M4 MOTORWAY

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.

Contents

Special features

Notable junctions

Speed checks

Developments

Route

See also

References

External links

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.^[1] 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.^[2]
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.
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 is 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).
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".
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^[3].

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 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 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 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".

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.^[4]
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, and 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.^[5]

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^[6].
Plans are progressing for the M4 Toll Newport bypass, scheduled for completion by 2014.
A new junction is to be constructed near Reading commencing in Spring 2008.^[7]

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 '(M1, M23, M3)'	4B	'M25': Heathrow (Terminals 4, 5 & Cargo), Gatwick Airport, Watford, Oxford, Winchester '(M1, M23, M3)'
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: Newport city centre B4237: Newport A449: Monmouth '(M50)': The MIDLANDS	24	A48: Newport 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	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''		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	45	A4067: Swansea, Pontardawe
B4489: Swansea, Llangyfelach	46	B4489: Swansea, Llangyfelach
A483: Swansea	47 ''Swansea Services''	A483: Swansea A48: Pontarddulais
A4138: Pontarddulais, Llanelli	48	A4138: Pontarddulais, Llanelli
''Start of motorway''	49 ''Pont Abraham Services''	''End of motorway'' A48: Carmarthen A483: Ammanford

See also

★ M4 Toll

★ A404(M) motorway

★ M4 corridor

★ List of motorways in the United Kingdom

★ A48(M) motorway

★ Transport in Wales

References

1. M-way bus lane gets green light
2. M4 Bus Lane
3. http://www.hanson.biz/files/pdf/magazine/2006/2006Q4_op.pdf
4.
5.
6. www.m4widening29to32.com
7.

External links

★ CBRD Motorway Database - M4

★ British Road Database: Motorways - M4 Junction 8/9

★ The Motorway Archive

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★ Junctions 1 to 5

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★ Junctions 5 to 7

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★ Junctions 8 to 14

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★ Junctions 14 to 21

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★ Junctions 21 to 23

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★ Junctions 23 to 49