Monday, 16 December 2013

Junction 8/9

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.


  1. The M27 has no junction 6, but I don't know why. The gap between 5 & 7 is too short to fit another junction in. Still, it's always a laugh to direct people to come off the motorway at junction 6.

    1. Thanks, I never noticed that...apparently there was a planned M273 spur going into Southampton but it was never built so the need for a J6 became redundant.

  2. There's an entire society called SABRE where discussion of such subjects is meat and drink. 680,000 forum posts too!

  3. This is all very worrying how are the new wave of migrants going to find their way west past J8/9

  4. If I remember correctly, Arizona has distance based exit numbering. I have always thought this was eminently sensible. You get on at junction 12 and get off at junction 34. How far is that, well 34 - 12 = 22, so 22 miles. Not only that it stops stupid junction numbering like 21a on the M1 in Leicestershire.
    I'm sorry, I'll get my coat and head over to SABRE.

  5. Ah yes, but picking up another favourite SABRE theme, there would then be an almighty row over whether to number the junctions in miles or kilometres. And no doubt the EU would have a view on the subject...

  6. There's an ex-junction just to the east here that was actually J8.

  7. Perhaps it's to hide exit 9 3/4 for those people attending Hogwarts by car?

  8. Godo motorway the M4.

    Earlier this year, we went for a three-day break in Swansea, and we live in London, so we took the M4 all the way. That was nice and simple.

    Next question - why do you have to pay a toll to drive over the bridge into Wales but it is free to go the other way.

  9. @Mark There are a number of reasons; they learnt from the experiences of the first Severn crossing (now M48) which used to have booths in both directions.

    The main reason is that most of the traffic is communter which as a consequence uses the bridge twice in one day. So in cost terms it's cheaper to man them one side and effectively charge double.

    The second is that the bridges are often exposed to high winds - the original used to close regularly - so if there's a lot of traffic on the bridge it's safer to have queue in one direction only.

  10. I always understood that reason the original route was abandoned was that the route going north of Reading and Newbury would upset the racing community who used the Newbury Downs as gallops.

  11. TBF: "most of the traffic is commuter which as a consequence uses the bridge twice in one day. So in cost terms it's cheaper to man them one side and effectively charge double"

    Seems fair enough, unless it encourages people to make the detour to the other bridge once day.

    "if there's a lot of traffic on the bridge it's safer to have queue in one direction only."

    Easily fixed - have the toll booth before you drive onto the bridge, which enables them to regulate the amount of traffic on the bridge itself.