Dual mode versus Single Mode: Which Approach is More Likely to Succeed?

by Francis D. Reynolds

Cost vs. performance (user satisfaction) should be the criterion for deciding between single-mode (SM) and dual-mode (DM) transportation systems. It seems unlikely that "Leave your car at home and walk in the rain to the nearest transit stop" will ever be as satisfying to the customers as "You can take your own car all the way with no highway traffic jams." But what about relative costs? Some people have advocated single-mode (walk in the rain or blizzard at the start of and at the end of the trip) systems on the assumption that they would be cheaper. Actually, the reverse would be true. Guideway costs per mile should be about the same for either a single-mode or a dual- mode system, but "per mile" is the catch. In this case cost-per-mile is a misleading unit to use in comparing total system costs.

Some hypothetical numbers: Assume that most people would be willing to walk up to a quarter mile in driving rain or subzero weather to the nearest PRT station (I'm not willing, by the way). And assume we would build a DM system with entries and exits on an average of every two miles. Assume both the SM and the DM systems would be built as idealized square grids. The SM stations will need to be four times closer together than the DM stations. But since this is a two-dimensional or area problem, there will need to be sixteen times as many SM stations as we would need DM stations. Since the guideways are linear, only four times as many miles of SM guideway will be required as miles of DM system. So an adequate single-mode system should cost somewhere between four and sixteen times as much as a dual-mode system, and use between four and sixteen times as much land.

It has been suggested that we should start with a SM system, and later convert it to a DM system. Never! We couldn't afford to put in enough stations (with acceleration and deceleration ramps), and guideways to satisfy most of the walkers. Even if we did, most of these stations and guideways would be surplus when the system converted to DM. And if we didn't put in the huge number of stations and large number of guideways required for SM, without dual-mode vehicles only the closest people would walk and use it. If people farther away could be talked into using it, they would drive their cars to the PRT station, and we would still be in the messy land-consuming and time-consuming park-and-ride business.

In a DM system such as HiLoMag, the customers (both private and commercial) would acquire and own all of the vehicles; while in a SM system, be it trains, buses, PRT, or whatever, the system has to provide all of the vehicles. This is another major reason why a dual-mode system will be far cheaper to build and operate than an adequate single-mode system of any kind. The DM cars will be built by other companies, and sold to individuals and commercial transportation companies while the guideways are being built. The guideway companies will never need to design, build, or own any DM cars, except possibly for initial testing of the system. Even the test cars will probably be provided by car makers, just as Buick provided the cars for the National Automated Highway System Consortium (NAHSC) development and demonstrations in San Diego.

DM cars may be sold below cost initially, to encourage people to start buying them. But once the rest of the population sees how the early birds are zipping along the guideways while they are stuck in traffic, the rush for DM cars will be on and their prices will rise to the profit side of the ledger. The latest example comparable to the expected DM explosion is the current explosion of the Internet and e-mail usage.

Pallets could be used to put conventional automobiles on dual-mode guideways, but this writer does not recommend them. Our present cars will be worn out by the time the guideways can be built, for one thing. And providing pallets would increase the cost of the system in the same way that providing vehicles in a SM system would. A palletized system would also be much more expensive in another major way: The logistics requirements for assuring that an empty pallet was always available where it was needed would be very expensive to meet.

While HiLoMag or a comparable system is getting started many families may own a conventional automobile and an electric DM vehicle, rather than two autos or two DMs. The gridwork of guideways will go together gradually, and some highway travel will still be necessary in that interim. Present battery-powered cars can't cut it on the highways. If, however, internal-combustion, (or fuel-cell or advanced-battery) powered DM cars are used initially, one-car families won't be kept off the new guideways. Families without a car won't be kept off either, because there will be guideway buses, guideway taxis, and guideway rental cars.

SM systems, including proposed PRTs, tend to encourage high-density living, since users have to walk to the closest transit station or stop. The extensive revival and use of any type of transit system, including PRT, would be regressing to earlier times when people walked because there were no automobiles. A universal dual-mode system, on the other hand, will tend to reduce population density, and further encourage the wonderful suburban living that the automobile made possible in the first place.

And the lower the density the fewer the parking problems. There will be no intermediate parking with DM--none at bus depots, train stations, much less at airports, and no park-and-ride lots. Only final destinations will need to provide parking. Only a slight increase in destination parking will be required, over what we already have, since a very low percentage of the population now uses transit of any form.

The idea that we need to build a new SM, or expand old SM systems, or build more SM before we go to DM, is all wrong. More SM systems of any kind are simply more patches on our obsolete transportation systems; they would cost a lot and do little in the long run. Worst of all they would greatly delay our getting on with the major overhaul of our transportation concepts and hardware that are desperately needed as soon as possible. The situation gets worse around the world every year, yet we continue to try to make do with late 19th and early 20th century systems, while casually mentioning some nebulous future system. Thousands of people are working on patches, but almost no work is being done on the design and development of an adequate future system.

It is clear to a growing number of investigators that by far the best future system is dual mode. All of the old systems are now obsolete, even the latest cars and the maglev trains. But if we combine cars and synchronous maglev into an integrated automatic dual-mode system such as HiLoMag (which is in the public domain, and is disclosed on this Web site), we will completely solve or greatly reduce a surprising number of our transportation problems. Let's either get on with it or determine that it wouldn't work. All lesser new systems simply delay the inevitable and increase the final cost.

I feel a bit humble in arguing these points with people who have been in the ground transportation business for many years. (My career was in aerospace engineering, and my favorite vehicle, until recently, was the airplane). However, as I stressed in my recent book on inventing, people who are not experts in the field of their inventions more often make the major innovations of the world. The amateur is not handicapped with the "knowledge" of all the things that "can't be done," or "won't work," and the expert may no longer be able to see the forest for the trees. I urge all transportation professionals to broaden their thinking, look at the whole picture, and not just promote the particular mode of transportation they are most familiar with. Our many transportation problems are serious; we shouldn't allow their solution to be impeded by the "not invented here" factor.

Let me relate how I happened to think of the HiLoMag concept, since the story illustrates the point I just tried to make. I was writing another book three years ago, called Nutopia, in other words, a new Utopia. I was going through all aspects of society one by one, and trying to "invent" what I considered would be the optimum solutions for everything. I didn't let myself worry about whether my ideas were grossly different from the status quo, or whether we could get there from here; this was just a story, an expression of idealized thoughts. When I came to the topic of transportation I sat down and, in about an hour, conceived of the HiLoMag system.

When I now reread what I first wrote, it is remarkably close to what the HiLoMag team proposes today; nothing basic has been changed, only a few details have been filled in. Had my thinking been limited by concerns over the problems of changing from what we have to what we should have, I probably never would have conceived of HiLoMag, but my mind was completely free of constraints. Within hours of coming up with these DM ideas (and at that time I had never heard of DM transportation) I began to realize that this fictional concept had great promise for the real world, and I felt obligated to try to promote it.

When I later researched the future transportation field, mostly working from Schneider's ITT web site, I found a lot of other DM systems (including some bad ones). I have found three or four other people who have proposed or mentioned the possibility of DM using maglev guideways. One other person casually mentioned using linear induction or synchronous motors in the guideway (emphasis mine), but he took no note of the fact that a synchronous system will allow very close vehicle spacing at very high speed, providing remarkably high guideway capacity. All this without any need for expensive, troublesome, and unsafe proximity sensors and velocity-control systems in the cars. Those are some of the reasons why the NAHSC ITS system would have been a nightmare. USDOT was very wise to terminate their support of it.


Last modified: November 15, 1998