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The A6400 is listed in the DfT Roads List as Wakefield, West Yorkshire Between A61 and A642 link road. which would make it a short road north of the city centre.
The exact route of the A6400 is unclear as it does not appear on any maps so far known. One possibility is that the road ran along Jacob's Well Lane which linked the A61 and A642 on the northern edge of the city centre when those roads still had their original routes; the road has now been taken over by the two roads. Another option is that the road was planned to run further north, such as Bar Lane, which is not shown on 1980s maps. In this case either the road scheme was later downgraded so the A6400 number was never actually used, or the number was short-lived and the road is now unclassified.
The route of the "A61-A642 Link Road", as evidenced in a Wakefield MDC planning document from December 2002, did become Bar Lane though it was seemingly never classified (and the number not mentioned).