From Roader's Digest: The SABRE Wiki
The M4 is the London to South Wales Motorway. Running for over 180 miles from Chiswick in the west of the capital to a point some 12 miles southeast of Carmarthen in west Wales, it passes such important centres of population and industry as Reading, Swindon, Bristol, Cardiff, and Swansea. Perhaps surprisingly, it may be counted Great Britain's second oldest motorway, as the Chiswick Flyover (then numbered A4, but now the first section of the M4) opened in 1959, a couple of months before the M1, M10, and M45. Other parts of this motorway started life as the A4(M) and A48(M), whilst the Severn and Wye Bridges – originally on the M4 but bypassed since 1996 by the Second Severn Crossing – now lie on the M48.
Main Article: Colnbrook Interchange
The Colnbrook Interchange (junction 4b of the M4; junction 15 of the M25) is a large four-level stack interchange between the M4 and M25 motorways. It is unusual in that, in addition to the various different roads that make up the junction, there is also a railway line running through it.
Main Article: Almondsbury Interchange
The Almondsbury Interchange (junction 20 of the M4; junction 15 of the M5) is an important junction allowing direct interchange between the M4 and M5, and is additionally linked to junction 16 of the M5, which provides access to and from the A38. This junction was the first example of a four-level stack in the country. The link roads leading from the M4 to the M5 pass over the top level of the interchange whilst those from the M5 to the M4 pass underneath all other levels.
Main Article: Chieveley services
These are situated where the M4 crosses the A34 near the village of Chieveley in Berkshire.
Leigh Delamere services
Main Article: Leigh Delamere services
Leigh Delamere services are located in Wiltshire between junctions 17 and 18. Originally, only the eastbound side was developed; the westbound side followed around ten years later.
Pont Abraham services
Main Article: Pont Abraham services
Pont Abraham services are a single-site MSA located on the western terminal roundabout of the M4, at junction 49. It is operated by RoadChef.
Second Severn Crossing
Main Article: Second Severn Crossing
The Second Severn Crossing was constructed between 1992 and 1996 to provide relief for the Severn Bridge and Wye Bridge between England and Wales. Upon completion, the M4 moved across from the original bridges (which became part of the M48) to the new bridge.
Main Article: Brynglas Tunnels
The Brynglas Tunnels are situated on the Newport bypass, between junctions 25a and 26. They are twin-bored tunnels, each carrying two traffic lanes and opened in 1967. Until recently, the eastbound tunnel was lit by orange lighting, the westbound by white. Both tunnels have overhead lane control signals for contraflow purposes.
- The section of the M4 between junctions 19 and 23 has the the highest number of consecutive motorway-to-motorway junctions in the United Kingdom.
- In 1999 a controversial bus lane was established on a 5.6-km stretch of the eastbound carriageway of the M4 between junction 3 and the start of the elevated section of the motorway near Brentford. Suspended in December 2010, it will be re-instated temporarily for the duration of the 2012 London Olympics.