A Proposed Evolutionary Transition Path to an Advanced Dualmode System

Excerpts from a 1994 paper by Larry Stern, edited and updated by Jerry Schneider

Editor's Note: While this paper was written more than a decade ago, the need to create millions of new jobs in the U.S. is once again a major national issue and the urban transportation problems addressed are much worse now than they were a decade ago. The evolutionary approach that is suggested is a sensible and realistic approach to dealing with both of those problems and their many associated problems. The evolutionary approach suggested is deemed to be very compatible with the interests and needs of the auto industry which is another important attribute given its importance to the U.S. economy. It   also offers some hope for reducing the nation's dependency on foreign sources of oil. My updates are displayed in red text.

Creating a New Market

Our society needs an entirely new mode of transport and a new industry to build it. This new mode should be dependent on higher technology and should address visible requirements left unsatisfied by both traditional rail and the automobile: pollution, traffic congestion, efficiency and convenience [Numark, 1993]. To be viable it must also be affordable and present significant advantages to the user.

Improvements made with current technologies are unlikely to lead to satisfactory solutions. If we solve traffic congestion problems associated with the automobile, our pollution problems will increase. If we increase the convenience of mass transit (by extending the hours of operation for example) efficiency drops and becomes unacceptable. To avoid the pitfalls of existing technology, our new form of transportation must not be subject to these same tradeoffs.

It is neither likely nor reasonable to expect such a system to replace existing modes of transportation on any immediate basis. The financial and systemic upheaval implied would simply not be acceptable. The target should therefore be a system which initially augments and over time replaces existing forms of transportation. This implies that system characteristics must heavily overlap with current forms of transport we seek to augment and ultimately replace.

To be useful to our society, the development of a new mode must be subject to an additional constraint. It must create jobs in a timely manner, not to be deferred to the end of a long and uncertain technology development phase. It should:

1. Create some jobs for the near term.

2. Project and provide a vision for jobs in the mid term.

3. Provide an upgrade/growth path for the long term advanced system.

The benefits we may expect from implementing such a development plan should be no less than:

  • A successful development plan should make full use of technology, knowledge and eventually manufacturing technology from the aerospace and automotive sector, transferring it as appropriate.
  • Create a whole new paradigm in public transportation and create the first truly successful public transportation system in the cities.
  • Significantly reduce transportation congestion while also cleaning up the air in major cities.

Characteristics of an Improved System

The defining characteristics of an advanced transportation system can be built up from the most advantageous features of the automobile and mass transit combined with solutions or improvements to known deficiencies. A rank ordered list of these characteristics is offered below. The more of these elements the new system embodies and the more it satisfies items towards the top of the list, the more likely it is to become a successful augmentation or replacement for either public or private transport.

  • The automobile is a de-facto public/private partnership. Public money is used to buy and maintain the roads, while private money is used to buy and maintain a majority of the vehicles in use. The offers enormous leverage on public money. Mass transit does not attract similar financial resources. The new system must be designed to be a public/private financial partnership.
  • The creation of mass transit does not occur in small increments. It is frequently created in increments of multiple millions of dollars and more likely billions. Automobiles/roads can be bought so to speak 'by the yard'. The new system should be capable of modular, incremental acquisition and it must be scalable. It should offer an increasing degree of efficiency and remain economical where traffic density reaches a certain level.
  • The automobile system supports mixed use in that cargo and passengers are transported alternately on its pathways with minimal coordination or advance global planning. Mass transit most often does not conveniently do so (except for heavy rail). The new system must preserve the mixed use characteristic of the internal combustion vehicle.
  • The automobile with its current internal combustion engine causes serious air pollution. The new system should use zero (or low) emissions vehicles.
  • Automobile usage is scheduled by the user 24 hours per day, 7 days a week. Mass transportation is scheduled by the public authority and cannot reasonably be made user scheduled. The new system should be completely user scheduled.
  • Service flexibility is maximal with the automobile and minimal for conventional rail based mass transit. Once the basic costs are covered, the incremental cost of travel by automobile is relatively small. This allows the user to make instantaneous choices and tradeoffs between a variety of cost and service levels.
  • The concept of being in a hurry, or priority service, does not apply to mass transit at any price. It is a legitimate (and sometimes extralegal) option with the automobile. The new system should implement a cost and function-based priority scheme.
  • The automobile offers wide area intra-urban, suburban and interurban service. Mass transit typically does not offer interurban service, only (incomplete) connectivity. The new system should offer metropolitan-region scale service.
  • The automobile stops only at the end points of interest for the user. By contrast, mass transit stops for the entire population of a train (often several hundred people). To cover like distances in the same amount of time, mass transit speeds must be considerably higher. Even then for certain distances it may simply not be possible to achieve comparable transit times between the car and mass transit. The new system should minimize transit time.
  • A road system for the automobile including rights-of-way to every doghouse already exists. Light and heavy rail currently do not offer that kind of coverage and possible extensions create significant land use issues. The new system should use existing and avoid creation of multiple, incompatible, parallel right of ways. Improvements required for use should be minimal.
  • Use of the automobile avoids vehicle changes required by mass transit for the purpose of delivering the passenger to a given destination. This insulates the users against weather discomforts, provides a convenient small cargo capability and secures the rider's possessions against accidental loss (umbrellas and small packages are typically at risk on mass transit).
  • The automobile leaves the user in relatively closer proximity to the ultimate destination when compared to mass transit. It is point-to-point or more nearly so than anything else.
  • Passengers like the relative security and privacy of the automobile as compared to mass transportation. For a new system this is desirable, at least on an individual request basis.
  • Climate selection is individual for the automobile, it is not available for mass transit.
  • The automobile has to be driven and requires personal concentration from point-to-point. Mass transit does not require user action or attention except at its end points. Personal concentration is a user inconvenience which should be done away with, or at least minimized.
  • The automobile requires personal attention and a secure location for parking when not in use. This is an inconvenience and in any new system should be eliminated. Parking should be automatic and should resemble mass transit in its convenience of ridding the user of the problem.
  • One gets to meet fellow travelers on mass transit, not so with the automobile. This is an advantage or disadvantage depending on point-of-view and personal preferences.
  • Low population density in suburban areas and the lure of the car have kept public transit systems from being efficient.
  • Low traffic makes mass transit inefficient or expensive after hours. The new system should be available and efficient around the clock.

The Choices

Characteristics of existing or proposed technologies are shown in the table below.

Characteristic Mass Transit* Auto ITS-AHS** PRT
Public/Private Financial Partnership No Yes Yes Basic - No

Extended - Yes

Modular Acquisition No Yes Yes Yes
Mixed Use No Yes Yes Yes
Zero Emissions Yes Electric/hybrid N/A Yes
User Scheduled No Yes Yes Yes
Variety of Cost and Service Levels No Yes Yes Basic - ?

Extended - Yes

Priority Capability No Yes Yes Yes
Minimize Transit Time No Yes Yes Yes
Right-of-Ways No Yes Yes ?/Yes
No Vehicle Changes No Yes Yes Yes
Point-to-Point No Yes Yes Yes
Security and Privacy ? Yes Yes Yes
Climate Control No Yes Yes Yes
No Driver Required Yes No Yes Partial
Automated Parking Yes No Yes Basic - No

Extended - Yes

Efficient Yes ? Yes Yes

* Mass Transit = Heavy and light rail transit and bus
** ITS-AHS - Intelligent Transportation System - Automated Highway System (smart cars, smart highways)

Close examination of the table shows that none of the existing modes of mass transportation hold the potential of being significantly improved through the use of technology. They exhibit limitations inherent in design choices selected and optimized in the previous century as required by the automation and technology level available at that time.

The technology that most nearly matches all the requirements is the Automated Highway System (AHS) concept which is based on electrical (or other zero emissions) technology. Unfortunately, the development of this technology has recently been largely defunded by the U.S. Congress.

Personal Rapid Transit (PRT) is an alternative that also matches many of these requirements [Pope, 1992]. While it is a poorer match, it seems to be technologically ripe and can perhaps be extended to become a precursor of a full AHS. Like AHS, PRT will also require a more sophisticated technology base than current rail, one that better matches available resources.

PRT may also offer advantages towards implementing a zero emissions vehicle as it offers a more supportive environment with easy adaptation to roadway supplied power. An advanced PRT and/or an extended hybrid version of it, are worthy of serious consideration as a technology of choice for the near and medium term [IEEP, 1993].

The question naturally arises as to why it has never been built ( see commentary on this point ). There are several answers to this question. One part of the answer lies in technology. Automation is the single major technology requirement for PRT. Significant affordable automation has only become economical in the past 10-15 years. Technologists have in fact experimented with PRT in the past but such experiments were stopped by limited vision and poor design. However, more recently, the Raytheon Company developed a PRT technology ( PRT 2000 ) that passed an an extensive test program but was terminated by the Raytheon Company in 1999 for a variety of reasons. Currently (early 2007), an initial line for the ULTra PRT system is being built at Heathrow Airport in the U.K and Vectus PRT is building a test track in Uppsala, Sweden.

In the past, technologists limited themselves to a vision which replaced metro and light rail service with PRT. A glance at the table will show that such a vision deals with only about half the criteria and crucially misses out on public/private financing. Engineering analyses of the test systems showed that it was technically possible to build the system and it would indeed function as designed [Advanced Transit Association ,1989]. The system would however be uneconomical because of the large number of vehicles which had to be purchased and maintained [LaFargue, 1992]. If one looks at the automobile it becomes immediately clear that differently financed, a large number of vehicles are in fact purchased and maintained at budgets which are beyond the comprehension of public transit designers. However in order to reach for this financing one cannot limit the goals to simple replacement of the rail elements of the transportation system. One has to additionally replace or at least augment portions of the transportation system which are serviced by cars and trucks. This was no longer possible in the narrow context of a rail replacement PRT.

Another part of the answer lies in land use. No one is ready to envision adding to existing right-of-ways a complete duplicate to the roadway system. It would cost too much and it would take too long. To the extent that PRT was envisioned as a grid laid out on a quarter-mile pitch, its land use requirements become oppressive and it is unlikely to be built on any kind of a large scale. If, on the other hand, one assumes a grid which is more sparse, much of the advantage if PRT over the rail it would be replacing disappears.

Again the possibilities are limited by narrow vision. If instead of a classical PRT one assumes a drive-on, drive-off hybrid system, the land use issue becomes easier. Some of the PRT paths would now be normal roads and thereby bypass the question of additional right-of-ways. The standard technological response is to point out the additional technical difficulties created by such a system with the introduction of poorly maintained or otherwise unfit vehicles. The reply to that argument has to be that such a system has been created and it works; just look out the window at the nearest street.

A Proposed Transition Path - A Hybrid PRT System

It is easier to describe the development sequence and functionality of a system which meets economic development criteria and results in a superior transportation medium than it is to describe those paths which lead to failure in the creation of such a system. In the interests of brevity I'll do the former.


PRT has a viable market as a niche system catering to campus and small community transportation systems. The vehicles are electrically powered and are consequently zero emissions. Several of such systems are being developed at this time. Examples include Skyweb Express, ULTra and Vectus PRT. The ULTra system has reached the demonstration stage, prototypes have been built by Skyweb Express and MicroRail.  A viable but limited business can be immediately created to build such systems. The enterprise would support a small engineering department as is characteristic of the typical commercial manufacturing enterprise. A manufacturing plant of this nature could be set-up and in production in a matter of a year or two [Higashi, 1992].


As a next step one would divert some part of the cash flow of the near-term enterprise and add government funding to create R&D for augmentation of the product line. This augmentation would seek to take the design out of niche markets into a full suburban transportation system. Design for the mid-term system could proceed in parallel with near-term design and production. Mid-term product should become available in two to five years.

The mid-term product should be compatible with the earlier niche system and should additionally allow vehicles to be driven on-and-off the PRT guideway. In this concept the PRT guideway becomes analogous to a freeway or expressway. Individual vehicles may be owned and maintained either by a public transit authority or by individuals. Public vehicles may be rented for the duration of a single trip or for a more extended period of time. The grid density and weight rating is variable but sparse compared to an optimum PRT. Some guideways- which service residential areas only--are limited to light weight passenger vehicles. Grid density varies with the type of housing in the area. Multiple lane traffic is available for heavy traffic areas and several types of vehicle coexist in the system.

There are several examples of companies around the world that are developing such systems, current referred to as dualmode systems. An overview of the current status of several of these dualmode systems is available on-line. One company, called MegaRail, is pursuing an evolutionary approach that is very similar to that defined in this paper. As expressed by Kirston Henderson, company president:

"At MegaRail, we have adopted a strategy to build guideways first as alternates to light rail that is likely to happen and funding will is being made available to build. Once those guideways are in place, we can add PRT capability intermingled with the GRT transit that we start with. At a later date, we can add dualmode entry/exit ramps at high-traffic points as people to start purchasing our auto-like, affordable dualmode vehicles. For MegaRail lines, we would first offer car ferry service to allow users to immediately use the system with conventional cars. Cargo services will also will be offered using carriers operating on the same or separate, special-purpose, heavier duty facilities.

The above evolutionary approach is probably the only one that will ever succeed in getting PRT and dualmode service in place and operating. We consider ourselves fortunate that we have developed an very low cost, multifunctional, system design that will enable us to succeed with the above approach. When I examine all of the other proposed systems out there, I don't see any system that has the essential configuration required to follow this evolutionary path."

Rental vehicles

The public-vehicle single trip rental is analogous to current metro, rail or bus usage. The vehicle becomes available to the user at a PRT station and is occupied to the destination, which must be another PRT station. Once the vehicle is vacated, it becomes available to any other user as assigned by the system.

In the extended duration rental, the public-vehicle is analogous to a rental car. It can be acquired at any PRT station and must eventually be returned to a PRT station. In the intervening interval it may however depart the PRT system and be driven along roads like a rental car. It may also re-enter the PRT system and travel within it to any station from where it can exist the system and complete its trip along a normal road. The user must drive the car during road travel. The vehicle is completely automatic for the PRT guideway portions of the trip.

When regulated and equipped with a driver these vehicles may also act as suburban cabs.

The individually owned vehicle is analogous to a normal current day car except it is electrically powered and carries significant automation-onboard. It can travel the PRT guideway, or normal roads to the extent of its battery storage capability. The vehicle can be recharged by traveling the guideway or otherwise.


The long term product adds incremental automation to the mid-term vehicle. It becomes available in five to seven years and is a precursor of a full AHS. The principal distinction to AHS is the degree of automation and the all-electric limitation of the advanced PRT system

The advanced system comprises the mid-term PRT augmented with teleoperation. A remote operator/driver can supervise the vehicle during its off guideway excursions. Teleoperator control would be optional at any time and may address the drunk-driver and remote parking problems. Teleoperation is accomplished through existing radio links and video technology. Military technology in existence, when commercialized, should allow a single operator, with the aid of vehicle automation, to safely control somewhere between six and eight vehicles during their off-track excursions.

The advanced system should be able to accommodate non-PRT vehicles that have been augmented for guideway operation and offer a seamless transition of the automobile to a partially automated, minimal emissions system, capable of handling six to eight times current day traffic, while using no additional real estate and providing improved traffic flow.

A Thousand Points of Light

A number of authors have noted the difficulty, time and expense involved with creating a new infrastructure for the support of any alternate transportation system. Ultimately the practicality of creating this infrastructure will dominate the implementation of any new mode of transport.

This concept seeks to allow the incremental creation of such a minimalist infrastructure on a market driven basis using a combination of public and private financing. It also seeks to finesse the range limitations imposed by current battery technology [Bindra, 1993] while extracting maximum leverage towards implementing an early form of AHS.

An essential role of the hybrid MID-TERM technology is the creation of a public/private financial partnership. Until vehicles can be driven to and from the homes of individual travelers, no concept of ownership and therefore no public/private partnership can be established. In time, improvements in energy storage may eliminate the technological need for a hybrid architecture [D.O.E., 1992]. Nevertheless a hybrid architecture will remain essential for social/financial reasons until a different paradigm can be found to continue private/public partnership support for the system.

Second, the infrastructure must be susceptible to incremental development. Any system dependent for its functionality on the a priori development of a nationwide network of tracks and charging stations is destined for the ash heap.

The proposal envisions growth and development by "the yard". Over time a thousand cities with congested downtown sections could adopt the NEAR-TERM (PRT) system. As vehicles are produced or upgraded to MID-TERM capability, drivers--and AHS vehicle computers--will learn to traverse gaps between adjacent systems to reach their intended destinations. In these gaps the vehicle would initially be negotiated on battery power under manual control. Eventually manual control would be replaced by the LONG-TERM system architecture.

As traffic and usage increase, the most frequently transited systems will be linked to allow travel over arbitrary distances. As AHS, battery and recharging technologies improve, distance between sections of track may become less relevant. At some point businesses should start seeing cost benefits from automated, point-to-point shipments.

Elimination of late night walks through rain, weather and poorly secured parking lots could be as valuable as commercial point-to-point automation. Automatic parking will increase the distance between the parking lot and the point of usage from the current maximum of about a quarter mile (even that is inconvenient) to about a mile and a half. In an automated system, a two to three minute wait, in secure, well lit, air conditioned comfort would likely be acceptable. At a vehicle speed of no more than thirty miles per hour this exceeds a one and one half mile parking radius. Manhattan users could theoretically park their vehicles across the bridge in Brooklyn.

National adoption of such a system would likely depend on the parallel development of battery technology and AHS. In the same way early telephone technology outgrew manual pegboards, an advanced PRT would outgrow its warts of teleoperation and specialized guideways. The purpose of this development concept is to increase usage and create a critical mass committed to the success of a zero emissions, automated system, before the supporting technology is fully-evolved. Equally crucial, it must not impede growth once the technology does become available.


Most studies conclude that limitations in battery technology and AHS capability limit the immediate development of an electrical transportation system to niche applications. This may not be a necessary conclusion.

Technology is not the dominant issue in the development of an individual vehicle transportation system. State-of-the-practice PRT systems could be used to implement a practical electrical vehicular system as described, and this path may not be unique. Development of an industrial capability and associated employment will be driven by market considerations that are at best tangential to the current state of technology. At issue may be an integrated solution to the transportation problem rather than the specific technology dependence or competitiveness issues insisted on by major car manufacturers and transportation technologists.


The author is grateful to the California Institute for Federal Policy Research, Washington, D.C. for access and opportunities related to the research for this article.


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Lynch, David, CALSTART Consortium, IEEE Spectrum (p. 54), July 1993.

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Project California, Advanced Transportation Systems, Industries and Jobs, presentation to Congress, June 1993.

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Regional Transportation Authority, Regional Transportation Authority Briefing Book 1991, PRT Personal Rapid Transit (One North Dearborn, Suite 1100, Chicago, Ill. 60602).

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This page contains excerpts from Larry Stern's paper that was originally published in the Journal of Advanced Transportation, V 28:1, Spring, 1994, pp 17-28.

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Last modified: March 24, 2007