Problems of Dual Mode Transportation
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
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
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
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  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.
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
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,
Last modified: September 20, 2002