M40 - A43

Where is it?

M40 junction 10, where the A43 splits off towards Northampton. This is one of the waypoints on the main freight route from Southampton to the north east.

The junction has been rebuilt since it was first listed in Bad Junctions. The description below was written about the previous layout, which ceased to operate in 2016.

What's wrong with it?

When the A43 between the M40 and M1 was upgraded to dual carriageway recently, this junction was updated too. It was built in 1991 as a regular two-roundabout dumbbell junction, but the improvements produced this. Amongst other things, it takes up more land than the previous junction, makes access in almost any direction difficult, and marginalises the most popular movement between M40 south and the A43. The previous set of four straight sliproads have been replaced by five sharply curved, narrow ones.

But what is most shocking about this junction is that at the northernmost roundabout, traffic from the A43 onto the M40 crosses traffic from the M40 north to the A43 — leaving north-eastbound traffic queuing northbound around two roundabouts and back onto the M40 northbound carriageway, even outside busy periods. It's such a stunningly shortsighted and dangerous mistake that it leaves me completely lost for words to describe how awful this new layout is.

Why is it wrong?

Well, here's the thing: it's hard to see any justification whatsoever for this layout. It's not smoother, it's not an improvement in width or speed of sliproads (in fact, lorries are now prone to tipping over on the sliproads), it doesn't make access to the services any easier... it makes, in short, no positive changes at all to the junction. How on earth did this design make it off the drawing board?

What would be better?

The most immediate solution would be to block the northernmost entry slip road to the M40 southbound, so A43 traffic no longer crosses at the northern roundabout. This would prevent almost all of the congestion at this junction. After that, I don't know. Maybe we could hunt down whoever designed this shameful mess.

Update: traffic lights have now been installed — no, not on the whole junction, just on the approach from the A43 south-westbound. Presumably when traffic gets bad enough northbound, these will act as 'ramp metering' — allowing bursts of traffic off the A43 southbound, creating gaps in the flow that will give northbound traffic a chance to get through.

There is also a new 50mph limit applying to the whole junction. This will make things much safer as one of the major problems it suffers is people tearing round those three queue-free roundabouts at dangerously high speeds.