Overcoming J.E. Anderson's Arguments Against
Dualmode Transportation
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.
Last modified: September 30, 1998