The big underlying factor is population increase, both permanent increases of the local population and temporary increases from visitors. Though I imagine there are also other factors even beyond that one that also increase traffic. It's probably safe to say that increased income results in more activity involving transport, for instance. I'd agree there's definitely some theoretic cutoff where it becomes practically physically impossible to squeeze any more people into an area, but I'm not sure how practical it is to consider that since you're hitting major dystopia long before you get to that point.