Comment Re:we already fixed it. its called 'trains'. (Score 1, Insightful) 603
The optimal and sustainable population density is extremely dependent on the social structure of said population. With social norms loosely or not at all enforced, living close together with millions of anti-socials is Hell in almost a literal sense. With social norms strongly enforced, the tenable population density goes up quickly, but the social control then will bring with it other problems.
In other words, the suburb layout e.g. in California may be the primary reason that mass transit is non-existent there, but the suburb layout is the result of a social structure and (lack of) social norm enforcement that would make living in tight spaces untenable for the myriads of the completely different cultures there. Wasting fuel and space on roads is the downside of that.
Counter-example would of course be Tokyo, where the social structure is totally uniform (1% non-Japanese). The Japanese culture probably has the strictest and strongly enforced social norms worldwide, with exception of N. Korea, so - for Japanese - it's perfectly possible to live in extreme population densities. Mass transit is totally feasible and in fact indispensable, private vehicles insanely wasteful. But there's downsides to that as well, with social norms (from a Western perspective) being untenable and overly strict, people shut themselves in or commit suicide much more often than elsewhere.
Clash of cultures in the US, overbearing control in JP. The existing transit systems are only a secondary consequence of that.