When the A1 was moved a little to the west between Highgate in Outer London and Hatfield in Hertfordshire, the old course through Barnet and Potters Bar was renumbered A1000. In fact about a mile of the old A1 in Hatfield was declassified altogether, leaving the as the A1000 ending abruptly on the B556.

Later, when road improvements were made in Welwyn Garden City, sections of the B556, B190 and B1000 through the town were renumbered as an extension of the A1000, which now ends on the A1 junction at Welwyn village, to the north of WGC.

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