Secret (number not signed) motorway near Huntingdon, at the top of a spur of the A14. Legislatively it bears the number A604(M), however, for administration purposes, it is the A14(M). Given that it links the A14 spur to the A1(M), it would be a better number. However, when first built in 1993, the road numbers around there were in a state of flux, with the coming of the new A14 M1 to A1 link road upgrading the old A604 - this goes some way to explaining the original number.
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