The A404(M) is a spur from the M4 to the A4 and A404 running to the west of Maidenhead, but it was once the main road to Bristol and the west, and is now on its third road number.
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, weirdly numbered to echo the two junctions it replaced. This little spur motorway is what was left behind, the old end of the M4 travelling to White Waltham and Maidenhead Thicket. Meanwhile, junction 8/9 has now existed for decades longer than junctions 8 and 9 ever did.
This short section of road is a strange time machine, 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 early-1960s charms, the A404(M) offers 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 were sacrificed in the 1970s to create a third running lane.
When it first gained its own identity, 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, with the old one crossed out, but it's been the A404(M) for about 30 years.
Despite looking and sometimes feeling like a museum piece, the A404(M) remains busy, serving a very useful purpose, and is still a trunk road. In fact it's more useful now than it ever was, linking to the A404, which continues as a fast dual carriageway route north to Handy Cross. Together they make a very well-used connection between the M4 and the M40 and carry significant amounts of traffic that would otherwise clog up Reading or the M25.