TheM4, originally theLondon-South Wales Motorway, is amotorway in the United Kingdom running fromwest London tosouthwest Wales. The English section to theSevern Bridge was constructed between 1961 and 1971; the Welsh element was largely complete by 1980, though a non-motorway section aroundBriton Ferry bridge remained until 1993. On the opening of theSecond Severn Crossing in 1996, the M4 was rerouted over it.
The line of the motorway fromLondon toBristol runs closely in parallel with theA4. After crossing theRiver Severn, toll-free since 17 December 2018, the motorway follows theA48, to terminate at thePont Abraham services inCarmarthenshire.
The M4 is the only motorway inWales apart from its twospurs: theA48(M) and theM48. The major towns and cities along the route—a distance of approximately 189 miles (304 km)—includeSlough,Reading,Swindon, Bristol,Newport,Cardiff,Bridgend,Port Talbot andSwansea.
A new road from London to South Wales was first proposed in the 1930s. In 1956 theMinistry of Transport announced the plans for the first major post-war road improvement projects.[1]
TheChiswick 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]
TheMaidenhead 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 theSevern Bridge. ThePort Talbot by-pass, also built in the 1960s and now part of the M4, was originally theA48(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 atTredegar Park west of Newport, and following the creation of theWelsh Office that the Government became committed to a high-standard dual carriageway toCarmarthenshire.[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 andSwindon) was opened to traffic.[6] The Welsh section was largely completed between 1970 and 1980, though a non-motorway section remained across theBriton Ferry bridge until 1993, when a second motorway-only bridge opened.
In 1996, theSecond 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 theM48, and theM49 was opened to link the new crossing with theM5 atAvonmouth.[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.
In April 2005, speed checks carried out bypolice 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 fromCastleton (junction 29) toCoryton (junction 32) was widened to six lanes.[9] The scheme was formally opened on 25 January 2010 byIeuan Wyn Jones theDeputy 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 firsthydrogen highway withhydrogen stations provided along the route, with an aspiration for further stations to be provided along the M4 intoSouth 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 nearShinfield, 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 localHighways Agency andFire Service offices, and the construction of a long footbridge network, a newbus lane and a newgyratory. Sound barriers for nearby residential areas were also installed.[14] In April 2008, the decision to preserve a rareVickers machine gunpillbox and turn it into abat roost was announced by the developers.[15]
Tolls were charged on the Severn Bridge(s) from opening until 2018. In 1966 the toll was 2s 6d (post-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 theRiver Severn were eliminated on 17 December 2018,[16] and the former toll booths were removed in 2019.
Year opened | 1961 | 1963 | 1965 | 1966 | 1967 | 1971 | 1972 | 1977 | 1980 | 1994 |
---|---|---|---|---|---|---|---|---|---|---|
Section(s) | J7–9 | J5–7 | J1–5 | J18–23 J39–41 | J22–28 | J9–18 | J44–46 | J28–29 J32–35 J37–39 J46–49 | J29–32 J35–37 | J41–44 |
Maintenance of the Second Severn Crossing and the 123 miles (198 km) of motorway in England is the responsibility ofNational Highways.[24] The 76 miles (122 km) in Wales is the responsibility of theSouth Wales Trunk Road Agent.[25]
For the majority of its length, thenational speed limit applies. Exceptions include the following:
The first section ofsmart 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 asmart 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]
TheBrynglas Tunnels carry the M4 underBrynglas Hill in Newport. The 360-metre-long (390 yd) tunnels are the first and only twin–bored tunnels in the UK motorway network (theDartford Tunnel is not classified as part of theM25 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]
In June 1999, theM4 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] The3+1⁄2-mile (5.6 km) bus lane was on the eastbound carriageway; from the western end of the Chiswick Flyover nearBrentford to junction 3 (A312), covering part of the 15-mile (24 km) journey betweenHeathrow 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 the2012 Summer Olympics and then permanently decommissioned.[39]
Near junction 35 of the M4, there is a stretch of the motorway that has a surfacing of porousasphalt that improves drainage and reduces noise. When driving in heavy rain drivers notice a reduction inroad spray from other vehicles and improved visibility. This special surface was publicised in an episode of the BBC'sTomorrow's World programme. This was the site of the first trial of the new road surface when it was laid down in 1993.[40]
The elevated section of the M4 in West London, built in the 1960s, is mostly directly above the A4 and extends over parts ofBrentford'sGolden Mile. This section was designed to have a heated road surface to reduce icing in winter.[41][42]
The M4 has two of the three four-levelstack 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.
Junction 8/9 nearMaidenhead,Berkshire, andHigh 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 aroundabout interchange with theA4. 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 laterA404(M), and the junction with the A4 became 9B. Junction 9A is the exit forCox Green andWhite 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 toRAF 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 theThornhillinterchange, for whichplanning 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 toCardiff Council.[44] The developers of the business park,St Modwen Developments, would likely fund the new junction, which would be on theA469.[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 inBriton Ferry, formerly known as junction 41a, and the original bridge over theRiver 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 towardsPort 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 theslip 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]
On 5 June 2019, theWelsh Government scrapped the proposal for a proposed motorway south ofNewport.[55] TheWelsh Assembly Government had revived the scheme as a tolled bypass in 2007 and later abandoned it for financial reasons.[56] An extension to the NewportSouthern Distributor Road through the oldCorus steel works was considered. This road is already a dual carriageway. A public consultation exercise on options for improving the capacity of theM4 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]
County | Location | mile | km | Jct | Destinations (EB) | Destinations (WB) | Notes |
---|---|---|---|---|---|---|---|
Greater London | London Borough of Ealing/London Borough of Hounslow/London Borough of Hillingdon | 7.3 | 11.8 | 1 | Road continues as![]() | Start of motorway | |
8.3 8.5 | 13.4 13.7 | 2 | ![]() ![]() ![]() | ![]() | |||
12.5 12.8 | 20.1 20.6 | Heston services | |||||
13.4 13.7 | 21.5 22.1 | 3 | Heathrow![]() | ||||
15.2 15.7 | 24.5 25.2 | 4 | Heathrow![]() UxbridgeA408 | ||||
16.8 17.7 | 27.0 28.5 | 4b Thorney Interchange [coord 1] | ![]() ![]()
| ||||
Berkshire/ Buckinghamshire | Slough–Windsor–High Wycombe boundary andMaidenhead | 19.1 19.5 | 30.7 31.4 | 5 | Colnbrook,Langley,Slough (East)A4 Eton,DatchetB470 | ||
22.9 23.2 | 36.8 37.4 | 6 | Slough (Central)A355 WindsorA322 | ||||
24.7 25.0 | 39.8 40.3 | 7 Trumpet interchange | Slough (West)A4 | ||||
27.8 28.2 | 44.7 45.4 | 8/9 | High Wycombe,HenleyA404(M) MaidenheadA308(M) | ||||
35.0 35.8 | 56.4 57.6 | Reading (East),Wokingham,BracknellA329(M) | J10 Partial cloverleaf interchange | Reading (East),Wokingham,BracknellA329(M) | |||
40.6 41.2 | 65.4 66.3 | Basingstoke,Reading (Central & South)A33 | J11 | Basingstoke,Reading (Central & South)A33 | |||
43.5 43.8 | 70.0 70.5 | Reading services | Services | Reading services | |||
45.3 45.7 | 72.9 73.5 | Reading (West),ThealeA4 | J12 | Reading (West),ThealeA4 | |||
56.9 57.2 | 91.5 92.1 | Newbury,OxfordA34 Chieveley services (Moto) | J13 Services [coord 2] | Newbury,OxfordA34 Chieveley services (Moto) | |||
64.4 64.8 | 103.7 104.3 | Hungerford,WantageA338 | J14 | Hungerford,WantageA338 | |||
68.7 | 110.5 | Membury services | Services | Membury services | |||
EnteringBerkshire | EnteringWiltshire | ||||||
76.4 77.1 | 122.9 124.0 | Swindon (Central & East)A419 MarlboroughA346 Oxford (A420) | J15 | Swindon (Central & East)A419,MarlboroughA346 | |||
82.4 82.8 | 132.6 133.2 | Swindon (West),Royal Wootton Bassett,Wroughton,MoD Lyneham,CalneA3102 | J16 | Swindon (West),Royal Wootton Bassett,Wroughton,MoD Lyneham,CalneA3102 | |||
94.8 95.3 | 152.5 153.4 | ChippenhamA350 CirencesterA429 | J17 | ChippenhamA350 CirencesterA429 | |||
96.7 | 155.7 | Leigh Delamere services | Services | Leigh Delamere services | |||
EnteringWiltshire | EnteringSouth Gloucestershire | ||||||
104.9 105.6 | 168.8 170.0 | Bath,StroudA46 | J18 | Bath,StroudA46 | |||
112.8 113.2 | 181.5 182.1 | Bristol M32 | J19 | BristolM32 | |||
115.8 116.3 | 186.3 187.2 | The South West,Bristol (West),The Midlands,Gloucester,Bristol Airport![]() | J20 Almondsbury Interchange [coord 3] | The South West, Bristol (West) M5(S) The Midlands, Gloucester M5(N) | |||
117.7 189.5 | 189.5 190.0 | No access | J21 | ChepstowM48 | |||
121.3 121.6 | 195.2 195.7 | Avonmouth,Bristol Airport![]() | J22 | Avonmouth M49 | |||
121.7 124.1 | 195.9 199.8 | EnteringSouth Gloucestershire | Second Severn Crossing [coord 4] | Entry into Wales | |||
Entry into England | EnteringMonmouthshire | ||||||
127.0 | 204.4 | No toll plaza | Bridge Toll | Former toll plaza (closed Dec 2018) | |||
129.2 | 208.0 | ChepstowM48 | J23 | No access | |||
131.4 132.2 | 211.4 212.8 | Magor,CaldicotA4810 Magor services | J23A Services | Magor,CaldicotA4810 Magor services | |||
EnteringMonmouthshire | Entering theCity and County Borough of Newport | ||||||
134.8 135.6 | 217.0 218.3 | City centreA48 Newport (East)B4237 MonmouthA449 The Midlands (M50) | J24 | City centreA48 Newport (East)B4237 MonmouthA449 | |||
137.9 | 222.0 | No access | J25 | CaerleonB4596 | |||
138.3 | 222.6 | No access | J25A | Newport (East),CwmbranA4042 | |||
138.8 139.1 | 223.4 223.8 | Brynglas Tunnels | Tunnel | Brynglas Tunnels | |||
139.1 139.5 | 223.8 224.5 | Newport (North & South),Cwmbran,CaerleonA4051 | J26 | Newport (North & South)A4051 | |||
140.8 141,4 | 226.6 227.6 | High CrossB4591 | J27 | High CrossB4591 | |||
142.0 142.9 | 228.5 230.0 | Newport (West)A48 Risca,BrynmawrA467 | J28 | Newport (West)A48 Risca,BrynmawrA467 | |||
143.6 | 231.1 | No access | J29 | Cardiff (South & East)A48(M) | |||
Entering theCity and County Borough of Newport | Entering theCity 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)A470 | J32 | Cardiff (North & Central),Merthyr TydfilA470 | |||
155.4 156.0 | 250.1 251.1 | Cardiff (West),Barry,PenarthA4232 Cardiff Airport ![]() Cardiff West services | J33 Services | Cardiff (West),Barry,PenarthA4232 Cardiff Airport ![]() Cardiff West services | |||
157.5 158.1 | 253.5 254.4 | Llantrisant,RhonddaA4119 Royal Glamorgan Hospital | J34 | Llantrisant,RhonddaA4119 Royal Glamorgan Hospital | |||
Entering theCity and County of Cardiff | Entering theVale of Glamorgan, theRhondda Cynon Taf,Bridgend County Borough andNeath Port Talbot | ||||||
163.9 164.4 | 263.7 264.5 | Pen-coedA473 | J35 | Bridgend,Pen-coedA473 | |||
167.5 168.3 | 269.6 270.8 | BridgendA4061 MaestegA4063 Princess of Wales Hospital Sarn Park services | J36 Services | BridgendA4061 MaestegA4063 Princess of Wales Hospital Sarn Park services | |||
172.8 173.5 | 278.1 279.3 | Pyle,PorthcawlA4229 | J37 | Porthcawl,PyleA4229 | |||
Entering theVale of Glamorgan, theRhondda Cynon Taf,Bridgend County Borough andNeath Port Talbot | Entering theCity and County of Swansea | ||||||
177.0 177.3 | 284.9 285.3 | Port TalbotA48 | J38 | Port TalbotA48 | |||
178.3 | 286.9 | No access (on-ramp only) | J39 | No access | |||
179.7 179.9 | 289.2 289.6 | Port TalbotA4107 | J40 | Port TalbotA4107 | |||
180.9 181.3 | 291.1 291.8 | Port TalbotA48 | J41 | Briton FerryA48 | |||
183.9 184.9 | 295.9 297.3 | Briton FerryA48 Swansea (South)A483 | J42 | Swansea (South)A483 | |||
184.9 185.7 | 297.5 298.8 | Neath,Merthyr TydfilA465 | J43 | Neath,Merthyr TydfilA465 | |||
187.0 187.4 | 300.9 301.6 | Swansea (East)A48 | J44 | Swansea (East)A48 | |||
188.8 189.3 | 303.9 304.7 | Pontardawe,Swansea (North & Central)A4067 | J45 | Pontardawe,Swansea (North & Central)A4067 | |||
190.8 191.4 | 307.0 308.0 | LlangyfelachB4489 Felindre Morriston Hospital | J46 | LlangyfelachB4489 Felindre Morriston Hospital | |||
192.4 193.1 | 309.6 310.7 | Swansea (West)A483 GorseinonA48 Swansea services | J47 Services | Swansea (West)A483 GorseinonA48 Swansea services | |||
Entering theCity and County of Swansea | EnteringCarmarthenshire | ||||||
196.4 197.1 | 316.0 317.2 | Pontarddulais,LlanelliA4138 | J48 | Pontarddulais,LlanelliA4138 | |||
199.2 | 320.6 | Start of motorway | J49 Terminus Services [coord 5] | Motorway terminates at a roundabout: CarmarthenA48 Ammanford,LlandeiloA483 Pontarddulais A48 Pont Abraham services | |||
Data[59][60][61][62] fromdriver 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) betweenCork in Ireland andOmsk in Russia.
The showbiz opening on 30 September 1959 was therefore a shrewd publicity stunt by Alderton's managing director, J E Dayton. It worked.
The suspension of the M4 Bus Lane is being carried out under an Experimental Order under the Road Traffic Regulation Act 1984
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
Select Telephone & marker post locations.
Geographic data related toM4 motorway atOpenStreetMap
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