M4 Motorway Guide | Go West

The M4 began life as the London–South Wales Motorway and is now the third-longest motorway in the United Kingdom, running from west London to southwest Wales. It was built in pieces across nearly four decades: the Chiswick Flyover in 1959, sections numbered A4(M) in 1961 and A48(M) in 1966, with the English run to the Severn Bridge constructed between 1961 and 1971 and the Welsh element largely complete by 1980.

The most interesting history is at the river. When the Second Severn Crossing opened in 1996, the M4 was rerouted over it — which is why the motorway now leaves and rejoins the M48 at Junctions 21 and 23, the older bridge having been demoted to a number most drivers never think about.

One stubborn gap held out longest of all: a non-motorway section around Briton Ferry bridge survived until 1993. West of there the road finally runs out at Pont Abraham, north of Swansea, 189 miles from where it started.