M4 motorway

The M4 motorway links London with South 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. Originally referred to as the London-South Wales Motorway, the English section was constructed between 1965 and 1971, the Welsh section was completed in 1993 and the Second Severn Crossing opened in 1996. The M4 runs close to the A4 from London to Bristol. After crossing the River Severn it follows the A48 through Wales, using the Brynglas Tunnels at Junction 25a, Newport and terminates just north of Pontarddulais. The route of the M4 is known as the M4 corridor.

History
A new road from London to South Wales was first proposed in the 1930s, and the Ministry of Transport announced plans for the M4 as one of the first major post-war trunk road improvement projects in 1956. The motorway was built in several stages. In the 1960s two sections were built, one extending from the London end to near Maidenhead, one from north of Bath to west of Newport, including the Severn Bridge (opened in 1966 and now part of the M48). The Port Talbot by-pass, also built in the 1960s and now part of the M4, was originally the A48(M) 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 it was only following the creation of the Welsh Office that the Government became committed to a high-standard dual carriageway to Pont Abraham in Carmarthenshire. The English section of the motorway was completed on 22 December 1971 when the 50 mi stretch between junctions 9 and 15 (Maidenhead and Swindon) was opened to traffic. The Welsh section was completed in 1993, when the Briton Ferry motorway bridge opened. The Second Severn Crossing opened in 1996, together with new link motorways on either side of the estuary to divert the M4 over the new crossing. The existing route over the Severn Bridge was redesignated the M48, and the new M49 was opened to connect the new crossing to the M5. In June 1999 the section of the third lane (the lane nearest the central reservation) between junctions 2 and 3 was converted to a bus lane and opened as a pilot scheme. The scheme was made permanent in 2001. A lower speed limit was introduced along the bus lane section at the same time. The bus lane was scrapped at the end of 2010 and the third lane was returned to all-traffic use. In April 2005 speed checks carried out by police camera vans between junction 14 and junction 18 resulted in 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 (Junction 29) to and Coryton (Junction 32) was widened to 6 lanes. The scheme was officially opened in 25 January 2010 by the Deputy First Minister. Subsequent to opening there were occasional works with associated lane restrictions. 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. A similar claim was made for a 30 mi section of road in Scotland close to Aberdeen in September 2009 with refueling points at Bridge of Don, Ellon and Peterhead. In October 2010 the new transport secretary, Philip Hammond announced that the bus lane would be suspended for 18 months from 24 December 2010 to be brought back for the 2012 Summer Olympics after which it was likely to be scrapped permanently. Opening timeline

Toll bridge


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, now the M48. Tolls in Wales are charged in one direction only - westbound. Drivers therefore have to pay to enter Wales, but not to enter England.

Speed limits
For the majority of its length the speed limit is the National speed limit. Exceptions include:-
 * 40 mph on the elevated section within London.
 * 60 mph between the Heathrow turnoff and the elevated section
 * 50 mph when approaching the toll plaza on the Severn Crossing to protect tolling staff moving between the tolling booths,
 * 50 mph temporarily during road works between Junction 24 and Junction 28 enforced with average-speed-check cameras.
 * 50 mph on the Port Talbot elevated section between junction 40 and junction 41.

M4 bus lane


There was a controversial 3.5 mi bus lane on the eastbound (London-bound) carriageway from junction 3 (A312) to the start of the elevated 2-lane section near Brentford, covering part of the 15 mi journey between Heathrow Airport and central London. The lane which had no intermediate exits was for use by buses, coaches, motorbikes, emergency vehicles and licensed taxis but not mini-cabs. It was used by 7% of vehicles which carried 21% of the people. The lane was restored for normal motorway running at the end of 2010 for 18 months using a Experimental Traffic Order and will be re-established for the duration of the 2012 Summer Olympics with the intention that it will then be removed permanently.

Porous road surface
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 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 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, however, due to the high costs in preventing icing by this method the heating is no longer used.

Four level stack interchanges
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). Junction 4B also has to make provision for a railway line passing beneath the M4. Due to the nature of such junctions, it is impossible to make a U-turn at J20 or J4B.

Tunnels
The M4 passes through the Brynglas Tunnels at Junction 25a, Newport, which is the only two bored tunnel on the UK motorway network.

Notable junctions
Junction '8/9' (the only one in the UK with dual-numbers) is the turn off for Maidenhead, Berkshire. 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. Junction 27 (High Cross) is a normal grade-separated roundabout junction. However the on-bound slip roads point in the opposite direction to the intended direction of travel. Due to the topology of the landscape, both sets of slip roads are conjoined on either side of the roundabout. To travel westbound the driver must use an eastbound-facing slip road before looping around 180 degrees to join the westbound carriageway. Junctions 30-31 (East Cardiff) were set aside for intermediate additional interchanges at the time on construction. Junction 30 (Cardiff Gate) has since been added but there are no current plans to construct Junction 31 (A469 road). Junction 39 does actually exist, but 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 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 mi in the same direction having both joined and subsequently left the motorway at "Junction 41". Junction 44 is unusual in that the eastbound entrance dives under the inside of the junction, effectively a creating a "right-turn" on a roundabout.

M4 Junction 11 Improvement Scheme
Junction 11, near Reading, is in the process of being extensively developed with a new four-lane motorway junction and the construction of two extra road bridges around the existing junction and other works. Work started in 2008 and is expected to be completed by summer 2010. The £65m scheme includes work to the Mereoak roundabout and part of the A33 Swallowfield Bypass in Shinfield conversion, and also the conversion of the two existing bridges, one of which will be available only for pedestrians and cyclists and the other for buses. It will also involve the movement of the local Highways Agency and Fire Service offices, build a long footbridge network, a special bus-lane and a new gyratory. It will also install sound barriers for nearby residential areas, some of which have already been completed. 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.

Variable speed limit scheme between Junctions 24 and 28
Due to be opened by June 2011 is the variable speed limit scheme between junction 24 and junction 29. This section of the motorway was built in 1967 and has many non-standard gradients and a number of tight bends. It is hoped will smooth flow of traffic and increase motorway capacity for the 13 km between the two junctions and also save money through fewer accidents. Widening this section of the motorway is not possible because of the two-lane Brynglas Tunnels and existing housing close to the motorway.

M4 Junction 31
Plans for the "missing" Junction 31, also known as the Thornhill interchange, which was originally granted planning permission in September 1991 (but subsequently expired) have been rekindled after proposals for a new business park on a 125 acre site north of the M4 were submitted to Cardiff Council. The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469.

Other proposals
Plans for an 'M4 Relief Road' around Newport were first announced by the Welsh Office in 1991, but made little progress. The Welsh Assembly Government revived the scheme as the 'New M4' tolled bypass in 2007 but later abandoned it for financial reasons. An extension to the Newport Southern Distributor Road through the old Corus steel works is being considered. This road is already a dual carriageway but not open to the public. There have been calls to close the slip roads at Junction 40 and 41 (at Port Talbot) to 'improve traffic flow'. The motorway is only two lanes in this stretch and is a major traffic congestion blackspot. Junctions 40 and 41 (at Port Talbot) have very short slip roads which are not up to modern standards. The Port Talbot peripheral distributor road is under development, which should divert local traffic away from the M4. In future, it is hoped to extend the M4 to Carmarthen, but this depends on financing.

Junctions
Data  from driver location signs are used to provide  distance and carriageway identification information. Where a junction span several hundred metres and the data is available, both the start and finish values for the junction are shown.