Where is it?
M1 junction 6a, M25 junction 21. The country's principal north-south motorway meets the M25, London's ring road.
What's wrong with it?
It's a nice three-level free flowing interchange, except for the minor gripe that the M1 to the south is totally ignored. It may as well end here. For M25 traffic to reach the M1 into London, or vice versa, requires leaving a junction or two earlier and tootling along the A41 or A405 through houses and shops.
Why is it wrong?
This is what's so annoying. There is no real reason why this is the case. There's no shortage of land to the south for the rest of the movements to be built on, which is often the reason for a silly layout. One reason that seemed plausible was that when the M25 was built the M1 south of here was still two lanes wide and it would have been thought better to prevent any more traffic from using the road. But in fact the M1 widening was carried out before this section of M25 was built, so that's not it. It seems that to save a few pennies, whoever planned the junction was perfectly happy to allow the streets of Watford to be used as rat-runs.
What would be better?
A couple of left-turn slip roads in the southern half of the interchange would be a simple and quick start. Adding the other movements in is harder — it would probably involve demolishing what's there already.
Personally I hate how when heading South on the M1...when you reach this junction there's only a single lane to exit onto the M25 and three to continue on the M1. It really should be two for the M25 and two for the M1, with the former divided into "South Mimms/Dartford" for lane 1 and "Heathrow/Gatwick" for lane 2.
All that happens is traffic starts queuing about three miles before the junction as everybody who doesn't want to go to London crams into a single lane.