Overcoming J.E. Anderson's Arguments Against Dualmode Transportation

by Tad Winiecki

Most of J. Edward Anderson's arguments against dual mode can be overcome in a PRT/palletized dual mode system with the following ten characteristics:

1. It is a palletized dual mode system, with bogies which stay on the guideways and are owned by the PRT system . (There are no pallets, just bogies with means for attaching to street vehicles). The bogies can operate on the guideways empty or carrying cargo containers or street vehicles. The bogies are longer than the street vehicles so that when bogies bump each other the street vehicles don't bump.

2. Dual mode is an addition to the PRT system. There is a requirement that the dual mode vehicles (captive bogie plus street vehicle) not exceed the size, weight or aerodynamic drag of the largest, heaviest PRT vehicles on the system.

3. The vehicles which operate on the street are owned by the users who are responsible for maintenance of their vehicles.

4. The interfaces between bogie and street vehicle are limited to mechanical and communications, not power or fluids. The bogie locks onto the street vehicle structure, not its tires. There are no street vehicle/guideway interfaces.

5. The street vehicles enter and leave the system at suburban PRT stations (ground level, not elevated) with "landings".

6. Passengers may enter or leave the vehicles at any PRT station, but if they leave the vehicle on the guideway, the vehicle owner will have to pay valet parking rates for the bogie to take the street vehicle to a garage.

7. The bogies can detach from the street vehicles in a garage and leave the street vehicles there.

8. If a street vehicle is in a garage, the owner will have to go to a PRT station and enter his card in the card reader (or equivalent communications technology) to ask for his vehicle to be brought back. This will take a while.

9. The street vehicles have retractable wheels and computers with vehicle health monitoring and communications functions. The wheels retract like aircraft landing gear to reduce vehicle size and aerodynamic drag.

10. Mating/demating operation:

a. The street vehicle enters a landing and aligns itself to the guideway.

b. The street vehicle communicates to the bogie computer (by radio, infrared or other technology) the owner/vehicle identification, desired destination station, and vehicle health status (battery charge, doors locked, etc.).

c. The bogie computer communicates to the central PRT routing/billing computer for confirmation of readiness to accept the street vehicle into the system.

d. The central computer accepts the street vehicle and gives the bogie routing information.

e. The bogie moves into position to mate with the street vehicle and communicates readiness for mating.

f. The bogie locks on and the street vehicle retracts its wheels. The computers confirm that the doors and street vehicle suspension are locked. (The sequence of e and f is slightly different for supported and suspended PRT systems).

g. The bogie goes to the destination station in the same way as any other PRT vehicle.

h. After demating in a landing the street vehicle backs away from the landing and turns onto the street. (In a suspended system the street vehicle wouldn't have to back away.)

i. Street vehicle failure responses:

Failure - wheels won't retract on landing. Response - demate from bogie and drive to maintenance facility

Failure - wheels won't deploy on landing. Response - bogie take street vehicle to maintenance facility on guideway.

Failure - suspension stuck part way down so mate or demate can't be accomplished. Response - send emergency repair team to clear landing.

Assuming a dualmode system with the above characteristics, here are some specific answers to Anderson's arguments (A1 throught A12)

A1: 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 any period of time and may be in any condition.

Answer - In PRT/palletized dual mode, the bogie is part of the PRT system and maintained by the same organization that maintains the PRT bogies. The bogie confirms the condition of the street vehicle by communicating with the street vehicle computer, which must have status monitoring functions and sensors.

A2:. 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.

Answer - The bogie confirms the condition of the street vehicle by communicating with the street vehicle computer, which must have status monitoring functions and sensors. The important street vehicle status information includes street vehicle identity, desired destination, battery charge, door lock, suspension position and locks. This status communicaton can be done in much less than a second with no personnel. The only time personnel would be needed is for a suspension failure (which could be caused by a street vehicle electrical failure) which blocks a landing. If the street vehicle fails the status check the driver will drive it to a maintance station for repair.

A3:. 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 can have its guideways farther apart than in a PRT network 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.

Answer - Adding dual mode landings to PRT stations will increase station throughput for those stations because there will be more places for people to enter the system. For those stations without dual mode landings, it shouldn't take any longer to leave a dual mode street vehicle than a PRT vehicle. Calling a dual mode vehicle from a garage to a PRT station will mean a wait for the dual mode vehicle user, but it shouldn't reduce station throughput by PRT riders any more than if the dual mode owner took a PRT vehicle instead.

A4: 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.

Answer - The landing ramp area for dual mode street vehicles could double the land used for a ground-level PRT station. Additional guideway including merge/diverge sections would be required, also. Inspection stations are not needed as explained previously. Adding a dual mode landing would not double the cost of a ground-level PRT station, and would not double the throughput, either, but I think it would add enough market share and political pressure to expand the system that it would be cost-effective.

A5: 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.

Answer - Make a requirement that dual mode not be the design driver and this argument goes away. Limit the weight, size, and aerodynamic drag of the captive bogie/street vehicle combination to be less than the largest PRT vehicle. This implies that the street vehicle will have a much smaller payload than the largest PRT vehicle. For a PRT system sized to meet the minimum requirements of the Americans with Disabilities Act, the street vehicle would only carry one person, the driver, very little luggage space, and it would have short range if battery powered. The wheels would have to retract to reduce width and aerodynamic drag in a high speed system.

A6: 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.

Answer - The bogie will cost the same as PRT bogies and be almost identical, except for communication and mechanical interfaces to the street vehicle or cargo container. The street vehicles will be individually owned. The cost will depend on how well it is designed and manufactured and the production quantities. If several manufacturers build the street vehicles to standard requirements and they sell a large quantity the cost will be about the same as small car

A7: 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.

Answer - I advocate a PRT/ palletized dual mode system. The line throughput would be diminished by having empty bogies on the line and the dual mode vehicles would only carry one person each. I believe that the fares from additional market share would pay for more stations and guideways so that the overall system throughput would be increased.

A8:. 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.

Answer - Don't put dual mode landings in central business districts. Let the dual mode drivers get off at a PRT station and send the street vehicles to a garage where the bogies would leave them until needed again. Usually PRT stations in CBDs would be elevated and it would not be practical to add dual mode landings. The main market for dual mode is suburb to suburb, not to or from CBDs.

A9: 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.

Answer - Since the dual mode vehicle is smaller than the largest PRT vehicle, it shouldn't use as much space to park as a car. This implies that drivers never go to the garage with their vehicles and the street vehicle doors never open in the garage. The garage will require a lot of merge/diverge guideway sections because each individually-owned street vehicle must be capable of individual retrieval, and special racks will be required to hold the street vehicles if the wheels remain retracted in the garage. Since the PRT system is quick, the garages don't have to be located downtown to have reasonably quick retrieval time for a street vehicle to a downtown PRT station. Anderson's statement "The retrieval time would be substantially greater than the wait time for the next available PRT vehicle..." is true, but the time may be made up by not having to wait for another transportation mode (such as a bus) at a suburban station.

A10: 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 in PRT. 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.

Answer - While additional bogies are required to add palletized dual mode to the PRT system, they can be used for carrying cargo containers, also. Thus their utilization can be nearly the same as the PRT vehicles. The street vehicles would be individually owned and what Anderson says about utilization applies. Palletized dual mode could still be the most economical travel mode for many people in lower population density areas if the cost of travel time is considered. Palletized dual mode may take some people where they want to go significantly quicker than anything else including PRT/station cars or private automobiles and motorcycles on surface streets.

A11: 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 as a novelty, but hardly enough to support the construction of the 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.

Answer - I would like to believe that the economics of the PRT system would be so good that profits will cause its expansion without "the tax burden required to build the guideways". The government should not subsidize individually-owned dual mode vehicles unless they are subsidizing other battery-powered vehicles. (I assume the street vehicles are battery powered). The street vehicles should be affordable for families which can afford to operate two vehicles now. Politics is always a consideration in transit systems. I believe palletized dual mode can be politically advantageous to a PRT system if dual mode landings are only put at ground-level PRT stations and if the parking garages and other additions necessary are profitable. Begin the system by putting ground-level PRT/dual mode stations at shopping malls, schools, and large apartment complexes. Put PRT stations at large office buildings, jails, airports, train stations, intercity bus stations, stadiums, hospitals. Build guideways in a grid system between the stations. If a station becomes congested, build more guideway and more stations closer to where people want to go. Add station cars to stations with enough room to park them. Provide for people to park bicycles at PRT stations and carry them on PRT vehicles.

The political advantage comes from appealing to a larger population segment in a wider geographic area (including rich people who are in a hurry). If dual mode landings separate from PRT stations are not allowed, dual mode congestion at a PRT station will lead to political pressure for more ground-level PRT stations and guideways. This will help to expand the PRT system faster and could result in lower fares and better service for poor PRT riders. The advantage of adding this type of dual mode is increased market share in the suburbs which will allow the economical expansion of the PRT system to areas with lower population density. The dual-mode users will be those people whose destinations are approximately 0.5 to 10 km from PRT stations with dual-mode landings and who will save time by using the system compared to using their car or motorcycle instead.

A12:. 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.

Answer - The street vehicles in my palletized dual mode system are like tiny station cars with computers and retractable wheels. For some people, especially in inclement weather, the convenience of driving from their garage to the dual mode landing, being carried on the PRT guideway over all the traffic and then driving several kilometers to their workplace and parking at a recharger without leaving their vehicle is better than the alternative. For the commuter, the ordinary automobile/PRT/station car alternative involves two vehicle transfers, finding a parking space at the PRT station, and renting a station car for approximately ten hours per workday. There may be a parking fee and uncertainty in parking place availability at the PRT station and uncertainty in obtaining a station car and uncertainty in the reliability of the station car. The utilization rate for the station cars may be no higher than the dual mode street vehicles, so the cost of renting one each workday may be just as much as the costs for the dual mode vehicle.

Contact Information: winiecki@pacifier.com


Last modified: September 30, 1998