The Advantages of Dual-Mode Transportation

With some specific references to HiLo


Francis Reynolds , P.E.

My thanks to Prof. Jerry Schneider for his invitation to join this Dual-mode Debate.


The automobile has become a dearly loved, delightfully private, and practically indispensable door-to-door vehicle. Very few people would be willing to give up their car. With dual mode we won't have to; we will still have private cars, but they will be different and better. Dennis Manning, in his " Dual Mode Musings ," said it well: "Dual Mode has a far more personal aspect--owning ones own PRT vehicle. .... .... Unless single mode can offer private ownership, which seems a difficult hurdle, dual mode should prove to be more socially adoptable."

But, from a practical standpoint, the pride and satisfactions of private-vehicle ownership are secondary: The from-home-to-final-destination capability and the instant availability of the automobile are its most important advantages over trains, buses, single-mode PRTs , airlines, and all other forms of transportation except bikes and "dual mode." We don't want to have to walk to the bus stop; especially in the dark, wind, rain, snow, heat, or in dangerous neighborhoods. What was acceptable a hundred years ago (because there was less technology and fewer choices) is no longer acceptable. We will refuse to go back.

But driving our present automobiles to the bus, train, airport, (or a PRT stop) causes pollution, takes extra time, and requires intermediate parking where the cars may be stolen or vandalized. With a dual-mode system we will drive to the nearest guideway, but we won't park there. We will continue our journey in the same vehicle, in privacy, at much higher speed, without polluting, without driving stresses, without traffic jams, and we will still have our own car for street use at our destination. With PRTs, as it is now with trains and buses, we would have to walk in the rain again at the destination end, or hire a taxi, or rent an automobile. Dr. Anderson said, "Dual mode has the advantages for auto drivers that the same vehicle can be taken from home to any destination, similar to driving an automobile." I would add, "but far better than an automobile in speed, reduced stress, and reduced environmental damage."

The street mode of any dual-mode system is the easy part; the streets are already there. So including their use as an integral part of any new high-tech system makes sense. Excluding them would degrade the usefulness, and therefore decrease the use, of the system tremendously, for the reasons noted above.

As our roads improved and expanded, automobiles also improved, they became more essential, and their numbers increased at a great rate. Then the traffic jams and automobile-related environmental problems developed. That we all know. But the point that I feel is being missed by most people is that the concept of having and using private vehicles is not the root of the problem. Our automobile woes stem from the type of cars we have and the type of system in which we use them, rather than from the existence of a lot of private cars per se. Most dual-mode-system efforts are attempts to retain the advantages of the automobile while at the same time solving at least some of the problems it now causes. It appears to the writer that some critics of dual mode have locked onto one or more poor dual-mode concepts then falsely associated the resulting problems with all dual-mode concepts.


In studying the dual-mode-debate material already on the website, namely the contributions of Dr. J. Edward Anderson , Dennis Manning and Joe Palen , I came to the conclusion that one could not logically debate dual mode generically, since there are so many dual-mode transportation concepts and they differ so markedly. Some arguments that would apply to one concept would be meaningless, or even reversed, when applied to a different dual-mode concept. These observations are particularly pertinent to HiLo , the dual-mode concept I have been formulating and am proposing. It is the only PRT-type system I am aware of which would use maglev rather than wheels of some kind for vehicle support in the automatic high-speed mode. It also appears to be the only one to use synchronous linear motors to hold constant vehicle spacing on the guideways. The following arguments and comments will therefore apply to HiLo or similar systems. Those who need to ask, "What is a HiLo?" are urged to read the two HiLo articles before continuing to read this contribution.

The HiLo articles referred to have been on the Internet since early November 1997 without revision. In the meantime minor changes have been made in the system. The most significant are changes in its speed. The articles were written around a constant-100-mph guideway mode, with suggestions that the system speed might be increased at a later time. The revised system uses constant-60-mph maglev guideways in and around cities. These will connect with a constant-200-mph guideway network between cities throughout the United States. The advantage of the lower-speed for local guideways is great: The distance required to achieve a given velocity at a constant acceleration is proportional to the square of that velocity. Therefore 60-mph guideways will require only 36% as much expensive real estate for the acceleration and deceleration ramps as 100-mph local guideways would require. Also the 60-mph system will be much quieter for business and residential areas. The 60-mph local guideways will have a capacity equal to that of twelve freeway lanes--more than ample. The 200-mph (well within current maglev state-of-the-art) transcontinental guideway network will halve the time that would have been required for long trips at 100 mph. Two-hundred-mph guideways will have a single-lane capacity equal to forty freeway lanes--while using only one freeway-lane-width worth of land. The constant loss of good land for more highway lanes will stop.

Edward Anderson compared dual mode to the Automated Highway Systems work, and pointed out that dual-mode systems (other than AHS) can use narrower (cheaper and less land-gobbling) guideways. True. I consider AHS a dual-mode system, but a poor one compared to HiLo or something comparable. The AHS "intelligent" or "smart cars" would not solve any environmental problems since they would still use internal-combustion engines, and we would still continue to need more freeway lanes. Smart cars would be much less safe than HiLo at high speeds, since they would depend upon the tires, engines, transmissions, steering, and a lot of automatic sensing, guidance, and control equipment in each car. Maglev dual-mode cars will use little such safety-degrading equipment in the guideway mode.

Perhaps the most important high-speed safety aspect of HiLo is the use of synchronous linear motors. These will make possible very close spacing between cars for very high capacity, and they will hold this spacing constant without proximity or velocity sensors and control systems. The cars will be fixed with respect to each other like boxes on a conveyer belt. This simple, inexpensive, and effective synchronous-drive and spacing control concept has been strangely neglected. Dr. Anderson may have had it in mind: he used the words linear induction motors in his brief TAXI 2000 PRT disclosure on this website, but mentioned both induction and synchronous linear motors in item 8 of his dual-mode debate write-up. (Anderson's excellent article on the capacity of a Personal Rapid Transit System, which also appears in this website, is recommended reading. The capacity principles he discusses are equally applicable to both dual and single-mode systems.)

Also in item 8 Anderson makes the statement, "In pure DMS more guideways would be needed than in an optimally designed PRT system, not fewer." I am unable to determine what he is calling a pure dual-mode system; but with HiLo, which I consider to be pure dual-mode, the number of guideway lanes required is one, for any traffic likely in the foreseeable future. And it will be explained later under "costs" why the number of miles of guideway required for dual-mode will be very much less than that for single-mode.

In his items 9 and 10 Anderson discussed the jams and bottlenecks which would occur in central business districts due to the presence of a large number of dual-mode cars. These, I agree, are major concerns. The dual-mode-car street traffic jams, ramp bottlenecks, and parking problems in the big cities will be very similar to the problems we already have there with our present automobiles. The basic problem is the density of the cities, and no transportation system will permit us to pack an unlimited number of vehicles into them. New York City and others reached the saturation point a great many years ago. Much more recently Bangkok, Singapore, Taipei, and others have joined the list.

There are no easy answers to this problem. I am an advocate of decentralization and further urban sprawling in the long term. For the nearer future, as Anderson suggested, people could leave their dual-mode cars at stations in the outskirts (like our current park-and-ride lots), and take single-mode (guideway-only) PRT or LRT vehicles into the central business district and back. But note that these dense traffic problems are self-limiting, and decentralization occurs naturally. When traffic gets worse fewer people take their automobiles (or will take their dual-mode cars) into the cities. Or they will seldom go to the central district. Then the businesses move branches to the suburbs, and the necessary decentralization takes place. Dense cities developed when (and because) walking, horses, and later, bicycles were the only transportation modes. The advantages of high density are now gone. City densities will decrease with time, but it will take a long time.


In items 2, 3, and 4 of his contribution to this debate, Anderson expressed several concerns over how the vehicle-safety-inspection requirements that he foresaw would degrade the usefulness of dual-mode systems in general. Safety is a primary consideration, and we will have special concerns for our safety on a high-speed computer-controlled system. It is natural to mistrust and fear the new and unknown. Actually the high-speed or automatic mode of a good dual-mode system will be much safer than our present freeways: There will be no driver errors on the guideways. And with synchronous maglev very close spacing (very brief headway) between cars will decrease the danger of serious collisions, even at very high speeds. Since dual mode will be less dangerous, we might argue that our present mandatory automobile inspections (and driver tests) could be relaxed in the future with dual mode. But this is not my position. Certainly prudence dictates that dual-mode vehicles pass some kind of rigid inspection and test procedure before they are allowed to enter the guideways. These mandatory inspections and tests need not limit the usefulness of the system in any way, however.

Referring back to my point that treating "dual mode" generically may lead us to false conclusions, the safety-inspection problems discussed by Edward Anderson would apply to some types of dual-mode systems, but there will be no such problems with HiLo. Since HiLo cars will be magnetically levitated off of their wheels while on the guideways, the condition of the wheels, tires, transmission system, motor or engine, steering system, and most other moving parts would be of no concern in the high mode of the system. Also, since HiLo vehicles will be electric-powered in both modes, present requirements for periodic emission tests will not apply. Some states or cities may require periodic mechanical inspections, but on HiLo vehicles these will apply chiefly to the low-speed street mode and need be no more extensive or frequent than they are for ordinary automobiles.


HiLo vehicles will be tested for safety at the beginning of every guideway trip. I use the word "tested" rather than "inspected," because the latter word implies the presence of an expensive, slow, mistake-making, and sometimes bribable human being. Fortunately a human inspector won't be required. The testing will be done automatically, and almost instantaneously, every time a car seeks to enter one of the maglev guideways. You may recall that HiLo drivers wishing to use the guideways will drive onto an "entry stop" where the vital statistics of the vehicle will be electronically read, and the desired destination exit number will be punched into a keyboard on the dash. At the same time, equipment installed in the entry stop will be automatically testing all of the important aspects of the car's levitation/linear-motor magnets, and the guidance/switching magnets. These magnets do not make physical contact with anything on the guideways; therefore they can also be tested without physical contact. The car communication systems will also be tested.

The one mechanical system in the car that is essential to guideway travel is the "either/or" lateral guidance and switching mechanism. It must work positively, reliably, and rapidly. Since the signals to operate this mechanism come to the cars on the guideway without physical contact, and the useful output of the mechanism is magnetic, the mechanism, like the magnets, can be automatically tested in a second or two without physical contact. The onboard battery that supplies power for the operation of this switching mechanism will also be tested without contact. If a car passes all of the safety tests, and other requirements for guideway use, such as up-to-date insurance and guideway-bill payment, it will be automatically levitated, accelerated, and merged with the guideway traffic. If it fails to pass any of the requirements for guideway use the driver will be so informed, by recorded voice and a message on a dashboard screen, and asked to leave the entry stop and return to the streets. The driver will have no means for entering the guideway if the car is rejected; the cars will have only street-mode manual controls.

The guideways themselves will of course be inspected and tested on a regular basis; and critical systems such as the computer(s) will incorporate redundancy and will be designed to "fail safe." But these same safety requirements apply to single-mode systems as well as dual, so they are not pertinent to this debate. Individual sections of the HiLo guideways may be shut down for inspection or maintenance without disrupting the rest of the system.


It should be noted that dual mode is much more versatile than single mode. A dual-mode guideway can and will be used not only by several types of dual-mode vehicles, but by several types of single-mode vehicles; while a single-mode guideway could never accommodate dual modes. HiLo will be mostly used for dual-mode private car traffic, but it will also have dual-mode taxis and rental cars, dual mode school buses, dual mode mail and other delivery trucks, and small dual-mode private or rental trucks. In addition the HiLo guideways will carry single-mode driverless PRTs, buses, and cargo-container vehicles. Since all of these single-mode HiLo vehicles will operate only on the maglev guideways they will have no wheels, transmissions, engines, or steering; and since they will have no use for drivers they will have no instrument panels, manual controls, steering wheels, or driver's seats. The elimination of all these subsystems will greatly reduce the cost of the vehicles as well as increase their capacity for passengers or cargo, and increase their safety. The elimination of the drivers themselves will further reduce costs and increase safety.

The net result of all of this will be cheaper bus fares and cheaper freight rates. The inclusion of all of these other types of vehicles on the guideways will absorb more guideway capacity, the HiLo bonds will be paid off faster, and the guideway use fees will be reduced. A good dual-mode system will be highly popular because it will solve our problems by giving us better, faster, safer, cleaner transportation than we now have, yet let us still have private cars. The extent to which the system will be used for charitable or subsidized transportation will be left to the people and the politicians, but the guideway system as a whole does not need to be and should not be subsidized; it should not be added to the public debt. It has the potential to pay for itself, or even make a profit. In my opinion that would not be possible with any single-mode system.


Dennis Manning, in his " Dual Mode Musings ", expressed some opinions which I can't agree with. For instance, Dennis said, "Attainable single mode speeds would surely be far higher than dual mode." Again we need to know what dual-mode concept we are talking about. The proposed and readily achievable constant speed for the mainline HiLo guideways is 200 mph, whether they are being used by dual-mode or single-mode vehicles. And the system could be designed for 300 mph, if a higher speed was considered worth the extra cost. The local HiLo guideways could also be faster than the proposed constant 60 mph, as we have already discussed, but there are tradeoffs as usual.


Manning seems to feel that whether we end up with a major dual-mode system, or a single-mode system will depend mostly upon which system gets there first; and he was inclined to think that single mode would be the winner. Yet he seemed to agree that dual mode had many advantages. I certainly think that dual-mode is far more desirable, but I don't see the single-or-dual race in the same light that he does. Our existing ground transportation systems are (with minor exceptions) single-mode.

The addition of an extensive single-mode PRT to the mix would entice some users. But the great majority would continue to use their automobiles, either to get to the transit stops or for their entire trips.

Dual-mode offers the greatest incentives for us to clean up our act. It alone will provide private door-to-door origin to destination service without inconveniences, interruptions, delays, possible discomforts, or possible exposure to unsafe situations. If we can get the public and the politicians to see this, and I think we can if we work hard at it, then there is no contest. We will have dual mode because it is the system that will solve the most of our transportation problems. Joe Palen wrote, "Few things could provide more incentive to get single occupancy vehicle [drivers] into a PRT than to have the PRT vehicles whiz by them every morning as they are stuck in traffic." Joe Palen might have added that far more of those SOV drivers would switch to a dual-mode system, knowing that the people whizzing by them were in their own cars, that they started from their own garages, and that they would still have their cars to use after they got to their destinations.


The acceleration and deceleration ramps required on a constant-speed dual-mode system would not be required on a single-mode system; however the guideway or track network for any adequate single-mode system will always cost far more than that for a comparably-adequate dual-mode system built for similar speeds and load capacity. The word "adequate" is emphasized because the guideway network for single mode needs to be much more extensive in order to significantly reduce ordinary automobile traffic. Several times as much single-mode guideway would be required, with far more switching interconnects, because most people won't use public transit, PRT, or whatever unless it will pick them up at or very near their own homes. Requiring them to use their cars to get to the transit stops is not popular and therefore transit has not, and PRT cannot, greatly reduce our traffic problems. But building a guideway system extensive enough to pick all people up at their homes would be extremely costly. It would mean installing guideways or tracks everyplace we now have streets. A very much more modest dual-mode system will let anyone (who can now afford a car) get into his or her vehicle at home and stay in it. Decreasing the quality of our transportation will never be tolerated, other than in a dictatorship. Dual mode will allow us to increase transportation quality as well as solve our environmental problems and reduce fossil-fuel consumption.

Dual-mode vehicles will cost considerably more than single-mode vehicles of the same speed and capacity, but the dual-mode vehicles will be privately purchased and will not be seen as part of the system cost. We now own our automobiles and don't consider them part of public street and highway costs. But with a single-mode PRT, the huge number of public vehicles required would be added to the cost of the very extensive, and therefore expensive, guideway system; making the total cost of an adequate single-mode PRT system enormous and next to impossible to sell to the politicians and the public.


The dual-mode system we so badly need will be cheaper than single-mode would be, but still very expensive. Necessary things, like good health care for instance, often are. But limiting a big new transportation system to single mode to save money would be as penny wise and pound foolish as incomplete medical care would be. "You are going to die, but the good news is that we saved money by not running that test." With regard to the vehicles, dual mode cars will cost a lot to develop, but millions of them will be produced, and mass production will do wonders in getting their price down. Joe Palen observed, "You can't beat the purchase price of autos." But Joe asked, "How do you get a conventional auto vehicle design to run on an automated PRT guideway at low headways with high safety?" I think it would be very foolish to try. That would be a pound-foolish patch job. The conventional automobile is not only subject to traffic jams on the highways, it is an environmental disaster. And I include the constant addition of more freeway lanes as part of that disaster. The conventional automobile has served us well, but like railway trains it is now obsolete--even if a new model does out every year. Our present automobiles are dinosaurs--or at least related to cloth-covered biplanes with reciprocating engines. But Detroit (or Japan) will be more than happy to build us dual-mode electric cars that are designed from scratch for this new transportation system. We won't have to scrap our existing cars, because they will be worn out before the guideways can be completed, and they will continue to be needed during the changeover.

However we look at it, and whatever it will consist of, a national guideway system will be very expensive; but this is a very big country, and a very large number of people would be using and paying for the system. Roughly a century ago our national railroad network, an equally huge undertaking for its day, was built by private enterprise. But the railroads were helped a lot by federal land grant right of ways. Hopefully the same type of help will come about on our national guideways; many people believe that in most areas dual-mode guideways could be installed above freeway median strips, and within the railroad right of ways.

Single-mode offers so much less than dual toward solving our total transportation problems that, in my opinion, it would have to be highly subsidized like all of our existing public transportation systems are. But a good and adequately promoted dual-mode system would have much popular appeal. It would serve us so well that it could not only pay for itself but even make money. Therefore it would also appeal to big business. The railroad barons of a century ago took a lot of flak, but they did us a great service. The present equally valuable entrepreneurial barons are Steve Jobs, Bill Gates and similar people. Now we need some dual-mode-transportation-system barons.

Francis Reynolds lives in Bellevue, Washington and can be reached at 425-885-2647 or 3802-127th Avenue, N.E., Bellevue, WA 98005-1346.

HOME | Dualmode Debate

Last modified: May 8, 1998