Easily one of the most varied motorways in Britain, from urban chaos in west London and Heathrow Airport, out through the rolling hills of Gloucestershire, across the Severn Estuary and through industrial South Wales, the M4 does it all.
Before the M4 Severn Bridge was built, there was no road crossing of the Severn south of Gloucester, meaning that traffic between South Wales and southern England had to drive the A40, far out of its way to the north and frequently congested at Gloucester, or go via Bristol and Chepstow, choosing between the Severn ferry or the railway tunnel. The train and boat options were not particularly well used, but the bridge certainly is today, which suggests that the lack of a decent crossing was preventing a great number of journeys from happening at all.
In 1996, the Second Severn Crossing opened, carrying a widened and improved M4 over the estuary. The original Severn Bridge and Wye Bridge, along with a substantial length of ex-M4 at each side, now form the M48. Built and opened at the same time, the M49 was designed to cut congestion at the M5 junction, cutting the corner between the south-west, Bristol and the new bridge. Because of these multiple motorway connections (M48, M49, M48 again, M5 and M32), the M4 holds the UK record with five consecutive motorway-to-motorway interchanges.
Given that the first section of this motorway was opened in 1959 - the Chiswick Flyover, which was initially part of the A4 but was designed to become part of the motorway a few years later - it is surprising to learn that it remained in two disconnected parts until 1994. Plugging the gap was a six-mile section of the A48 at Briton Ferry in South Wales, with a motorway bypass delayed again and again due to the difficult marshy land and a navigable river. The problem was only solved very late in the day with a large and horrendously expensive viaduct over the top of the whole thing.