M4 motorway
Encyclopedia : M : M4 : M4M : M4 motorway
- For other uses, see M4 motorway (disambiguation)}}}.
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| M4 motorway | |
| Length | 189.5 miles 305.0 km |
| Direction | East - West |
| Start | London Chiswick |
| Primary destinations | Heathrow Airport Maidenhead Reading Newbury Swindon Chippenham Bristol Newport Cwmbran Cardiff Bridgend Port Talbot Neath Swansea Llanelli |
| End | Pont Abraham services |
| Construction dates | 1961 - 1996 |
| Motorways joined | 4B -  M25 motorway 8/9 - A308(M) motorway 8/9 - A404(M) motorway 10 - A329(M) motorway 19 -  M32 motorway 20 -  M5 motorway 21 -  M48 motorway 22 -  M49 motorway 23 - M48 motorway 29 - A48(M) motorway |
The M4 motorway is a motorway in England and Wales linking London and west Wales via Bristol. Other major places directly accessible from M4 junctions are Reading, Swindon, Newport, Cardiff and Swansea.
Notes
In the construction of the section around Cardiff in 1980, numbers were set aside for intermediate interchanges that were not built at the time. 33 had been set for the link road to the west (A4232), 32 for the A470, with 31 intended for the A469 to Caerphilly, and 30 for the great "Hook Road" - a plan which was later shelved. Junction 30 was later added to serve the link road in the east, which will eventually form a ring road from junctions 33 to 30, via Cardiff Bay. Number 31 remains unused as of 2005, and there are no immediate plans to build a junction with the A469.
Contrary to popular belief, junction 39 does exist, however, it consists of a single slip road completing the missing movement onto the westbound carriageway from the A48 at junction 38, thus there is no exit from the motorway at this junction.
Junction 41 refers to two different junctions in the 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 km) in the same direction having both joined and subsequently left the motorway at "junction 41".
Special features
The M4 has several special features. In London, the eastbound (London-bound) carriageway has a bus lane from junction 3 until the elevated section. This lane caused much [controversy] when it was created.
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.
25 miles (40 km) west, the motorway is the only one in the UK to feature a 'dual-numbered' junction. Junction '8/9' is the turn off for Maidenhead, Berkshire.
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 (the original route of the M4, now the M48).
There is a slip road west of the A34 to Newbury from the eastbound carriageway which leads to RAF Welford. This slip road is marked by a red and white roadsign.
Speed checks
Speed checks are carried out between junction 14 and junction 18 (in both directions). These are carried out by police camera vans situated on bridges over the motorway. These measure the average speed of vehicles between set points. The speed can be calculated because the distance between vans and the time taken for a vehicle to get from one point to another are known.
Traffic heading into London travels at a maximum speed of 60 mph (97 km/h) after the Heathrow turnoff, the limit is enforced by cameras. The elevated section is also enforced by speed cameras at 40 mph (64 km/h). Traffic heading out of London is limited to 40 mph (64 km/h) on the elevated section, but there are no cameras.
Speed checks are also carried out by mobile Safety Camera Partnership vans on the toll plaza on the Welsh side of the M4 after the Second Severn Bridge. There are also mobile camera vans on sections of the M4 between the toll plaza and Cardiff.
The section between junction 40 and junction 41 has a maximum speed of 50 mph (80 km/h), and is enforced by a permanent speed camera (Gatso), to be found near junction 41 (in both directions).
Junctions
See also
External links
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