> The fundamental prerequisite for functional mass transit is a high population density. What does "high" mean? I think this should be quantified, preferably b

> The fundamental prerequisite for functional mass transit is a high population density. What does "high" mean? I think this should be quantified, preferably by someone qualified. Unqualified opinion: It seems to me that if we took all of the money spent on personal vehicles, time spent driving, fuel and emission costs, infrastructure maintenance, and costs to human happiness + safety, we would be able to provide a better transportation system.