The A404(M) is a spur from the M4 to the A4 and A404 running to the west of Maidenhead. Along with its little brother the A308(M), it has the dubious honour of starting at Britain's only double-numbered junction, M4 junction 8/9.
This little spur started life as the end of the M4. Travelling west from London, the thrilling new motorway ran as far as Maidenhead, turning north and dropping traffic back onto the old A4 at Maidenhead Thicket. Junction 8 connected to the A308 and junction 9 was the local interchange at White Waltham. When the M4 was extended further west, towards Reading, it branched off its old route at what is now junction 8/9, leaving behind a little spur to White Waltham and Maidenhead Thicket. Junction 8/9 has now existed for longer than junctions 8 and 9 ever did.
What's left behind is a short section of road that is barely modified from the days when it was part of the M4, offering a snapshot of what this major route looked like when its first sections opened. Among its quaint, early-60s charms, the A404(M) features some toe-curlingly tight corners on the sliproads at junction 9A, and bridges and other structures with hard shoulders complete, unlike the widened sections of the M4 to the east where they have been sacrificed to create a third running lane.
When it was first bypassed, the spur was given the number A423(M), in recognition of it connecting to the A423 at Maidenhead Thicket. The A423 was then absorbed into the A404, and the junction at Maidenhead Thicket gained an underpass, at which point the motorway was renumbered A404(M). There are still a couple of signs on and around the spur indicating the change to its "new" number.
Despite looking and sometimes feeling like an empty museum piece, the A404(M) continues to serve a very useful purpose and is still a trunk road. Together with the A404, which continues north to Handy Cross, it's part of a very well-used connection between the M4 and the M40 and carries significant amounts of traffic that would otherwise clog up Reading or the M25.