|Location Map ( geo)|
|From:||Chiswick, Middlesex (TQ19578)|
|To:||Pont Abraham, Carmarthenshire (SN575071)|
|Via:||Slough, Reading, Swindon, Bath, Bristol, Newport, Cardiff, Port Talbot, Swansea|
|Distance:||191.9 miles (308.8 km)|
|Meets:||M5, M25, M32, M48, M49, A33, A34, A4, A46, A48(M), A48, A308(M), A312, A329(M), A346, A350, A404(M), A419, A429, A449, A465, A470, A483, A4042, A4138, A4232|
|Former Number(s):||A4, A4(M), A48(M) (Port Talbot and Morriston bypasses)|
|Old route now:||M48, A404(M), A48(M) (Cardiff spur)|
HE Area 2 • HE Area 3 • HE Area 5 • South Wales TRA
|Route outline (key)|
|Former junctions are shown on a thick grey background.|
The M4 is the London to South Wales Motorway. Running for about 190 miles from Chiswick in the west of the capital to a point some 12 miles southeast of Carmarthen in west Wales, it passes such important centres of population and industry as Reading, Swindon, Bristol, Newport, Cardiff, and Swansea. The route forms the bulk of the E30's Fishguard to Felixstowe route in the UK.
- 1 Route
- 2 History
- 3 Improvements
- 4 Junctions
- 5 Structures
- 6 Services
- 7 Traffic data
- 8 Miscellanea
- 9 Links
London – Reading
The road into London is one of few examples of truly urban major motorways which were built as intended. It was also a pioneer of modern features, boasting three CCTV cameras, detectors to report congestion and a never-used underfloor heating system which claimed there would never be a frost on the road. The elevated section claims to have been built without "any disruption" to the Great West Road beneath it.
The tensioned wire central reservation barrier fitted to the road at Langley was the first in the UK, and was used by TRL to test its effectiveness before the road opened.
Reading – Bath
After Reading, the M4 becomes more rural and picturesque - travelling first through the North Wessex Downs, and then the southern Cotswolds. The motorway runs a few miles north of the A4 and the towns along it that, save for Swindon, are the main places that it serves. At junction 13 the motorway meets the major A34 north-south route, serving Newbury. At junction 14, Hungerford is served via the A338. Swindon has two junctions: junction 15, which serves also serves Marlborough via an extended A346, as well as the A417/A419 trunk dual carriageway to the M5 at Gloucester and junction 16, which serves western suburbs. Junction 17 serves Chippenham, with the A429 south of the junction having been renumbered A350 upon the opening of the motorway. Junction 18 with the A46 serves Bath.
Bath – Cardiff
After the junction for Bath, the road continues over some rolling hills before we reach the peripheral areas of Bristol, and Junction 19 for the M32. The road continues onwards for a couple of miles as a Smart Motorway, before coming to Almondsbury Interchange, the Four Level Stack interchange with the M5. The original route of the M4 for the Severn Bridge goes along what is now the M48, with main line traffic taking the newer Second Severn Crossing at Awkley Interchange. The M49 has a junction just before the Second Severn Crossing, which one can now cross without having to pay a toll. The old toll office has now been turned into a nondescript "works unit." We meet again with the M48 at Rogiet Interchange, before being shunted with Magor Interchange two thirds of a mile later, with access to the A4810. Junction 24 is for the Coldra Interchange, with access to the A48, A449 and B4237. Another smart motorway section starts shortly before this section, in an attempt to avail the hopelessly under-capacity Newport Bypass. We meet two limited-access interchanges at junctions 25 and 25A, before entering the D2 pinch point of the Brynglas Tunnels. Almost immediately after the tunnels, we encounter Junction 26, Malpas Interchange, with access to the A4051. We then have a couple of miles without junctions, before we get to juntion 27, with access to the B4591. The bends around this junction are sharper than are typical for a motorway, hence warnings on their approach. After Tredegar Park Interchange for the A48, we meet the A48(M) at Castleton Interchange, providing one-way access to the only motorway that is solely in Wales. The motorway bypasses Cardiff to the north, meeting Pentwyn Interchange for the A4232 and Coryton Interchange, one of the biggest roundabouts in the UK, providing access to the A470 and the A4054.
Cardiff – Pont Abraham
The motorway continues west from Coryton, passing J33 Capel Llanilltern Interchange before running along the southern fringe of the South Wales Valleys past Llantrisant to Pencoed. Then the motorway runs to the north of Bridgend before crossing the Ogwr Viaduct to the west of Sarn Interchange. Continuing west, the motorway soon turns northwards at Pyle, passing over a long exposed embankment and Kenfig bypass before reaching the former A48(M) (Port Talbot Bypass). At junction 38 the motorway reduces to two lanes in each direction, and remains in this configuration along its remaining length to Pont Abraham, with a couple of localised exceptions.
The Port Talbot bypass was opened in 1965 as the A48(M) and was the first section of motorway in Wales. It follows a sinuous route between the eastern fringe of the town , some times built into the steep hills east of the town. This section was renumbered M4 in 1977 when the extension to Pyle was built, as part of the plan to extend the M4 from Newport to Pont Abraham. The elevated section skirts around chapels, rugby grounds and houses. In the 1980s, the original parapets were raised and strengthened to meet current standards. For many years, the M4 had a gap between the Baglan roundabout and Lon-Las Interchange, and became famous for having the only traffic signals on the main line of a motorway at Baglan. In the early 90s the missing link was built in three contracts (two phases) comprising an extension east from Lonlas to Earlswood Interchange on the west side of the original Briton Ferry Bridge, with the second phase completing the link through construction of the M4 Briton Ferry Bridge and link to Baglan. The Briton Ferry bridge was designed in an era of tall shipping passing up the River Neath, but by the time the road opened, the shipping had long since stopped.
The section between Earlswood and Lon-Las made use of the existing A48 interchange with the A465, and the short link between the A465 and Earlswood has three lanes running in each direction.
Continuing west, the section from Lonlas to the Llangyfelach Interchange opened in 1972 and includes a climbing lane running west from Ynysforgan Interchange towards Llangyfelach. Some maps from the early 1970s show this road as the A48(M), but contemporary records shown that it opened as part of the M4. From Llangyfelach, the motorway continues west via turnings for Swansea West and Llanelli before arriving at its western terminus at roundabout Pont Abraham Interchange where the two onward routes are the A48 towards Carmarthen and the A483 which is one of the three principal north-south routes across Wales.
Perhaps surprisingly, the M4 may be counted Great Britain's second oldest motorway, as the Chiswick Flyover (then numbered A4, but now the first section of the M4) opened in 1959, a couple of months before the M1, M10, and M45. Other parts of this motorway started life as the A4(M) and A48(M), whilst the Severn and Wye Bridges – originally on the M4 but bypassed since 1996 by the Second Severn Crossing – now lie on the M48.
2005 – M4 Jct 18 widening
The eastbound carriageway was widened from D3M to D4M on the approach to Junction 18 at Tormarton. The fourth lane was added to act as a crawler lane for slow vehicles travelling up the steep hill.
2010 – M4 Jct 29 to 32 widening
The motorway was widened from D2M to D3M between Junction 29 at Castleton to Junction 32 at Coryton. The scheme started in 2007 and was completed in 2010, costing around £99.2 million in total.
2011 – M4 Jct 24 to 28 variable speed limit scheme
The motorway between Junction 24 at Coldra and Junction 28 at Tredegar Park received variable speed limits in order to help relieve congestion in and around Newport. This section of the motorway saw the installation of new gantries and speed cameras, along with the replacement of the street lighting in the central reservation.
2014 – M4 Jct 19 to 20 smart motorway scheme
The motorway was upgraded to become a smart motorway between Junction 19 at Hambrook and Junction 20 at Almondsbury. The scheme started in 2012 and was completed in 2014. Features of this smart motorway include a dynamic hard shoulder and variable speed limits.
The Brentford Interchange (junction 2) is a compact 'ironing-board' interchange.
Thorney Interchange (junction 4b of the M4; junction 15 of the M25) is a large four-level stack interchange between the M4 and M25 motorways. It is unusual in that, in addition to the various different roads that make up the junction, there is also a railway line running through it.
The Chieveley Interchange (junction 13) was upgraded in the late 2000s to a 3-level roundabout.
The Almondsbury Interchange (junction 20 of the M4; junction 15 of the M5) is an important junction allowing direct interchange between the M4 and M5, and is additionally linked to junction 16 of the M5, which provides access to and from the A38. This junction was the first example of a four-level stack in the country. The link roads leading from the M4 to the M5 pass over the top level of the interchange whilst those from the M5 to the M4 pass underneath all other levels.
The Coryton Interchange (junction 32) is a very large 3-level roundabout, with the A470. The circulatory carriageway of the roundabout is one of the longest in the country, clocking in at over a mile. The A470 mainline goes over the roundabout's north side, but under its south side.
Pont Abraham Interchange
Pont Abraham is the westernmost motorway junction in Britain. Consisting of a roundabout with the A48 and A483, junction 49 is only one of two (junction 48 being the other one) motorway junctions signed with Welsh destinations first.
Second Severn Crossing
The Second Severn Crossing was constructed between 1992 and 1996 to provide relief for the Severn Bridge and Wye Bridge between England and Wales. Upon completion, the M4 moved across from the original bridges (which became part of the M48) to the new bridge.
The Brynglas Tunnels are situated on the Newport bypass, between junctions 25a and 26. They are twin-bored tunnels, each carrying two traffic lanes and opened in 1967. Until recently, the eastbound tunnel was lit by orange lighting, the westbound by white. Both tunnels have overhead lane control signals for contraflow purposes.
These are situated where the M4 crosses the A34 near the village of Chieveley in Berkshire.
Leigh Delamere services
Leigh Delamere services are located in Wiltshire between junctions 17 and 18. Originally, only the eastbound side was developed; the westbound side followed around ten years later.
Pont Abraham services
Pont Abraham services are a single-site MSA located on the western terminal roundabout of the M4, at junction 49. It is operated by RoadChef.
|Link||2002 AADF||2006 AADF||2010 AADF||2014 AADF|
- The section of the M4 between junctions 19 and 23 has the the highest number of consecutive motorway-to-motorway junctions in the United Kingdom.
- In 1999 a controversial bus lane was established on a 5.6-km stretch of the eastbound carriageway of the M4 between junction 3 and the start of the elevated section of the motorway near Brentford. Suspended in December 2010, it was re-instated temporarily for the duration of the 2012 London Olympics.
- The M4 Motorway (Port Talbot, Neath Port Talbot) (50 Mph Speed Limit) Regulations 2004 - This legislation is revoked by WSI/2019/1079
- The M4 Motorway (Junctions 19 to 20) and the M5 Motorway (Junctions 15 to 17) (Actively Managed Hard Shoulder and Variable Speed Limits) Regulations 2013
- The M4 Motorway (West of Junction 23A (Magor) to East of Junction 29 (Castleton)) (Variable Speed Limits) Regulations 2011 - these were revoked by the 2015 regulations
- The M4 Motorway (West of Junction 23A (Magor) to East of Junction 29 (Castleton)) (Variable Speed Limits) Regulations 2015 - these were revoked by the 2021 regulations
- The M4 Motorway (Junction 28 (Tredegar Park) to Junction 24 (Coldra)) (50 mph Speed Limit) Regulations 2021 - These Regulations impose a maximum speed limit of 50 miles per hour (in place of the national speed limit) on the lengths of the M4 motorway specified in the Schedule to these Regulations. These Regulations also revoke the M4 Motorway (West of Junction 23A (Magor) to East of Junction 29 (Castleton)) (Variable Speed Limits) Regulations 2015.
- The M4 Motorway (Junction 41 (Pentyla) to Junction 42 (Earlswood), Neath Port Talbot) (Temporary 50 mph Speed Limit) Order 2018 - This legislation is revoked by WSI/2019/1079
- The M4 Motorway (Junction 40 (Tai Bach) to Junction 42 (Earlswood), Neath Port Talbot) (50 mph Speed Limit) Regulations 2019
- Post Opening Project Evaluation: M4 J19 –20 & M5 J15 –17 Smart Motorway - One Year After January 2020 (archive.org)
- Severn bridges: M4 Severn crossing reopens toll free (17.12.2018)
Construction and early photographs