The Motorway
The Motorway was part of the mass transit infrastructure of New New York. An immense sealed tunnel, the vehicles within it permamently trapped in an enormous traffic jam, vehicals were not allowed access to the Fast Lane unless they had more than two people on board, if they lied to the computer system the wheel would lock.
After the population of the upper levels of the city was destroyed by a mutant plague from a mood drug, "Bliss", the travellers in the Motorway were unable to leave the system and simply thought that the exits were closed for repair and kept driving found the system, thinking that on their second circit that the exits would have been reopened. Access was still unrestricted eventually resulting in a tremendous traffic jam. The situation was exacerbated by the presence of several apparently devolved Macra in the lower reaches of the system. The Doctor was able to open the Motorway exits and allow the drivers trapped there to repopulate New New York. (DW: Gridlock)