The A604(M) was the secret (number not signed) motorway near Huntingdon, at the top of a spur of the A14. The Scheme authorising its construction referred to it by this number; however, for administration purposes, it is the A14(M). Given that it links the A14 spur to the A1(M), that 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. To make things worse, some maps claim the road is a spur of the A1(M).