I was reading and interview Marsden Burger http://www.metrotimes.com/editorial/story.asp?id=8756
quite a good read but I noticed a counter comment by Vukan R. Vuchic.
"A decade ago, Vukan R. Vuchic, a professor of transportation engineering at the University of Pennsylvania, had this to say about personal rapid transit in an article published in the magazine Urban Transport International: "The PRT concept is imagined to capture the advantages of personal service by private car with the high efficiency of rapid transit. Actually, the PRT concept combines two mutually incompatible elements of these two systems: very small vehicles with complicated guideways and stations. Thus, in central cities, where heavy travel volumes could justify investment in guideways, vehicles would be far too small to meet the demand. In suburbs, where small vehicles would be ideal, the extensive infrastructure would be economically unfeasible and environmentally unacceptable."
Kind of interesting and for once coherent critique. Naturally being a family of technologies this makes the idea of 'complicated guideways and stations' hard to truly pin down. Some PRT systems use magnetically levitated track - high speed. This can be simple as a flat plate of metal - it can be more complex. The Ultra system uses roadway - infact this isn't even the kind of expensive ( 10$ millilon per mile) you get under your car but the kind of $10,000 per mile you get under leisure/pedestrian trails. Why so cheap ?The Ultra system is like a golf cart moderately low weight (I've had a golf cart run over my foot and walked away from it). Leasure trails don't have to cope with the huge peak loads provided by giant 16 wheeler trucks. Once elevated the costs go up naturally. For Ultra the stations are bus shelters with a hardened computer terminal.
The computer evaluations I've seen put PRT on a par if not above light rail for high volumes. Again this is not EVERY PRT system. If super high volumes are a problem you can train the PRT cars together. Using sensors digital signalling you can push the vehicles together ( separated by a few inches/cm). This can also reduce wind resistance ( hence energy consumption).
You might this this is a technological impossibly but I knew a guy a GRTA who worked on a DOT(US) project to do this for cars in the 80's. ( see http://www.memagazine.org/backissues/may98/features/smarter/smarter.htm for a description). This concept of platooning ( as it is known technically) was designed to run normal cars on augmented roads to help increase capacity. Its a kind of super smart cruse control. Did the project fail ? No not technically. The problems where institutional. It required a standard set of sensors on each car in the system and the car manufactures would have to agree to standardize on( they wouldn't). The big problem I was told was one of insurance if the guy at the front suddenly switches to manual ( this is just cruise control) and does something out of the normal. If this causes the guy behind you to hit you then who is liable ? The guy at the front, the guy behind, the software company?
This is solved by one authority (company, city) owning the separate track and cars. Again if capacity is a
real problem then you really do need a flat escalator /travelator. These have the highest capacities for any form of transport.
