The Hangar Lane Gyratory System is, despite its fancy name, simply a roundabout. It entangles the North Circular Road with the A40, the major road heading due west out of London.
The Gyratory System is poignant because soon after it opened in the late 70s, the designers admitted that they had made a mistake and it would never work. God knows how you can mistake either of those two roads or which directions they go in.
The result in 1995 is this: a roundabout which is designed to avoid a traffic light snarl-up is now controlled by a carousel of traffic lights. Say you want to turn right: you may have to wait at four red lights as you gyrate. This is instead of the one traffic light the System replaced. The Gyratory System regularly achieves gridlock, which is excellent considering that all the through traffic on the A40 passes under it via a tunnel.