-time hard-shoulder. Completion was in summer 2014. The section between junctions 24 and 29 in Newport has variable speed limits.
In 2010, it was announced that the motorway would be upgraded to a smart motorway between junctions 3 and 12. With a length of 32 miles (51 km), this will be the longest smart motorway scheme in the United Kingdom. Work started in autumn 2018 and is expected to be completed in March 2022 at a cost of £848 million.
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.
In June 1999, the M4 bus lane was created on the third lane between junctions 2 and 3—initially, a pilot scheme—permanently in 2001. A lower speed limit was introduced along the section at the same time. The 3 1⁄2-mile (5.6 km) bus lane was on the eastbound carriageway; from the western end of the Chiswick Flyover near Brentford to junction 3 (A312) covering part of the 15-mile (24 km) 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.
The bus lane was used by 7% of vehicles which carried 21% of the people. At the end of 2010, the M4 was restored to normal motorway use for 18 months. The bus lane was maintained temporarily just for the 2012 Summer Olympics.
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.
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 junction 4b with the M25. Junction 4b has to make provision for the railway line passing beneath the M4. Due to the nature of these junctions, one cannot make a U-turn at either of them.
Junction 8/9 near Maidenhead, Berkshire, and High Wycombe, Buckinghamshire 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 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 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 125 acres (51 ha) 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) 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 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, one can travel for almost 2 miles (3.2 km) 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 a 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 roads 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.
On 5 June 2019, the Welsh Government 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 but not open to the public. 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."
List of junctions
|mile||km||Eastbound exits (B carriageway)||Junction||Westbound exits (A carriageway)|
|7.3||11.8||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)
||Heathrow (Terminals 4, 5&Cargo), Gatwick, Watford (M3), (M23), (M40), (M1), M25|
|Colnbrook, Langley, Slough (East) A4
Eton, Datchet B470
|J5||Colnbrook, Langley, Slough (East) 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 & South) A33||J11||Basingstoke, Reading (Central & South) A33|
|Reading services||Services||Reading services|
|Reading (West), Theale A4||J12||Reading (West), Theale A4|
|Newbury, Oxford A34
Chieveley services (Moto)
|Newbury, Oxford A34|
Chieveley services (Moto)
|Hungerford, Wantage A338||J14||Hungerford, Wantage A338|
|68.7||110.5||Membury services||Services||Membury services|
|Swindon (East) A419
|J15||Swindon A419, Marlborough A346|
|Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham, Calne A3102||J16||Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham, Calne A3102|
|96.7||155.7||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||J20
|The SOUTH WEST, Bristol (West) M5(S)|
The MIDLANDS, Gloucester M5(N)
|No access||J21||Chepstow M48|
|Avonmouth M49||J22||Avonmouth M49|
|Entry into England||Second Severn Crossing
||Entry into Wales|
|127.0||204.4||No toll plaza||Bridge Toll||Former Toll plaza (closed Dec 2018)|
|129.2||208.0||Chepstow M48||J23||No access|
|Magor, Caldicot A4810
|Magor, Caldicot A4810 |
|City centre A48
Newport (East) B4237
The MIDLANDS (M50)
|J24||City centre A48|
Newport (East) B4237
|137.9||222.0||No access||J25||Caerleon B4596|
|138.3||222.6||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|
Risca, Brynmawr A467
Risca, Brynmawr A467
|143.6||231.1||No access||J29||Cardiff (East and South) A48(M)|
|Cardiff (East) A4232
Cardiff Gate services
|Cardiff (East) A4232|
Cardiff Gate services
|Merthyr Tydfil, Cardiff A470||J32||Cardiff (North), Merthyr Tydfil A470|
|Cardiff (West), Barry, Penarth A4232
Cardiff West services
|Cardiff (West), Barry, Penarth A4232|
Cardiff West services
|Llantrisant, Rhondda A4119
Royal Glamorgan Hospital
|J34||Llantrisant, Rhondda A4119|
Royal Glamorgan Hospital
|Pen-coed A473||J35||Bridgend, Pen-coed A473|
Princess of Wales Hospital
Sarn Park services
Princess of Wales Hospital
Sarn Park services
|Pyle, Porthcawl A4229||J37||Porthcawl, Pyle A4229|
|Port Talbot A48||J38||Port Talbot A48|
|178.3||286.9||No access (on-ramp only)||J39||No access|
|Port Talbot A4107||J40||Port Talbot A4107|
|Port Talbot A48||J41||Briton Ferry A48|
|Briton Ferry A48
Swansea (South) A483
|J42||Swansea (South) A483|
|Neath, Merthyr Tydfil A465||J43||Neath, Merthyr Tydfil A465|
|Swansea (East) A48||J44||Swansea (East) A48|
|Pontardawe, Swansea (North) A4067||J45||Pontardawe, Swansea (North) A4067|
|Swansea (West) A483
|Swansea (West) A483|
|Pontarddulais, Llanelli A4138||J48||Pontarddulais, Llanelli A4138|
|199.2||320.6||Start of motorway||J49
|Motorway terminates at a roundabout:|
Ammanford, Llandeilo A483
Pont Abraham services
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|
- 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.
Although not signed, European route E30 includes most of the M4.
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 April 1998, drummer Cozy Powell died following a car accident on the M4 near Bristol.
- In July 2002, Gus Dudgeon, a music producer known for his work with Elton John, 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.
- "The M4 London to South Wales Motorway. Holyport to Tormarton". The Motorway Archive Trust. Retrieved 3 October 2010.
- Curtis, Nick (1 October 2009). "The 'sweet little Chiswick Flyover' hits 50". This Is London. Archived from the original on 21 March 2011. Retrieved 3 March 2012.
The showbiz opening on 30 September 1959 was therefore a shrewd publicity stunt by Alderton's managing director, J E Dayton. It worked.
- "THE OLDEST MOTORWAY". The Motorway Archive Trust. Archived from the original on 22 January 2010. Retrieved 3 March 2012.
- "The M4 in Wales". The Motorway Archive Trust. Archived from the original on 22 January 2011. Retrieved 1 October 2010.
- "On the road". The Motor. nbr 3625: Page 30. 23 December 1971.
- "The Motorway Archive: M4 Second Severn Crossing". Iht.org. 28 April 1992. Archived from the original on 7 February 2012. Retrieved 31 December 2011.
- "Drivers hold M4 speed camera demo". BBC News. 30 April 2005.
- ": : M4 Motorway Widening : :". web.archive.org. Archived from the original on 15 January 2008. Retrieved 20 November 2010.
- "M4 in Wales to be 'hydrogen highway,' ministers to say". BBC News. 12 February 2010. Retrieved 12 February 2010.
- Fraser, Douglas (8 September 2009). "'Hydrogen highway' plans backed". BBC News. Retrieved 12 February 2010.
- "M4 Junction 11 Improvement Scheme". Highways Agency. Archived from the original on 26 June 2009.
- "M4 Junction 11 Improvements". Reading Borough Council. Archived from the original on 13 July 2010.
- "Reading is the fastest-growing economic centre in UK". Reading Chronicle. 10 July 2007.
- "M4 VICKERS MACHINE GUN PILLBOX, PILLBOX STUDY GROUP". Pillbox-study-group.org.uk. 11 April 2008. Archived from the original on 8 September 2012. Retrieved 31 December 2011.
- "Dates:M4. Chiswick to Slough By-pass (J1 to J5)". The Motorway Archive Trust. Retrieved 13 March 2013.
- "Dates:M4 Slough-Maidenhead By-pass (Junctions 5 to 7) Statistics and options". The Motorway Archive Trust. Retrieved 13 March 2013.
- "Dates:M4. Maidenhead to Wickham (J8 to J14) Statistics and options". The Motorway Archive Trust. Retrieved 13 March 2013.
- "Dates:The Aust (J21) to Wickham (J14) section of M4 Statistics and options". The Motorway Archive Trust. Retrieved 13 March 2013.
- "Dates:M4 in Wales Statistics and options". The Motorway Archive Trust. Retrieved 13 March 2013.
- "History Overview". Severn River Crossing Plc. 2011. Archived from the original on 15 March 2013. Retrieved 16 March 2013.
- "Second Severn Crossing, England and Wales – Halcrow Group". Halcrow.com. 29 October 2009. Archived from the original on 10 March 2010. Retrieved 16 March 2013.
- Highways Agency: Our network Archived 14 March 2013 at the Wayback Machine. Retrieved 8 March 2013
- Welsh Government: Roads. Retrieved 8 March 2013
- "Average speed cameras for Port Talbot M4 stretch". BBC South West Wales. 14 July 2014. Retrieved 14 July 2014.
- "M4 speed cameras generate half a million pounds". itv.com. 16 June 2015. Retrieved 5 March 2016.
- "M4 J19-20 and M5 J15-17 Managed Motorways". Highways Agency. Retrieved 18 February 2012.
- "M4 junctions 3 to 12 smart motorway" (PDF). Highways Agency. Retrieved 27 December 2015.
- "M4 junctions 3-12: smart motorway".
- "Traffic chaos after M4 Brynglas tunnel lorry blaze". BBC News. 26 July 2011. Retrieved 9 April 2013.
- "M4 tunnel fire: Brynglas tunnel reopens". BBC News. 29 July 2011. Retrieved 9 April 2013.
- "M4 tunnel fire sparks relief road debate". BBC News. 27 July 2011. Retrieved 9 April 2013.
- "U-turn". BBC. 18 January 2001. Retrieved 12 May 2010.
- Edwards, Tom (11 September 2009). "M4 bus lane is 'barely enforced'". BBC News. Retrieved 12 May 2010.
- "Stretch of M4 bus lane opens to all motorists". BBC News. 16 November 2010. Retrieved 16 November 2010.
- "Work starts to remove M4 Bus Lane". Highways Agency. Archived from the original on 7 July 2012.
The suspension of the M4 Bus Lane is being carried out under an Experimental Order under the Road Traffic Regulation Act 1984
- "Government to scrap M4 bus lane". BBC News. 1 October 2010. Retrieved 1 October 2010.
The controversial M4 bus lane is due to be scrapped at the end of the year. Under the plans, all motorists will be able to use the 3.5 miles (5.6 km) lane which operates on the London-bound carriageway from near Heathrow
- "2006 Fourth quarter foundation magazine – Operations and products" (PDF). Hanson.biz. 31 May 2011. Archived from the original (PDF) on 30 May 2008. Retrieved 31 December 2011.
- "One More Link to the West". New Scientist: 772. 25 March 1965.
- Thornes, John Edward (1984). The Prediction of Ice Formation on Motorways in Britain (PDF) (Ph. D.). University College London. Retrieved 1 November 2015.
- "This Sceptred Isle". Notes and Queries. Guardian. Retrieved 14 February 2016.
- "Council will look again at case for M-way link". icwales. 23 June 2007.
- "Business park plan 'would hit green land'". icwales. 19 December 2007.
- Salmon, Jeff. "M4 J31 Freedom of information act". www.whatdotheyknow.com. Retrieved 3 January 2016.
- "M4 Exit List". CBRD. Archived from the original on 25 January 2016. Retrieved 14 February 2016.
- "The M4 Motorway (Junction 41, Westbound Exit Slip Road, Sunnycroft Roundabout, Baglan, Neath Port Talbot) (50 MPH Speed Limit) Regulations 2010" (PDF). Assembly for Wales. Retrieved 14 February 2016.
- "Are M4 plans on the right track?". South Wales Evening Post. 12 December 2007.
- "Twice-a-day trial closure of Port Talbot M4 junction starts". BBC News. 4 August 2014.
- "Controversial twice-day closure of M4 junction in Port Talbot starts". Wales Online. 4 August 2014.
- "200 attend meeting over M4 junction 41 Port Talbot closure". BBC News. 12 March 2015.
- "M4 Port Talbot junction 41 trial closures end". BBC News. 29 May 2015.
- "Temporary closures of Port Talbot's M4 junction 41 are called off but slip road's future remains in doubt". Wales Online. 29 May 2015.
- "Plans for the M4 Toll (Newport)". BBC News. 20 September 2007. Retrieved 12 May 2010.
- M4 corridor enhancement measures: Overview. Accessed 5 March 2012
- "The New M4 Project – Magor to Castleton". Welsh Assembly Government. 13 July 2009.
- 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. Archived from the original on 10 February 2012. Retrieved 5 November 2009.
- "Resources". Traffig Cymru/Traffic Wales. Archived from the original on 3 September 2011. Retrieved 22 August 2011.
Select Telephone & marker post locations.
- "Death toll on British roads". Daily Mail. London.
13 died in a crash on the M4 near Maidenhead, Berks, in June 1984.
- "Record producer dies in crash". BBC News Online. 22 July 2002. Retrieved 13 February 2015.
- Jackson, Mike (2005). The M4 Sights Guide. Severnpix. ISBN 978-0954540227.
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