Or perhaps for a large enough city, you can't build enough roads, because you'd need like >70% of all area for roads and parking.

The scaling doesn't work.

For another similar example, think of how much percentsge space you need in a 2 story building devoted to elevators (almost none) vs a 200 story building (a significant percentage of the middle core of the building is nothing but elevators). Take that and scale it even worse, and you have roughly the small town vs big city traffic issue.