(Dujon) Watford Gap is not a gap, pass, col, gorge or anything so dramatic. It's just another dreary motorway service station about 80 miles NW of London, in pretty flat country. It's nowhere near Watford, which is on the NW edge of London. The original phrase used contemptuously by Londoners to denote provinciality was "North of Watford", ie outside London, and in particular to the north of it. When Watford Gap service station on the M1 was built people started talking about "north of Watford Gap" thus changing the meaning, then it became "north of the Watford Gap", which ain't there, as I said. This is how place-names and language in general changes; misunderstanding by stupid people. One reason the country grinds to a halt is that many roads are at full capacity, more or less, which means that the slightest hazard slows everything down and in no time at all the whole thing congeals. Another reason is that we are now the soppiest country in the world.
Actually, as Herr Bratsche pointed out when this last arose about 5 years ago, there is a geographical significance to Watford Gap as the attached map shows. Within about 400 metres you have a railway line, a roman road (Watling Street - now the A5), and a canal all passing through the same minor low point. The motorway just follows the rest, showing that the romans knew a thing or two about roads. When you drive through on the motorway the surrounding geography is not evident.
[INJ] Ah - seem to remember my late father saying as much way back in the 60s when we travelled north regularly, so thanks for that map ref for Watford Gap. I would so hate to be one of Rosie's 'stupid people' who misunderstand the term :-)