M4 motorway


The M4, a motorway in the United Kingdom running from west London to southwest Wales, was originally referred to as the London-South Wales Motorway. The English section to the Severn Bridge was constructed between 1961 and 1971; the Welsh element was completed in 1993. The construction of the Second Severn Crossing, officially renamed the Prince of Wales Bridge and inaugurated on 5 June 1996 by the Prince of Wales, caused the M4 to be rerouted.
The line of the motorway from London to Bristol runs closely in parallel with the A4. After crossing the River Severn, toll-free since 17 December 2018, the motorway follows the A48, to terminate at the Pont Abraham services in Carmarthenshire.
The M4 is the only motorway in Wales apart from its two spurs: the A48 and the M48. The major towns and cities along the routea distance of approximately include Slough, Reading, Swindon, Bristol, Newport, Cardiff, Bridgend, Port Talbot and Swansea.

History

A new road from London to South Wales was first proposed in the 1930s. In 1956 the Ministry of Transport announced the plans for the first major post-war road improvement projects.
The Chiswick flyover, a short section of elevated dual-carriageway, not originally classed as a motorway, opened in 1959 to reduce the impact of traffic travelling between central London and the west.

original bridge
The Maidenhead bypass opened in 1961, the section from Slough to Maidenhead opened in 1963 and J1-J5 opened on 24 March 1965 incorporating the Chiswick Flyover. The stretch from J18 to the west of Newport was opened in 1966, including the Severn Bridge. 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 Ministry of Transport originally intended that the M4 would terminate at Tredegar Park west of Newport, and following the creation of the Welsh Office that the Government became committed to a high-standard dual carriageway to Carmarthenshire.
The English section of the motorway was completed on 22 December 1971 when the stretch between junctions 9 and 15 was opened to traffic. The Welsh section was completed in 1993, when the Briton Ferry motorway bridge opened.
In 1996, the Second Severn Crossing opened with new link motorways on either side of the estuary to divert the M4 over the new crossing. At the same time, the original route over the Severn Bridge was redesignated the M48 and the M49 was opened to link the new crossing with the M5 at Avonmouth.

After completion

In April 2005, speed checks carried out by police camera vans between junction 14 and junction 18 led to a public protest, involving a "go-slow" of several hundred vehicles along the affected sections of the motorway.
Between 2007 and January 2010, the section from Castleton to Coryton was widened to six lanes. The scheme was formally opened on 25 January 2010 by Ieuan Wyn Jones the Deputy First Minister for Wales.
During 2009, the Newport section of the motorway between junctions 23a and 29 was upgraded with a new concrete central barrier. In February 2010 it was proposed that the M4 in South Wales would become the first hydrogen highway with hydrogen stations provided along the route, with an aspiration for further stations to be provided along the M4 into South West England over time.
Between 2008 and 2010, junction 11 was extensively remodelled with a new four-lane junction, two new road bridges and other works. The £65m scheme included work on the Mereoak roundabout and part of the A33 Swallowfield Bypass near Shinfield, and also the conversion of the two existing bridges, one of which is available only to pedestrians and cyclists and the other to buses. It also involved the movement of the local Highways Agency and Fire Service offices, and the construction of a long footbridge network, a new bus-lane and a new gyratory. Sound barriers for nearby residential areas were also installed. 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.
The M4 crosses the River Severn on the Second Severn Crossing, toll-free since 17 December 2018.

Timeline of construction

Year opened1961196319651966196719711972197719801994
SectionJ7–9J5–7J1–5J18–23
J39–41		J22–28J9–18J44–46J28–29
J32–35
J37–39
J46–49		J29–32
J35–37		J41–44

was opened in 1996, the M4 was re-routed and the section of motorway between Junctions 21 to 23 became the M48.

Maintenance

Maintenance of the Second Severn Crossing and the of motorway in England is the responsibility of Highways England. The in Wales is the responsibility of the Welsh Government.

Features

Speed limits

For the majority of its length, the national speed limit applies. Exceptions include the following:
The M4 has two sections of smart motorway. The one between junctions 19 and 20 north of Bristol has variable speed limits and a part-time hard-shoulder. Completion was in summer 2014. The section between junctions 24 and 29 in Newport has variable speed limits.
In 2010, it was announced that the motorway would be upgraded to a smart motorway between junctions 3 and 12. With a length of, this will be the longest smart motorway scheme in the United Kingdom. Work started in autumn 2018 and is expected to be completed in March 2022 at a cost of £848 million.

Brynglas tunnels

The Brynglas Tunnels carry the M4 under Brynglas Hill in Newport, Wales. The tunnels are the first and only twin–bored tunnels in the UK motorway network.
In July 2011, a lorry fire in one tunnel closed the motorway. Although there were no injuries and no deaths, the tunnel remained closed and a contraflow system was in place in the remaining tunnel for about one month, causing major travel delays.

Bus lane

In June 1999, the M4 bus lane was created on the third lane between junctions 2 and 3initially, a pilot schemepermanently in 2001. A lower speed limit was introduced along the section at the same time. The bus lane was on the eastbound carriageway; from the western end of the Chiswick Flyover near Brentford to junction 3 covering part of the journey between Heathrow Airport and central London. The lane which had no intermediate exits was for use by buses, coaches, motorcycles, emergency vehicles and licensed taxis but not mini-cabs.
The bus lane was used by 7% of vehicles which carried 21% of the people. At the end of 2010, the M4 was restored to normal motorway use for 18 months.
The bus lane was maintained temporarily just for the 2012 Summer Olympics.

Porous road surface

Near Junction 35 of the M4, 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 drivers notice a reduction in road spray from other vehicles and improved visibility. This special surface was publicised in an episode of the BBC's 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 of the M4 in West London, built in the 1960s, is mostly directly above the A4 and extends over parts of Brentford's Golden Mile. This section was designed to have a heated road surface to reduce icing in winter.

Four level stack interchanges

The M4 has two of the three four-level stack interchanges in the UK, including the first UK example at junction 20, the "Almondsbury Interchange" with the M5. The other is junction 4b with the M25. Junction 4b has to make provision for the railway line passing beneath the M4. Due to the nature of these junctions, one cannot make a U-turn at either of them.

Notable junctions

Junction 8/9 near Maidenhead, Berkshire, and High Wycombe, Buckinghamshire is the only one in the UK with dual numbers. This arose at the time when the M4 turned north near junction 8, where it met the A308, and headed for the original junction 9, where the motorway ended at a roundabout interchange with the A4. When the westward extension was opened, junction 8 was closed and a new junction built a little to the west, taking both numbers. The road to the A4 became A423 and later A404, and the junction with the A4 became 9B. Junction 9A is the exit for Cox Green and White Waltham. To the 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 Welford, a Second World War airfield and now an RAF/USAF military installation mainly used for storing munitions. The M4 entrance allows easier access for the large vehicles used to carry the munitions.
Plans for the "missing" Junction 31, also known as the Thornhill interchange, for which planning permission was originally granted in September 1991, were rekindled after proposals for a new business park on a site north of the M4 were submitted in 2007 to Cardiff Council. The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469. A freedom of information request in 2010 to Cardiff Council shows that whilst the land that would enable this junction should continue to be strategically protected, the decision to formally abandon the proposed Junction 31 Thornhill was made in October 2007 and there had been no subsequent mention of it in Cardiff Council Strategic or Planning meetings since.
In South Wales, the M4 has to thread its way through mountainous terrain and built-up areas, so there are some unusual junction layouts. Junction 27 is a normal grade-separated roundabout junction, but has severe space constraints: traffic joining the motorway must initially travel in the opposite direction to the intended direction of travel, before making a sharp left-hand turn from the slip road onto the motorway. At the time of construction, junction numbers 30 and 31 were reserved for future intermediate interchanges. Junction 30 has since been added, but there are no current plans to construct Junction 31. Junction 39 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 comprises two different junctions; one for local traffic to and from the west and one from the east. The former leads to and from a spur leading to the roundabout in Briton Ferry, formerly known as junction 41a, and the original bridge over the River Neath, which would allow access onto the stretch of the M4 from junction 43 westward. The second, eastern junction leads to and from the A48 towards Port Talbot. As a result, one can travel for almost on the motorway in either direction, both joining and then leaving the motorway at junction 41. Junction 44 is unusual in that the eastbound entrance dives under the inside of the junction, effectively creating a "right-turn" on a roundabout. Similarly, slip roads pass under or over the main motorway at junctions 41 and 42.
There have been calls to close the slip roads at junctions 40 and 41 to improve traffic flow. The motorway has only two lanes on this stretch and is a major traffic congestion blackspot. The short slip roads have not been modernised. A small-scale trial of closing the westbound on-slip of junction 41 between 7 am and 9 am and from 4 pm to 6 pm on weekdays started on 4 August 2014 but following heavy criticism from local businesses and residents, was stopped on 29 May 2015.

Relief road

On 5 June 2019, the Welsh Government scrapped the proposal for a proposed motorway south of Newport. The Welsh Assembly Government had revived the scheme as a tolled bypass in 2007 and later abandoned it for financial reasons. An extension to the Newport Southern Distributor Road through the old Corus steel works was considered. This road is already a dual carriageway but not open to the public. A public consultation exercise on options for improving the capacity of the M4 corridor around Newport opened on 5 March 2012. Its website states that: "the motorway around Newport does not conform to today’s motorway standards. It lacks continuous hard shoulders, has closely spaced junctions with sub-standard slip road visibility and narrows to a restricted two lane section through the Brynglas Tunnels. Heavy congestion occurs along this stretch and either side of it at peak hours."

List of junctions

E30

Although not signed, European route E30 includes most of the M4.

Major incidents and accidents