M4 motorway

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.[8]

Between 2007 and January 2010, the section from Castleton (junction 29) to Coryton (junction 32) was widened to six lanes.[9] 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.[10][11]

Between 2008 and 2010, junction 11 was extensively remodelled with a new four-lane junction, two new road bridges and other works.[12] 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.[13] 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.[14] 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.[15]

The M4 crosses the River Severn on the Second Severn Crossing,[16] toll-free since 17 December 2018.[17]

[18][19][20][21][22][23]

Animated map showing build progress at five-years (or greater) intervals.
Note: When the Second Severn Crossing was opened in 1996, the M4 was re-routed and the section of motorway between Junctions 21 to 23 became the M48.[24]

Maintenance of the Second Severn Crossing and the 123 miles (198 km) of motorway in England is the responsibility of Highways England.[25] The 76 miles (122 km) in Wales is the responsibility of the South Wales Trunk Road Agent.[26]

For the majority of its length, the national speed limit applies. Exceptions include the following:

• 40 miles per hour (64 km/h) on the Chiswick Flyover within London in both directions.
• 60 miles per hour (97 km/h) between junction 4 and the Chiswick Flyover eastbound only.
• 50 miles per hour (80 km/h) on the Port Talbot elevated section between junction 40 and junction 41. The fixed speed camera was removed in 2006 as it was believed to be causing tailbacks. In July 2014, an average speed camera system was installed; it generated around £500,000 in fines in the first six months.[27][28]

Construction of electronic indicator signs for the variable speed limit scheme at junction 27 and a new concrete reservation (2010)

The M4 has two sections of smart motorway. The one between junctions 19 (M32) and 20 (M5) 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.[29]

In 2010, it was announced that the motorway would be upgraded to a smart motorway between junctions 3 and 12. With a length of 32 miles (51 km), 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.[30][31]

Brynglas Tunnels – western portals

The Brynglas Tunnels carry the M4 under Brynglas Hill in Newport, Wales. The 360-metre-long (390 yd) tunnels are the first and only twin–bored tunnels in the UK motorway network (the Dartford Tunnel is not classified as part of the M25 motorway). 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.[32][33][34]

M4 bus lane near Norwood Green

In June 1999, the M4 bus lane was created on the third lane between junctions 2 and 3—initially, a pilot scheme—permanently in 2001. A lower speed limit was introduced along the section at the same time.[35] The 3 ¹⁄₂-mile (5.6 km) bus lane was on the eastbound carriageway; from the western end of the Chiswick Flyover near Brentford to junction 3 (A312) covering part of the 15-mile (24 km) 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.[36]

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.[37][38] The bus lane was maintained temporarily for the 2012 Summer Olympics.[39]

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.[40]

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.[41][42]

M4 junction with the M25 near Heathrow Airport

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.

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(M) and later A404(M), 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.[43]

Plans for the "missing" Junction 31, also known as the Thornhill interchange, for which planning permission was originally granted in September 1991 (but subsequently expired), were rekindled after proposals for a new business park on a 125 acres (51 ha) site north of the M4 were submitted in 2007 to Cardiff Council.[44] The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469.[45] 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.[46] 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 (High Cross) 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 (Cardiff Gate) has since been added, but there are no current plans to construct Junction 31 (A469 road). 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.[47]

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 2 miles (3.2 km) 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.[48]

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.[49] 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[50][51] but following heavy criticism from local businesses and residents,[52] was stopped on 29 May 2015.[53][54]

Although not signed, European route E30 includes most of the M4. The entire route runs 6,530 kilometres (4,060 mi) between Cork in Ireland and Omsk in Russia.