William G. Turnbull

There is considerable controversy over the question of true dualmode versus palleted dualmode. It should be noted that my use of the term pallet is inclusive of such terms as carrier, car-ferry, and the like. Quite recently Mr. Arthur has provided us with what might be described as a "real-politik" assessment of the benefits of palleted dualmode. I agree almost completely with his observations; however I think more should be made of his suggestion of "Let a thousand flowers bloom!" While his emphasis was on the practical and marketing issues, technical arguments also obtain. Thus this paper should be viewed as (what in a judicial context would be termed) a concurring opinion.

In previous discussions, the most common argument used to advance palleted dualmode is the "chicken and egg" problem. These were primarily economic; and while I regard them as valid and substantial, I submit that "getting it right" is even more important.

Most, if not all, readers of this page enthusiastically support the societal and financial advantages of some sort of a dualmode system. However, presently there is no such system in revenue service. I have no doubt that, at the corresponding stage of development, those who argued for light rail where equally enthusiastic. However, revenue service has not been kind to their enthusiasm. I am not suggesting that dualmode advocates are similarly wrong (my own view is that they are substantially correct), rather it is that there is no real operating experience upon which to base their enthusiasm. Moreover, there are now several competing systems with substantial differences, and in varying degrees of development. Absent any empirical evidence, and with the champions of each claiming superiority; how is one to decide to which should we pledge our enthusiasm?

An Optimum Approach Not Apparent

There have been calls to come together and agree on a common operational standard. I believe this is shortsighted. The Los Angeles freeway system has been under construction for over five decades, with apparently no end in sight. Thus it certainly is not unreasonable to assume that to effect the kind of radical change envisioned here will require several decades, at the minimum. Recalling, among others, the Denver Airport debacle, and the early advocacy of light rail; I submit that none of us, either individually or collectively, has the necessary knowledge or prescience to select the optimum technology for the mid twenty-first century.

Underlying all of this is the assumption that no matter how careful we are in design; practice will uncover serious flaws, or obvious improvements. I am confident that anyone with experience in large and/or complex projects would wholeheartedly agree.

Accordingly, it is essential that, for each project, a single agency control all aspects. This, so that as changes are identified, either required or desired, they can be effected as expeditiously as practicable with a minimum of jurisdictional squabble.

Given this, and for the reasons detailed below, I argue that all dualmode systems should be designed as palleted dualmode, certainly for the present. I have no quarrel with those who would look ahead to the possibility of true dualmode, and thus may wish to design so as not to preclude this; I simply argue that no great weight should attach to it. Any final design will most likely be substantially different.

Several Approaches Required

Dr. Hopkins has been quoted extensively in these pages. I believe none more to the point than when he wrote, "Without very limited small-scale revenue-producing applications to establish credibility, there doesn't seem much likelihood of getting anyone to take a really large-scale program seriously." Considering the novelty and scale of this undertaking, I believe it prudent, more likely essential, to consider multiple exploratory projects. To encourage and facilitate an optimum outcome, we should impose an absolute minimum of standards or other impedimenta to future development. In this, I refer only to system standards, certainly not safety standards.

Accordingly, it is imperative that as many users as possible actually use these systems as early and as frequently as practicable. To accommodate this, a minimum of special apparatus should be required of the user’s vehicle. It is also necessary that many users have the opportunity to use multiple systems. In my view, only palleted dualmode has the flexibility that adequately satisfies all these requirements. Developers or local governments must be free to provide the system thought to be best suited to the local need. It is only after the experience of substantial revenue service, with more than just a single system, that we can begin to make informed choices. It is only then that we can even begin to contemplate any national standards.

Pallets Do Not Hinder Development

Mr. Goltermann is concerned that if an adequate pallet system is developed, this might inhibit the development of a true dualmode (and by implication a superior) system. Thus he urges that we must go directly to true dualmode. Again, the greater danger is in making any final decision absent empirical evidence of any kind. Moreover, if the difficulties he posits turn out to be true, he will have precisely the evidence he now lacks.

I am not persuaded that the use of pallets somehow compromises optimal development. It is almost always true that a design optimized for a single task is superior to one that must perform several. Moreover, aside from the unfortunate emergence of SUV’s; private vehicles, computers and communication equipment, and almost everything else tend to be smaller and more able in subsequent models. Thus the design of a true dualmode vehicle, should we choose to do so, would benefit by earlier experience with pallets. I see little reason that the advantages of a specific guideway, the superiority of linear versus on-board motors, or the value of magnetic suspension versus the use of rail, cannot be profitably explored with pallets.

Central Agency Is Essential

Moreover, there is no more important interface than that required between the general system and the vehicles, whether pallet or not. It is essential that all vehicles be in good order. There can be no ambiguity about the responsibility for assuring this.

It has been suggested that as true dualmode vehicles enter the system, the good order can be insured by an instant test of some sort. Perhaps later, but for the present I believe that only thorough and frequent, off-line, non-revenue testing will insure the integrity of the system. This can only be accomplished if the system is in complete control of its own destiny i.e., owns all the operational pieces. If it is ever to turn over partial control, this can be only after extensive development and testing in a real operational environment. There is simply no margin here for an "oops".

It is unfortunate but nevertheless true that we are a litigious society (at least in the United States, I can't speak for the world). Divided responsibility (even the appearance of it) invites legal disputes and thus delay, and quite possibly a less than optimum system.

Number of Pallets Needed

Looking to the future, I would disagree mildly with Mr. Arthur’s assertion that true dualmode cars ". . . are more expensive and less elegant than pure dualmode." Less elegant is in the eye of the beholder, but we might try to quantify the former. In particular, this would depend critically on the ratio of user vehicles to the number of required pallets.

It would also depend on the extent of commonality between propulsion schemes used on the guideway and on the street. Further, it would depend on the extent that redundant safety apparatus and controls are thought to be required on the vehicle. Initially, at least, this could be considerable. If it turns out that the ratio of vehicles to pallets is in the range of ten to twenty, it is not at all clear that that providing the additional apparatus to each street vehicle is an economic positive.

Mr. Goltermann has estimated that one pallet would be required for every two vehicles using a dualmode system. This estimate was derived from data generated by Mr. Andreasson in his Gothenburg simulation. I believe a fair reading of the data suggests a somewhat larger number.

If we consider Table 2 and take the ratio of 'passengers departed per cab hour' to 'loaded cabs departed per cab hour' (4.3 / 2.1 = 2.05) we get a number slightly greater than two. That is there is one cab for every two departing passengers. The difference of this quantity from the average of 1.76 is easily explained by the fact that all passengers do not exit at the same station. Nevertheless, If we apply this ratio (2.05) to the total estimated daily traffic i.e., 600,000 passengers, we obtain something over 293,000 cab departures per day. This undoubtedly underestimates the actual number as it would seem likely that the number of passengers carried per cab during off hours would be less than those for peak hours. Nevertheless, having no better number, I will continue its use. As there are 12,795 cabs, this would suggest that, on average, each cab makes 22.9 revenue departures per day. It is interesting to note that if we extend the peak value of 2.1 departures per hour, we obtain something over 50 revenue departures per day.

Palleted dualmode journeys are likely to be a round trip i.e., a minimum of 2 journeys a day for each vehicle. Thus to extend the above number to dualmode, the appropriate ratio of vehicles to pallets is more like half of the cab departures, or 11.5.

This would not necessarily directly apply to a real dualmode system. There are a number of factors that differ between some proposed systems and the Gothenburg simulations. For instance, the simulation contemplated a system speed of 21.6 miles per hour (36 kilometers per hour). Many proposed dualmode systems posit speeds of 60 to over 100 hundred miles per hour. To first order, leaving everything else the same, the ratio would scale with system speed. Thus an average of 80 mph would suggest an increase in the ratio by approximately four. On the other hand, the average distance traveled in the Gothernburg study is 3.78 miles (6.3 kilometers); my guess is that in the United States this would be significantly higher, thus tending to reduce the ratio.

Another factor that must be considered is the so-called "empty pallet" problem. Data for Los Angeles freeways (1998 Traffic Volumes on the California State Freeway Systems, State of California, Business, Transportation and Housing Agency, Department of Transportation, June 1999), provides that the split, or D factor (i.e., the percentage going in the peak direction during the peak hour), is typically in the in the mid 50 percent range and only occasionally rising to the low sixties. If we assume that the demand is represented by a 55 percent split, then an ideal solution would require only an additional 10 percent of empties travelling in the non-peak direction; resulting in approximately 91 percent of the travelling pallets providing revenue. Undoubtedly, real operation involving depots, waiting times and the like would reduce this quantity.

The data generated in the Gothenburg study represented a typical demand matrix representative of PRT system wherein traffic is mostly in one direction. In a typical dualmode environment, the above discussion underscores that the traffic is decidedly not simply one-way. Thus one would reasonably suspect that the utilization of pallets in a dualmode system would be significantly higher than would be suggested by an extrapolation from PRT data.

Total Cost

I don't know what the correct ratio is - that must await a comprehensive simulation. But I feel confident in expressing the opinion that it is quite considerably higher than two. As stated previously, this ratio will have a profound impact on the comparative cost. Thus it may be a tad premature to characterize palleted dualmode as more expensive, or at least grossly so.

Some have suggested that the extra cost of outfitting a street-legal vehicle to transit the guideway is not really part of the system cost (i.e. that these should be assumed by the vehicle owner). Others have suggested that a separate entity assume the cost and operational responsibility for the pallets. This is patent nonsense. The system is the system; and the cost is the cost. It will be paid for by the taxpayer/user no matter in what form. No creative financing is going to reduce it. It may make it more palatable (please pardon the pun); but it will not reduce it.

Must Be Available To All

I also quarrel with Mr. Arthur's statement that the initial service can be ". . . targeted to the Neimen Marcus/BMW/Lexus crowd." As he himself suggests, there will be substantial governmental involvement in this. No government, and certainly no politician, will allow the perception that they are creating a system only for the very rich. It must be available to all, even the occasional user. This only reinforces the argument that there must be only a minimum of additional apparatus required for a street vehicle to use the system.


There are surely additional issues that bear on this question, but I believe the central point has been identified and, in my opinion, established. At least initially, dualmode systems must be palleted. Moreover, it remains to be established that a true dualmode system is optimum.


Last modified: December 12, 2000