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M4
Prince of Wales Bridge looking towards Wales
Route information
Part of
Maintained by Highways England South Wales Trunk Road Agent
Length	189 mi[1] (304 km)
Existed	1963–present
History	Opened: 1959 (Chiswick Flyover), 1961 (as A4(M)), 1963, 1966 (as A48(M)) Completed: 1996
Major junctions
East end	Chiswick 51°29′23″N 0°16′41″W / 51.4897°N 0.2781°W / 51.4897; -0.2781 (M4 motorway (eastern end))
	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
To	Pont Abraham 51°44′42″N 4°03′54″W / 51.7451°N 4.0651°W / 51.7451; -4.0651 (M4 motorway (western end))
Location
Counties	Greater London, Berkshire, Wiltshire, Gloucestershire, Monmouthshire, Newport, Cardiff, Vale of Glamorgan, Rhondda Cynon Taf, Bridgend, Neath Port Talbot, Swansea, Carmarthenshire
Primary destinations	London Heathrow Airport Slough Maidenhead Reading Newbury Swindon Bristol Newport Cardiff Bridgend Port Talbot Neath Swansea
Road network
Roads in the United Kingdom Motorways A and B road zones ← M3 M5 →

The M4 is a motorway in the United Kingdom running from West London to South Wales. Towns and cities along the route include Slough, Reading, Swindon, Bristol, Newport, Cardiff and Swansea. Originally referred to as the London-South Wales Motorway, the English section to the Severn Bridge over the River Severn was constructed between 1961 and 1971; the Welsh element was completed in 1993. Another Severn bridge, known as the Second Severn Crossing, was opened in 1996 and the M4 was rerouted.

From London to Bristol, the M4 runs close to the A4, and after crossing the River Severn, following the A48 through South Wales, using the Brynglas Tunnels in Newport, it terminates north of Pontarddulais. The M4 is one of only three motorways in Wales; the others being the A48(M) and M48 spurs. Although not signed, European route E30 includes most of the M4.

History

A new road from London to South Wales was first proposed in the 1930s, and the Ministry of Transport announced plans for the M4 as one of the first major post-war trunk road improvement projects in 1956.^[2]

The Chiswick flyover, a short section of elevated dual-carriageway built to reduce the impact of traffic travelling between central London and the west, opened in 1959;^[3] it was not originally classed as a motorway.^[4]
The Maidenhead bypass opened in 1961 whilst J1-J5 opened in 1965. The stretch from J18 to the west of Newport was opened in 1966, including the Severn Bridge (now part of the M48). 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 it was only following the creation of the Welsh Office that the Government became committed to a high-standard dual carriageway to Pont Abraham in Carmarthenshire.^[5]

The English section of the motorway was completed on 22 December 1971 when the 50-mile (80 km) stretch between Junctions 9 and 15 (Maidenhead and Swindon) was opened to traffic.^[6] The Welsh section was completed in 1993, when the Briton Ferry motorway bridge opened. The Second Severn Crossing opened in 1996, together with new link motorways on either side of the estuary to divert the M4 over the new crossing. The existing route over the Severn Bridge was redesignated the M48, and the new M49 was opened to connect the new crossing to the M5.^[7]

The Second Severn Crossing

In June 1999 the section of the third lane (the lane nearest the central reservation) between Junctions 2 and 3 was converted to a bus lane, first as a pilot scheme and then permanently in 2001. A lower speed limit was introduced along the bus lane section at the same time^[8] (the lane was however reverted to general traffic use by the incoming coalition government in late 2010).^[9]

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.^[10]

Between 2007 and January 2010 the section from Castleton (Junction 29) to Coryton (Junction 32) was widened to six lanes.^[11] 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.^[12] A similar claim was made for a 30-mile (48 km) section of road in Scotland close to Aberdeen in September 2009 with refuelling points at Bridge of Don, Ellon and Peterhead.^[13]

The original (A48(M)) bridge over the River Neath is to the right, the new M4 bridge is to the left

In October 2010 the new Secretary of State for Transport, Philip Hammond, announced that the bus lane in the London section would be suspended for 18 months from 24 December 2010, be brought back into use temporarily for the 2012 Summer Olympics, and then be removed.^[14]

Between 2008 and 2010, Junction 11, near Reading, was extensively remodelled with a new four-lane motorway junction, two new road bridges and other works.^[15] 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.^[16] 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.^[17] 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.^[18]

The table below shows the timeline for the construction of the motorway on a section by section basis.^{[19][20][21][22][23][24]}

An animated map showing build progress at five-year (or greater) intervals.
Note: When the Second Severn Crossing was opened in 1996 the M4 was re-routed to cross the new bridge; the previous section of motorway between Junctions 21 to 23 was renamed the M48.^[25]

Features

Toll bridge

The westbound carriageway tolls (left), near Rogiet, and the three-lane eastbound carriageway (right).

The M4 crosses the River Severn on a toll bridge, the last of two on the UK motorway network. The other being the Severn Bridge carrying the M48. Tolls are charged travelling only westbound. Drivers therefore have to pay to enter Wales, but not to enter England.^[26]

Speed limits

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

• 40 miles per hour (64 km/h) on the Chiswick Flyover within London in both directions.


• 60 miles per hour (97 km/h) between junction 4 and the Chiswick Flyover eastbound only.


• 50 miles per hour (80 km/h) when approaching the toll plaza after the Severn Crossing.^[27]
  


• 50 miles per hour (80 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 average speed camera system was installed; it generates around £500,000 in fines each six months.^[28][29]
  

M4 bus lane

M4 bus lane near Norwood Green, Ealing

Between 1999 and 2010 there was a controversial 3.5-mile (5.6 km) bus lane on the eastbound (London-bound) carriageway from Junction 3 (A312) to the western end of the Chiswick Flyover near Brentford, 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.^[30] It was used by 7% of vehicles which carried 21% of the people.^[8]

The lane was restored for normal motorway running at the end of 2010 for 18 months^[9] using an Experimental Traffic Order^[31] and was re-established for the duration of the 2012 Summer Olympics with the intention that it would then be removed permanently.^[9]

Porous road surface

Near Junction 35 of the M4, there is a stretch of the motorway that has a surfacing of porous asphalt that improves drainage and reduces noise. When driving in heavy rain drivers notice a reduction in road spray from other vehicles and improved visibility. This special surface was publicised in an episode of the BBC's Tomorrow's World programme. This was the site of the first trial of the new road surface when it was laid down in 1993.^[32]

Elevated and heated section

The elevated section of the M4 in West London, built in the 1960s, is mostly directly above the A4 and extends over parts of Brentford's Golden Mile. This section was designed to have a heated road surface to reduce icing in winter.^[33][34]

Four level stack interchanges

M4 Junction 4b / M25 Junction 15 near Heathrow Airport

The M4 has two of the three four-level stack interchanges in the UK, including the first UK example at junction 20, the "Almondsbury Interchange" with the M5, The other is 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, it is impossible to make a u-turn at either junction.

Tunnels

The M4 passes through the Brynglas Tunnels at Junction 25a, Newport. These are the only twin–bored tunnels on the United Kingdom motorway network.

Notable junctions

Junction 8/9 near Maidenhead, Berkshire, and High Wycombe, Buckinghamshire is the only one in the UK with dual numbers. This a relic from when the M4 used to turn north after junction 8, where it met the A308, and head for the original Junction 9, where it ended on 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. 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.^[35]

The section of the M4 in South Wales 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 subject to 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, junctions 30 and 31 were set aside for intermediate additional 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.^[36]

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, it is possible to travel in both directions for almost 2 miles (3.2 km) on the motorway, 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 a creating a "right-turn" on a roundabout. Similarly, slip roads pass under or over the main motorway at Junctions 41 and 42.^[37]

Maintenance

Maintenance of the 123 miles (198 km) section of the motorway in England is the responsibility of the Highways Agency.^[38] The 76 miles (122 km) section in Wales is the responsibility of the Welsh Government.^[39]

Smart motorway

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

The M4 motorway has two sections of smart motorway. The one between junctions 19 (M32) and 20 (M5) north of Bristol has variable speed limits and a part-time hard-shoulder. Completion was in summer 2014. The section between junctions 24 and 29 in Newport has variable speed limits.^[40]

In 2010 it was announced that a smart motorway would be constructed between junctions 3 and 12, with work starting in Autumn 2018. This will be the longest smart motorway scheme in the United Kingdom, with a length of 51km (32 miles). Work is expected to be completed in March 2022 at a cost of £848 million.^[41][42]

New junctions: 15a, 18a

A new Junction 15a is being considered^[when?] by Wiltshire Council, which would give access to South and Central Swindon as well as to Wroughton, Marlborough and Devizes via the existing A4361.^{[citation needed]} In South Gloucestershire, a possible Junction 18a would connect with the A432 and A4174 giving better access to Bristol, via Mangotsfield, and also a direct link with Yate.^{[43][not in citation given]}

M4 Junction 31

Plans for the "missing" Junction 31, also known as the Thornhill interchange, for which planning permission was originally granted in September 1991 (but subsequently expired), were rekindled after proposals for a new business park on a 125 acres (51 ha) site north of the M4 were submitted in 2007 to Cardiff Council.^[44] The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469.^[45]

A freedom of information request in 2010 to Cardiff Council shows that whilst the land that would enable this junction should continue to be strategically protected, the decision to formally abandon the proposed Junction 31 Thornhill was made in October 2007 and there had been no subsequent mention of it in Cardiff Council
Strategic or Planning meetings since.^[46]

M4 relief road

Plans for an 'M4 Relief Road' around Newport were first announced by the Welsh Office in 1991, but made little progress. The Welsh Assembly Government revived the scheme as the M4 relief road tolled bypass in 2007 but later abandoned it for financial reasons.^[47] An extension to the Newport Southern Distributor Road through the old Corus steel works is being considered.^[48] This road is already a dual carriageway but not open to the public. 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:^[49] "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."

Port Talbot, Junctions 40 and 41

There have been calls to close the slip roads at Junction 40 and 41 (at Port Talbot) to improve traffic flow. The motorway has only two lanes on this stretch and is a major traffic congestion blackspot. Junctions 40 and 41 (at Port Talbot) have very short slip roads which have not been modernised.^[50] 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^[51][52] but following heavy criticism from local businesses and residents,^[53] was stopped on 29 May 2015.^[54][55]

Junctions

M4 motorway
mile	km	Eastbound exits (B carriageway)	Junction	Westbound exits (A carriageway)
7.3	11.8	Road continues as A4 to Central London	J1 [coord 1]	Start of motorway
8.3 8.5	13.4 13.7	North Circular A406 South Circular A205 Chiswick A315	J2	Staines, Hounslow, Brentford A4
12.5 12.8	20.1 20.6	Heston services	Services	Heston services
13.4 13.7	21.5 22.1	Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow A312	J3	Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow A312
15.2 15.7	24.5 25.2	Heathrow (Terminals 1, 2 & 3) (A4) Uxbridge A408	J4a	Heathrow (Terminals 1, 2 & 3) (A4) Uxbridge A408
16.8 17.7	27.0 28.5	Heathrow (Terminals 4, 5 & Cargo) , Gatwick Airport , Watford (M3), (M23), (M40), (M1) M25	J4b [coord 2]	Heathrow (Terminals 4, 5&Cargo), Gatwick, Watford (M3), (M23), (M40), (M1), M25
19.1 19.5	30.7 31.4	Colnbrook, Langley, Slough (East) A4 Eton, Datchet B470	J5	Colnbrook, Langley, Slough (East) A4, Datchet B470
22.9 23.2	36.8 37.4	Slough (Central) A355 Windsor A322	J6	Slough (Central) A355 Windsor A322
24.7 25.0	39.8 40.3	Slough (West) A4	J7	Slough (West) A4
27.8 28.2	44.7 45.4	High Wycombe, Henley A404(M) Maidenhead A308(M)	J8/9	High Wycombe, Henley A404(M) Maidenhead A308(M)
35.0 35.8	56.4 57.6	Reading (East), Wokingham, Bracknell A329(M)	J10	Reading (East), Wokingham, Bracknell A329(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), Theale A4	J12	Reading (West), Theale A4
56.9 57.2	91.5 92.1	Newbury, Oxford A34 Chieveley services (Moto)	J13 Services [coord 3]	Newbury, Oxford A34 Chieveley services (Moto)
64.4 64.8	103.7 104.3	Hungerford, Wantage A338	J14	Hungerford, Wantage A338
68.7	110.5	Membury services	Services	Membury services
76.4 77.1	122.9 124.0	Swindon (East) A419 Marlborough A346 Oxford (A420)	J15	Swindon A419, Marlborough A346
82.4 82.8	132.6 133.2	Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham, Calne A3102	J16	Swindon (West), Royal Wootton Bassett, Wroughton, MoD Lyneham, Calne A3102
94.8 95.3	152.5 153.4	Chippenham A350 Cirencester A429	J17	Chippenham A350 Cirencester A429
96.7	155.7	Leigh Delamere services	Services	Leigh Delamere services
104.9 105.6	168.8 170.0	Bath, Stroud A46	J18	Bath, Stroud A46
112.8 113.2	181.5 182.1	Bristol M32	J19	Bristol M32
115.8 116.3	186.3 187.2	The SOUTH WEST, Bristol (West), The MIDLANDS, Gloucester M5	J20 Almondsbury Interchange [coord 4]	The SOUTH WEST, Bristol (West) M5(S) The MIDLANDS, Gloucester M5(N)
117.7 189.5	189.5 190.0	No access	J21	Chepstow M48
121.3 121.6	195.2 195.7	Avonmouth M49	J22	Avonmouth M49
121.7 124.1	195.9 199.8	Entry into England	Second Severn Crossing [coord 5]	Entry into Wales
127.0	204.4	No toll plaza	Bridge Toll	Toll plaza
129.2	208.0	Chepstow M48	J23	No access
131.4 132.2	211.4 212.8	Magor, Caldicot A4810 Magor services	J23A Services	Magor, Caldicot A4810 Magor services
134.8 135.6	217.0 218.3	City centre A48 Newport (East) B4237 Monmouth A449 The MIDLANDS (M50)	J24	City centre A48 Newport (East) B4237 Monmouth A449
137.9	222.0	No access	J25	Caerleon B4596
138.3	222.6	No access	J25A	Newport, Cwmbran A4042
138.8 139.1	223.4 223.8	Brynglas Tunnels	Tunnel	Brynglas Tunnels
139.1 139.5	223.8 224.5	Newport, Cwmbran, Caerleon A4051	J26	Newport A4051
140.8 141,4	226.6 227.6	High Cross B4591	J27	High Cross B4591
142.0 142.9	228.5 230.0	Newport A48 Risca, Brynmawr A467	J28	Newport A48 Risca, Brynmawr A467
143.6	231.1	No access	J29	Cardiff (East and South) A48(M)
146.6 147.3	236.0 237.0	Cardiff (East) A4232 Cardiff Gate services	J30 Services	Cardiff (East) A4232 Cardiff Gate services
151.9 244.5	244.5 246.1	Merthyr Tydfil, Cardiff A470	J32	Cardiff (North), Merthyr Tydfil A470
155.4 156.0	250.1 251.1	Cardiff (West), Barry, Penarth A4232 Cardiff Airport Cardiff West services	J33 Services	Cardiff (West), Barry, Penarth A4232 Cardiff Airport Cardiff West services
157.5 158.1	253.5 254.4	Llantrisant, Rhondda A4119 Royal Glamorgan Hospital	J34	Llantrisant, Rhondda A4119 Royal Glamorgan Hospital
163.9 164.4	263.7 264.5	Pen-coed A473	J35	Bridgend, Pen-coed A473
167.5 168.3	269.6 270.8	Bridgend A4061 Maesteg A4063 Princess of Wales Hospital Sarn Park services	J36 Services	Bridgend A4061 Maesteg A4063 Princess of Wales Hospital Sarn Park services
172.8 173.5	278.1 279.3	Pyle, Porthcawl A4229	J37	Porthcawl, Pyle A4229
177.0 177.3	284.9 285.3	Port Talbot A48	J38	Port Talbot A48
178.3	286.9	No access (on-ramp only)	J39	No access
179.7 179.9	289.2 289.6	Port Talbot A4107	J40	Port Talbot A4107
180.9 181.3	291.1 291.8	Port Talbot A48	J41	Briton Ferry A48
183.9 184.9	295.9 297.3	Briton Ferry A48 Swansea (South) A483	J42	Swansea (South) A483
184.9 185.7	297.5 298.8	Neath, Merthyr Tydfil A465	J43	Neath, Merthyr Tydfil A465
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) A4067	J45	Pontardawe, Swansea (North) A4067
190.8 191.4	307.0 308.0	Llangyfelach B4489 Felindre Morriston Hospital	J46	Llangyfelach B4489 Felindre Morriston Hospital
192.4 193.1	309.6 310.7	Swansea (West) A483 Gorseinon A48 Swansea services	J47 Services	Swansea (West) A483 Gorseinon A48 Swansea services
196.4 197.1	316.0 317.2	Pontarddulais, Llanelli A4138	J48	Pontarddulais, Llanelli A4138
199.2	320.6	Start of motorway	J49 Terminus Services [coord 6]	Motorway terminates at a roundabout: Carmarthen A48 Ammanford, Llandeilo A483 Pontarddulais A48 Pont Abraham services
Data[56][57][58][59] 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.
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Incomplete access       Tolled

Coordinate list

Incidents and accidents

• In June 1984 a crash near Maidenhead resulted in 13 deaths.^[60]
  


• In March 1991 ten people died in a series of crashes in 1991 involving 51 vehicles near Hungerford.^[60]
  


• In May 1995 a coach carrying Royal British Legion members left the road close to the Severn Bridge, resulting in 10 deaths.^[60]
  


• 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.^[61]
  


• In July 2011 a lorry fire in the Brynglas Tunnels closed the motorway.^[62] Although there were no injuries and no deaths, one tunnel remained closed and a contraflow system was in place in the remaining tunnel for about one month, causing major travel delays.^[63][64]
  

See also

• M4 corridor


• M4 Thames Bridge, Maidenhead


• List of motorways in the United Kingdom


• Transport in Wales

References

Further reading

• 
  Jackson, Mike (2005). The M4 Sights Guide. Severnpix. ISBN 978-0954540227.
  

External links

Wikimedia Commons has media related to M4 motorway.

Route map:

• CBRD Motorway Database – M4


• British Road Database: Motorways – M4 Junction 8/9


• The Motorway Archive
  
  
  
  • Junctions 1 to 5
  
  
  • Junctions 5 to 7
  
  
  • Junctions 8 to 14
  
  
  • Junctions 14 to 21
  
  
  • Junctions 21 to 23
  
  
  • Junctions 23 to 49
  
  
  
  


• Map of the preferred route around Newport (relief road)