View source for M4 motorway

{{short description|Major motorway in England and Wales}}
{{other uses|M4 motorway (disambiguation)}}
{{Use British English|date=May 2013}}
{{Use dmy dates|date=February 2016}}

{{infobox road
| country=GBR
| type=M
| route=4
| maint = <br />[[National Highways]]<br />[[South Wales Trunk Road Agent]]
| map = {{switcher
  | {{maplink-road|from=M4 motorway.map}}M4 highlighted in blue{{hr}}
  | Show interactive map
  | [[File:M4 motorway (Great Britain) map.svg|frameless|x323px]]<br />Shown with UK motorway network{{hr}}
   | Show UK motorways map
   }}
| map_custom = yes
| map_notes =
| photo= New Severn Bridge - panoramio.jpg
| photo_notes= [[Prince of Wales Bridge (United Kingdom)|Prince of Wales Bridge]] looking towards [[Wales]], 2009
| length_mi=189
| length_ref=
| established= 1963
| history=Opened: 1959 ([[Chiswick Flyover]]), 1961 (as [[A4 road (England)|A4]](M)), 1963, 1966 (as [[A48(M) motorway|A48(M)]])<br />Completed: 1996
| direction_a= East
| direction_b= West
| terminus_a= [[Chiswick]]<br /><small>{{coord|51.4897|-0.2781|type:landmark_region:GB|display=inline|name=M4 motorway (eastern end)}}</small>
| terminus_b= [[Pont Abraham services|Pont Abraham]]<br /><small>{{coord|51.7451|-4.0651|type:landmark_region:GB|display=inline|name=M4 motorway (western end)}}</small>
| counties= [[Greater London]], [[Buckinghamshire]], [[Berkshire]], [[Wiltshire]], [[South Gloucestershire]], [[Monmouthshire]], [[Newport, Wales|Newport]], [[Cardiff]], [[Vale of Glamorgan]], [[Rhondda Cynon Taf]], [[Bridgend County Borough|Bridgend]], [[Neath Port Talbot]], [[Swansea]], [[Carmarthenshire]]
| destinations= <br />[[London]]<br />[[Heathrow Airport]]<br />[[Slough]]<br />[[Maidenhead]]<br />[[Reading, Berkshire|Reading]]<br />[[Newbury, Berkshire|Newbury]]<br />[[Swindon]]<br />[[Bristol]]<br />[[Newport, Wales|Newport]]<br />[[Cardiff]]<br />[[Bridgend]]<br />[[Port Talbot]]<br />[[Neath]]<br />[[Swansea]]
| e-road= {{Jct|country=EUR|E|30}}
| junction= [[File:Junction 4b.svg|1000x20px]] [[File:UK-Motorway-M25.svg|1000x20px]]<br />J4b → [[M25 motorway]]<br />[[File:Junction 8-9.svg|1000x20px]] [[File:UK-Motorway-A308 (M).svg|1000x20px]]/[[File:UK-Motorway-A404 (M).svg|1000x20px]]<br />J8/9 → [[A308(M) motorway]]/[[A404(M) motorway]]<br />[[File:Junction 10.svg|1000x20px]] [[File:UK-Motorway-A329 (M).svg|1000x20px]]<br />J10 → [[A329(M) motorway]]<br />[[File:Junction 19.svg|1000x20px]] [[File:UK-Motorway-M32.svg|1000x20px]]<br />J19 → [[M32 motorway]]<br />[[File:Junction 20.svg|1000x20px]] [[File:UK-Motorway-M5.svg|1000x20px]]<br />J20 → [[M5 motorway]]<br />[[File:Junction 21.svg|1000x20px]] [[File:UK-Motorway-M48.svg|1000x20px]]<br />J21 → [[M48 motorway]]<br />[[File:Junction 22.svg|1000x20px]] [[File:UK-Motorway-M49.svg|1000x20px]]<br />J22 → [[M49 motorway]]<br />[[File:Junction 23.svg|1000x20px]] [[File:UK-Motorway-M48.svg|1000x20px]]<br />J23 → [[M48 motorway]]<br />[[File:Junction 29.svg|1000x20px]] [[File:UK-Motorway-A48 (M).svg|1000x20px]]<br />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 the third-longest [[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 road (England)|A4]]. After crossing the [[River Severn]], toll-free since December 2018, the motorway follows the [[A48 road (Great Britain)|A48]], to terminate at the [[Pont Abraham services]] in [[Carmarthenshire]].

The M4 is the only motorway in [[Wales]] apart from its two [[Spur route|spurs]]: the [[A48(M) motorway|A48(M)]] and the [[M48 motorway|M48]]. The major towns and cities along the route{{mdash}}a distance of approximately {{convert|189|mi|km|0}}{{mdash}}include [[Slough]], [[Reading, Berkshire|Reading]], [[Swindon]], Bristol, [[Newport, Wales|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 (United Kingdom)|Ministry of Transport]] announced the plans for the first major post-war road improvement projects.<ref name=J8_J18>{{cite web|title=The M4 London to South Wales Motorway. Holyport to Tormarton|url=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|publisher=The Motorway Archive Trust|access-date=3 October 2010|archive-url=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|archive-date=24 June 2013|url-status=dead}}</ref>

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.<ref name="Curtis">{{cite web |url=http://www.thisislondon.co.uk/lifestyle/article-23751213-the-sweet-little-chiswick-flyover-hits-50.do |title=The 'sweet little Chiswick Flyover' hits 50 |first=Nick |last=Curtis |date=1 October 2009 |publisher=[[Evening Standard|This Is London]] |access-date=3 March 2012 |quote=The showbiz opening on 30 September 1959 was therefore a shrewd publicity stunt by Alderton's managing director, J E Dayton. It worked. |url-status=dead |archive-url=https://web.archive.org/web/20110321084329/http://www.thisislondon.co.uk/lifestyle/article-23751213-the-sweet-little-chiswick-flyover-hits-50.do |archive-date=21 March 2011 |df=dmy-all }}</ref><ref name="motorwayarchive1">{{cite web |title=The Oldest Motorway |url=http://www.ciht.org.uk/motorway/stats.htm |url-status=dead |archive-url=https://web.archive.org/web/20100122131318/http://www.ciht.org.uk/motorway/stats.htm |archive-date=22 January 2010 |access-date=3 March 2012 |publisher=The Motorway Archive Trust}}</ref>
[[File:M4 River Neath Crossing.jpg|thumb|left|M4 bridge over the [[River Neath]] (left) and 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.<ref>{{cite news |url=https://link.gale.com/apps/doc/CS270231671/TTDA?u=kccl&sid=TTDA&xid=a68c2f80 |title=Vandals' Damage To Motorway Cost Thousands Of Pounds |work=[[The Times]] |date=23 March 1965 |issue=56277 |page=16 |access-date=1 March 2020}}</ref> The stretch from J18 to the west of Newport was opened in 1966, including the [[Severn Bridge]].

[[File:Motorway construction (geograph 6936486).jpg|thumb|right|Construction in April 1980]]
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, but following the creation of the [[Welsh Office]] the government became committed to a high-standard dual carriageway to [[Carmarthenshire]].<ref>{{cite web|title=The M4 in Wales|url=http://www.motorwayarchive.ihtservices.co.uk/m4wales.htm|publisher=The Motorway Archive Trust|access-date=1 October 2010|url-status=dead|archive-url=https://web.archive.org/web/20110122052452/http://www.motorwayarchive.ihtservices.co.uk/m4wales.htm|archive-date=22 January 2011|df=dmy-all}}</ref>

[[File:Motorway construction - geograph.org.uk - 6936496.jpg|thumb|right|Construction in May 1980]]
The English section of the motorway was completed on 22 December 1971 when the {{convert|50|mi|km|adj=on}} stretch between junctions 9 and 15 (Maidenhead and [[Swindon]]) was opened to traffic.<ref name=Motor197112>{{cite journal |title = On the road| journal =[[The Motor (magazine)|The Motor]]| volume = nbr 3625| page = 30| date = 23 December 1971}}</ref> 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 motorway|M48]], and the [[M49 motorway|M49]] was opened to link the new crossing with the [[M5 motorway|M5]] at [[Avonmouth]].<ref>{{cite web|url=http://www.iht.org/motorway/m4sscscheme.htm |title=The Motorway Archive: M4 Second Severn Crossing |publisher=Iht.org |date=28 April 1992 |access-date=31 December 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120207181941/http://www.iht.org/motorway/m4sscscheme.htm |archive-date= 7 February 2012 }}</ref> 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 [[Mobile speed camera|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.<ref name="m4protest">{{cite news
|url=http://news.bbc.co.uk/1/hi/england/wiltshire/4498551.stm
|work=BBC News
|title=Drivers hold M4 speed camera demo
|date=30 April 2005
}}</ref>

Between 2007 and January 2010, the section from [[Castleton, Newport|Castleton]] (junction 29) to [[Coryton, Cardiff|Coryton]] (junction 32) was widened to six lanes.<ref>{{cite web |url=http://www.m4widening29to32.com/overview.htm  |title=M4 Motorway Widening |access-date=20 November 2010 |archive-url = https://web.archive.org/web/20080115074004/http://www.m4widening29to32.com/overview.htm |archive-date = 15 January 2008}}</ref> 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 station]]s provided along the route, with an aspiration for further stations to be provided along the M4 into [[South West England]] over time.<ref>{{cite news|url=http://news.bbc.co.uk/1/hi/wales/8511319.stm|title=M4 in Wales to be 'hydrogen highway,' ministers to say|work=BBC News|date=12 February 2010|access-date=12 February 2010}}</ref><ref>{{cite news|url=http://news.bbc.co.uk/1/hi/scotland/north_east/8245093.stm|title='Hydrogen highway' plans backed|work=BBC News|date=8 September 2009|access-date=12 February 2010|first=Douglas|last=Fraser}}</ref>

Between 2008 and 2010, junction 11 was extensively remodelled with a new four-lane junction, two new road bridges and other works.<ref>{{cite web|url=http://www.highways.gov.uk/roads/projects/19089.aspx|title=M4 Junction 11 Improvement Scheme|publisher=Highways Agency|url-status=dead|archive-url=https://web.archive.org/web/20090626122652/http://www.highways.gov.uk/roads/projects/19089.aspx|archive-date=26 June 2009|df=dmy-all}}</ref> The £65{{nbsp}}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.<ref>{{cite web|url=http://www.reading.gov.uk/ltp/General.asp?id=SX9452-A7832CD2|title=M4 Junction 11 Improvements|publisher=Reading Borough Council|url-status=dead|archive-url=https://web.archive.org/web/20100713000524/http://www.reading.gov.uk/ltp/General.asp?id=SX9452-A7832CD2|archive-date=13 July 2010|df=dmy-all}}</ref> It also involved the movement of the local [[Highways Agency]] and [[Fire service in the United Kingdom|Fire Service]] offices, and the construction of a long footbridge network, a new [[bus lane]] and a new [[Gyratory#Gyratory system|gyratory]]. Sound barriers for nearby residential areas were also installed.<ref name="m4juncreading">{{cite news|url=http://icberkshire.icnetwork.co.uk/chronicle/headlines/tm_headline=reading-is-the-fastest-growing-economic-centre-in-uk&method=full&objectid=19433560&siteid=50102-name_page.html|publisher=Reading Chronicle|title=Reading is the fastest-growing economic centre in UK|date=10 July 2007}}</ref> In April 2008, the decision to preserve a rare [[Vickers machine gun]] [[Bunker#Pillbox|pillbox]] and turn it into a [[Bat#Conservation|bat roost]] was announced by the developers.<ref>{{cite web |url=http://www.pillbox-study-group.org.uk/m4vmgpillboxpage.htm |title=M4 VICKERS MACHINE GUN PILLBOX, PILLBOX STUDY GROUP |publisher=Pillbox-study-group.org.uk |date=11 April 2008 |access-date=31 December 2011 |url-status=dead |archive-url=https://archive.today/20120908073038/http://www.pillbox-study-group.org.uk/m4vmgpillboxpage.htm |archive-date=8 September 2012 |df=dmy-all }}</ref>

===Former bridge tolls===
Tolls were charged on the Severn Bridge(s) from opening until 2018. In 1966 the toll was 2s&nbsp;6d (post-[[Decimal Day|decimalisation]] equivalent £0.125) 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,<ref>{{Cite web|url=https://www.bbc.co.uk/news/uk-wales-45467656|title=Severn tolls to be axed earlier than planned on 17 December|publisher=[[BBC News]]|date=2 October 2018|access-date=21 January 2022}}</ref> and the former toll booths were removed in 2019.

===Timeline of construction===
<ref>{{cite web  |title = Dates:M4. Chiswick to Slough By-pass (J1 to J5)  |url = http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/m4-chiswick-to-slough-bypass-j1-to-j5/dates.cfm  |access-date = 13 March 2013  |publisher = The Motorway Archive Trust  |archive-url = 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  |archive-date = 24 June 2013  |url-status = dead  }}</ref><ref>{{cite web |title = Dates:M4 Slough-Maidenhead By-pass (Junctions 5 to 7) Statistics and options |url = http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/m4-sloughmaidenhead-bypass-junctions-5-to-7/dates.cfm |access-date = 13 March 2013 |publisher = The Motorway Archive Trust |archive-url = 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 |archive-date = 24 June 2013 |url-status = dead }}</ref><ref>{{cite web |title = Dates:M4. Maidenhead to Wickham (J8 to J14) Statistics and options |url = http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/m4-maidenhead-to-wickham-j8-to-j14/dates.cfm |access-date = 13 March 2013 |publisher = The Motorway Archive Trust |archive-url = 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 |archive-date = 24 June 2013 |url-status = dead }}</ref><ref>{{cite web |title = Dates:The Aust (J21) to Wickham (J14) section of M4 Statistics and options |url = http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/the-aust-j21-to-wickham-j14-section-of-m4/dates.cfm |access-date = 13 March 2013 |publisher = The Motorway Archive Trust |archive-url = 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 |archive-date = 24 June 2013 |url-status = dead }}</ref><ref>{{cite web |title = Dates:M4 in Wales Statistics and options |url = http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/m4-wales/dates.cfm |access-date = 13 March 2013 |publisher = The Motorway Archive Trust |archive-url = https://web.archive.org/web/20130624052654/http://motorwayarchive.ihtservices.co.uk/en/motorways/motorway-listing/m4/m4-wales/dates.cfm |archive-date = 24 June 2013 |url-status = dead }}</ref><ref>{{cite web|url=http://www.severnbridge.co.uk/history.shtml |title=History Overview |publisher=Severn River Crossing Plc |year=2011 |access-date=16 March 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130315074917/http://www.severnbridge.co.uk/history.shtml |archive-date=15 March 2013 }}</ref>
{| class="wikitable"
|-
!Year opened
|1961||1963||1965||1966||1967||1971||1972||1977||1980||1994
|- valign="top"
!Section(s)
|J7–9||J5–7||J1–5||J18–23<br />J39–41||J22–28||J9–18||J44–46||J28–29<br />J32–35<br />J37–39<br />J46–49||J29–32<br />J35–37||J41–44
|}
[[File:M4 motorway map animated.gif|thumb|500px|Animated map showing build progress at five-years (or greater) intervals.<br />''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 motorway|M48]].<ref>{{cite web|url=http://www.halcrow.com/our-projects/project-details/second-severn-crossing-england-and-wales/ |title=Second Severn Crossing, England and Wales – Halcrow Group |publisher=Halcrow.com |date=29 October 2009 |access-date=16 March 2013 |url-status=dead |archive-url=https://web.archive.org/web/20100310164651/http://www.halcrow.com/Our-projects/Project-details/Second-Severn-Crossing-England-and-Wales/ |archive-date=10 March 2010 }}</ref>]]

=== Maintenance ===
Maintenance of the Second Severn Crossing and the {{convert|123|mi}} of motorway in England is the responsibility of [[National Highways]].<ref>[http://www.highways.gov.uk/our-road-network/our-network/  Highways Agency: Our network] {{webarchive|url=https://web.archive.org/web/20130314104707/http://www.highways.gov.uk/our-road-network/our-network |date=14 March 2013 }}. Retrieved 8 March 2013</ref> The {{convert|76|mi}} in Wales is the responsibility of the [[South Wales Trunk Road Agent]].<ref>{{cite web |title=South Wales Trunk Road Agent (SWTRA) |url=https://traffic.wales/south-wales-trunk-road-agent-swtra |website=Traffic Wales |access-date=24 October 2020}}</ref>

==Features==
===Speed limits===
[[File:Port Talbot & the M4 Corridor - geograph-3685831-by-Kevin-Corcoran (1).jpg|thumb|The M4 around Port Talbot]]
For the majority of its length, the [[national speed limit]] applies. Exceptions include the following:
* {{convert|40|mph|km/h}} on the [[Chiswick Flyover]] within London in both directions.
* {{convert|60|mph|km/h}} between junction 4 and the Chiswick Flyover, eastbound only.
* {{convert|50|mph|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 [[SPECS (speed camera)|average speed camera]] system was installed; it generated around £500,000 in fines in the first six months.<ref>{{cite news|title=Average speed cameras for Port Talbot M4 stretch|publisher=BBC South West Wales|date=14 July 2014|url=https://www.bbc.co.uk/news/uk-wales-south-west-wales-28265463|access-date=14 July 2014}}</ref><ref>{{cite news | title = M4 speed cameras generate half a million pounds | publisher = [[itv.com]] | date = 2015-06-16 | url = http://www.itv.com/news/wales/update/2015-06-16/m4-speed-cameras-generate-half-a-million-pounds/ | access-date = 2016-03-05}}</ref>
* {{convert|50|mph|km/h}} between junctions 24 and 28 at [[Newport, Wales|Newport]].

=== Smart motorway ===
[[File:Newport M4 junction 27.jpg|thumb|left|Construction of electronic indicator signs for the [[smart motorway|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 {{Convert | 50 | mph | abbr = on}} limit with average speed cameras.<ref name="j19-20 atm - ha main">{{cite web|url=https://www.highways.gov.uk/roads/projects/23382.aspx|publisher=[[Highways Agency]]|title=M4 J19-20 and M5 J15-17 Managed Motorways|access-date=18 February 2012|archive-date=9 February 2012|archive-url=https://web.archive.org/web/20120209162511/http://www.highways.gov.uk/roads/projects/23382.aspx|url-status=dead}}</ref>

In 2010, it was announced that the motorway would be changed to a [[smart motorway]] between junctions 3 and 12. With a length of {{convert|32|mi}}, 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&nbsp;million. Further conversions to smart motorways have been halted indefinitely.<ref>{{Cite web|url= https://www.newcivilengineer.com/latest/m4-smart-motorway-upgrade-05-12-2022/|access-date=2 May 2024 | title = Final section of M4 smart motorway upgrade completed | website = [[New Civil Engineer]]}}</ref><ref>{{cite web|title=M4 junctions 3 to 12 smart motorway | url = https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/372310/S140566_M4_junction3_to_12_consultation_brochure__for_web_-_final_.pdf|publisher=Highways Agency|access-date=27 December 2015}}</ref><ref>{{Cite web|url=https://highwaysengland.co.uk/our-work/south-east/m4-junctions-3-12-smart-motorway/|title=M4 junctions 3-12: smart motorway - Highways England|first=Highways|last=England|date=20 May 2020|website=highwaysengland.co.uk}}</ref>

===Brynglas tunnels===
[[File:M4 Motorway - Brynglas tunnels, western portals 2079471 3c135e6c.jpg|thumb|[[Brynglas Tunnels]] – western portals]]

The [[Brynglas Tunnels]] carry the M4 under [[Brynglas]] Hill in Newport. The {{convert|360|m|yd|adj=mid|-long}} 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.<ref>{{Cite web | url = https://www.bbc.co.uk/news/uk-wales-south-east-wales-14288312 | title = Traffic chaos after M4 Brynglas tunnel lorry blaze| date = 26 July 2011 | access-date = 9 April 2013 | work = BBC News}}</ref><ref>{{Cite web |url = https://www.bbc.co.uk/news/uk-wales-south-east-wales-14344483 | title = M4 tunnel fire: Brynglas tunnel reopens| date = 29 July 2011 | access-date = 9 April 2013 | work = BBC News}}</ref><ref>{{Cite web | url = https://www.bbc.co.uk/news/uk-wales-14318891 | title = M4 tunnel fire sparks relief road debate | date = 27 July 2011 | access-date = 9 April 2013 | work = BBC News}}</ref>

===Bus lane===
[[File:M4 bus lane 1017020.jpg|thumb|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.<ref name=bbc2001>{{cite news|title=U-turn|publisher=BBC|url=http://news.bbc.co.uk/1/hi/uk/1119193.stm | date=18 January 2001 | access-date=12 May 2010}}</ref> The {{convert|3+1/2|mi|adj=on}} 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 {{convert|15|mi|adj=on}} 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.<ref name=bbc2009>{{cite news| url=http://news.bbc.co.uk/2/hi/uk_news/england/london/8249919.stm | work=BBC News | title=M4 bus lane is 'barely enforced' | date=11 September 2009 | access-date=12 May 2010 | first=Tom | last=Edwards}}</ref>

In December 2010, the bus lane was suspended for 18 months.<ref name=buslaneremoval>{{cite news |title=Stretch of M4 bus lane opens to all motorists |date=16 November 2010 |work=BBC News |url=https://www.bbc.co.uk/news/uk-england-london-11763882 |access-date=16 November 2010}}</ref><ref>{{cite web|date=12 November 2010|title=Work starts to remove M4 Bus Lane|url=http://www.highways.gov.uk/news/pressrelease.aspx?pressreleaseid=416512|url-status=dead|archive-url=https://web.archive.org/web/20120707142823/http://www.highways.gov.uk/news/pressrelease.aspx?pressreleaseid=416512|archive-date=7 July 2012|publisher=Highways Agency|quote=The suspension of the M4 Bus Lane is being carried out under an Experimental Order under the Road Traffic Regulation Act 1984|df=dmy-all}}</ref> It was reinstated temporarily for the [[2012 Summer Olympics]] and then permanently decommissioned.<ref name=bbc_scrap>{{cite news|url=https://www.bbc.co.uk/news/uk-england-london-11451350|title=Government to scrap M4 bus lane|work=BBC News|date=1 October 2010|access-date=1 October 2010|quote=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 {{Convert|3.5|mi}} lane which operates on the London-bound carriageway from near Heathrow}}</ref>

===Porous road surface===
Near junction 35 of the M4, there is a stretch of the motorway that has a surfacing of porous [[asphalt concrete|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.<ref>{{cite web |url=http://www.hanson.biz/files/pdf/magazine/2006/2006Q4_op.pdf |title=2006 Fourth quarter foundation magazine – Operations and products |publisher=Hanson.biz |date=31 May 2011 |access-date=31 December 2011 |url-status=dead |archive-url=https://web.archive.org/web/20080530154411/http://www.hanson.biz/files/pdf/magazine/2006/2006Q4_op.pdf |archive-date=30 May 2008 |df=dmy-all }}</ref>

===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 (Brentford)|Golden Mile]]. This section was designed to have a heated road surface to reduce icing in winter.<ref>{{cite journal|title=One More Link to the West|journal=New Scientist|date=25 March 1965|page=772|url=https://books.google.com/books?id=57SmliUoH-QC&q=M4+motorway+London+heated+surface&pg=PA772}}</ref><ref>{{cite thesis |last=Thornes |first=John Edward |date=1984 |title=The Prediction of Ice Formation on Motorways in Britain |type=Ph. D. |publisher=University College London |url=http://discovery.ucl.ac.uk/1348922/1/336707.pdf |access-date=1 November 2015}}</ref>

===Four-level stack interchanges===
[[File:M4 M25 junction.jpg|thumb|M4 junction with the [[M25 Motorway|M25]] near [[Heathrow Airport]]]]
The M4 has two of the three four-level [[stack interchange]]s 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.{{cn|date=January 2023}} This dates from 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 road (England)|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) motorway|A404(M)]], and the junction with the A4 became 9B. Junction 9A is the exit for [[Cox Green, Berkshire|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/[[United States Air Force|USAF]] military installation mainly used for storing munitions. The M4 entrance allows easier access for the large vehicles used to carry the munitions.<ref>{{cite web|title=This Sceptred Isle|url=https://www.theguardian.com/notesandqueries/query/0,5753,-11482,00.html|website=Notes and Queries|publisher=Guardian|access-date=14 February 2016}}</ref>

Plans for the "missing" Junction 31, also known as the [[Thornhill, Cardiff|Thornhill]] [[Interchange (road)|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 {{convert|125|acre|ha|0}} site north of the M4 were submitted in 2007 to [[Cardiff Council]].<ref name="J31">{{cite news|url=http://icwales.icnetwork.co.uk/news/cardiff-news/2007/06/23/council-will-look-again-at-case-for-m-way-link-91466-19345314/|publisher=icwales|title=Council will look again at case for M-way link|date=23 June 2007}}</ref> The developers of the business park, [[St. Modwen Properties|St Modwen Developments]], would likely fund the new junction, which would be on the [[A469 road|A469]].<ref name="J31Dev">{{cite news|url=http://icwales.icnetwork.co.uk/business-in-wales/business-news/2007/12/19/business-park-plan-would-hit-green-land-91466-20266885/|publisher=icwales|title=Business park plan 'would hit green land'|date=19 December 2007}}</ref> 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.<ref>{{cite web|url=https://www.whatdotheyknow.com/request/m4_junction_31|last1=Salmon|first1=Jeff|title=M4 J31 Freedom of information act|website=www.whatdotheyknow.com|date=11 October 2010|access-date=3 January 2016}}</ref>

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, Newport|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.<ref>{{cite web|title=M4 Exit List|url=http://www.cbrd.co.uk/motorway/m4/exitlist|publisher=CBRD|access-date=14 February 2016|url-status=dead|archive-url=https://web.archive.org/web/20160125221609/http://www.cbrd.co.uk/motorway/m4/exitlist|archive-date=25 January 2016|df=dmy-all}}</ref>

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 {{convert|2|mi}} 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.<ref>{{cite web|title=The M4 Motorway (Junction 41, Westbound Exit Slip Road, Sunnycroft Roundabout, Baglan, Neath Port Talbot) (50 MPH Speed Limit) Regulations 2010|url=http://www.assembly.wales/Laid%20Documents/SUB-LD8271%20-%20The%20M4%20Motorway%20(Junction%2041,%20Westbound%20Exit%20Slip%20Road,%20Sunnycroft%20Roundabout,%20Baglan,%20Neath%20Port%20-28102010-201185/sub-ld8271-e-English.pdf|publisher=Assembly for Wales|access-date=14 February 2016}}</ref>

There have been calls to close the [[slip road]]s 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.<ref name="m4junc4041">{{
cite news|url=http://www.thisissouthwales.co.uk/displayNode.jsp?nodeId=161818&command=displayContent&sourceNode=161644&contentPK=19049034&folderPk=88499&pNodeId=161375
|publisher=[[South Wales Evening Post]]
|title=Are M4 plans on the right track?
|date=12 December 2007
}}</ref> A small-scale trial of closing the westbound on-slip of junction 41 between 7&nbsp;am and 9&nbsp;am and from 4&nbsp;pm to 6&nbsp;pm on weekdays started on 4&nbsp;August 2014<ref name="m4-41-start1">{{
cite news|url=https://www.bbc.co.uk/news/uk-wales-south-west-wales-28632091
|work=[[BBC News]]
|title=Twice-a-day trial closure of Port Talbot M4 junction starts
|date=4 August 2014
}}</ref><ref name="m4-41-start2">{{
cite news|url=http://www.walesonline.co.uk/news/wales-news/m4-port-talbot-controversial-twice-day-7557261
|publisher=[[Wales Online]]
|title=Controversial twice-day closure of M4 junction in Port Talbot starts
|date=4 August 2014
}}</ref> but following heavy criticism from local businesses and residents,<ref name="m4-41-critisism">{{
cite news|url=https://www.bbc.co.uk/news/uk-wales-south-west-wales-31837531
|work=[[BBC News]]
|title=200 attend meeting over M4 junction 41 Port Talbot closure
|date=12 March 2015
}}</ref> was stopped on 29 May 2015.<ref name="m4-41-stop1">{{
cite news|url=https://www.bbc.co.uk/news/uk-wales-south-west-wales-32932347
|work=[[BBC News]]
|title=M4 Port Talbot junction 41 trial closures end
|date=29 May 2015
}}</ref><ref name="m4-41-stop2">{{
cite news|url=http://www.walesonline.co.uk/news/wales-news/trial-closures-port-talbots-m4-9356682
|publisher=[[Wales Online]]
|title=Temporary closures of Port Talbot's M4 junction 41 are called off but slip road's future remains in doubt
|date=29 May 2015
}}</ref>

==Relief road==
{{main|M4 relief road}}
On 5 June 2019, the [[Welsh Government]] scrapped the proposal for a proposed motorway south of [[Newport, Wales|Newport]].<ref>{{Cite news|url=https://www.bbc.co.uk/news/uk-wales-politics-48519010|title=M4 relief road: Proposals cost £114m before scheme was axed|work=BBC News|date=5 June 2019}}</ref> The [[Welsh Assembly Government]] had revived the scheme as a tolled bypass in 2007 and later abandoned it for financial reasons.<ref>{{cite news|url=https://news.bbc.co.uk/2/hi/uk_news/wales/7003690.stm|title=Plans for the M4 Toll (Newport) | work=BBC News | date=20 September 2007 | access-date=12 May 2010}}</ref> An extension to the Newport [[Southern Distributor Road]] through the old [[Corus Group plc|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:<ref>[https://www.m4cem.com/overview.html M4 corridor enhancement measures: Overview].  Accessed 5 March 2012</ref> "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."<ref name="wag1">{{
cite news
|url=https://wales.gov.uk/publications/accessinfo/drnewhomepage/transportdrs2/transportdrs2009/m4projmagor-castleton/?lang=en
|publisher=Welsh Assembly Government
|title=The New M4 Project – Magor to Castleton
|date=13 July 2009
}}</ref>

==Junctions==
{{GeoGroup|maplink=yes|section=Junctions}}
{{-}}
{| class="plainrowheaders wikitable"
|-
!scope=col|County
!scope=col|Location
!scope=col|mile
!scope=col|{{tooltip|km|Kilometres}}
!scope=col|{{tooltip|Jct|Junction}}
!scope=col|Destinations (EB)
!scope=col|Destinations (WB)
!scope=col|Notes
|-
|rowspan=6|[[Greater London]]
|rowspan=6|[[London Borough of Ealing]]/[[London Borough of Hounslow]]/[[London Borough of Hillingdon]]
|style="background:#eaecf0;text-align:right;"|7.3
|style="background:#eaecf0;text-align:right;"|11.8
|style="text-align:center;"|1
|''Road continues as {{jct|country=GBR|A|4|dir2=east|location1=[[Central London]]}}''
| ''Start of motorway''
|- style="text-align:center;"
|style="background:#eaecf0;text-align:right;"|8.3<br />8.5
|style="background:#eaecf0;text-align:right;"|13.4 <br /> 13.7
|style="text-align:center;"|2
|{{jct|country=GBR|A|406|dir2=clockwise|name2=North Circular}}<br />{{jct|country=GBR|A|205|dir2=anti-clockwise|name2=South Circular|location2=Chiswick|to2=to|A|315}}
|{{jct|country=GBR|A|4|dir2=west|location1=[[Staines-upon-Thames|Staines]]|location2=[[Hounslow]]|location3=[[Brentford]]}}
|- style="text-align:center;"
|style="background:#eaecf0;text-align:right;"|12.5<br />12.8
|style="background:#eaecf0;text-align:right;"|20.1<br />20.6
|style="text-align:center" colspan=3|''[[Heston services]]''
|- style="text-align:center;"
|style="background:#eaecf0;text-align:right;"|13.4<br />13.7
|style="background:#eaecf0;text-align:right;"|21.5 <br /> 22.1
|style="text-align:center;"|3
|style="text-align:center" colspan=2|[[Heathrow Airport|Heathrow]] [[File:20 airtransportation.svg|15px]] (Terminals [[Heathrow Terminal 4|4]], [[Heathrow Terminal 5|5]] & Cargo), [[Hayes, Hillingdon|Hayes]], [[Harrow, London|Harrow]], [[Hounslow]] [[A312 road|A312]]
|- style="text-align:center;"
|style="background:#eaecf0;text-align:right;"|15.2<br />15.7
|style="background:#eaecf0;text-align:right;"|24.5 <br /> 25.2
|style="text-align:center;"|4
|style="text-align:center" colspan=2|[[Heathrow Airport|Heathrow]] [[File:20 airtransportation.svg|15px]] (Terminals [[Heathrow Terminal 2|2]] & [[Heathrow Terminal 3|3]]) [[A4 road (England)|(A4)]]<br /> [[Uxbridge]] [[A408 road|A408]]
|- style="text-align:center;"
|style="background:#eaecf0;text-align:right;"|16.8<br />17.7
|style="background:#eaecf0;text-align:right;"|27.0 <br /> 28.5
|style="text-align:center;"|4b<br />''[[Thorney Interchange]]''<br /><ref group=coord>{{coord|51|29|41|N|0|29|44|W|name=Junction 4b of M4}} Thorney Interchange – Intersection of M4 and M25</ref>
|style="text-align:center" colspan=2|{{jct|country=GBR|M|25|to2=to|M|40|noshield2=y|M|1|noshield3=yto2=to|M|11|noshield4=y|extra=airport|location1=[[London Luton Airport|Luton Airport]]|location2=[[Stansted Airport]]|city3=Oxford|city4=Watford|city5=Harlow}} <br />
{{jct|country=GBR|M|25|to2=to|M|3|dab2=Great Britain|noshield2=y|M|23|noshield3=y|extra=airport|location1=[[Heathrow Airport]] Terminals 4, 5  & Cargo|location2=[[London Gatwick Airport|Gatwick Airport]]|city3=Southampton}}
|- style="text-align:center;"
|rowspan=4|[[Berkshire]]/<br/>[[Buckinghamshire]]
|rowspan=4|[[Slough]]–[[Windsor, Berkshire|Windsor]]–[[High Wycombe]] boundary and [[Maidenhead]]
|style="background:#eaecf0;text-align:right;"|19.1<br />19.5
|style="background:#eaecf0;text-align:right;"|30.7 <br />31.4
|style="text-align:center;"|5
|style="text-align:center" colspan=2|[[Colnbrook]], [[Langley, Slough|Langley]], [[Slough|Slough (East)]] [[A4 road (England)|A4]]<br />[[Eton, Berkshire|Eton]], [[Datchet]] [[List of B roads in Great Britain|B470]]
|- style="text-align:center;"
|style="background:#eaecf0;text-align:right;"|22.9<br />23.2
|style="background:#eaecf0;text-align:right;"| 36.8 <br /> 37.4
|style="text-align:center;"|6
|style="text-align:center" colspan=2|[[Slough|Slough (Central)]] [[A355 road|A355]]<br />[[Windsor, Berkshire|Windsor]] [[A322 road|A322]]
|- style="text-align:center;"
|style="background:#eaecf0;text-align:right;"|24.7<br />25.0
|style="background:#eaecf0;text-align:right;"|39.8 <br /> 40.3
|style="text-align:center;"|7<br/><small>''[[Trumpet interchange]]''</small>
|style="text-align:center" colspan=2|[[Slough|Slough (West)]] [[A4 road (England)|A4]]
|- style="text-align:center;"
|style="background:#eaecf0;text-align:right;"|27.8<br />28.2
|style="background:#eaecf0;text-align:right;"|44.7 <br /> 45.4
|style="text-align:center;"|8/9
|style="text-align:center" colspan=2|[[High Wycombe]], [[Henley-on-Thames|Henley]] [[A404(M) motorway|A404(M)]]<br />[[Maidenhead]] [[A308(M) motorway|A308(M)]]
|- style="text-align:center;"
|rowspan=5|[[Berkshire]]
|rowspan=5|Bracknell Forest, Reading, Newbury
|35.0<br />35.8
| 56.4 <br /> 57.6
| [[Reading, Berkshire|Reading (East)]], [[Wokingham]], [[Bracknell]] [[A329(M) motorway|A329(M)]]
|J10<br/><small>''[[Partial cloverleaf interchange]]''</small>
| [[Reading, Berkshire|Reading (East)]], [[Wokingham]], [[Bracknell]] [[A329(M) motorway|A329(M)]]
|- style="text-align:center;"
|40.6<br />41.2
|65.4 <br />66.3
| [[Basingstoke]], [[Reading, Berkshire|Reading (Central & South)]] [[A33 road|A33]]
|J11
| [[Basingstoke]], [[Reading, Berkshire|Reading (Central & South)]] [[A33 road|A33]]
|- style="text-align:center;"
|43.5<br />43.8
|70.0 <br />70.5
|''[[Reading services]]''
|''Services''
|''[[Reading services]]''
|- style="text-align:center;"
|45.3<br />45.7
| 72.9 <br />73.5
| [[Reading, Berkshire|Reading (West)]], [[Theale]] [[A4 road (England)|A4]]
|J12
| [[Reading, Berkshire|Reading (West)]], [[Theale]] [[A4 road (England)|A4]]
|- style="text-align:center;"
|56.9<br />57.2
|91.5 <br /> 92.1
| [[Newbury, Berkshire|Newbury]], [[Oxford]] [[A34 road (England)|A34]]<br />''[[Chieveley services]] ([[Moto Hospitality|Moto]])''
|J13<br />''Services''<br /><ref group=coord>{{coord|51|27|14|N|1|18|52|W|name=Junction 13 of M4}} Intersection of M4 and A34 (E05)</ref>
| [[Newbury, Berkshire|Newbury]], [[Oxford]] [[A34 road (England)|A34]]<br />''[[Chieveley services]] ([[Moto Hospitality|Moto]]) ''
|- style="text-align:center;"
|64.4<br />64.8
|103.7 <br /> 104.3
| [[Hungerford]], [[Wantage]] [[A338 road|A338]]
|J14
| [[Hungerford]], [[Wantage]] [[A338 road|A338]]
|- style="text-align:center;"
|68.7
|110.5
|''[[Membury services]]''
|''Services''
|''[[Membury services]]''
|- style="text-align:center;"
|
|
|  ''Entering [[Berkshire]]''
|
|  ''Entering [[Wiltshire]]''
|- style="text-align:center;"
|76.4<br />77.1
|122.9 <br /> 124.0
| [[Swindon]] (Central & East) [[A419 road|A419]]<br />[[Marlborough, Wiltshire|Marlborough]] [[A346 road|A346]]<br />[[Oxford]] ([[A420 road|A420]])
|J15
| [[Swindon|Swindon (Central & East)]] [[A419 road|A419]], [[Marlborough, Wiltshire|Marlborough]] [[A346 road|A346]]
|- style="text-align:center;"
|82.4<br />82.8
|132.6 <br /> 133.2
| [[Swindon]] (West), [[Royal Wootton Bassett]], [[Wroughton]], [[MoD Lyneham]], [[Calne]] [[A3102 road|A3102]]
|J16
| [[Swindon|Swindon (West)]], [[Royal Wootton Bassett]], [[Wroughton]], [[MoD Lyneham]], [[Calne]] [[A3102 road|A3102]]
|- style="text-align:center;"
|94.8<br />95.3
|152.5 <br /> 153.4
| [[Chippenham]] [[A350 road|A350]]<br />[[Cirencester]] [[A429 road|A429]]
|J17
| [[Chippenham]] [[A350 road|A350]]<br />[[Cirencester]] [[A429 road|A429]]
|- style="text-align:center;"
|96.7
|155.7
|''[[Leigh Delamere services]]''
|''Services''
|''[[Leigh Delamere services]]''
|- style="text-align:center;"
|
|
|  ''Entering [[Wiltshire]]''
|
|  ''Entering [[South Gloucestershire]]''
|- style="text-align:center;"
|104.9<br />105.6
|168.8 <br /> 170.0
| [[Bath, Somerset|Bath]], [[Stroud]] [[A46 road (Great Britain)|A46]]
|J18
| [[Bath, Somerset|Bath]], [[Stroud]] [[A46 road (Great Britain)|A46]]
|- style="text-align:center;"
|112.8<br />113.2
|181.5 <br /> 182.1
| Bristol M32
|J19
| [[Bristol]] [[M32 motorway|M32]]
|- style="text-align:center;"
|115.8<br />116.3
|186.3 <br /> 187.2
| [[South West England|The South West]], [[Bristol]] (West), [[Midlands|The Midlands]], [[Gloucester]], [[Bristol Airport]] [[File:20 airtransportation.svg|15px]], [[M5 motorway|M5]]    
|J20<br />''[[Almondsbury Interchange]]''<br /><ref group=coord>{{coord|51|33|01|N|2|33|11|W|name=J20 of M4}} Almondsbury Interchange – Intersection of M4 and M5</ref>
| The South West, Bristol (West) M5(S)<br /> The Midlands, Gloucester M5(N)
|- style="text-align:center;"
|117.7<br />189.5
|189.5 <br /> 190.0
|{{no2}}No access
|J21
| [[Chepstow]] [[M48 motorway|M48]]
|- style="text-align:center;"
|121.3<br />121.6
|195.2 <br /> 195.7
| [[Avonmouth]], [[Bristol Airport]] [[File:20 airtransportation.svg|15px]], [[Weston-super-Mare]] [[M49 motorway|M49]]
|J22
| Avonmouth M49
|- style="text-align:center;"
| rowspan=2 | 121.7<br />124.1
| rowspan=2 | 195.9<br />199.8
|  ''Entering [[South Gloucestershire]]''
| rowspan=2 | ''[[Second Severn Crossing]]''<br /><ref group=coord>{{coord|51|34|21|N|2|41|31|W|name=M4 Severn Crossing}} Second Severn Crossing</ref>
| ''Entry into Wales''
|- style="text-align:center;"
| ''Entry into England''
|  ''Entering [[Monmouthshire]]''
|- style="text-align:center;"
|127.0
|204.4
| ''No toll plaza''
| ''Bridge Toll''
| style="background:#dcdcfe;"| ''Former toll plaza (closed Dec 2018)''
|- style="text-align:center;"
|129.2
|208.0
| [[Chepstow]] [[M48 motorway|M48]]
|J23
|{{no2}}No access
|- style="text-align:center;"
|131.4<br />132.2
|211.4<br />212.8
| [[Magor, Monmouthshire|Magor]], [[Caldicot, Monmouthshire|Caldicot]] [[List of A roads in Great Britain|A4810]]<br />''[[Magor services]]''
|J23A <br /> ''Services''
| [[Magor, Monmouthshire|Magor]], [[Caldicot, Monmouthshire|Caldicot]] [[List of A roads in Great Britain|A4810]] <br />''[[Magor services]]''
|- style="text-align:center;"
|
|
|  ''Entering [[Monmouthshire]]''
|
|  ''Entering the [[Newport, Wales|City and County Borough of Newport]]''
|- style="text-align:center;"
|134.8<br />135.6
|217.0<br />218.3
| [[Newport city centre|City centre]] [[A48 road (Great Britain)|A48]]<br />[[Newport, Wales|Newport]] (East) [[List of B roads in Great Britain|B4237]]<br />[[Monmouth]] [[A449 road|A449]]<br />[[Midlands|The Midlands]] ([[M50 motorway (Great Britain)|M50]])
|J24
| [[Newport city centre|City centre]] [[A48 road (Great Britain)|A48]]<br />[[Newport, Wales|Newport]] (East) [[List of B roads in Great Britain|B4237]]<br />[[Monmouth]] [[A449 road|A449]]
|- style="text-align:center;"
|137.9
|222.0
|{{no2}}No access
|J25
| [[Caerleon]] [[List of B roads in Great Britain|B4596]]
|- style="text-align:center;"
|138.3
|222.6
|{{no2}}No access
|J25A
| [[Newport, Wales|Newport]] (East), [[Cwmbran]] [[A4042 road|A4042]]
|- style="text-align:center;"
|138.8<br />139.1
|223.4<br />223.8
|''[[Brynglas Tunnels]]''
|''Tunnel''
|''[[Brynglas Tunnels]]''
|- style="text-align:center;"
|139.1<br />139.5
|223.8<br />224.5
| [[Newport, Wales|Newport]] (North & South), [[Cwmbran]], [[Caerleon]] [[A4051 road|A4051]]
|J26
| [[Newport, Wales|Newport]] (North & South) [[A4051 road|A4051]]
|- style="text-align:center;"
|140.8<br />141,4
|226.6<br />227.6
| [[High Cross, Newport|High Cross]] [[List of B roads in Great Britain|B4591]]
|J27
| [[High Cross, Newport|High Cross]] [[List of B roads in Great Britain|B4591]]
|- style="text-align:center;"
|142.0<br />142.9
|228.5<br />230.0
| [[Newport, Wales|Newport]] (West) [[A48 road|A48]]<br />[[Risca]], [[Brynmawr]] [[A467 road|A467]]
|J28
| [[Newport, Wales|Newport]] (West) [[A48 road|A48]]<br />[[Risca]], [[Brynmawr]] [[A467 road|A467]]
|- style="text-align:center;"
|143.6
|231.1
|{{no2}}No access
|J29
| [[Cardiff]] (South & East) [[A48(M) motorway|A48(M)]]
|- style="text-align:center;"
|
|
|  ''Entering the [[Newport, Wales|City and County Borough of Newport]]''
|
|  ''Entering the [[Cardiff|City and County of Cardiff]]''
|- style="text-align:center;"
|146.6<br />147.3
|236.0<br />237.0
| Cardiff (East) [[A4232 road|A4232]]<br />''[[Cardiff Gate services]]''
|J30 <br /> ''Services''
| Cardiff (East) [[A4232 road|A4232]]<br />''[[Cardiff Gate services]]''
|- style="text-align:center;"
|
|
| {{no2}} ''Non- existent''
| {{no2}} J31
| {{no2}} ''Non-existent''
|- style="text-align:center;"
|151.9<br />244.5
|244.5<br />246.1
| [[Merthyr Tydfil]], [[Cardiff]] (North & Central) [[A470 road|A470]]
|J32
| Cardiff (North & Central), [[Merthyr Tydfil]] [[A470 road|A470]]
|- style="text-align:center;"
|155.4<br />156.0
|250.1<br />251.1
| Cardiff (West), [[Barry, Vale of Glamorgan|Barry]], [[Penarth]] [[A4232 road|A4232]]<br />[[Cardiff Airport]] [[File:20 airtransportation.svg|15px]]<br />''[[Cardiff West services]]''
|J33<br />''Services''
| Cardiff (West), [[Barry, Vale of Glamorgan|Barry]], [[Penarth]] [[A4232 road|A4232]]<br />[[Cardiff Airport]] [[File:20 airtransportation.svg|15px]]<br />''[[Cardiff West services]]''
|- style="text-align:center;"
|157.5<br />158.1
|253.5<br />254.4
| [[Llantrisant]], [[Rhondda]] [[A4119 road|A4119]]<br />[[Royal Glamorgan Hospital]]
|J34
| [[Llantrisant]], [[Rhondda]] [[A4119 road|A4119]]<br />[[Royal Glamorgan Hospital]]
|- style="text-align:center;"
|
|
|  ''Entering the [[Cardiff|City and County of Cardiff]]''
|
|  ''Entering the [[Vale of Glamorgan]], the [[Rhondda Cynon Taf]], [[Bridgend County Borough]] and [[Neath Port Talbot]]''
|- style="text-align:center;"
|163.9<br />164.4
|263.7<br />264.5
| [[Pencoed|Pen-coed]] [[A473 road|A473]]
|J35
| [[Bridgend]], [[Pencoed|Pen-coed]] [[A473 road|A473]]
|- style="text-align:center;"
|167.5<br />168.3
|269.6<br />270.8
| [[Bridgend]] [[A4061 road|A4061]]<br />[[Maesteg]] [[A4063 road|A4063]]<br />[[Princess of Wales Hospital]]<br />''[[Sarn Park services]]''
|J36<br />''Services''
| [[Bridgend]] [[A4061 road|A4061]]<br />[[Maesteg]] [[A4063 road|A4063]]<br />[[Princess of Wales Hospital]]<br />''[[Sarn Park services]]''
|- style="text-align:center;"
|172.8<br />173.5
|278.1<br />279.3
| [[Pyle]], [[Porthcawl]] [[A4229 road|A4229]]
|J37
| [[Porthcawl]], [[Pyle]] [[A4229 road|A4229]]
|- style="text-align:center;"
|
|
|  ''Entering the [[Vale of Glamorgan]], the [[Rhondda Cynon Taf]], [[Bridgend County Borough]] and [[Neath Port Talbot]]''
|
|  ''Entering the [[Swansea|City and County of Swansea]]''
|- style="text-align:center;"
|177.0<br />177.3
|284.9<br />285.3
| [[Port Talbot]] [[A48 road|A48]]
|J38
| [[Port Talbot]] [[A48 road|A48]]
|- style="text-align:center;"
|178.3
|286.9
|{{no2}}No access (on-ramp only)
|J39
|{{no2}}No access
|- style="text-align:center;"
|179.7<br />179.9
|289.2<br />289.6
| [[Port Talbot]] [[A4107 road|A4107]]
|J40
| [[Port Talbot]] [[A4107 road|A4107]]
|- style="text-align:center;"
|180.9<br />181.3
|291.1<br />291.8
| [[Port Talbot]] [[A48 road|A48]]
|J41
| [[Briton Ferry]] [[A48 road|A48]]
|- style="text-align:center;"
|183.9<br />184.9
|295.9<br />297.3
| [[Briton Ferry]] [[A48 road|A48]]<br />[[Swansea|Swansea (South)]] [[A483 road|A483]]
|J42
| [[Swansea|Swansea (South)]] [[A483 road|A483]]
|- style="text-align:center;"
|184.9<br />185.7
|297.5<br />298.8
| [[Neath]], [[Merthyr Tydfil]] [[A465 road|A465]]
|J43
| [[Neath]], [[Merthyr Tydfil]] [[A465 road|A465]]
|- style="text-align:center;"
|187.0<br />187.4
|300.9<br />301.6
| [[Swansea|Swansea (East)]] [[A48 road|A48]]
|J44
| [[Swansea|Swansea (East)]] [[A48 road|A48]]
|- style="text-align:center;"
|188.8<br />189.3
| 303.9<br />304.7
| [[Pontardawe]], [[Swansea|Swansea (North & Central)]] [[A4067 road|A4067]]
|J45
| [[Pontardawe]], [[Swansea|Swansea (North & Central)]] [[A4067 road|A4067]]
|- style="text-align:center;"
|190.8<br />191.4
| 307.0<br />308.0
| [[Llangyfelach]] [[List of B roads in Great Britain|B4489]]<br />[[Felindre, Swansea|Felindre]]<br />[[Morriston Hospital]]
|J46
| [[Llangyfelach]] [[List of B roads in Great Britain|B4489]]<br />[[Felindre, Swansea|Felindre]]<br />[[Morriston Hospital]]
|- style="text-align:center;"
|192.4<br />193.1
|309.6<br />310.7
| [[Swansea|Swansea (West)]] [[A483 road|A483]]<br />[[Gorseinon]] [[A48 road|A48]]<br />''[[Swansea services]]''
|J47<br />''Services''
| [[Swansea|Swansea (West)]] [[A483 road|A483]]<br />[[Gorseinon]] [[A48 road|A48]]<br />''[[Swansea services]]''
|- style="text-align:center;"
|
|
|  ''Entering the [[Swansea|City and County of Swansea]]''
|
|  ''Entering [[Carmarthenshire]]''
|- style="text-align:center;"
|196.4<br />197.1
|316.0<br />317.2
| [[Pontarddulais]], [[Llanelli]] [[A4138 road|A4138]]
|J48
| [[Pontarddulais]], [[Llanelli]] [[A4138 road|A4138]]
|- style="text-align:center;"
|199.2
| 320.6
| ''Start of motorway''
|J49<br />Terminus<br />''Services''<br /><ref group=coord>{{coord|51|44|42|N|4|03|54|W|name=End of M4}} Western end of M4</ref>
| Motorway terminates at a roundabout:<br />[[Carmarthen]] [[A48 road|A48]]<br />[[Ammanford]], [[Llandeilo]] [[A483 road|A483]]<br />Pontarddulais A48<br />''[[Pont Abraham services]]''
|-
{{jctbtm|keys=incomplete|conv=no}}

;Coordinate list
<references group=coord/>
Data<ref>Area 3 Driver Location Signs (map) – Highway Authority, 2009</ref><ref>Driver Location Signs, M5 J18-11, M4 J22-15 (map) Highway Authority 2009</ref><ref>{{cite web
 |url=http://www.trafficengland.com/map.aspx
 |title=Traffic England Live Traffic Condition Map ''(selected Popups)''
 |publisher=Highways Agency
 |access-date=5 November 2009
 |url-status=dead
 |archive-url=https://web.archive.org/web/20120210221222/http://www.trafficengland.com/map.aspx
 |archive-date=10 February 2012
}}</ref><ref>{{cite web
 |url=http://www.traffic-wales.com/resources
 |title=Resources
 |quote=Select Telephone & marker post locations.
 |publisher=Traffig Cymru/Traffic Wales
 |access-date=22 August 2011
 |url-status=dead
 |archive-url=https://web.archive.org/web/20110903120036/http://www.traffic-wales.com/resources
 |archive-date= 3 September 2011
}}</ref> from [[Driver location signs|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.

===E30===
Although not signed, [[European route E30]] includes most of the M4. The entire route runs {{convert|6,530|km}} between [[Cork (city)|Cork]] in Ireland and [[Omsk]] in Russia.

==Major incidents and accidents==
* In June 1984 a crash near [[Maidenhead]] resulted in 13 deaths.<ref name="Telegraph 2003"/>
* In March 1991, ten people died in the [[1991 M4 motorway crash]] near [[Hungerford]]. The crash involved 51 vehicles in foggy conditions.<ref>{{cite news |last1=Holt |first1=Gerry |title=M5 motorway crash: How safe are Britain's motorways? |url=https://www.bbc.co.uk/news/uk-15608217 |access-date=4 June 2020 |work=BBC News |date=6 November 2011}}</ref>
* In May 1995 a coach carrying [[Royal British Legion]] members left the M4 close to the Severn Bridge, resulting in 10 deaths.<ref name="Telegraph 2003">{{cite news |title=Road death toll mounts |url=https://www.telegraph.co.uk/news/1435738/Road-death-toll-mounts.html |archive-url=https://ghostarchive.org/archive/20220112/https://www.telegraph.co.uk/news/1435738/Road-death-toll-mounts.html |archive-date=12 January 2022 |url-access=subscription |url-status=live |work=[[The Daily Telegraph]] |date=10 July 2003 |location=London}}{{cbignore}}</ref>
* 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.<ref>{{cite news | title=Record producer dies in crash | url=http://news.bbc.co.uk/1/hi/entertainment/2144817.stm | publisher=[[BBC News Online]] | access-date=13 February 2015 | date=22 July 2002}}</ref>
* In May 2025 a white Ford Transit minibus overturned on the link road at junction 10 of the M4 to the A329(M) near Reading, Berkshire, at about 14:25 BST on 11 May. A six-year-old boy was also killed in the crash. Five other children injured in the incident have since been discharged from hospital.

==See also==
{{portal|Roads|Geography}}

* [[M4 corridor]]
* [[M4 Thames Bridge]]
* [[List of motorways in the United Kingdom]]
* [[Transport in Wales]]

==References==
{{reflist}}

==Further reading==
* {{cite book|last=Jackson|first=Mike|title= The M4 Sights Guide|year=2005|publisher=Severnpix|isbn=978-0954540227}}

==External links==
{{OSM relation|13007988|M4 motorway|link=yes}}
{{Commons category}}
{{GeoGroup}}
{{Attached KML|display=inline,title}}
* [http://www.cbrd.co.uk/motorway/m4/ CBRD Motorway Database&nbsp;– M4]
* [http://www.cbrd.co.uk/motorway/a404m/ British Road Database: Motorways&nbsp;– M4 Junction 8/9]
* The Motorway Archive
** [https://web.archive.org/web/20070127094705/http://www.iht.org/motorway/m4chisslou.htm Junctions 1 to 5]
** [https://web.archive.org/web/20070311002900/http://www.iht.org/motorway/m4slomai.htm Junctions 5 to 7]
** [https://web.archive.org/web/20070102140113/http://www.iht.org/motorway/m4londswind.htm Junctions 8 to 14]
** [https://web.archive.org/web/20071008114756/http://www.iht.org/motorway/m4lswscheme.htm Junctions 14 to 21]
** [https://web.archive.org/web/20071008115613/http://www.iht.org/motorway/m4sscscheme.htm Junctions 21 to 23]
** [https://web.archive.org/web/20081231035742/http://www.iht.org/motorway/m4wales.htm Junctions 23 to 49]
*  [http://www.m4newport.com/assets/preferred-route-context-plan.pdf Map of the preferred route around Newport (relief road)]

{{UK motorways}}
{{Roads in London}}
{{Navboxes
|title = [[Transport in England]] by local authority
|list  =
{{Transport in Bristol}}
{{Transport in Buckinghamshire}}
{{Transport in London}}
}}
{{Motorways and Trunk Roads in England|state=collapsed}}
{{Motorways and Trunk Roads in Wales}}
{{Navboxes
|title = [[Transport in Wales]] by local authority
|list  =
{{Transport in Bridgend county borough}}
{{Transport in Cardiff}}
{{Transport in Carmarthenshire}}
{{Transport in Monmouthshire}}
{{Transport in Neath Port Talbot}}
{{Transport in Newport}}
{{Transport in Rhondda Cynon Taf}}
{{Transport in Swansea}}
}}

{{Transport in Gloucestershire}}

{{DEFAULTSORT:4-0004m}}
[[Category:M4 motorway| ]]
[[Category:Constituent roads of European route E30]]
[[Category:Motorways in England]]
[[Category:Motorways in London]]
[[Category:Motorways in Wales]]
[[Category:Roads in Berkshire]]
[[Category:Roads in Bristol]]
[[Category:Transport in Bridgend County Borough]]
[[Category:Transport in Cardiff]]
[[Category:Transport in Carmarthenshire]]
[[Category:Transport in the London Borough of Hillingdon]]
[[Category:Transport in the London Borough of Hounslow]]
[[Category:Transport in Monmouthshire]]
[[Category:Transport in Neath Port Talbot]]
[[Category:Transport in Newport, Wales]]
[[Category:Transport in Rhondda Cynon Taf]]
[[Category:Transport in South Gloucestershire District]]
[[Category:Transport in Swansea]]
[[Category:Transport in Wiltshire]]