There is a conference at Cornell about the viability of Personal Rapid Transit as a future transportation technology. The proponents of PRT argue that their preferred mode is environmentally friendly and generally a forward looking progressive approach to personal transportation. While I admire their goals, there is no reason that their main concerns can't be addressed on the existing transport system.
Mostly we shouldn't be looking at a system that requires brand new infrastructure. We can better manage what we have and simultaneously encourage cleaner and more efficient vehicles. Overhead lines will harm the sidewalk and road environment by reducing the views and sunshine while increasing noise. In addition, the energy expended to build a brand new transportation infrastructure will greatly diminish any potential environmental gains, even if we (wrongly) assume that the only way to have a system of electric vehicles is through PRT.
Perhaps PRT has a role for niche markets such as airports and maybe colleges. As a large scale transit system, however, it will likely fail to achieve the stated goals, and it will divert precious resources in the process leaving us with a transport system that is worse off than we have now.