I think improving bus systems in the United States (and probably other countries) could be a plausible Cause X.
Importance: Improving bus service would:
Neglectedness: City buses probably don't get much attention because most people don't think very highly of them, and focus much more on novel transportation technologies like electric vehicles.
Tractability: According to Higashide, ... (read more)
I don't have statistics, but my best guess is that if you sample random points across all public buses running in America, in over 3/4 of the time, less than half of the seats are filled.
This is extremely unlike my experiences in Asia (in China or Singapore).