I fear with this post I may expose myself to accusations of geekisim. Yet despite that, being a regular commuter on the M4 motorway (which runs from London to South Wales), I’ve always been slightly puzzled by the unusually numbered junction 8/9 for Maidenhead. It’s the only junction on the entire UK motorway network that is dual numbered and there appeared to be no obvious reason why.
Minded to find out I discover that the reason lies in initial construction of the motorway in the 1960s. At that time, the M4 construction started from London and was built out towards the west. The original plan was to construct it around the southern side of Maidenhead and then curve it to the west of Maidenhead with the intention of sweeping north of Reading. Maidenhead itself was to be junction 8 and the junction with the A4, west of Maidenhead was to be 9 (shown below).
However at this point they stopped building it because the planners discovered a problem; they correctly identified that sending the motorway to the north of Reading would send it north of the River Thames. This posed a significant problem because the vast majority of Reading lies south of the river. Thus this would very likely place an unsustainable strain on the two poxy river crossings in the centre of Reading itself. The two river crossings would be overwhelmed by all of the traffic from the south trying to access the motorway.
So they changed the plans, and in 1971 sent the motorway south of Reading instead. This though meant that the final stretch of the M4 that had been constructed in the 1960s to the west of Maidenhead was now redundant, so it was renumbered as the A404(M). Junction 9 then became superfluous due to the re-route – J10 was already allocated to Reading so to keep number consistency and as not to confuse drivers the original J8 was renumbered as J8/9.
As one can see from the map above the original route of the M4 motorway is now marked the A404(M) going northwards off junction 8/9; a motorway until it becomes a simple dual carriageway after junction 9(B) – the original junction 9.
All in all an idiosyncratic quirk - if that's not a tautology.
