As mentioned throughout, my preference for powering a national personal transit system is to use a combination of solar and wind generation with a hydrogen or compressed air backup. The most obvious reasons for shifting over 1/4 of Americas current energy use to some other method are the environmental and geopolitical problems associated with the widespread use of oil. These include; pollution, global warming, uncertainty of supplying the "pipeline" from volatile regions of the world, and oil "profits" largely going to entities Americans should definitely not be giving money to. In addition, as other forms of cleaner energy become cheaper, competition for increasingly hard to get oil will make its price keep rising. It is no mistake that folks like T. Boone Pickens are investing their own money in things like wind powered generators, changes must be made soon or our children and grandchildren are slated to pay an even heavier price than they are aleady likely end up with. This is the same reason I believe that coal, nuclear, and (current)bio-fuels are not good options. Coal is one of the dirtiest fuels ever used by humans and even if the pollution from usage is worked out you still have to mine, transport, and store it. Nuclear* power seems to hold promise for relatively cheap energy but to my knowledge nobody has ever put forth a potentially successful plan for either controlling proliferation (with enough skilled people in the world talented in refining and using nuclear materiel for industrial use to potentially misuse their skill) or what to do with waste (which is among the most toxic and radioactive substances ever created by humans and will last many times longer than our civilization has been in existence). Biofuels as are now being produced use almost as much or more energy than the final product yeilds depending on the source and method and are as dependent on market fluctuations as oil. Other methods of production like geothermal or tidal generators might be appropriate for parts of the new system but would have to be sent over long distances (where much of the electricity generated is lost) to much of the system to power it entirely. The main reason I can see for anyone advocating any of the power generating methods discussed above is to make sure it stays centralized for better control by fewer people (can you say Enron or market manipultion?). That brings us to my preferred generators, the sun and wind (still the sun technically but different form of energy.....kinetic vs radiation). Solar power alone could probably provide enough energy for the system if it has solar cells over the length of it. According to "The Scientific American" 30,000 square miles (which also happens to be about how many square miles of road there are in America according to U.S. D.oT.) of solar cells in the desert southwest would provide half of our energy needs in 2050. While it is true that most of the system will be somewhere else (and presumably less effecient), the system will not consume 50% of our energy either. Unfortunately, a solar panel only produces usable energy during daylight hours, so some sort of supplement is needed. That supplement would be the wind, which is in every location in the world like the sun. Also, like the sun, it is not a steady source. During the times of production the system should have much excess capacity which could be used to compress air or make hydrogen for generators (can you imagine a strategic hydrogen reserve like todays oil reserve?). The excess could also be used to charge batteries but that would be very cumbersome and expensive and may not be practical with current battery technology. Once installed the energy source is free, making it very competitive cost-wise over the long haul and less subject to market vagaries and manipulation plus the technology is in a usable form today.
*I would like for someone to show me how I am wrong about nuclear power but I will warn you I know firsthand a great deal about the technology and industry.