M4 shield
MapM4 highlighted in blue

Shown with UK motorway network
New Severn Bridge - panoramio.jpg
Route information
Part of E30
Maintained by
National Highways
South Wales Trunk Road Agent
Length189 mi (304 km)
HistoryOpened: 1959 (Chiswick Flyover), 1961 (as A4(M)), 1963, 1966 (as A48(M))
Completed: 1996
Major junctions
East endChiswick
51°29′23″N 0°16′41″W / 51.4897°N 0.2781°W / 51.4897; -0.2781 (M4 motorway (eastern end))
Major intersections
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
West endPont Abraham
51°44′42″N 4°03′54″W / 51.7451°N 4.0651°W / 51.7451; -4.0651 (M4 motorway (western end))
CountryUnited Kingdom
CountiesGreater London, Buckinghamshire, Berkshire, Wiltshire, South Gloucestershire, Monmouthshire, Newport, Cardiff, Vale of Glamorgan, Rhondda Cynon Taf, Bridgend, Neath Port Talbot, Swansea, Carmarthenshire

Heathrow Airport
Port Talbot
Road network
M3 M5

The M4, originally the London-South Wales Motorway, is a motorway 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 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 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 the plans for the first major post-war road improvement projects.[1]

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.[2][3]

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

The Maidenhead 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.[4] 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.[5]

The English section of the motorway was completed on 22 December 1971 when the 50-mile (80 km) stretch between junctions 9 and 15 Maidenhead and Swindon) was opened to traffic.[6] The Welsh section was largely completed between 1970 and 1980, though a non-motorway section remained across the Briton Ferry 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.[7] 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.[8]

Between 2007 and January 2010, the section from Castleton (junction 29) to Coryton (junction 32) was widened to six lanes.[9] 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.[10][11]

Between 2008 and 2010, junction 11 was extensively remodelled with a new four-lane junction, two new road bridges and other works.[12] The £65 million 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.[13] 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.[14] 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.[15]

Former bridge tolls

Tolls were charged on the Severn Bridge(s) from opening until 2018. In 1966 the toll was 12+12 pre-decimal pence each-way 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 River Severn were eliminated on 17 December 2018,[16] and the former toll booths were removed in 2019.

Timeline of construction


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
Animated map showing build progress at five-years (or greater) intervals.
Note: When the Second Severn Crossing was opened in 1996, the M4 was re-routed and the section of motorway between Junctions 21 to 23 became the M48.[23]


Maintenance of the Second Severn Crossing and the 123 miles (198 km) of motorway in England is the responsibility of National Highways.[24] The 76 miles (122 km) in Wales is the responsibility of the South Wales Trunk Road Agent.[25]


Speed limits

The M4 around Port Talbot

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

Smart motorway

Construction of electronic indicator signs for the variable speed limit scheme at junction 27 and a new concrete reservation (2010)

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 50 mph (80 km/h) limit with average speed cameras.[28]

In 2010, it was announced that the motorway would be changed to a smart motorway between junctions 3 and 12. With a length of 32 miles (51 km), 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.[29][30][31]

Brynglas tunnels

Brynglas Tunnels – western portals

The Brynglas Tunnels carry the M4 under Brynglas Hill in Newport. The 360-metre-long (390 yd) 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). 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.[32][33][34]

Bus lane

M4 bus lane near Norwood Green

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.[35] The 3+12-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.[36]

In December 2010, the bus lane was suspended for 18 months.[37][38] It was reinstated temporarily for the 2012 Summer Olympics and then permanently decommissioned.[39]

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.[40]

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.[41][42]

Four-level stack interchanges

M4 junction with the M25 near Heathrow Airport

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 Maidenhead, Berkshire, and High Wycombe, Buckinghamshire is the only one in the UK with dual numbers.[citation needed] 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.[43]

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.[44] The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469.[45] 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.[46] 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.[47]

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 creating a "right-turn" on a roundabout. Similarly, slip roads pass under or over the main motorway at junctions 41 and 42.[48]

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.[49] 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[50][51] but following heavy criticism from local businesses and residents,[52] was stopped on 29 May 2015.[53][54]

Relief road

Main article: M4 relief road

On 5 June 2019, the Welsh Government scrapped the proposal for a proposed motorway south of Newport.[55] The Welsh Assembly Government had revived the scheme as a tolled bypass in 2007 and later abandoned it for financial reasons.[56] 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 around Newport opened on 5 March 2012. Its website states that:[57] "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."[58]

List of junctions

This article contains a bulleted list or table of intersections which should be presented in a properly formatted junction table. Please consult this guideline for information on how to create one. Please improve this article if you can. (December 2021)
M4 motorway
mile km Eastbound exits (B carriageway) Junction Westbound exits (A carriageway)
7.3 11.8 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 2 & 3) (A4)
Uxbridge A408
J4a Heathrow (Terminals 2 & 3) (A4)
Uxbridge A408
Heathrow (Terminals 4, 5 & Cargo) , Gatwick Airport , Dartford, Oxford, Watford, Stansted Airport , (M3), (M23), (M20), (M40), (M1), (A1(M)), (M11), M25 J4b
Thorney Interchange
[coord 2]
Heathrow (Terminals 4, 5 & Cargo), Gatwick, Dartford, Oxford, Watford, Stansted, (M3), (M23), (M20), (M40), (M1), (A1(M)), (M11), M25
Entering Greater London Entering Berkshire
Colnbrook, Langley, Slough (East) A4
Eton, Datchet B470
J5 Colnbrook, Langley, Slough (East) A4, Datchet B470
Slough (Central) A355
Windsor A322
J6 Slough (Central) A355
Windsor A322
Entering Berkshire Entering Buckinghamshire
Slough (West) A4 J7
Trumpet interchange
Slough (West) A4
Entering Buckinghamshire Entering Berkshire
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
Partial cloverleaf interchange
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)
[coord 3]
Newbury, Oxford A34
Chieveley services (Moto)
Hungerford, Wantage A338 J14 Hungerford, Wantage A338
68.7 110.5 Membury services Services Membury services
Entering Berkshire Entering Wiltshire
Swindon (Central & East) A419
Marlborough A346
Oxford (A420)
J15 Swindon (Central & East) A419, Marlborough A346
Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham, Calne A3102 J16 Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham, Calne A3102
Chippenham A350
Cirencester A429
J17 Chippenham A350
Cirencester A429
96.7 155.7 Leigh Delamere services Services Leigh Delamere services
Entering Wiltshire Entering South Gloucestershire
Bath, Stroud A46 J18 Bath, Stroud A46
Bristol M32 J19 Bristol M32
The South West, Bristol (West), The Midlands, Gloucester, Bristol Airport , M5 J20
Almondsbury Interchange
[coord 4]
The South West, Bristol (West) M5(S)
The Midlands, Gloucester M5(N)
No access J21 Chepstow M48
Avonmouth, Bristol Airport , Weston-super-Mare M49 J22 Avonmouth M49
Entering South Gloucestershire Second Severn Crossing
[coord 5]
Entry into Wales
Entry into England Entering Monmouthshire
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 services
Magor, Caldicot A4810
Magor services
Entering Monmouthshire Entering the City and County Borough of Newport
City centre A48
Newport (East) B4237
Monmouth A449
The Midlands (M50)
J24 City centre A48
Newport (East) B4237
Monmouth A449
137.9 222.0 No access J25 Caerleon B4596
138.3 222.6 No access J25A Newport (East), Cwmbran A4042
Brynglas Tunnels Tunnel Brynglas Tunnels
Newport (North & South), Cwmbran, Caerleon A4051 J26 Newport (North & South) A4051
High Cross B4591 J27 High Cross B4591
Newport (West) A48
Risca, Brynmawr A467
J28 Newport (West) A48
Risca, Brynmawr A467
143.6 231.1 No access J29 Cardiff (South & East) A48(M)
Entering the City and County Borough of Newport Entering the City and County of Cardiff
Cardiff (East) A4232
Cardiff Gate services
Cardiff (East) A4232
Cardiff Gate services
Non- existent J31 Non-existent
Merthyr Tydfil, Cardiff (North & Central) A470 J32 Cardiff (North & Central), Merthyr Tydfil A470
Cardiff (West), Barry, Penarth A4232
Cardiff Airport
Cardiff West services
Cardiff (West), Barry, Penarth A4232
Cardiff Airport
Cardiff West services
Llantrisant, Rhondda A4119
Royal Glamorgan Hospital
J34 Llantrisant, Rhondda A4119
Royal Glamorgan Hospital
Entering the City and County of Cardiff Entering the Vale of Glamorgan, the Rhondda Cynon Taf, Bridgend County Borough and Neath Port Talbot
Pen-coed A473 J35 Bridgend, Pen-coed A473
Bridgend A4061
Maesteg A4063
Princess of Wales Hospital
Sarn Park services
Bridgend A4061
Maesteg A4063
Princess of Wales Hospital
Sarn Park services
Pyle, Porthcawl A4229 J37 Porthcawl, Pyle A4229
Entering the Vale of Glamorgan, the Rhondda Cynon Taf, Bridgend County Borough and Neath Port Talbot Entering the City and County of Swansea
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 & Central) A4067 J45 Pontardawe, Swansea (North & Central) A4067
Llangyfelach B4489
Morriston Hospital
J46 Llangyfelach B4489
Morriston Hospital
Swansea (West) A483
Gorseinon A48
Swansea services
Swansea (West) A483
Gorseinon A48
Swansea services
Entering the City and County of Swansea Entering Carmarthenshire
Pontarddulais, Llanelli A4138 J48 Pontarddulais, Llanelli A4138
199.2 320.6 Start of motorway J49
[coord 6]
Motorway terminates at a roundabout:
Carmarthen A48
Ammanford, Llandeilo A483
Pontarddulais A48
Pont Abraham services
Coordinate list
  1. ^ 51°29′22″N 0°16′40″W / 51.48944°N 0.27778°W / 51.48944; -0.27778 (Start of M4) Eastern end of M4
  2. ^ 51°29′41″N 0°29′44″W / 51.49472°N 0.49556°W / 51.49472; -0.49556 (Junction 4b of M4) Thorney Interchange – Intersection of M25 and M4
  3. ^ 51°27′14″N 1°18′52″W / 51.45389°N 1.31444°W / 51.45389; -1.31444 (Junction 13 of M4) Intersection of M4 and A34 (E05)
  4. ^ 51°33′01″N 2°33′11″W / 51.55028°N 2.55306°W / 51.55028; -2.55306 (J20 of M4) Almondsbury Interchange – Intersection of M4 and M5
  5. ^ 51°34′21″N 2°41′31″W / 51.57250°N 2.69194°W / 51.57250; -2.69194 (M4 Severn Crossing) Second Severn Crossing
  6. ^ 51°44′42″N 4°03′54″W / 51.74500°N 4.06500°W / 51.74500; -4.06500 (End of M4) Western end of M4

Data[59][60][61][62] 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. The entire route runs 6,530 kilometres (4,060 mi) between Cork in Ireland and Omsk in Russia.

Major incidents and accidents

See also


  1. ^ "The M4 London to South Wales Motorway. Holyport to Tormarton". The Motorway Archive Trust. Archived from the original on 24 June 2013. Retrieved 3 October 2010.
  2. ^ 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.
  3. ^ "The Oldest Motorway". The Motorway Archive Trust. Archived from the original on 22 January 2010. Retrieved 3 March 2012.
  4. ^ "Vandals' Damage To Motorway Cost Thousands Of Pounds". The Times. No. 56277. 23 March 1965. p. 16. Retrieved 1 March 2020.
  5. ^ "The M4 in Wales". The Motorway Archive Trust. Archived from the original on 22 January 2011. Retrieved 1 October 2010.
  6. ^ "On the road". The Motor. nbr 3625: 30. 23 December 1971.
  7. ^ "The Motorway Archive: M4 Second Severn Crossing". Iht.org. 28 April 1992. Archived from the original on 7 February 2012. Retrieved 31 December 2011.
  8. ^ "Drivers hold M4 speed camera demo". BBC News. 30 April 2005.
  9. ^ "M4 Motorway Widening". Archived from the original on 15 January 2008. Retrieved 20 November 2010.
  10. ^ "M4 in Wales to be 'hydrogen highway,' ministers to say". BBC News. 12 February 2010. Retrieved 12 February 2010.
  11. ^ Fraser, Douglas (8 September 2009). "'Hydrogen highway' plans backed". BBC News. Retrieved 12 February 2010.
  12. ^ "M4 Junction 11 Improvement Scheme". Highways Agency. Archived from the original on 26 June 2009.
  13. ^ "M4 Junction 11 Improvements". Reading Borough Council. Archived from the original on 13 July 2010.
  14. ^ "Reading is the fastest-growing economic centre in UK". Reading Chronicle. 10 July 2007.
  15. ^ "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.
  16. ^ "Severn tolls to be axed earlier than planned on 17 December". BBC News. 2 October 2018. Retrieved 21 January 2022.
  17. ^ "Dates:M4. Chiswick to Slough By-pass (J1 to J5)". The Motorway Archive Trust. Archived from the original on 24 June 2013. Retrieved 13 March 2013.
  18. ^ "Dates:M4 Slough-Maidenhead By-pass (Junctions 5 to 7) Statistics and options". The Motorway Archive Trust. Archived from the original on 24 June 2013. Retrieved 13 March 2013.
  19. ^ "Dates:M4. Maidenhead to Wickham (J8 to J14) Statistics and options". The Motorway Archive Trust. Archived from the original on 24 June 2013. Retrieved 13 March 2013.
  20. ^ "Dates:The Aust (J21) to Wickham (J14) section of M4 Statistics and options". The Motorway Archive Trust. Archived from the original on 24 June 2013. Retrieved 13 March 2013.
  21. ^ "Dates:M4 in Wales Statistics and options". The Motorway Archive Trust. Archived from the original on 24 June 2013. Retrieved 13 March 2013.
  22. ^ "History Overview". Severn River Crossing Plc. 2011. Archived from the original on 15 March 2013. Retrieved 16 March 2013.
  23. ^ "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.
  24. ^ Highways Agency: Our network Archived 14 March 2013 at the Wayback Machine. Retrieved 8 March 2013
  25. ^ "South Wales Trunk Road Agent (SWTRA)". Traffic Wales. Retrieved 24 October 2020.
  26. ^ "Average speed cameras for Port Talbot M4 stretch". BBC South West Wales. 14 July 2014. Retrieved 14 July 2014.
  27. ^ "M4 speed cameras generate half a million pounds". itv.com. 16 June 2015. Retrieved 5 March 2016.
  28. ^ "M4 J19-20 and M5 J15-17 Managed Motorways". Highways Agency. Retrieved 18 February 2012.
  29. ^ "Final section of M4 smart motorway upgrade completed". New Civil Engineer. Retrieved 2 May 2024.
  30. ^ "M4 junctions 3 to 12 smart motorway" (PDF). Highways Agency. Retrieved 27 December 2015.
  31. ^ England, Highways (20 May 2020). "M4 junctions 3-12: smart motorway - Highways England". highwaysengland.co.uk.
  32. ^ "Traffic chaos after M4 Brynglas tunnel lorry blaze". BBC News. 26 July 2011. Retrieved 9 April 2013.
  33. ^ "M4 tunnel fire: Brynglas tunnel reopens". BBC News. 29 July 2011. Retrieved 9 April 2013.
  34. ^ "M4 tunnel fire sparks relief road debate". BBC News. 27 July 2011. Retrieved 9 April 2013.
  35. ^ "U-turn". BBC. 18 January 2001. Retrieved 12 May 2010.
  36. ^ Edwards, Tom (11 September 2009). "M4 bus lane is 'barely enforced'". BBC News. Retrieved 12 May 2010.
  37. ^ "Stretch of M4 bus lane opens to all motorists". BBC News. 16 November 2010. Retrieved 16 November 2010.
  38. ^ "Work starts to remove M4 Bus Lane". Highways Agency. 12 November 2010. 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
  39. ^ "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
  40. ^ "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.
  41. ^ "One More Link to the West". New Scientist: 772. 25 March 1965.
  42. ^ Thornes, John Edward (1984). The Prediction of Ice Formation on Motorways in Britain (PDF) (Ph. D.). University College London. Retrieved 1 November 2015.
  43. ^ "This Sceptred Isle". Notes and Queries. Guardian. Retrieved 14 February 2016.
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Further reading

Geographic data related to M4 motorway at OpenStreetMap

KML is from Wikidata