The Motorway was part of the mass transit infrastructure of New New York.
The Motorway was an immense sealed tunnel, divided vertically into around fifty lanes, each lane apparently comprising one layer of traffic. The Fast Lane, which occupied the lowest level, was restricted to cars with three or more occupants. Attempts to break these rules caused the steering wheel of the vehicle to lock. However, everyone who used the fast lane would be killed by the Macra lurking beneath. (TV: Gridlock)
The population of the upper levels of the city was destroyed by a mutant plague as a result of widespread use of the mood drug Bliss. The travellers in the Motorway were unaware of this. Unable to leave, they thought the exits were closed for repair and kept on driving around the system, thinking that by the end of their second circuit the exits would be reopened.
Access was still unrestricted, eventually resulting in a tremendous traffic jam and journeys lasting decades. The situation was exacerbated by the presence of large numbers of devolved Macra in the lower reaches of the system. The Tenth Doctor was able to open the Motorway exits and allow the drivers trapped there to escape and repopulate New New York. They sang "Abide with Me" as they escaped. (TV: Gridlock)