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M4 motorway
Road of the United Kingdom
Length	189.5 miles (305 km)
Direction	East - West
Start	Chiswick
Primary destinations	Hounslow Heathrow Airport Slough Maidenhead Reading Newbury Swindon Chippenham Bristol Newport Cardiff Bridgend Port Talbot Swansea Llanelli
End	Pont Abraham services
Construction dates	1961 - 1996
Motorways joined	M25 motorway A308(M) motorway A404(M) motorway A329(M) motorway M32 motorway M5 motorway M48 motorway M49 motorway M48 motorway A48(M) motorway
Euroroute(s)	E 30
Opening timeline

The M4 motorway is a motorway in Great Britain linking 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

The original bridge over River Neath (the A48), is to the right, the new M4 bridge is to the left

The M4 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 English section of the motorway was completed on 22 December 1971 when the 50 miles (80 km) stretch between junctions 9 and 15 (Maidenhead and Swindon) was opened to traffic^[1]. 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.^[2]

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 during April 2005.^[3]

Features

The Second Severn Crossing

The M4/M25 motorway junction, near Heathrow Airport

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. Unusually for a toll road, tolls are charged in one direction only - westbound. Drivers therefore have to pay to enter Wales but not to enter England.

M4 bus lane

In London, the eastbound (London-bound) M4 carriageway has a bus lane, which opened in June 1999, from junction 3 until the elevated section. This lane caused some controversy when it was created.^[4] However, the bus lane has improved traffic flow onto the 2-lane elevated section which follows it. A study by the Transport Research Laboratory has shown that the lane has reduced rush hour journey times by 3.5 minutes for buses and one minute for cars, even though the speed limit was cut from 70 miles per hour (110 km/h) to 50 miles per hour (80 km/h).^[5] There is some speculation that the original plan involved closing one lane from junction 3 to the elevated section to prevent congestion caused by the filtering of three lanes of traffic into two (the motorway is already temporarily down to two lanes at junction 3 because the left lane is exclusively for traffic leaving the motorway, so the effect of the bus lane is to make the 3-to-2 lane-transition seamless). Making the "spare" lane available for buses - and now taxis and motorcycles - was an afterthought.^[6]

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 conditions drivers notice a reduction in spray from other vehicles and improved visibility. This special surface was publicised in an episode of the BBCs Tomorrow's World programme. This was the site of the first trial of the new road surface when it was laid down in 1993.^[7]

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 has a heated road surface to reduce icing in winter.

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). Both are impressive feats of engineering, particularly junction 4B which also has to make provision for a railway line passing beneath the M4. However, due to the nature of such junctions, drivers should bear in mind that it is impossible to make a U-turn at J20 or J4B.

Tunnels

The M4 passes through the Brynglas Tunnels at Junction 25a, Newport.

Notable junctions

Junction '8/9' (only one in the UK to feature a 'dual-numbered' junction) 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 installment 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 30-33 (Cardiff area) were set aside for intermediate additional interchanges at the time on construction. Junction 30 has since been added but there are no current plans to construct Junction 31 (A469 road), Junction 32 (A470) or Junction 33 (A4232).

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 miles (3.2 km) in the same direction having both joined and subsequently left the motorway at "Junction 41".

Speed limits

The M4 near Bristol (England) between junctions 19 and 20, looking west towards Wales.

For the majority of its length the speed limit is the National speed limit. Exceptions include:-

• 40 miles per hour (64 km/h) on the elevated section within London.
• 60 miles per hour (97 km/h) between the Heathrow turnoff and the elevated section
• 50 miles per hour (80 km/h) when approaching the toll plaza on the Severn Crossing to protect tolling staff moving between the tolling booths,^[8]
• 50 miles per hour (80 km/h) during road widening between Junction 24 and Junction 28 enforced with average speed check cameras.^{[9 ]}
• 50 miles per hour (80 km/h) between Junction 28 and Junction 32 during development enforced by speed cameras.^[10]
• 50 miles per hour (80 km/h) between junction 40 and junction 41 enforced by a permanent Gatso speed camera.

Current developments

Current widening works (J29-J32)

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.^[11] The £65m scheme includes work to the Mereoak roundabout and part of the A33 Swallowfield Bypass in Shinfield conversion and the conversion of the two existing existing bridges, one of which be available only for pedestrians and cyclists and the only for buses.^[12] 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.^[13] 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.^[14]

M4 J23a-J29 Upgrade

As of April 2009, the Newport section of the motorway between junctions 23a and 29 is being upgraded with a new concrete central barrier; from 2010, a variable speed limit will be introduced.^[15]

M4 J29-J32 Widening

As of spring 2007, the stretch between Junction 29 Castleton and J32 Coryton is being widened to 6 lanes in order to improve journey reliability and add capacity. Work is expected to be completed by December 2009.^[16]

Proposed developments

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 acres (51 ha) site north of the M4 were submitted to Cardiff Council.^[17] The developers of the business park, St Modwen Developments, would likely fund the new junction, which would be on the A469.^[18]

Other proposals

A 'New M4' tolled bypass for Newport was proposed in 2007^[19] but later abandoned for financial reasons. An extension to the Newport Southern Distributor Road through the old Corus steel works is being considered.^[20]. 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.^[21] The Port Talbot peripheral distributor road is under development, which should divert local traffic away from the M4.

Junctions

Data^[22][23][24] from driver location signs are used to provide distance and carriageway identification information.

See also

• Media related to M4 motorway at Wikimedia Commons
• New M4
• M4 Thames Bridge, Maidenhead
• A404(M) motorway
• M4 corridor
• List of motorways in the United Kingdom
• A48(M) motorway
• Transport in Wales

References

External links

• CBRD Motorway Database - M4
• British Road Database: Motorways - M4 Junction 8/9
• The Motorway Archive
  • Junctions 1 to 5
  • Junctions 5 to 7
  • Junctions 8 to 14
  • Junctions 14 to 21
  • Junctions 21 to 23
  • Junctions 23 to 49

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