M4 motorway Totally Explained

Everything about M4 Motorway totally explained

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The M4 motorway is a motorway in Great Britain linking London with Wales. It is part of the unsigned European route E30. Other major places directly accessible from M4 junctions are Reading, Swindon, Bristol, Newport, Cardiff and Swansea.
The M4 runs close to the A4 from London to Bristol. After crossing the River Severn it follows the A48 through Wales, until its termination just north of Pontarddulais.

Special features

In London, the eastbound (London-bound) M4 carriageway has a bus lane 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. 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.
   The elevated section in West London, built in the 1960s, is mostly directly above the A4 and extends over parts of Brentford's Golden Mile. This section has a heated road surface to reduce icing in winter.
   The motorway is the only one in the UK to feature a 'dual-numbered' junction. Junction '8/9' is the turn off for Maidenhead, Berkshire.
   There is a special slip road west of the A34 to Newbury from the eastbound carriageway which leads to RAF Welford. This slip road is signposted in red lettering (the normal military signing standard), "Works Unit Only".
   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 Interchange") 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's impossible to make a U-turn at J20 or J4B.
   The M4 crosses the River Severn via a toll bridge, the second of only two on the UK motorway network – the first was the original Severn Bridge (the original route of the M4, now the M48).
   The M4 passes through the Brynglas Tunnels at Junction 25a, Newport.
   Near Junction 35, there's 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.

Notable junctions

Junction 27 at High Cross is a normal roundabout B-road junction, however the slip roads point in the wrong direction to the destination. Due to the topology of the landscape, both sets of sliproads are conjoined on opposite sides of the roundabout. To go to Cardiff, the driver must use an eastbound-facing sliproad before looping around 180 degrees to join the westbound carriageway.
   During the construction of the section around Cardiff in 1980, junction numbers were set aside for intermediate interchanges that were not built at the time. 33 had been set for the link road to the west (A4232), 32 for the A470, with 31 intended for the A469 to Caerphilly, and 30 for the great "Hook Road" - a plan which was later shelved. Junction 30 was later added to serve the A4232 link road, in the east, which will eventually form a ring road from junctions 33 to 30, via Cardiff Bay. Number 31 remains unused as of 2006, and there are no immediate plans to build a junction with the A469.
   Contrary to popular belief, junction 39 does exist. However, it consists of a single slip road completing the missing movement onto the westbound carriageway from the A48 at junction 38, thus there's 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 Ferry formerly known as 41A, and the original bridge over the River Neath, 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 Talbot. As a result, it's 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".

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 (97 km/h) after the Heathrow 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 29 and Junction 32, there are 2 speed cameras on the westbound carriageway, one just before Junction 30 (it is clearly visible against the junction exit sign), and another under a bridge approximately 1 mile before Junction 32. On the eastbound carriageway, there are 2 cameras, one approximately 1.5 miles after Junction 32 and the other is approximately 1 mile before Junction 30.

Developments

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.
   Junction 11, near Reading, will be extensively developed commencing in Spring 2008 and lasting until well into 2010 in an attempt to relieve congestion. The development won't only build two extra bridges, and re-route many roads by the junction, but 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. Redevelopment of roundabouts on the A33 and the installation of a new junction off that road will be part of the works. 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.
   There have been calls to close the slip roads at Junction 40 and 41 (at Port Talbot) to improve traffic flow in the area. The motorway is only two lanes in this stretch and this stretch 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 "missing" Junction 31, also known as the Thornhill interchange, which was originally granted planning permission in September 1991 (but subsequently expired) have been rekindled after plans 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.

New M4

Plans are progressing for the New M4 Newport bypass. The Welsh Assembly Government Transport Minister Ieuan Wyn Jones said "We could start the work in 2010, with an anticipated completion date of 2013."

Route

M4 Motorway
Eastbound exits	Junction	Westbound exits
A4 from/to Central London becomes M4
End of motorway	1	Start of motorway A406: North Circular A205: South Circular A315: Chiswick
A406: North Circular A205: South Circular A315: Chiswick	2	A4: Staines, Hounslow, Brentford
Heston Services
A312: Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow	3	A312: Heathrow (Terminals 4, 5 & Cargo), Hayes, Harrow, Hounslow
(A4): Heathrow (Terminals 1, 2 & 3) (A408): Uxbridge	4	(A4): Heathrow (Terminals 1, 2 & 3) (A408): Uxbridge
M25: Heathrow (Terminals 4, 5 & Cargo), Gatwick Airport, Watford, Oxford, Winchester (M40, M1, M3, M23)	4B	M25: Heathrow (Terminals 4, 5 & Cargo), Gatwick Airport, Watford, Oxford, Winchester (M40, M1, M3, M23)
A4: Colnbrook, Langley B470: Eton, Datchet	5	A4: Colnbrook, Langley B470: Eton, Datchet
A355: Slough (Central) A322: Windsor	6	A355: Slough (Central) A322: Windsor
A4: Slough (West)	7	A4: Slough (West)
A404(M): High Wycombe, Henley A308(M): Maidenhead	8/9	A404(M): High Wycombe, Henley A308(M): Maidenhead
A329(M): Reading (East), Wokingham, Bracknell	10	A329(M): Reading (East), Wokingham, Bracknell
A33: Basingstoke, Reading (Central)	11	A33: Basingstoke, Reading (Central)
Reading Services
A4: Reading (West), Theale	12	A4: Reading (West), Theale
A34: Newbury, Oxford	13 Chieveley Services	A34: Newbury, Oxford
A338: Hungerford, Wantage	14	A338: Hungerford, Wantage
Membury Services
A419: Swindon (East) A346: Marlborough (A420): Oxford	15	A419: Swindon (East), Gloucester A346: Marlborough
A3102: Swindon (West), Wootton Bassett, RAF Lyneham, Calne	16	A3102: Swindon (West), Wootton Bassett, RAF Lyneham, Calne
A350: Chippenham A429: Cirencester	17	A350: Chippenham A429: Cirencester
Leigh Delamere Services
A46: Bath, Stroud	18	A46: Bath, Stroud
M32: Bristol	19	M32: Bristol
M5 S: The SOUTH WEST, Bristol (West) M5 N: The MIDLANDS, Gloucester	20 Almondsbury Interchange	M5 S: The SOUTH WEST, Bristol (West), Exeter M5 N: The MIDLANDS, Gloucester
Access only	21	M48: Chepstow Exit only
M49: Avonmouth (M5 S): The SOUTH WEST, Bridgwater, Exeter	22	M49: Avonmouth
ENGLAND
Second Severn Crossing
WALES
No tolls	Tolls	Toll Booth
M48: Chepstow Exit only	23	Access only
B4245: Magor, Caldicot	23A Magor Services	B4245: Magor, Caldicot
A48: City centre B4237: Newport A449: Monmouth (M50): The MIDLANDS	24	A48: City centre B4237: Newport A449: Monmouth
Access only	25	B4596: Caerleon Exit only
Access only	25A	A4042: Newport, Cwmbran Exit only
Brynglas Tunnels
A4051: Newport, Cwmbran, Caerleon	26	A4051: Newport
B4591: High Cross	27	B4591: High Cross
A48: Newport A467: Risca, Brynmawr	28	A48: Newport A467: Risca, Brynmawr
Access only	29	A48(M): Cardiff East and South Exit only
A4232: Cardiff East	30 Cardiff Gate Services	A4232: Cardiff East
A470: Cardiff North, Merthyr Tydfil	32	A470: Cardiff North, Merthyr Tydfil
A4232: Cardiff West, Cardiff International Airport, Barry, Penarth	33 Cardiff West Services	A4232: Cardiff West, Cardiff International Airport, Barry, Penarth
A4119: Llantrisant, Rhondda	34	A4119: Llantrisant, Rhondda
A473: Pen-coed	35	A473: Pen-coed, Bridgend
A4061: Bridgend A4063: Maesteg	36 Sarn Park Services	A4061: Bridgend A4063: Maesteg
A4229: Porthcawl, Pyle	37	A4229: Porthcawl, Pyle
A48: Port Talbot	38	A48: Port Talbot Exit only
No access or exit	39	Access only
A4107: Port Talbot	40	A4107: Port Talbot
A48: Port Talbot Exit only	41	Access only
Access only	41	A48: Briton Ferry Exit only
A483: Swansea A48: Briton Ferry	42	A483: Swansea
A465: Neath, Merthyr Tydfil	43	A465: Neath, Merthyr Tydfil
A48: Swansea	44	A48: Swansea
A4067: Swansea, Pontardawe, Morriston, Clydach	45	A4067: Swansea, Pontardawe, Morriston, Clydach
B4489: Swansea, Llangyfelach	46	B4489: Swansea, Llangyfelach
A483: Swansea A48: Pontarddulais A4240: Penllergaer, Gorseinon	47 Swansea Services	A483: Swansea A48: Pontarddulais A4240: Penllergaer, Gorseinon
A4138: Pontarddulais, Llanelli	48	A4138: Pontarddulais, Llanelli
Start of motorway	49 Pont Abraham Services	End of motorway A48: Carmarthen A483: Ammanford

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