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.
|Part of E30|
|Maintained by |
South Wales Trunk Road Agent
|Length||189 mi (304 km)|
|History||Opened: 1959 (Chiswick Flyover), 1961 (as A4(M)), 1963, 1966 (as A48(M))|
J4b → M25 motorway
J8/9 → A308(M) motorway/A404(M) motorway
J10 → A329(M) motorway
J19 → M32 motorway
J20 → M5 motorway
J21 → M48 motorway
J22 → M49 motorway
J23 → M48 motorway
J29 → A48(M) motorway
|Counties||Greater London, Buckinghamshire, Berkshire, Wiltshire, Gloucestershire, Monmouthshire, Newport, Cardiff, Vale of Glamorgan, Rhondda Cynon Taf, Bridgend, Neath Port Talbot, Swansea, Carmarthenshire|
The line of the motorway from London to Bristol runs closely in parallel with the A4. After crossing the River Severn, toll-free since 17 December 2018, the motorway follows the A48, to terminate at the Pont Abraham services in Carmarthenshire.
The M4 is the only motorway in Wales apart from its two spurs: the A48(M) and the M48. The major towns and cities along the route—a distance of approximately 189 miles (304 km)—include Slough, Reading, Swindon, Bristol, Newport, Cardiff, Bridgend, Port Talbot and Swansea.
The Chiswick flyover, a short section of elevated dual-carriageway, not originally classed as a motorway, opened in 1959 to reduce the impact of traffic travelling between central London and the west.
The Maidenhead bypass (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. The stretch from J18 to the west of Newport was opened in 1966, including the Severn Bridge. The Port Talbot by-pass, also built in the 1960s and now part of the M4, was originally the A48(M) motorway, a number now allocated to a short section of motorway near Cardiff. The Ministry of Transport originally intended that the M4 would terminate at Tredegar Park west of Newport, and following the creation of the Welsh Office that the Government became committed to a high-standard dual carriageway to Carmarthenshire.
The English section of the motorway was completed on 22 December 1971 when the 50-mile (80 km) stretch between junctions 9 and 15 (Maidenhead and Swindon) was opened to traffic. The Welsh section was 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. 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 by police camera vans between junction 14 and junction 18 led to a public protest, involving a "go-slow" of several hundred vehicles along the affected sections of the motorway.
Between 2007 and January 2010, the section from Castleton (junction 29) to Coryton (junction 32) was widened to six lanes. The scheme was formally opened on 25 January 2010 by Ieuan Wyn Jones the Deputy First Minister for Wales.
During 2009, the Newport section of the motorway between junctions 23a and 29 was upgraded with a new concrete central barrier. In February 2010 it was proposed that the M4 in South Wales would become the first hydrogen highway with hydrogen stations provided along the route, with an aspiration for further stations to be provided along the M4 into South West England over time.
Between 2008 and 2010, junction 11 was extensively remodelled with a new four-lane junction, two new road bridges and other works. The £65m scheme included work on the Mereoak roundabout and part of the A33 Swallowfield Bypass near Shinfield, and also the conversion of the two existing bridges, one of which is available only to pedestrians and cyclists and the other to buses. It also involved the movement of the local Highways Agency and Fire Service offices, and the construction of a long footbridge network, a new bus lane and a new gyratory. Sound barriers for nearby residential areas were also installed. In April 2008, the decision to preserve a rare Vickers machine gun pillbox and turn it into a bat roost was announced by the developers.
Tolls were charged on the Severn Bridge(s) from opening until 2018. In 1966 the toll was 12.5p each-way for cars, rising to 1 pound 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, and the former toll booths were removed in 2019.
Maintenance of the Second Severn Crossing and the 123 miles (198 km) of motorway in England is the responsibility of National Highways. The 76 miles (122 km) in Wales is the responsibility of the South Wales Trunk Road Agent.
For the majority of its length, the national speed limit applies. Exceptions include the following:
The M4 has one section of smart motorway, between junctions 19 (M32) and 20 (M5) north of Bristol, where there are variable speed limits and 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 limit with average speed cameras.
In 2010, it was announced that the motorway would be upgraded to a smart motorway between junctions 3 and 12. With a length of 32 miles (51 km), this will be the longest smart motorway scheme in the United Kingdom. Work started in autumn 2018 and is expected to be completed in March 2022 at a cost of £848 million.
The Brynglas Tunnels carry the M4 under Brynglas Hill in Newport, Wales. The 360-metre-long (390 yd) tunnels are the first and only twin–bored tunnels in the UK motorway network (the Dartford Tunnel is not classified as part of the M25 motorway). In July 2011, a lorry fire in one tunnel closed the motorway. Although there were no injuries and no deaths, the tunnel remained closed and a contraflow system was in place in the remaining tunnel for about one month, causing major travel delays.
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. The 3+1⁄2-mile (5.6 km) bus lane was on the eastbound carriageway; from the western end of the Chiswick Flyover near Brentford to junction 3 (A312), covering part of the 15-mile (24 km) journey between Heathrow Airport and central London. The lane which had no intermediate exits was for use by buses, coaches, motorcycles, emergency vehicles and licensed taxis but not mini-cabs.
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.
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.
The M4 has two of the three four-level stack interchanges in the UK, including the first UK example at junction 20, the "Almondsbury Interchange" with the M5. The other is junction 4b with the M25. Junction 4b has to make provision for the railway line passing beneath the M4. Due to the nature of these junctions, one cannot make a U-turn at either of them.
Junction 8/9 near Maidenhead, Berkshire, and High Wycombe, Buckinghamshire is the only one in the UK with dual numbers. This arose at the time when the M4 turned north near junction 8, where it met the A308, and headed for the original junction 9, where the motorway ended at a roundabout interchange with the A4. When the westward extension was opened, junction 8 was closed and a new junction built a little to the west, taking both numbers. The road to the A4 became A423(M) and later A404(M), and the junction with the A4 became 9B. Junction 9A is the exit for Cox Green and White Waltham. To the west of junction 13 on the eastbound carriageway there are a set of sliproads signposted "Works Unit Only". The signs have red borders, implying a military exit. It is a back entrance to RAF Welford, a Second World War airfield and now an RAF/USAF military installation mainly used for storing munitions. The M4 entrance allows easier access for the large vehicles used to carry the munitions.
Plans for the "missing" Junction 31, also known as the Thornhill interchange, for which planning permission was originally granted in September 1991 (but subsequently expired), were rekindled after proposals for a new business park on a 125 acres (51 ha) site north of the M4 were submitted in 2007 to Cardiff Council. The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469. A freedom of information request in 2010 to Cardiff Council shows that whilst the land that would enable this junction should continue to be strategically protected, the decision to formally abandon the proposed Junction 31 Thornhill was made in October 2007 and there had been no subsequent mention of it in Cardiff Council Strategic or Planning meetings since. In South Wales, the M4 has to thread its way through mountainous terrain and built-up areas, so there are some unusual junction layouts. Junction 27 (High Cross) is a normal grade-separated roundabout junction, but has severe space constraints: traffic joining the motorway must initially travel in the opposite direction to the intended direction of travel, before making a sharp left-hand turn from the slip road onto the motorway. At the time of construction, junction numbers 30 and 31 were reserved for future intermediate interchanges. Junction 30 (Cardiff Gate) has since been added, but there are no current plans to construct Junction 31 (A469 road). Junction 39 can only be used to access the motorway from a single slip road onto the westbound carriageway from the A48 at junction 38. There is no exit from the motorway at this junction.
Junction 41 comprises two different junctions; one for local traffic to and from the west and one from the east. The former leads to and from a spur leading to the roundabout in Briton Ferry, formerly known as junction 41a, and the original bridge over the River Neath, which would allow access onto the stretch of the M4 from junction 43 westward. The second, eastern junction leads to and from the A48 towards Port Talbot. As a result, one can travel for almost 2 miles (3.2 km) on the motorway in either direction, both joining and then leaving the motorway at junction 41. Junction 44 is unusual in that the eastbound entrance dives under the inside of the junction, effectively creating a "right-turn" on a roundabout. Similarly, slip roads pass under or over the main motorway at junctions 41 and 42.
There have been calls to close the slip roads at junctions 40 and 41 to improve traffic flow. The motorway has only two lanes on this stretch and is a major traffic congestion blackspot. The short slip roads have not been modernised. A small-scale trial of closing the westbound on-slip of junction 41 between 7 am and 9 am and from 4 pm to 6 pm on weekdays started on 4 August 2014 but following heavy criticism from local businesses and residents, was stopped on 29 May 2015.
On 5 June 2019, the Welsh Government scrapped the proposal for a proposed motorway south of Newport. The Welsh Assembly Government had revived the scheme as a tolled bypass in 2007 and later abandoned it for financial reasons. An extension to the Newport Southern Distributor Road through the old Corus steel works was considered. This road is already a dual carriageway. A public consultation exercise on options for improving the capacity of the M4 corridor around Newport opened on 5 March 2012. Its website states that: "the motorway around Newport does not conform to today’s motorway standards. It lacks continuous hard shoulders, has closely spaced junctions with sub-standard slip road visibility and narrows to a restricted two lane section through the Brynglas Tunnels. Heavy congestion occurs along this stretch and either side of it at peak hours."
|mile||km||Eastbound exits (B carriageway)||Junction||Westbound exits (A carriageway)|
|7.3||11.8||Road continues as A4 to Central London||J1
|Start of motorway|
|North Circular A406
South Circular A205
|J2||Staines, Hounslow, Brentford A4|
|Heston services||Services||Heston services|
|Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow A312||J3||Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow A312|
|Heathrow (Terminals 2 & 3) (A4)
|J4a||Heathrow (Terminals 2 & 3) (A4)|
|Heathrow (Terminals 4, 5 & Cargo) , Gatwick Airport , Watford (M3), (M23), (M40), (M1) M25||J4b
Harmondsworth Interchange[coord 2]
|Heathrow (Terminals 4, 5&Cargo), Gatwick, Watford (M3), (M23), (M40), (M1), M25|
|Entering Greater London||Entering Berkshire|
|Colnbrook, Langley, Slough (East) A4
Eton, Datchet B470
|J5||Colnbrook, Langley, Slough (East) A4, Datchet B470|
|Slough (Central) A355
|J6||Slough (Central) A355|
|Entering Berkshire||Entering Buckinghamshire|
|Slough (West) A4||J7||Slough (West) A4|
|Entering Buckinghamshire||Entering Berkshire|
|High Wycombe, Henley A404(M)
|J8/9||High Wycombe, Henley A404(M)|
|Reading (East), Wokingham, Bracknell A329(M)||J10||Reading (East), Wokingham, Bracknell A329(M)|
|Basingstoke, Reading (Central & South) A33||J11||Basingstoke, Reading (Central & South) A33|
|Reading services||Services||Reading services|
|Reading (West), Theale A4||J12||Reading (West), Theale A4|
|Newbury, Oxford A34
Chieveley services (Moto)
|Newbury, Oxford A34|
Chieveley services (Moto)
|Hungerford, Wantage A338||J14||Hungerford, Wantage A338|
|68.7||110.5||Membury services||Services||Membury services|
|Entering Berkshire||Entering Wiltshire|
|Swindon (East) A419
|J15||Swindon A419, Marlborough A346|
|Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham, Calne A3102||J16||Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham, Calne A3102|
|96.7||155.7||Leigh Delamere services||Services||Leigh Delamere services|
|Entering Wiltshire||Entering Gloucestershire|
|Bath, Stroud A46||J18||Bath, Stroud A46|
|Bristol M32||J19||Bristol M32|
|The South West, Bristol (West), The Midlands, Gloucester, Bristol Airport , M5||J20
|The South West, Bristol (West) M5(S)|
The Midlands, Gloucester M5(N)
|No access||J21||Chepstow M48|
|Avonmouth M49||J22||Avonmouth M49|
|Entering Gloucestershire||Second Severn Crossing
|Entry into Wales|
|Entry into England||Entering Gwent|
|127.0||204.4||No toll plaza||Bridge Toll||Former toll plaza (closed Dec 2018)|
|129.2||208.0||Chepstow M48||J23||No access|
|Magor, Caldicot A4810
|Magor, Caldicot A4810 |
|City centre A48
Newport (East) B4237
The Midlands (M50)
|J24||City centre A48|
Newport (East) B4237
|137.9||222.0||No access||J25||Caerleon B4596|
|138.3||222.6||No access||J25A||Newport, Cwmbran A4042|
|Brynglas Tunnels||Tunnel||Brynglas Tunnels|
|Newport, Cwmbran, Caerleon A4051||J26||Newport A4051|
|High Cross B4591||J27||High Cross B4591|
Risca, Brynmawr A467
Risca, Brynmawr A467
|143.6||231.1||No access||J29||Cardiff (East and South) A48(M)|
|Entering Gwent||Entering South Glamorgan|
|Cardiff (East) A4232
Cardiff Gate services
|Cardiff (East) A4232|
Cardiff Gate services
|Merthyr Tydfil, Cardiff A470||J32||Cardiff (North), Merthyr Tydfil A470|
|Cardiff (West), Barry, Penarth A4232
Cardiff West services
|Cardiff (West), Barry, Penarth A4232|
Cardiff West services
|Llantrisant, Rhondda A4119
Royal Glamorgan Hospital
|J34||Llantrisant, Rhondda A4119|
Royal Glamorgan Hospital
|Entering South Glamorgan||Entering Mid Glamorgan|
|Pen-coed A473||J35||Bridgend, Pen-coed A473|
Princess of Wales Hospital
Sarn Park services
Princess of Wales Hospital
Sarn Park services
|Pyle, Porthcawl A4229||J37||Porthcawl, Pyle A4229|
|Entering Mid Glamorgan||Entering West Glamorgan|
|Port Talbot A48||J38||Port Talbot A48|
|178.3||286.9||No access (on-ramp only)||J39||No access|
|Port Talbot A4107||J40||Port Talbot A4107|
|Port Talbot A48||J41||Briton Ferry A48|
|Briton Ferry A48
Swansea (South) A483
|J42||Swansea (South) A483|
|Neath, Merthyr Tydfil A465||J43||Neath, Merthyr Tydfil A465|
|Swansea (East) A48||J44||Swansea (East) A48|
|Pontardawe, Swansea (North) A4067||J45||Pontardawe, Swansea (North) A4067|
|Swansea (West) A483
|Swansea (West) A483|
|Entering West Glamorgan||Entering Dyfed|
|Pontarddulais, Llanelli A4138||J48||Pontarddulais, Llanelli A4138|
|199.2||320.6||Start of motorway||J49
|Motorway terminates at a roundabout:|
Ammanford, Llandeilo A483
Pont Abraham services
Data from driver location signs and location marker posts are used to provide distance and carriageway identification information. Where a junction spans several hundred metres and the data is available, both the start and finish values for the junction are shown.
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.
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