The usual rebuttals are: Overhead rails are smaller and cheaper than highways for the capacity, standardized pods can be lighter and smaller-scale, and the auto

The usual rebuttals are: Overhead rails are smaller and cheaper than highways for the capacity, standardized pods can be lighter and smaller-scale, and the automated guidance allows shorter timings for acceleration and braking. However, when transit planners run the numbers, they come up against a different problem, which is that building this isn't really a substitute for a new metro line. The peak capacity is still more along the lines of what a highway can provide, and that wouldn't do much to improve the rush hour commute - plus, if they wanted a highway, there's always funding around for those. So the PRT approach has mostly been implemented, to date, in airport systems and private campuses.