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The M4 motorway is a motorway in Great Britainmarker linking Londonmarker with South Walesmarker. It is part of the unsigned European route E30. Other major places directly accessible from M4 junctions are Readingmarker, Swindonmarker, Bristolmarker, Newportmarker, Cardiffmarker and Swanseamarker. Originally referred to as the London-South Wales Motorway, the English section was constructed between 1965 and 1971, the Welsh section was completed in 1993 and the Second Severn Crossingmarker opened in 1996.

The M4 runs close to the A4marker from London to Bristol. After crossing the River Severn it follows the A48 through Walesmarker, using the Brynglas Tunnelsmarker at Junction 25a, Newportmarker and terminates just north of Pontarddulaismarker. The route of the M4 is known as the M4 corridormarker.


The M4 was built in several stages. In the 1960s two sections were built, one extending from the London end to near Maidenheadmarker, one from north of Bathmarker to west of Newport, including the Severn Bridgemarker (opened in 1966 and 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 motorway, a number now allocated to a short section of motorway near Cardiff.

The English section of the motorway was completed on 22 December 1971 when the 50 mile (80 km) stretch between junctions 9 and 15 (Maidenheadmarker and Swindonmarker) was opened to traffic. The Welsh section was completed in 1993, when the Briton Ferrymarker motorway bridge opened. The Second Severn Crossingmarker 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.


Toll bridge

The M4 crossesmarker the River Severn via a toll bridge, the second of only two on the UK motorway network – the first was the original Severn Bridge, now the M48. Unusually for a toll road, tolls are charged in one direction only - westbound. Drivers therefore have to pay to enter Wales but not to enter England.

M4 bus lane

In London, the eastbound (London-bound) M4 carriageway has a bus lane, which opened in June 1999, from junction 3 until the elevated section. This lane caused some controversy when it was created. However, the bus lane has improved traffic flow onto the 2-lane elevated section which follows it. A study by the Transport Research Laboratorymarker has shown that the lane has reduced rush hour journey times by 3.5 minutes for buses and one minute for cars, even though the speed limit was cut from 70mph to 50mph. There is some speculation that the original plan involved closing one lane from junction 3 to the elevated section to prevent congestion caused by the filtering of three lanes of traffic into two (the motorway is already temporarily down to two lanes at junction 3 because the left lane is exclusively for traffic leaving the motorway, so the effect of the bus lane is to make the 3-to-2 lane-transition seamless). Making the "spare" lane available for buses - and now taxis and motorcycles - was an afterthought.

Porous road surface

Near Junction 35, 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 conditions drivers notice a reduction in spray from other vehicles and improved visibility. This special surface was publicised in an episode of the BBCs Tomorrow's World programme. This was the site of the first trial of the new road surface when it was laid down in 1993.

Elevated and heated section

The elevated section in West London, built in the 1960s, is mostly directly above the A4marker and extends over parts of Brentfordmarker's Golden Milemarker. This section has a heated road surface to reduce icing in winter.

Four level stack interchanges

It has two of only three four-level stack interchanges in the UK, including the first UK example at the junction with the M5 (J20/"Almondsbury Interchangemarker") and the other at the junction with the M25 (J4B). Both are impressive feats of engineering, particularly junction 4B which also has to make provision for a railway line passing beneath the M4. However, due to the nature of such junctions, drivers should bear in mind that it is impossible to make a U-turn at J20 or J4B.


The M4 passes through the Brynglas Tunnelsmarker at Junction 25a, Newportmarker.

Notable junctions

Junction '8/9' (only one in the UK to feature a 'dual-numbered' junction) is the turn off for Maidenheadmarker, Berkshire.

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 Welfordmarker, a Second World War airfield and now an RAF/USAF military installment mainly used for storing munitions. The M4 entrance allows easier access for the large vehicles used to carry the munitions.

Junction 27 (High Crossmarker) is a normal grade-separated roundabout junction. However the on-bound slip roads point in the opposite direction to the intended direction of travel. Due to the topology of the landscape, both sets of slip roads are conjoined on either side of the roundabout. To travel westbound the driver must use an eastbound-facing slip road before looping around 180 degrees to join the westbound carriageway.

Junctions 30-33 (Cardiff area) were set aside for intermediate additional interchanges at the time on construction. Junction 30 has since been added but there are no current plans to construct Junction 31 (A469 road), Junction 32 (A470) or Junction 33 (A4232).

Junction 39 does actually exist, but 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 refers to two different junctions in two different directions, their movements not complementary. In the westbound direction, 41 is indicated as a spur leading to the roundabout in Briton Ferrymarker formerly known as 41A, and the original bridge over the River Neathmarker, which would allow access onto the stretch of the M4 from junction 43 westward. In the eastbound direction, 41 is indicated as an exit-only route to the A48 towards Port Talbotmarker. As a result, it is possible in both directions to travel almost 2 miles (3 km) in the same direction having both joined and subsequently left the motorway at "Junction 41".

Current developments

M4 Junction 11 Improvement Scheme

Junction 11, near Readingmarker, is in the process of being extensively developed with a new four-lane motorway junction and the construction of two extra road bridges around the existing junction and other works. Work started in 2008 and is expected to be completed by summer 2010. The £65m scheme includes work to the Mereoak roundabout and part of the A33 Swallowfield Bypass in Shinfield conversion and the conversion of the two existing existing bridges, one of which be available only for pedestrians and cyclists and the only for buses. It will also involve the movement of the local Highways Agency and Fire Service offices, build a long footbridge network, a special bus-lane and a new gyratory. It will also install sound barriers for nearby residential areas, some of which have already been completed. 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.

M4 J23a-J29 Upgrade

As of April 2009, the Newport section of the motorway between junctions 23a and 29 is being upgraded with a new concrete central barrier; from 2010, a variable speed limit will be introduced.

M4 J29-J32 Widening

As of spring 2007, the stretch between Junction 29 Castleton and J32 Coryton is being widened to 6 lanes in order to relieve congestion that plagues this stretch. Work is expected to be completed by December 2009.

Proposed developments

M4 Junction 31

Plans for the "missing" Junction 31, also known as the Thornhillmarker interchange, which was originally granted planning permission in September 1991 (but subsequently expired) have been rekindled after proposals for a new business park on a site north of the M4 were submitted to Cardiff Council. The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469.

Other proposals

A 'New M4' tolled bypass for Newportmarker was proposed in 2007 but later abandoned for financial reasons. An extension to the Newport Southern Distributor Road through the old Corus steel works is being considered.. This road is already a dual carriageway but not open to the public.

There have been calls to close the slip roads at Junction 40 and 41 (at Port Talbot) to 'improve traffic flow'. The motorway is only two lanes in this stretch and is a major traffic congestion blackspot. Junctions 40 and 41 (at Port Talbot) have very short slip roads which are not up to modern standards. The Port Talbot peripheral distributor road is under development, which should divert local traffic away from the M4.

Speed checks

Speed checks are carried out between junction 14 and junction 18 (in both directions). These are carried out by police camera vans situated on bridges over the motorway. These measure the speed of vehicles using laser detection equipment. The introduction of these measures proved controversial and resulted in a public protest, involving a go-slow of several hundred vehicles along the affected sections of the motorway during April 2005.

Traffic heading into London travels at a maximum speed of 60 mph (96 km/h) after the Heathrowmarker turnoff, the limit is enforced by cameras. The elevated section is also enforced by speed cameras at 40 mph (64 km/h). Traffic heading out of London is limited to 40 mph (64 km/h) on the elevated section, but there are no cameras.

Speed checks are also carried out by mobile Safety Camera Partnership vans on the toll plaza on the Welsh side of the M4 after the Second Severn Bridge. The speed limit is reduced to 50 mph (80 km/h) when approaching the toll plaza, the theory behind this is to protect tolling staff moving between the tolling booths, the limit is then raised back to the National speed limit after leaving the toll plaza. There are also mobile camera vans on sections of the M4 between the toll plaza and Cardiff.

Between Junction 34 and Junction 37, there are three bridges crossing over the motorway. These three bridges sometimes have mobile speed camera vans parked on them to catch speeding motorists on the motorway below. These bridges are located on fairly sharp bends in the motorway.

The section between junction 40 and junction 41 has a maximum speed of 50 mph (80 km/h), and is enforced by a permanent speed camera (Gatso), to be found near junction 41 in the eastbound direction. To improve traffic flow, the westbound speed camera was removed in late 2006 and replaced with an electronic warning sign.

Temporary average speed cameras are in operation around Junction 6 at a speed limit of 50 mph; operation commenced westbound on Monday 26 November 2007 while roadworks on the bridge are taking place (expected to last 40 weeks according to signs alongside the carriageway). Eastbound average speed cameras in the same location were operational a short time after that.

Between Junction 24 and Junction 28, an average speed camera system is in operation, in both directions. This is enforcing a speed limit of 50 mph between these junctions whilst roadworks are in progress.

Between Junction 28 and Junction 32, there are 2 speed cameras on the westbound carriageway, one camera located just after Junction 28, and another located approximately 1 mile before Junction 30. On the eastbound carriageway, there are 2 speed cameras, one approximately 1 mile before Junction 30, and another just before the entrance slip road for Junction 29. The speed limit at these camera sites is 50 mph due to ongoing development.


Datafrom driver location signs are used to provide distance and carriageway identification information.
M4 Motorway
km Eastbound exits (B Carriageway) Junction Westbound exits (A Carriageway)
Road becomes A4 to Central London J1 North Circular A406marker
South Circular A205
Chiswickmarker A315
Non-motorway traffic

11.8 A4 from Central London becomes the M4
Start of motorway
13.4 North Circular A406
South Circular A205
Chiswick A315

J2 Stainesmarker, Hounslowmarker, Brentfordmarker A4
Heston services Services Heston Services
21.8 Heathrowmarker (Terminals 4, 5 & Cargo), Hayesmarker, Harrowmarker, Hounslowmarker A312 J3 Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow A312
25.2 Heathrow (Terminals 1, 2 & 3) (A4)
Uxbridgemarker (A408)
J4a Heathrow (Terminals 1, 2 & 3) (A4)
Uxbridge (A408)
27.5 Heathrow (Terminals 4, 5 & Cargo), Gatwick Airport, Watfordmarker, Oxfordmarker, Stansted Airportmarker (M40, M1,M11, M3, M23) M25marker J4b Heathrow (Terminals 4, 5 & Cargo), Gatwick Airport, Maidstone, Watford, Oxford, Stansted Airport(M40, M1 ,M11, M3, M23, M20) M25
31.4 Colnbrookmarker, Langleymarker A4
Etonmarker, Datchetmarker B470
J5 Colnbrook, Langley A4
Eton, Datchet B470
36.8 Sloughmarker (Central) A355
Windsormarker A322
J6 Slough (Central) A355
Windsor A322
40.0 Slough (West) A4 J7 Slough (West) A4
44.8 High Wycombemarker, Henleymarker A404
Maidenheadmarker A308
J8/9 High Wycombe, Henley A404(M)
Maidenhead A308(M)
56.9 Readingmarker (East), Wokinghammarker, Bracknellmarker A329 J10 Reading (East), Wokingham, Bracknell A329(M)
65.6 Basingstokemarker, Reading (Central) A33 J11 Basingstoke, Reading (Central) A33
Reading services 'Services Reading Services
72.9 Reading (West), Thealemarker A4 J12 Reading (West), Theale A4
91.5 Newburymarker, Oxfordmarker A34
Chieveley services
Newbury, Oxford A34
Chieveley services
103.7 Hungerfordmarker, Wantagemarker A338 J14 Hungerford, Wantage A338
110.5 Membury services Services Membury Services
123.4 Swindonmarker (East) A419
Marlboroughmarker A346
Oxford (A420)

J15 Swindon (East), Gloucestermarker A419
Marlborough A346
132.8 Swindon (West), Wootton Bassettmarker, RAF Lynehammarker, Calnemarker A3102 J16 Swindon (West), Wootton Bassett, RAF Lyneham, Calne A3102
153.0 Chippenhammarker A350
Cirencestermarker A429
J17 Chippenham A350
Cirencester A429
155.7 Leigh Delamere services Services Leigh Delamere services
169.4 Bathmarker, Stroudmarker A46 J18 Bath, Stroud A46
181.8 Bristolmarker M32 J19 Bristol M32
186.7 The SOUTH WEST, Bristol (West), The MIDLANDSmarker, Gloucestermarker M5
Almondsbury Interchangemarker
J20 The SOUTH WEST, Bristol (West), Exetermarker, The MIDLANDS, Gloucester M5
Almondsbury Interchange
189.0 No access J21 Chepstowmarker M48
195.5 Avonmouthmarker M49
The SOUTH WEST, Bridgwatermarker, Exeter (M5 South)
J22 Avonmouth M49
Second Severn Crossingmarker
No tolls Tolls Toll Booth
Chepstow M48 J23 No access
Magormarker, Caldicotmarker B4245
Magor services
Magor, Caldicot B4245
Magor services
City centre A48
Newportmarker B4237
Monmouthmarker A449
The MIDLANDS (M50marker)

J24 City centre A48
Newport B4237
Monmouth A449

No access J25 Caerleonmarker B4596
No access J25a Newport, Cwmbranmarker A4042
Brynglas Tunnelsmarker Tunnel Brynglas Tunnels
Newport, Cwmbran, Caerleon A4051 J26 Newport A4051
High Crossmarker B4591 J27 High Cross B4591
Newport A48
Riscamarker, Brynmawrmarker A467
J28 Newport A48
Risca, Brynmawr A467
No access J29 Cardiff East and South A48
Cardiff East A4232
Cardiff Gate servicesmarker
Cardiff East A4232
Cardiff Gate services
Cardiff North, Merthyr Tydfilmarker A470 J32 Cardiff North, Merthyr Tydfil A470
Cardiff West, Cardiff International Airportmarker, Barrymarker, Penarthmarker A4232
Cardiff West services
Cardiff West, Cardiff International Airportmarker, Barrymarker, Penarthmarker A4232
Cardiff West services
Llantrisantmarker, Rhonddamarker A4119 J34 Llantrisant, Rhondda A4119
Pen-coedmarker A473 J35 Pen-coed, Bridgendmarker A473
Bridgend A4061
Maestegmarker A4063
Sarn Park services

Bridgend A4061
Maesteg A4063
Sarn Park services

Porthcawlmarker, Pylemarker A4229 J37 Porthcawl, Pyle A4229
Port Talbotmarker A48 J38 Port Talbot A48
No access J39 No access
Port Talbot A4107 J40 Port Talbot A4107
Port Talbot A48 J41 Port Talbot, Baglan A48
Swanseamarker A483
Briton Ferrymarker A48
J42 Swanseamarker A483
Neathmarker, Merthyr Tydfilmarker A465 J43 Neath, Merthyr Tydfil A465
Swansea A48 J44 Swansea A48
Swansea, Pontardawemarker, Morristonmarker, Clydachmarker A4067 J45 Swansea, Pontardawe, Morriston, Clydach A4067
Swansea, Llangyfelachmarker B4489 J46 Swansea, Llangyfelach B4489
Swansea A483
Pontarddulaismarker A48
Penllergaermarker, Gorseinonmarker A4240
Swansea services

Swansea A483
Pontarddulais A48
Penllergaer, Gorseinon A4240
Swansea services

Pontarddulais, Llanellimarker A4138 J48 Pontarddulais, Llanelli A4138
Start of motorway J49
Carmarthenmarker A48
Ammanfordmarker A483
Pont Abraham services

See also


  1. The Motorway Archive: M4 Second Severn Crossing
  2. 2006 Fourth quarter foundation magazine - Operations and products
  4. New speed limit on part of M4, South Wales Argus Saturday 4th April 2009
  6. Area 3 Driver Location Signs (map) - Highway Authority, 2009
  7. Driver Location Signs, M5 J18-11, M4 J22-15 (map) Highway Authority 2009

External links

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