Easily one of the most varied motorways in Britain, from urban chaos in west London and Heathrow Airport, out through the rolling hills of Gloucestershire, across the Severn Estuary and through industrial South Wales, the M4 does it all.

Before the M4 Severn Bridge was built, there was no road crossing of the Severn south of Gloucester, meaning that traffic between South Wales and southern England had to drive the A40, far out of its way to the north and frequently congested at Gloucester, or go via Bristol and Chepstow, choosing between the Severn ferry or the railway tunnel. The train and boat options were not particularly well used, but the bridge certainly is today, which suggests that the lack of a decent crossing was preventing a great number of journeys from happening at all.

In 1996, the Second Severn Crossing opened, carrying a widened and improved M4 over the estuary. The original Severn Bridge and Wye Bridge, along with a substantial length of ex-M4 at each side, now form the M48. Built and opened at the same time, the M49 was designed to cut congestion at the M5 junction, cutting the corner between the south-west, Bristol and the new bridge. Because of these multiple motorway connections (M48, M49, M48 again, M5 and M32), the M4 holds the UK record with five consecutive motorway-to-motorway interchanges.

Given that the first section of this motorway was opened in 1959 - the Chiswick Flyover, which was initially part of the A4 but was designed to become part of the motorway a few years later - it is surprising to learn that it remained in two disconnected parts until 1994. Plugging the gap was a six-mile section of the A48 at Briton Ferry in South Wales, with a motorway bypass delayed again and again due to the difficult marshy land and a navigable river. The problem was only solved very late in the day with a large and horrendously expensive viaduct over the top of the whole thing.

Start
London
End
Pont Abraham
Passes
Hounslow, Heathrow Airport, Slough, Maidenhead, Reading, Newbury, Swindon, Chippenham, Bristol, Newport, Cardiff, Bridgend, Port Talbot, Swansea
Length
192 miles
Open Junctions Section
Sep 1959 J1 Chiswick Flyover Formerly A4.
May 1961 J7-9 Maidenhead Bypass Opened as A4(M)
Mar 1963 J5-7 Slough → Maidenhead Bypass
Mar 1965 J1-5 Chiswick → Slough
Jan 1966 J18-20 Tormarton → Almondsbury
Jul 1966 J39-41* Port Talbot Bypass Opened as A48(M)
Sep 1966 J20-21 Almondsbury → Aust
May 1967 J24-28 Newport Bypass
Aug 1967 J23*-24 Crick → Newport Bypass
1971 J9-10 Holyport → Winnersh
Dec 1971 J14-18 Wickham → Tormarton
Dec 1971 J10-14 Winnersh → Wickham
1972 J44-46 Morriston Bypass
Apr 1977 J46-49 Pontardulais Bypass
Oct 1977 J28-29 Tredegar Park → Castleton
Nov 1977 J37*-39 Pyle Bypass
Dec 1977 J32-35 Coryton → Pencoed
1980 J35-37* Bridgend Bypass
Jul 1980 J29-32 Castleton → Coryton
Dec 1994 J41*-44 Baglan → Lon Las
Jun 1996 J21-23 Second Severn Crossing

Exit list

Junction   Eastbound               Westbound  
1
12.4 km
Central London
Hammersmith
A4


A315

A406
EAST
A4


A205
N/A
LanesLanesLanesLanesLanes SignsSigns LanesLanesLanesLanesLanes SignsSigns
1 mile, 2 lanes 1 mile, 2 lanes
2
13.4 km
North Circular Road
A406 Link
South Circular Road
A205
Chiswick
A315


A4
Staines
Hounslow
A4
LanesLanesLanesLanes LanesLanesLanesLanes
2 miles, 2 lanes 2 miles, 2 lanes
16.0 km
LanesLanesLanesLanesLanes Signs LanesLanesLanesLanes Signs
3 miles, 3 lanes 3 miles, 3 lanes
20.5 km Services Heston Services Heston
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
1 mile, 3 lanes 2 miles, 3 lanes
3
22.0 km
Feltham
A312
Hounslow
(A3006)
A312 A312 Heathrow Airport T4 & Cargo
Feltham
Hayes
A312
LanesLanesLanesLanesLanes Signs LanesLanesLanesLanesLanes
2 miles, 3 lanes 2 miles, 3 lanes
4
25.0 km
Uxbridge
Hayes
Heathrow Airport T1, 2 & 3
(A408)
(A408) (A4) Heathrow Airport T1, 2 & 3
Uxbridge
(A408)
LanesLanesLanesLanesLanesLanes LanesLanesLanesLanesLanesLanes
2 miles, 4 lanes 2 miles, 4 lanes
4B
27.5 km
Watford (M1 Link)
Birmingham (M40 Link)
Heathrow Airport T4 & Cargo
Gatwick Airport (M23 Link)
Winchester (M3 Link)
M25 Link
M25
(M1)
(M40)
M25
(M3)
(M23)
Heathrow Airport T4 & Cargo
Gatwick Airport (M23 Link)
Watford (M1 Link)
Birmingham (M40 Link)
M25 Link
LanesLanesLanesLanesLanesLanes LanesLanesLanesLanesLanesLanes
2 miles, 4 lanes 2 miles, 4 lanes
5
31 km
Langley
B470
Staines (A3044)
A4
A4 A4
(A3044)
B470
Langley
A4
Eton
B470
LanesLanesLanesLanesLanesLanes LanesLanesLanesLanesLanesLanes
4 miles, 3 lanes 4 miles, 3 lanes
6
37 km
Slough (C)
A335
Windsor
(A332)
A335 A335 Slough (C)
A335
Windsor
(A332)
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
2 miles, 3 lanes 2 miles, 3 lanes
7
40 km
Slough (W)
A4
(A4)

Slough (W)
A4
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
4 miles, 3 lanes  
River or canal Thames River or canal Thames
  4 miles, 3 lanes
8/9
45 km
High Wycombe
A404(M) Link
Maidenhead
A308(M) Link
A308(M)

A404(M)
High Wycombe
Henley
A404(M) Link
Maidenhead
A308(M) Link
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
7 miles, 3 lanes 7 miles, 3 lanes
10
57 km
Reading (E)
Wokingham
Bracknell
A329(M) Link
A329(M) A329(M) Reading (E)
Wokingham
Bracknell
A329(M) Link
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
6 miles, 3 lanes 6 miles, 3 lanes
11
66 km
Reading (C & S)
Basingstoke
A33
B3270

A33
A33 Reading (C & S)
Basingstoke
A33
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
3 miles, 3 lanes 3 miles, 3 lanes
70.2 km Services Reading Services Reading
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
2 miles, 3 lanes 2 miles, 3 lanes
12
73 km
Theale
Reading (W)
A4
A4 A4 Theale
Reading (W)
A4
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
12 miles, 3 lanes 12 miles, 3 lanes
13
92 km
Newbury
Oxford
A34
Services Chieveley
A34 A34 Newbury
Oxford
A34
Services Chieveley
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
8 miles, 3 lanes 8 miles, 3 lanes
14
104 km
Hungerford
Wantage
A338
A338 A338 Hungerford
Wantage
A338
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
4 miles, 3 lanes 4 miles, 3 lanes
110 km Services Membury Services Membury
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
8 miles, 3 lanes 8 miles, 3 lanes
15
123.5 km
Marlborough
A346
Swindon (E)
A419 Link
Oxford (A420)
A419
(A420)
A346 Marlborough
A346
Swindon
A419 Link
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
6 miles, 3 lanes 6 miles, 3 lanes
16
132.5 km
Swindon (W)
Wootton Bassett
RAF Lyneham
Calne
A3102
A3102 B4005

A3102
Swindon (W)
Wootton Bassett
RAF Lyneham
Calne
A3102
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
12 miles, 3 lanes 12 miles, 3 lanes
17
153 km
Cirencester
A429
Chippenham
A350
A429 B4122

A350
Cirencester
A429
Chippenham
A350
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
2 miles, 3 lanes 2 miles, 3 lanes
155.5 km Services Leigh Delamere Services Leigh Delamere
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
9 miles, 3 lanes 9 miles, 3 lanes
18
169.5 km
Bath
Stroud
A46
A46 A46 Bath
Stroud
A46
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
7 miles, 3 lanes 7 miles, 3 lanes
19
181.5 km
Bristol
M32 Link
M32 Bristol
M32 Link
LanesLanesLanesLanesLanesLanes Signs LanesLanesLanesLanesLanesLanes Signs
3 miles, 3 lanes + HSR 3 miles, 3 lanes + HSR
20
186.5 km
The MIDLANDS
Gloucester
The SOUTH WEST
Bristol & Airport
M5 Link
M5 M5 The SOUTH WEST
Bristol (W)
The MIDLANDS
Gloucester
M5 Link
LanesLanesLanesLanesLanesLanes
LanesLanesLanesLanesLanes Signs
LanesLanesLanesLanesLanesLanes Signs
2 miles, 3 lanes 2 miles, 2 lanes
21
189.5 km
M48 Chepstow
M48 Link
LanesLanesLanesLanesLanes LanesLanesLanesLanes
3 miles, 2 lanes 3 miles, 2 lanes
22
195.5 km
The SOUTH WEST
Avonmouth
M49 Link (M5 Link)
M49 Avonmouth
M49 Link
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
7 miles, 3 lanes  
River or canal Severn River or canal Severn
  6 miles, 3 lanes
Severn Bridge Toll
LanesLanesLanesLanesLanes Signs
   
23 Cas-gwent
Chepstow
M48 Link
M48
LanesLanesLanesLanesLanes
1 mile, 3 lanes 2 miles, 3 lanes
23A Magwyr
Magor
B4245

Services Magor
B4245 Magwyr
Magor
Caldicot
B4245
Services Magor
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
4 miles, 3 lanes 4 miles, 3 lanes
24 CANOLBARTH LLOEGR
The MIDLANDS
Trefynwy
Monmouth
A449 (M50) Link
Casnewydd
Newport A48
A48

A449


A48

B4237
Casnewydd (Dn)
Newport (E) A48
Trefynwy
Monmouth
A449 Link
LanesLanesLanesLanesLanes Signs LanesLanesLanesLanesLanes Signs
4.25 miles, 3 lanes 3 miles, 3 lanes
25 B4596 B4596 Caerleon
B4596
LanesLanesLanesLanesLanes Signs
  0.25 miles, 3 lanes
25A
A4042

A4042
Casnewydd
Newport
Cwmbran
A4042
LanesLanesLanesLanesLanes Signs LanesLanesLanesLanesLanes Signs
1 mile, 2 lanes  
Brynglas Tunnel Brynglas Tunnel
  1 mile, 2 lanes
26 Cwmbran
Casnewydd
Newport
Caerleon
A4051
A4051 A4051 Casnewydd
Newport
A4051
LanesLanesLanesLanesLanes Signs LanesLanesLanesLanesLanes Signs
2 miles, 3 lanes 2 miles, 3 lanes
27 High Cross
B4591
B4591 B4591 High Cross
B4591
LanesLanesLanesLanesLanes Signs LanesLanesLanesLanesLanes Signs
1 mile, 3 lanes 1 mile, 3 lanes
28 Casnewydd
Newport A48
Risca
Brynmawr
A467
A467 A48


A48
Casnewydd
Newport A48
Risca
Brynmawr
A467
LanesLanesLanesLanesLanes Signs LanesLanesLanesLanesLanes Signs
5 miles, 3 lanes 2 miles, 3 lanes
29 A48(M) Caerdydd (Dn a D)
Cardiff (E & S)
A48(M) Link
LanesLanesLanesLanesLanes
  3 miles, 3 lanes
30 Caerdydd (Dn)
Cardiff (E)
A4232
Services Cardiff Gate
A4232 Caerdydd (Dn)
Cardiff (E)
A4232
Services Cardiff Gate
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
5 miles, 3 lanes 5 miles, 3 lanes
32 Caerdydd (G)
Cardiff (N)
Merthyr Tudful
Merthyr Tydfil
A470
A4054

A470




A470

A4054
Caerdydd (G)
Cardiff (N)
Merthyr Tudful
Merthyr Tydfil
A470
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
3 miles, 3 lanes 3 miles, 3 lanes
33 Caerdydd (Gn) a Airport
Cardiff (W) & Airport
Barri
Barry
Penarth
A4232
Services Cardiff West
A4232 Caerdydd (Gn) a Airport
Cardiff (W) & Airport
Barri
Barry
Penarth
A4232
Services Cardiff West
LanesLanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
2 miles, 4 lanes 2 miles, 3 lanes
34 Llantrisant
Rhondda
A4119
A4119 Llantrisant
Rhondda
A4119
LanesLanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
6 miles, 3 lanes 6 miles, 3 lanes
35 Pen-y-bont
Bridgend
Pen-coed
A473
A473 A473 Pen-y-bont
Bridgend
Pen-coed
A473
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
4 miles, 3 lanes 4 miles, 3 lanes
36 Pen-y-bont
Bridgend
A4061
Maesteg
A4063
Services Sarn Park
A4061

A4063
A4061 Pen-y-bont
Bridgend
A4061
Maesteg
A4063
Services Sarn Park
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
6 miles, 3 lanes 6 miles, 3 lanes
37 Pîl
Pyle
Porthcawl
A4229
A4229
(A48)
A4229 Pîl
Pyle
Porthcawl
A4229
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
3 miles, 3 lanes 3 miles, 3 lanes
38 Port Talbot
A48
A48 Port Talbot
A48
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
4 miles, 2 lanes  
39 A48
  4 miles, 2 lanes
40 Port Talbot
A4107
A4107 A4107 Port Talbot
A4107
LanesLanesLanesLanes Signs LanesLanesLanesLanes Signs
1 mile, 2 lanes  
41 Port Talbot
A48
A48 B4286

A48
LanesLanesLanesLanes
1 mile, 2 lanes 2 miles, 2 lanes
41 (A48) Llansawel
Briton Ferry
A48
Signs LanesLanesLanesLanes Signs
3 miles, 2 lanes  
River or canal Nedd
River or canal Neath
River or canal Nedd
River or canal Neath
  3 miles, 2 lanes
42 A483 A483 Abertawe
Swansea
Vehicle Ferry
A483
LanesLanesLanesLanes
   
42 Llansawel
Briton Ferry A48
Abertawe
Swansea A483
(A48)
(A483)
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
2 miles, 3 lanes 2 miles, 3 lanes
43 Castell-nedd
Neath
Merthyr Tudful
Merthyr Tydfil
A465
A465 B4290 Castell-nedd
Neath
Merthyr Tudful
Merthyr Tydfil
A465
LanesLanesLanesLanesLanes LanesLanesLanesLanesLanes
2 miles, 2 lanes 2 miles, 2 lanes
44 Abertawe (D)
Swansea (E)
A48
A4230

B4291
A48 Abertawe (D)
Swansea (E)
A48
LanesLanesLanesLanes LanesLanesLanesLanes
2 miles, 2 lanes 2 miles, 2 lanes
45 Abertawe (G)
Swansea (N)
Pontardawe
A4067
A4067

B4603
A4067

B4603
Abertawe (G)
Swansea(N)
Pontardawe
A4067
LanesLanesLanesLanes LanesLanesLanesLanes
2 miles, 2 lanes 2 miles, 2 lanes
46 Llangyfelach
Felindre
B4489
A48

B4489

A48
Llangyfelach
Felindre
B4489
LanesLanesLanesLanes LanesLanesLanesLanes
2 miles, 2 lanes 2 miles, 2 lanes
47 Abertawe (Gn)
Swansea (W) A483
Services Swansea
A48

A483

A48
Abertawe (Gn)
Swansea (W) A483
Services Swansea
LanesLanesLanesLanes LanesLanesLanesLanes
4 miles, 2 lanes  
River or canal Loughor River or canal Loughor
  4 miles, 2 lanes
48 Pontarddulais
Llanelli
A4138
A4138 A4138 Pontarddulais
Llanelli
A4138
LanesLanesLanesLanes LanesLanesLanesLanes
3 miles, 2 lanes 3 miles, 2 lanes
49
35.3 km
N/A A48


A483











A48
WEST
Services Pont Abraham
Caerfyrddin
Carmarthen A48
Rhydaman
Ammanford
Llandeilo
A483
Pontarddulais A48
LanesLanesLanesLanes Signs LanesLanesLanesLanes Signs
Routes

Picture credits

With thanks to Maurice Turner, Michael Young, Dave Ross, Chris Jackson, Steve, Chris McKenna, Chris Boots, Tim Lidbetter, Neil Padgen, Jonathan, Chris Clifford, Chris (a different one) and Martin Wareham for information on this page.