M4 motorway explained

Country:GBR
Type:M
Route:4
Maint:
National Highways
South Wales Trunk Road Agent
Map Custom:yes
Photo Notes:Prince of Wales Bridge looking towards Wales
Length Mi:189
Established:1963
History:Opened: 1959 (Chiswick Flyover), 1961 (as A4(M)), 1963, 1966 (as A48(M))
Completed: 1996
Direction A:East
Direction B:West
Terminus A:Chiswick
51.4897°N -0.2781°W
Terminus B:Pont Abraham
51.7451°N -4.0651°W
Counties:Greater London, Buckinghamshire, Berkshire, Wiltshire, South Gloucestershire, Monmouthshire, Newport, Cardiff, Vale of Glamorgan, Rhondda Cynon Taf, Bridgend, Neath Port Talbot, Swansea, Carmarthenshire
Destinations:
London
Heathrow Airport
Slough
Maidenhead
Reading
Newbury
Swindon
Bristol
Newport
Cardiff
Bridgend
Port Talbot
Neath
Swansea
Junction:
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
Previous Type:M
Next Type:M
Previous Route:3
Next Route:5
Previous Dab:Great Britain

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 routea distance of approximately 189milesinclude Slough, Reading, Swindon, Bristol, Newport, Cardiff, Bridgend, Port Talbot and Swansea.

History

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]

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 50miles 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 £65million scheme included work on the Mereoak roundabout and part of the A33 Swallowfield Bypass near Shinfield, and also the conversion of the two existing bridges, one of which is available only to pedestrians and cyclists and the other to buses.[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 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

[17] [18] [19] [20] [21] [22]

Year opened1961196319651966196719711972197719801994
Section(s)J7–9J5–7J1–5J18–23
J39–41
J22–28J9–18J44–46J28–29
J32–35
J37–39
J46–49
J29–32
J35–37
J41–44

Maintenance

Maintenance of the Second Severn Crossing and the of motorway in England is the responsibility of National Highways.[23] The in Wales is the responsibility of the South Wales Trunk Road Agent.[24]

Features

Speed limits

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

Smart motorway

The first section of smart motorway on the M4, between junctions 19 (M32) and 20 (M5) north of Bristol, has variable speed limits and includes a part-time hard-shoulder. Completion was in summer 2014. Another section between junctions 24 and 29 in Newport had variable speed limits until 2021, when it was changed to a permanent 50mph limit with average speed cameras.[27]

In 2010, it was announced that the motorway would be changed to a smart motorway between junctions 3 and 12. With a length of, on completion it became the longest smart motorway scheme in the United Kingdom. Work started in autumn 2018 and was completed ahead of schedule in December 2021 at a cost of £848 million. Further conversions to smart motorways have been halted indefinitely.[28] [29] [30]

Brynglas tunnels

The Brynglas Tunnels carry the M4 under Brynglas Hill in Newport. The 360adj=midNaNadj=mid 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.[31] [32] [33]

Bus lane

In June 1999, the M4 bus lane was created on the third lane between junctions 2 and 3, initially as a pilot scheme and then a permanent arrangement from 2001. A lower speed limit was introduced along this section at the same time.[34] The NaNmiles 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 15miles 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.[35]

In December 2010, the bus lane was suspended for 18 months.[36] [37] It was reinstated temporarily for the 2012 Summer Olympics and then permanently decommissioned.[38]

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.[39]

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.[40] [41]

Four-level stack interchanges

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

Notable junctions

Junction 8/9 near 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.[42]

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 125acres site north of the M4 were submitted in 2007 to Cardiff Council.[43] The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469.[44] 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.[45] 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.[46]

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 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.[47]

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.[48] 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[49] [50] but following heavy criticism from local businesses and residents,[51] was stopped on 29 May 2015.[52] [53]

Relief road

See main article: M4 relief road. On 5 June 2019, the Welsh Government scrapped the proposal for a proposed motorway south of Newport.[54] The Welsh Assembly Government had revived the scheme as a tolled bypass in 2007 and later abandoned it for financial reasons.[55] 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:[56] "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."[57]

List of junctions

M4 motorway
scope=colmilescope=colkmscope=col abbr="Eastbound"Eastbound exits (B carriageway)scope=colJunctionscope=col abbr="Westbound"Westbound exits (A carriageway)
7.311.8Road continues as A4 to Central LondonJ1
[58]
Start of motorway
8.3
8.5
13.4
13.7
North Circular A406
South Circular A205
Chiswick A315
J2Staines, Hounslow, Brentford A4
12.5
12.8
20.1
20.6
Heston servicesServicesHeston services
13.4
13.7
21.5
22.1
Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow A312J3Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow A312
15.2
15.7
24.5
25.2
Heathrow (Terminals 2 & 3) (A4)
Uxbridge A408
J4aHeathrow (Terminals 2 & 3) (A4)
Uxbridge A408
16.8
17.7
27.0
28.5
Heathrow (Terminals 4, 5 & Cargo), Gatwick Airport, Dartford, Oxford, Watford, Stansted Airport, (M3), (M23), (M20), (M40), (M1), (A1(M)), (M11), M25J4b
Thorney Interchange
[59]
Heathrow (Terminals 4, 5 & Cargo), Gatwick, Dartford, Oxford, Watford, Stansted, (M3), (M23), (M20), (M40), (M1), (A1(M)), (M11), M25
style=background:skyblue Entering Greater Londonstyle=background:skyblue Entering Berkshire
19.1
19.5
30.7
31.4
Colnbrook, Langley, Slough (East) A4
Eton, Datchet B470
J5Colnbrook, Langley, Slough (East) A4, Datchet B470
22.9
23.2
36.8
37.4
Slough (Central) A355
Windsor A322
J6Slough (Central) A355
Windsor A322
style=background:skyblue Entering Berkshirestyle=background:skyblue Entering Buckinghamshire
24.7
25.0
39.8
40.3
Slough (West) A4J7
Trumpet interchange
Slough (West) A4
style=background:skyblue Entering Buckinghamshirestyle=background:skyblue Entering Berkshire
27.8
28.2
44.7
45.4
High Wycombe, Henley A404(M)
Maidenhead A308(M)
J8/9High Wycombe, Henley A404(M)
Maidenhead A308(M)
35.0
35.8
56.4
57.6
Reading (East), Wokingham, Bracknell A329(M)J10
Partial cloverleaf interchange
Reading (East), Wokingham, Bracknell A329(M)
40.6
41.2
65.4
66.3
Basingstoke, Reading (Central & South) A33J11Basingstoke, Reading (Central & South) A33
43.5
43.8
70.0
70.5
Reading servicesServicesReading services
45.3
45.7
72.9
73.5
Reading (West), Theale A4J12Reading (West), Theale A4
56.9
57.2
91.5
92.1
Newbury, Oxford A34
Chieveley services (Moto)
J13
Services
[60]
Newbury, Oxford A34
Chieveley services (Moto)
64.4
64.8
103.7
104.3
Hungerford, Wantage A338J14Hungerford, Wantage A338
68.7110.5Membury servicesServicesMembury services
style=background:skyblue Entering Berkshirestyle=background:skyblue Entering Wiltshire
76.4
77.1
122.9
124.0
Swindon (Central & East) A419
Marlborough A346
Oxford (A420)
J15Swindon (Central & East) A419, Marlborough A346
82.4
82.8
132.6
133.2
Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham, Calne A3102J16Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham, Calne A3102
94.8
95.3
152.5
153.4
Chippenham A350
Cirencester A429
J17Chippenham A350
Cirencester A429
96.7155.7Leigh Delamere servicesServicesLeigh Delamere services
style=background:skyblue Entering Wiltshirestyle=background:skyblue Entering South Gloucestershire
104.9
105.6
168.8
170.0
Bath, Stroud A46J18Bath, Stroud A46
112.8
113.2
181.5
182.1
Bristol M32J19Bristol M32
115.8
116.3
186.3
187.2
J20
Almondsbury Interchange
[61]
The South West, Bristol (West) M5(S)
The Midlands, Gloucester M5(N)
117.7
189.5
189.5
190.0
No accessJ21Chepstow M48
121.3
121.6
195.2
195.7
Avonmouth, Bristol Airport, Weston-super-Mare M49J22Avonmouth M49
121.7
124.1
195.9
199.8
style=background:skyblue Entering South GloucestershireSecond Severn Crossing
[62]
Entry into Wales
Entry into Englandstyle=background:skyblue Entering Monmouthshire
127.0204.4No toll plazaBridge TollFormer toll plaza (closed Dec 2018)
129.2208.0Chepstow M48J23No access
131.4
132.2
211.4
212.8
Magor, Caldicot A4810
Magor services
J23A
Services
Magor, Caldicot A4810
Magor services
style=background:skyblue Entering Monmouthshirestyle=background:skyblue Entering the City and County Borough of Newport
134.8
135.6
217.0
218.3
City centre A48
Newport (East) B4237
Monmouth A449
The Midlands (M50)
J24City centre A48
Newport (East) B4237
Monmouth A449
137.9222.0No accessJ25Caerleon B4596
138.3222.6No accessJ25ANewport (East), Cwmbran A4042
138.8
139.1
223.4
223.8
Brynglas TunnelsTunnelBrynglas Tunnels
139.1
139.5
223.8
224.5
Newport (North & South), Cwmbran, Caerleon A4051J26Newport (North & South) A4051
140.8
141,4
226.6
227.6
High Cross B4591J27High Cross B4591
142.0
142.9
228.5
230.0
Newport (West) A48
Risca, Brynmawr A467
J28Newport (West) A48
Risca, Brynmawr A467
143.6231.1No accessJ29Cardiff (South & East) A48(M)
style=background:skyblue Entering the City and County Borough of Newportstyle=background:skyblue Entering the City and County of Cardiff
146.6
147.3
236.0
237.0
Cardiff (East) A4232
Cardiff Gate services
J30
Services
Cardiff (East) A4232
Cardiff Gate services
Non- existent J31 Non-existent
151.9
244.5
244.5
246.1
Merthyr Tydfil, Cardiff (North & Central) A470J32Cardiff (North & Central), Merthyr Tydfil A470
155.4
156.0
250.1
251.1
Cardiff (West), Barry, Penarth A4232
Cardiff Airport
Cardiff West services
J33
Services
Cardiff (West), Barry, Penarth A4232
Cardiff Airport
Cardiff West services
157.5
158.1
253.5
254.4
Llantrisant, Rhondda A4119
Royal Glamorgan Hospital
J34Llantrisant, Rhondda A4119
Royal Glamorgan Hospital
style=background:skyblue Entering the City and County of Cardiffstyle=background:skyblue Entering the Vale of Glamorgan, the Rhondda Cynon Taf, Bridgend County Borough and Neath Port Talbot
163.9
164.4
263.7
264.5
Pen-coed A473J35Bridgend, Pen-coed A473
167.5
168.3
269.6
270.8
Bridgend A4061
Maesteg A4063
Princess of Wales Hospital
Sarn Park services
J36
Services
Bridgend A4061
Maesteg A4063
Princess of Wales Hospital
Sarn Park services
172.8
173.5
278.1
279.3
Pyle, Porthcawl A4229J37Porthcawl, Pyle A4229
style=background:skyblue Entering the Vale of Glamorgan, the Rhondda Cynon Taf, Bridgend County Borough and Neath Port Talbotstyle=background:skyblue Entering the City and County of Swansea
177.0
177.3
284.9
285.3
Port Talbot A48J38Port Talbot A48
178.3286.9No access (on-ramp only)J39No access
179.7
179.9
289.2
289.6
Port Talbot A4107J40Port Talbot A4107
180.9
181.3
291.1
291.8
Port Talbot A48J41Briton Ferry A48
183.9
184.9
295.9
297.3
Briton Ferry A48
Swansea (South) A483
J42Swansea (South) A483
184.9
185.7
297.5
298.8
Neath, Merthyr Tydfil A465J43Neath, Merthyr Tydfil A465
187.0
187.4
300.9
301.6
Swansea (East) A48J44Swansea (East) A48
188.8
189.3
303.9
304.7
Pontardawe, Swansea (North & Central) A4067J45Pontardawe, Swansea (North & Central) A4067
190.8
191.4
307.0
308.0
Llangyfelach B4489
Felindre
Morriston Hospital
J46Llangyfelach B4489
Felindre
Morriston Hospital
192.4
193.1
309.6
310.7
Swansea (West) A483
Gorseinon A48
Swansea services
J47
Services
Swansea (West) A483
Gorseinon A48
Swansea services
style=background:skyblue Entering the City and County of Swanseastyle=background:skyblue Entering Carmarthenshire
196.4
197.1
316.0
317.2
Pontarddulais, Llanelli A4138J48Pontarddulais, Llanelli A4138
199.2320.6Start of motorwayJ49
Terminus
Services
[63]
Motorway terminates at a roundabout:
Carmarthen A48
Ammanford, Llandeilo A483
Pontarddulais A48
Pont Abraham services

Notes and References

  1. Web site: The M4 London to South Wales Motorway. Holyport to Tormarton. The Motorway Archive Trust. 3 October 2010. https://web.archive.org/web/20130624044615/http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/m4-maidenhead-to-wickham-j8-to-j14/m4-london-to-south-wales-motorway-holyport-j89-to-tormarton-j18/index.cfm. 24 June 2013. dead.
  2. Web site: The 'sweet little Chiswick Flyover' hits 50 . Nick . Curtis . 1 October 2009 . . 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. . dead . https://web.archive.org/web/20110321084329/http://www.thisislondon.co.uk/lifestyle/article-23751213-the-sweet-little-chiswick-flyover-hits-50.do . 21 March 2011 . dmy-all .
  3. Web site: The Oldest Motorway . dead . https://web.archive.org/web/20100122131318/http://www.ciht.org.uk/motorway/stats.htm . 22 January 2010 . 3 March 2012 . The Motorway Archive Trust.
  4. News: Vandals' Damage To Motorway Cost Thousands Of Pounds . . 23 March 1965 . 56277 . 16 . 1 March 2020.
  5. Web site: The M4 in Wales. The Motorway Archive Trust. 1 October 2010. dead. https://web.archive.org/web/20110122052452/http://www.motorwayarchive.ihtservices.co.uk/m4wales.htm. 22 January 2011. dmy-all.
  6. On the road. The Motor. nbr 3625. 30. 23 December 1971.
  7. Web site: The Motorway Archive: M4 Second Severn Crossing . Iht.org . 28 April 1992 . 31 December 2011 . dead . https://web.archive.org/web/20120207181941/http://www.iht.org/motorway/m4sscscheme.htm . 7 February 2012 .
  8. News: Drivers hold M4 speed camera demo. BBC News. 30 April 2005.
  9. Web site: M4 Motorway Widening . 20 November 2010 . https://web.archive.org/web/20080115074004/http://www.m4widening29to32.com/overview.htm . 15 January 2008.
  10. News: M4 in Wales to be 'hydrogen highway,' ministers to say. BBC News. 12 February 2010. 12 February 2010.
  11. News: 'Hydrogen highway' plans backed. BBC News. 8 September 2009. 12 February 2010. Douglas. Fraser.
  12. Web site: M4 Junction 11 Improvement Scheme. Highways Agency. dead. https://web.archive.org/web/20090626122652/http://www.highways.gov.uk/roads/projects/19089.aspx. 26 June 2009. dmy-all.
  13. Web site: M4 Junction 11 Improvements. Reading Borough Council. dead. https://web.archive.org/web/20100713000524/http://www.reading.gov.uk/ltp/General.asp?id=SX9452-A7832CD2. 13 July 2010. dmy-all.
  14. News: Reading is the fastest-growing economic centre in UK. Reading Chronicle. 10 July 2007.
  15. Web site: M4 VICKERS MACHINE GUN PILLBOX, PILLBOX STUDY GROUP . Pillbox-study-group.org.uk . 11 April 2008 . 31 December 2011 . dead . https://archive.today/20120908073038/http://www.pillbox-study-group.org.uk/m4vmgpillboxpage.htm . 8 September 2012 . dmy-all .
  16. Web site: Severn tolls to be axed earlier than planned on 17 December. BBC News. 2 October 2018. 21 January 2022.
  17. Web site: Dates:M4. Chiswick to Slough By-pass (J1 to J5) . 13 March 2013 . The Motorway Archive Trust . https://web.archive.org/web/20130624060659/http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/m4-chiswick-to-slough-bypass-j1-to-j5/dates.cfm . 24 June 2013 . dead .
  18. Web site: Dates:M4 Slough-Maidenhead By-pass (Junctions 5 to 7) Statistics and options . 13 March 2013 . The Motorway Archive Trust . https://web.archive.org/web/20130624060152/http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/m4-sloughmaidenhead-bypass-junctions-5-to-7/dates.cfm . 24 June 2013 . dead .
  19. Web site: Dates:M4. Maidenhead to Wickham (J8 to J14) Statistics and options . 13 March 2013 . The Motorway Archive Trust . https://web.archive.org/web/20130624052809/http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/m4-maidenhead-to-wickham-j8-to-j14/dates.cfm . 24 June 2013 . dead .
  20. Web site: Dates:The Aust (J21) to Wickham (J14) section of M4 Statistics and options . 13 March 2013 . The Motorway Archive Trust . https://web.archive.org/web/20130624060550/http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/the-aust-j21-to-wickham-j14-section-of-m4/dates.cfm . 24 June 2013 . dead .
  21. Web site: Dates:M4 in Wales Statistics and options . 13 March 2013 . The Motorway Archive Trust . https://web.archive.org/web/20130624052654/http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/m4-wales/dates.cfm . 24 June 2013 . dead .
  22. Web site: History Overview . Severn River Crossing Plc . 2011 . 16 March 2013 . dead . https://web.archive.org/web/20130315074917/http://www.severnbridge.co.uk/history.shtml . 15 March 2013 .
  23. http://www.highways.gov.uk/our-road-network/our-network/ Highways Agency: Our network
  24. Web site: South Wales Trunk Road Agent (SWTRA) . Traffic Wales . 24 October 2020.
  25. News: Average speed cameras for Port Talbot M4 stretch. BBC South West Wales. 14 July 2014. 14 July 2014.
  26. News: M4 speed cameras generate half a million pounds . . 2015-06-16 . 2016-03-05.
  27. Web site: M4 J19-20 and M5 J15-17 Managed Motorways. Highways Agency. 18 February 2012.
  28. Web site: Final section of M4 smart motorway upgrade completed . 2 May 2024 . New Civil Engineer.
  29. Web site: M4 junctions 3 to 12 smart motorway . Highways Agency. 27 December 2015.
  30. Web site: M4 junctions 3-12: smart motorway - Highways England. Highways. England. 20 May 2020. highwaysengland.co.uk.
  31. Web site: Traffic chaos after M4 Brynglas tunnel lorry blaze. 26 July 2011 . 9 April 2013 . BBC News.
  32. Web site: M4 tunnel fire: Brynglas tunnel reopens. 29 July 2011 . 9 April 2013 . BBC News.
  33. Web site: M4 tunnel fire sparks relief road debate . 27 July 2011 . 9 April 2013 . BBC News.
  34. News: U-turn. BBC. 18 January 2001 . 12 May 2010.
  35. News: M4 bus lane is 'barely enforced' . BBC News . 11 September 2009 . 12 May 2010 . Tom . Edwards.
  36. News: Stretch of M4 bus lane opens to all motorists . 16 November 2010 . BBC News . 16 November 2010.
  37. Web site: 12 November 2010. Work starts to remove M4 Bus Lane. dead. https://web.archive.org/web/20120707142823/http://www.highways.gov.uk/news/pressrelease.aspx?pressreleaseid=416512. 7 July 2012. Highways Agency. The suspension of the M4 Bus Lane is being carried out under an Experimental Order under the Road Traffic Regulation Act 1984. dmy-all.
  38. News: Government to scrap M4 bus lane. BBC News. 1 October 2010. 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 lane which operates on the London-bound carriageway from near Heathrow.
  39. Web site: 2006 Fourth quarter foundation magazine – Operations and products . Hanson.biz . 31 May 2011 . 31 December 2011 . dead . https://web.archive.org/web/20080530154411/http://www.hanson.biz/files/pdf/magazine/2006/2006Q4_op.pdf . 30 May 2008 . dmy-all .
  40. One More Link to the West. New Scientist. 25 March 1965. 772.
  41. Thornes . John Edward . 1984 . The Prediction of Ice Formation on Motorways in Britain . Ph. D. . University College London . 1 November 2015.
  42. Web site: This Sceptred Isle. Notes and Queries. Guardian. 14 February 2016.
  43. News: Council will look again at case for M-way link. icwales. 23 June 2007.
  44. News: Business park plan 'would hit green land'. icwales. 19 December 2007.
  45. Web site: M4 J31 Freedom of information act. Salmon. Jeff. www.whatdotheyknow.com. 11 October 2010. 3 January 2016.
  46. Web site: M4 Exit List. CBRD. 14 February 2016. dead. https://web.archive.org/web/20160125221609/http://www.cbrd.co.uk/motorway/m4/exitlist. 25 January 2016. dmy-all.
  47. Web site: The M4 Motorway (Junction 41, Westbound Exit Slip Road, Sunnycroft Roundabout, Baglan, Neath Port Talbot) (50 MPH Speed Limit) Regulations 2010. Assembly for Wales. 14 February 2016.
  48. News: Are M4 plans on the right track?. South Wales Evening Post. 12 December 2007.
  49. News: Twice-a-day trial closure of Port Talbot M4 junction starts. BBC News. 4 August 2014.
  50. News: Controversial twice-day closure of M4 junction in Port Talbot starts. Wales Online. 4 August 2014.
  51. News: 200 attend meeting over M4 junction 41 Port Talbot closure. BBC News. 12 March 2015.
  52. News: M4 Port Talbot junction 41 trial closures end. BBC News. 29 May 2015.
  53. News: 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.
  54. News: M4 relief road: Proposals cost £114m before scheme was axed. BBC News. 5 June 2019.
  55. News: Plans for the M4 Toll (Newport) . BBC News . 20 September 2007 . 12 May 2010.
  56. https://www.m4cem.com/overview.html M4 corridor enhancement measures: Overview
  57. News: The New M4 Project – Magor to Castleton. Welsh Assembly Government. 13 July 2009.
  58. 51.4894°N -0.2778°W Eastern end of M4
  59. 51.4947°N -0.4956°W Thorney Interchange – Intersection of M25 and M4
  60. 51.4539°N -1.3144°W Intersection of M4 and A34 (E05)
  61. 51.5503°N -2.5531°W Almondsbury Interchange – Intersection of M4 and M5
  62. 51.5725°N -2.6919°W Second Severn Crossing
  63. 51.745°N -4.065°W Western end of M4