True Dualmode Cars vs. PRT and Palleted Dualmode Concepts
by Francis Reynolds
This commentary is intended to be a generic response to all who have written in favor of PRTs or Dualmode transportation with pallets
Pallets, ferries, carriers, trucks, dollies, or whatever we wish to call them have been proposed by a number of dualmode transportation proponents to carry unequipped cars on the automatic guideways. Others, including the HiLoMag team, propose cars that incorporate permanent provisions for both modes. I will call these "true dualmode cars." The pros and cons of true dual mode vs. pallets have been debated before, but I will rehash some of the old arguments here and add some new thoughts. New to me anyway.
A paper "Vehicle Distribution in Large Personal Rapid Transit Systems," by Ingmar Andréasson, (which may be read at http://faculty.washington.edu/jbs/itrans/ingsim.htm) begins:
"A personal rapid transit (PRT) system provides on-demand transport for individual passengers or small parties. The service is direct between off-line stations in a network of tracks and switches. The travel demand is generally unbalanced, particularly in the peak hours, with some stations serving mainly as origins and others mainly as destinations. Hence empty vehicles end up where they are not needed, with growing deficits at other stations. In a large network with long-distance commuting the task of balancing supply and demand of empty vehicles constitutes a major problem.
"In a PRT system with high demand link capacity is a limiting factor and over half of the vehicle flow on many links may be empty vehicles. The time to call an empty vehicle may be very long. It is therefore necessary to call vehicles before they are needed on the basis of the predicted supply and demand at each station. With random passenger arrivals and large call times vehicle buffers at stations would need to be quite large. Such large stations would be difficult to place in the city structure."
Getting the empty PRT cars back to where they are needed for someone else to use is exactly the same problem as getting empty pallets back to where they are needed in a pallet-type dualmode system. And Andreasson is right; "balancing supply and demand of empty vehicles [or pallets] constitutes a major problem." According to his calculations, "point-synchronous control," which he discusses later, would reduce these problems a few percent, but most of each of many different empty-car problems would still be there. I assume that the point-synchronous control system could also help some pallet-type dualmode systems. However, the major part of all of the problems due to empty-pallet routing, including degradation of system capacity, additional space required, additional pallet wear, waiting for pallets, wasted energy, added system cost, pallet storage, etc. would still be there.
There is a system running from Washington, DC to Orlando, FL that uses pallets -- Amtrak railroad cars to be specific. "Auto Train" hauls conventional automobiles and their occupants, and it is the only Amtrak service in the country to show a profit. Since it operates on existing railroad tracks it was cheap to build and was implemented rapidly. The black bottom line tells us that it is providing a needed service. Across the English Channel we have the channel-tunnel trains. These two systems carry automobiles and trucks inside of closed rail cars, or "pallets" to use dualmode terminology. But both put many automobiles or trucks in one pallet, and both are limited to one essentially straight-line route, so they are "dualmode" only to the extent that ferries can be considered dualmode. With these simple systems "pallets" are essential and make sense.
In Germany a visually similar but much higher tech service is being proposed. This one is called "Autoshuttle." It would use high-speed maglev guideways with linear-motor propulsion and one closed pallet-car per road vehicle. This system would have offline acceleration and deceleration and full-velocity switching, so it would be capable of providing high-speed high-capacity service to a large area through a complex grid system. Except for its closed-car pallets Autoshuttle would be much like several palleted dualmode systems being proposed in the United States and Germany (e.g. MegaRail, Autran, InTransSys , Autoshuttle). And it would suffer from all of the disadvantages of using pallets instead of true dualmode cars.
PRT systems are stuck with the need to route empty vehicles as required, and this is one of many disadvantages of all PRT systems. And the need to route empty pallets is the top disadvantage (out of many) of all palleted dualmode systems. Doubtless some pallet-system proponents have addressed the problems associated with returning the empties more than others have, and may have found the best answers, but even with the best empty-pallet routing and storage system the problems and added costs will still be major.
There is only one answer that completely eliminates all problems related to buying, routing, maintaining, and storing pallets, system performance degradation due to pallets, and customer waiting and other dissatisfactions due to pallets. That single universal answer is to eliminate the pallets. Dualmode systems with true dualmode cars would do just that -- they would have no pallets.
Pallet proponents usually argue that true dualmode cars would cost too much. But it is total cost that counts in the long run, and a true dualmode car will cost less than a single-mode car plus a pallet for it. Maybe we wouldn't need quite as many pallets as cars, but close to it. During rush hours a high percentage of all cars would be on the guideways, and a high percentage of all the pallets would be scurrying around empty, going where they are needed.
If pallets are used, they would be owned by the guideway system, and their cost would be added to the system cost, which would make it harder to sell the bonds, and would raise the guideway use fees. Ultimately it is the guideway users who will pay for all aspects of their transportation. Their total out-of-pocket would be less with a true dualmode system.
It has also been argued that we must use pallets so that existing cars can be carried on the guideways without modification. Who are we kidding? I would be happy to put my Dodge on the guideways, but that isn't going to happen. With optimistic assumptions, it would take at least twenty years to have a dualmode system ready to use. My present car will be worn out long before that (and so will I). There will be plenty of time for Detroit, Japan, and other countries to design and build millions of true-dualmode cars while the guideways are being designed and built. We must remember that we are planning for a somewhat distant tomorrow. Unfortunately the fact that we need the system today doesn't mean we can have it today. But it does mean that we need to get the ball rolling ASAP.
Without doubt a number of dualmode proponents had visions of personal financial gain, at least initially. Good luck guys. I feel that the only realistic reasons for being in the dualmode game at this point are the personal challenges it provides and the potential for helping future generations. The big corporations are not strongly into innovative transportation at this time because the break-even point is too far in the future to suit their stockholders, and government contracts on innovative transportation are few. I think that we little guys should recognize that we are amateurs in the financial sense, and we should not let personal needs affect our judgement on the type of system we should espouse for later generations.
I further think that recommending a simple less-expensive transportation system first, with later revision to a more optimum system, is usually the wrong thing to do. For instance, building a guideway system for wheel-supported cars with thoughts of upgrading to maglev later makes little sense since the cost of completely changing all of the infrastructure would come close to or probably exceed the initial system cost. And what do you tell the commuters, "Sorry your guideways will all be closed for the next ten years while we rebuild them, but in the meantime you can buy the new cars you will need for the new improved system." We must initially build the best system we know how to design and build.
Likewise, a plan to provide pallets at first, and then use true dualmode cars later doesn't make sense. A guideway system equipped to route and store empty pallets would be much more complex and costly than one for true-dualmode cars only, where no cars ever need to travel empty.
And don't worry about the added weight and drag in street mode due to the guideway travel provisions on true dualmode cars. Street mode will be slow and short-range. Worry about the extra weight and extra drag if pallets are used on the long-range high-speed guideways. There we will probably use ten times the energy that we will use in street mode.
Another argument that enters the pallet-use question is safety. I read one statement that pallets were essential because we could never allow possibly unsafe private cars to ride the automatic guideways directly. But what about possibly unsafe private cars on high-speed highways? And what about possibly unsafe pallets on the guideways? A dualmode system can and should provide automatic safety checks (taking a few seconds) of all features essential to safe guideway travel before each car is allowed to enter the guideways. In a palleted system each pallet should likewise be automatically safety-checked each time it picks up another car. Since pallets would add to the total complexity and total parts count of the system, it is likely that any palleted system would have a lower mean time between failures than a true-dualmode system.
With vehicles that are supported by wheels in the guideway mode there would be a large number of things that should be checked or tested prior to each guideway trip: tires, wheels, bearings, axles, power plants and transmissions, and mechanical steering in some systems. Some of these things would be quite difficult or impossible to check with an automatic testing system that cost less than the national debt. With maglev dualmode however, the tests required would be few and simple. Automatically test the strength of the levitation and propulsion magnets in the cars, and test the simple magnetic switching system, and we are done. Everything vital to safe travel on the guideways will have been tested.
And once on the guideways there is almost nothing in a maglev system to wear out, because nothing is rotating or even touching anything with relative motion.
Last modified: August 13, 2002