Junction 20 provides clues to one of several intriguing British motorway mysteries. Drivers today might rank events hereabouts as a remarkable folly, letting slip a great opportunity - or foundations for a long-dormant future opportunity. If you're of a sensitive environmental persuasion you might feel the opposite applies in both cases, but here's the story, judge for yourself.
This is a full-access intersection, notable for a roundabout amongst the motorway's biggest - yet with just one solitary exit, serving the small town of Clevedon. Experienced motorway observers might recognise far greater design capacity here than traffic demands even today, over forty years after it was built.
Continuing southbound, note how the two carriageways diverge, and the lanes seem wider - while the central reservation has grown very wide indeed. Six miles later, junction 21 offers escape to the fading Victorian splendour of Weston Super Mare, after which normal lane and central reservation width resumes. Intriguingly, this junction is involved in a separate mystery - but that's a story for another time.
Over twenty miles north-west, approaching Ashton Gate on Bristol's south west fringe, a complex high level interchange and part dual-carriageway link network begins. Executed in grim 1960's concrete, for the unfamiliar it’s a navigational nightmare, interconnecting two B roads, the A4 and A38, and several urban routes.
It also links traffic from the A370 Brunel Way, which points tantalisingly towards Weston Super Mare... marking the end of a stillborn dream. A mile south west from here this road becomes the Long Ashton bypass, another 1960's project, built with more in mind than easing congestion in a village community. Today its a curious retro-hybrid: brief single carriageway, three lane road alignments, a viaduct, a short two lane dual carriageway stretch - is that space for a hard shoulder as well? - book-ended by two over-engineered, grade-separated junctions. After a couple of miles it ends abruptly, and the A370 resumes its windingly tortuous way south west.
These apparently unconnected pieces of highway engineering all relate to the same fifty-year old puzzle, begun when Somerset County Council held regional highways responsibility to the Bristol city boundary.
Major roads then were largely planned on a so-called "predict and provide" model, as the 1960's were becoming the golden age of the car. Somerset's planners anticipated significant future traffic growth in both directions from south Bristol towards Exeter - and especially Weston Super Mare, where much new housing was expected.
A new high-capacity road linking south Bristol and the planned but then unbuilt M5 motorway route thus entered Somerset's plans. Its line ran from Bristol's Ashton Gate, including a new bypass for Long Ashton village, before striking away across country, passing just north of Nailsea, ultimately meeting the motorway line at what became Junction 20, the Clevedon interchange. The Long Ashton section was completed before the motorway arrived - and the route onwards to the M5 was protected from building until well after the motorway opened, but nothing else was ever constructed.