The Motorway
The Motorway was part of the mass transit infrastructure of New New York. An immense sealed tunnel, the vehicles within it were computer-controlled and allowed access to different lanes depending on their occupancy level.
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. 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)