A404(M) motorway
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In England, the A404(M) is the Maidenhead East Bypass, linking the M4 with the A404 dual carriageway to High Wycombe, Marlow and the M40.
It is wrongly numbered as it is south of the A4 and it is an A road with motorway regulations. If it was the M404 it would be fine. It doesn't need to be motorway either, as the A308(M), its sister motorway, can provide a non-motorway exit if also downgraded.
The reason why it is there dates back to the early 1960s. The A4(M) Maidenhead Bypass ended at what is now junction 9b of the M4. It was renumbered M4 when the Slough bypass was built, with junction 8 to the A308 (now not there) and junction 9 with the A4.
Then the route of the M4 was changed to go south of Reading, instead of north of it. A new junction was built near junction 8, and left as a spur. Another spur was added from the new junction to the A308. Old junction 8 was too close to this new junction, so it was closed. As the route had been moved, it was thought best that this junction should remain junction 8, and also so as not to confuse people junction 9 was moved to this junction as well and replaced with junction 9a. The old M4 was renumbered A423(M) as it was a direct continuation of the A423 to Oxford, and the new spur was numbered A308(M).
Over time the A423 was superseded and renumbered by the M40 and the A404 was made dual carriageway, with a new rerouting. Since it was going to lose its A423 status, the road became A404(M) and a new junction was added (called 9a with the old 9a now being 9b!) in the 1990s.