It's already implemented on the M25, the orbital motorway around London. It gets used quite frequently to slow commuter traffic down from a limit of 70mph to a

It's already implemented on the M25, the orbital motorway around London. It gets used quite frequently to slow commuter traffic down from a limit of 70mph to as low as 40. There's a particular incentive there because being a loop a bad traffic jam could (theoretically - I'm not sure it's ever happened) stretch so far that its head and tail collide. Plus, of course, the fact that the M25 was demonically planned in the shape of the mystic sigil Odegra, from the alphabet of the Black Priesthood of Ancient Mu.