Problems of Dual Mode Transportation


J. Edward Anderson , PhD, P. E.

Readers who are not familiar with the various dual mode concepts that are currently being worked on are urged to visit the Dualmode  page and browse the various system descriptions available there.  Another discussion of the merits of the dual mode approach is that by Joe Palen which is one of more than 100 contributions posted at the Dualmode Debate page.

1. Dual Mode Operation

Dual Mode (DM) transportation is a system, a DMS, in which the vehicles can run either on normal streets or on a guideway under fully automatic control. While its vehicles are on a network of guideways, a DMS operates exactly like a PRT system, i.e., the stations are all off line and all trips on the network are nonstop. A DMS may have stations similar to PRT stations , or they may be designed so that all vehicles leave the guideway before stopping, thus saving the cost of stations.

DM has the advantages for auto drivers that 1) the same vehicle may be taken from home to any destination, similar to driving an automobile, and 2) that congestion may be avoided because of the shorter headway and managed flow possible on the guideway. In many respects, DM is much like the system envisioned by advocates of Automated Highway Systems , except that special narrower guideways could be used.

DM does not come, however, without disadvantages, which must be overcome if it is to be practical. These disadvantages are discussed in the following paragraphs with the hope that further exchange of ideas between serious system designers can result in optimum system characteristics.

2. Variable Vehicle Condition

There must be an organization, private or public, assigned the task of maintaining safety and on-time performance of the automated guideway portion of a DM system. Such an organization must assume that a DM vehicle (DMV) requesting permission to enter the automated guideway may have been off the guideway for some period of time and may be in an unsafe condition.

3. Inspection at Entry Point

To insure safe operation on the automated guideway, off-guideway inspection stations are needed to determine if DMVs are fit to be permitted to enter the automated guideway. After leaving such an inspection station, the DMV must go directly to the guideway, for if it is permitted to drive around the streets for a period of time there is no way of knowing for certain, for the benefit of all users, that that vehicle is still fit to use the guideway. Thus there must be an inspection station at each entry point to the guideway. While it may be possible to automated much of the inspection procedure, it is difficult to see how such a station can be operated with no personnel at all, which increases operating costs.

4. Reduced Station Throughput

The time required for DMV inspection is key to determining station throughput. For practical purposes, one must envision an inspection station at ground level at the entry point to the guideway, with vehicles leaving it accelerating directly onto the guideway, usually up hill, as the guideway will generally be elevated. Study of PRT systems show that station-capacity requirements vary in the range from a few hundred passengers an hour up to perhaps 1800 passengers per hour, or from one person every 10 to 15 seconds to one every 2 seconds. In a comprehensive traffic survey of the Twin Cities Metropolitan Area of Minnesota in 1990, the Metropolitan Council found that the average auto occupancy in the rush periods is 1.08 persons per automobile. So for practical purposes, the above person headways are also vehicle headways.

A organization responsible for the safety and delay minimization of everyone using the automated guideway must perform an adequate inspection of every vehicle entering the guideway. Throughput of a DM entry point is thus determined by the mean time of such an inspection. Proponents of DM should therefore concentrate on how to minimize the time required for inspection. If that time cannot be proven to be in the range of only a few seconds, a DM system will have to have more entry points than a captive-vehicle PRT system carrying the same demand.

Some DM proponents argue, however, that a DM network (see Los Angeles and Seattle examples) can have its guideways farther apart than in a PRT network (see Los Angeles example ) because people will be able to drive to stations rather than walk. But, for a given area coverage and given ridership, the required station capacity is inversely proportional to the number of stations. Thus, a sparser DM network will require more frequent entry and exit points or higher capacity per entry point than a PRT system, yet the required time for inspection, albeit, unknown, is likely to cause DM entry points to have substantially less capacity than required of and practical at PRT stations.

5. Complex, Expensive Stations

An alternative way to design a DMS is to have PRT-like stations for persons who wish to use them, with on-off ramps for DMVs. This would be a way to increase the capacity of a DMS to a significant level. In the early 1970s, both Ford Motor Company and General Motors designed such stations and displayed models at a Conference on Dual Mode Transportation sponsored by the Transportation Research Board in Washington, D. C. These stations were complex and expensive, as can be envisioned by the requirement for an inspection station, on-and-off ramps, a PRT off-line guideway, and a PRT station.

6. Wide, Expensive Guideway

An advantage of PRT envisioned by many proponents is the possibility of designing a compact, minimally obtrusive guideway. Several such guideways have been designed, beginning with Ed Haltom's Monocab, Jack Irving's Aerospace Corporation PRT System , and Klaus Becker's Cabinentaxi . Early DM systems were assumed to have auto-like vehicles with one set of wheels that run on either the streets or the guideway. The designers of such systems assumed that the automated guideway would be like an elevated roadway. Unfortunately, such guideways turned out to be too visually intrusive, too expensive, and were not practical under winter conditions.

As a result, DMS designers have proposed either palletized DM , in which small automobiles would be clamped to captive pallets, or that the DMV would have a separate suspension system for operation on the guideway, thus permitting design of a compact guideway. In either of these cases, the DMV or combination of auto and pallet has more parts and is clearly more complex and heavier than a PRT vehicle captive to a guideway, which implies a more expensive guideway because guideway weight increases in direct proportion to vehicle weight. The alternative is the auto-type DMV operating on essentially an elevated roadway, thus accepting the disadvantages of such a guideway.

7. Expensive Vehicles

A DMV must be designed to operate on ordinary streets as well as on a system of guideways. To operate on ordinary streets it must be designed to withstand side and roll-over collisions and the suspension system must be designed for rough and pot-holed roads. These requirements add weight and expense not needed in a properly designed captive-vehicle PRT vehicle. To reduce guideway weight and cost, some DMV designs use a different suspension system on the guideway than on streets, but this further complicates and increases the weight of the vehicle.

8. Reduced Line Throughput

If the DMS uses auto-type vehicles, with propulsion and braking through wheels running on a normal roadway, the variability of friction on a wet [1] guideway will result in reduced minimum headway, as compared with the headways possible if propulsion and primary braking are performed through direct electromagnetic action between guideway and vehicle, for example with linear-induction or linear-synchronous motors. The improvement in capacity with linear motors is in the range of two to four and is an important factor in the economics of the system. Thus, in pure DMS, more guideways would be needed than in an optimally designed PRT system, not fewer. In palletized DM , throughput may not be diminished.

9. Downtown Congestion Beyond System Control

In a central business district (CBD), DMVs would leave the automated guideway and descend into the street system where the DMS cannot control congestion. Thus, DMVs may be blocked from leaving the guideway and cause vehicles behind to have to be rerouted to other off-ramps, thus increasing flow on what will probably be the busiest part of the network. With the same number of people wishing to enter the CBD as before, a DMS would decrease congestion only if it is designed with PRT-type stations in the CBD and if a significant portion of the people going to the CBD choose to stop and leave their vehicles at the PRT-type stations (see example of the dual mode station design for a dense urban center).

10. Downtown Vehicle Storage and Retrieval

Private DMVs will have to be stored in parking structures similar to those designed to store conventional automobiles. The space required for storage will be the same as required for conventional automobiles because any vehicle must be in a position to be removed from storage at any time. On the other hand, in captive-vehicle PRT, storage can be much more compact because it would not be necessary to remove any vehicle, only the first in line. The storage volume required for PRT vehicles is only a little greater than the volume of vehicles stored. DMV retrieval will be similar to that required with automobiles, except that each vehicle will have to be inspected before reentering the automated guideway. An alternative would be to have special DMV-storage facilities under control of the DMS so that the patron would enter such a facility to reclaim his vehicle. The retrieval time would be substantially greater than the wait time for the next available PRT vehicle in a PRT system (see illustration of a dual mode station design that includes parking and storage facilities).

11. Vehicle Usage and Amortization

In a DMS, each vehicle is the private property of an individual, and would be used only by that individual. That is the basic attraction of DM. Thus, the number of vehicles required in a fleet would not be diminished over that in use today. With captive-vehicle PRT, several estimates show that the number of vehicles required to move a given number of people is reduced by a factor between about six and ten, simply because each vehicle is immediately available for another trip as soon as one is finished. PRT vehicles will thus travel several times as many economically productive miles per year as DMVs (not six to ten times because of unavoidable deadheading), which means that, for the same guideway and vehicle costs, the total cost of capital and operation per passenger per unit of distance will be less by the same factor.

Consequently, for the same unit cost, DM guideways and vehicles would have to be cheaper than the corresponding elements of a PRT system in the same proportion. Yet, as has been shown, DM guideways and vehicles will be more expensive than PRT vehicles. If a DMS has no PRT-type stations, its economics is improved, but with the disadvantage that it is less available to people who cannot drive automobiles.

12. Elitist Solution

DMVs have all of the components needed to run on streets plus the automatic control system and possibly other components needed to operate on an automated guideway. They will thus, as mentioned, be more expensive than PRT vehicles, and will be available only to the wealthier segment of the population. Moreover, how much guideway must there be before even wealthy persons decide to purchase a DMV? At first, a few may purchase them as a novelty, but hardly enough to justify the construction of an extensive guideway. For political reasons, it would seem necessary for a government to subsidize the purchase of DMVs, so that they would be available to all who want them. For many years, many people would regard the DMS as an irrelevant novelty until there is enough guideway to serve a significant portion of their trips. Yet, in a democratic society, these people would be required to bear the tax burden required to build the guideways. DMS proponents need to describe in detail in a practical, understandable, and acceptable way how the system could expand into a viable alternative transportation mode.

13. The Alternative: Captive-Vehicle PRT + Small Electric Automobiles

An alternative to DM is a PRT network using captive vehicles. Such systems can start small, being used at first in specific major activity centers, and would be available to anyone regardless of ability to drive a car, just like a conventional transit system. While ordinary automobiles could be used to travel from home to origin station, the potential rider must wonder what to do at the other end of the trip. That problem could be solved by providing a fleet of small electric automobiles, now called "station cars," that could be rented for those needing to ride to or from a PRT station. Such vehicles need only a range of perhaps 10 to 20 miles, which is completely practical for battery-powered vehicles.

14. Conclusion

Dual Mode transportation is an appealing concept, but on close examination has a series of fundamental problems that do not seem amenable to technical solution. In the author's opinion, it is not a practical alternative. A practical automated guideway system (PRT) is a transit system, not a direct substitute for automobiles. It can take many of the burdens off the auto system, but would not replace it. Rather, PRT can augment and compliment the highway system if, for example, parking facilities on the periphery of the CBD are connected to the stations of a PRT circulator . Properly designed, it would consume far less land and would contribute little noise and air pollution. PRT thus has the potential of making the central city area more livable, giving it the potential of contributing to the solution of serious social and environmental problems.

As mentioned, PRT is a supplement to the auto system, not a replace- ment. It provides a level of service far superior to conventional-transit service, and is thus capable of attracting many more trips. PRT can genuinely contribute to reduction in congestion and air pollution. It provides necessary transit service in a humane society to all people including young, old and disabled regardless of ability to drive a car.


1. If propulsion and braking are through wheels, it is necessary to heat the running surface in conditions in which ice may form, so that the running surface is at worst wet, not icy.

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Last modified: September 20, 2002