The dual mode vs. single mode debate is extremely important. Not so much from
the standpoint of the economics of the systems themselves, but because of the vastly
different potential land use patterns each could produce. Dual mode, in this regard is
much more conservative. It promises to continue use of the auto related land use patterns
in the bulk of the urban and suburban areas of deployment. Single mode, on the other hand,
should it become widespread could begin to skew land use patterns away from auto based
patterns in order take advantage of PRT . An example might be
shopping centers with their current form of "island in the middle of the parking
lot". With PRT these shopping centers could be redesigned with PRTs unloading in the
center and the parking lots largely eliminated.
There are quite compelling arguments on both sides. Rather than arguing the ULTIMATE
merits of each, I would suggest that which system will dominate has more to do with which
system can get to an "explosive growth" stage first. I would have to give single
mode a slight edge.
One major reason is that whether one envisions single or dual mode, single mode will
be attempted first. For example, the well thought out RUF system,
in its beginnings, will be single mode. The reason is that in a starter configuration
there is no public vehicle fleet. The public fleet must wait until the system is extensive
enough to justify purchase of dual mode vehicles. The question becomes - how long and how
expensive will it be to carry dual mode design to the stage where the dual mode vehicles
can be sold? What ever expense and length of time is necessary it would be cheaper during
that time to design, operate, and expand a single mode system.
Even the Automated Highway System (AHS)will be single mode in the beginning. It has to
be tested in an isolated single mode environment first. The first real world system must
be an isolated single mode operation. Then comes the, I believe impossible task, of
building sufficient infrastructure while at the same time building the fully automated
fleet. Lastly, as this is written we are on the verge of seeing the first PRT 2000 system
funded. If it should go forward, single mode will definitely be out of the gate first.
I think in the system that Joe Palen advocates, he believes the
time period to develop the dual mode fleet will be relatively short as people watch PRT
vehicles race past them in the freeway corridor paradigm. Maybe he is correct. However,
while any dual mode plan is making headway towards deployment, single mode will be
operating. The explosive growth point for single mode is the point of profitability at the
farebox. Short of that, growth will be modest as PRT competes at the subsidy window.
(There could be an interim period of fairly significant growth if farebox ratios are
clearly better than light rail transit, but not quite profitable). This is why I give the
edge to single mode. It is just a guess, of course, but I believe profitability of single
mode can be achieved before significant fleet build up of dual mode can be achieved, and
at a lower cost.
I believe profitability will swiftly result in the elimination most LRTs. Why would
any municipality want to continue the financial drain of LRT when a PRT could quickly
replace it and return a profit and substantially improve the LOS. (I must inject here that
the so called "capacity" issue of PRT vs. LRT is a red herring. If PRT bumps up
against capacity problems, profitability will arrive even sooner.)
In the same way that dual mode systems need to progress to a scale that will encourage
purchase of dual mode vehicles, single mode has a size threshold that could evolve into
dual mode without any modifications to the automobiles. Enter the " pallet " concept. For a single mode system, at some given
network size, it might become practical to offer a pallet to carry autos. It would
probably be preceded by freight hauling. Given an extensive single mode network it remains
unknown if pallets for autos would be practical, but it would be relatively easy from a
technical point of view. The autos would need no special modifications and the rigorous
check in procedures eliminated. Quick loading methods should not be too formidable.
So the race is on. Can modest starter single mode PRT grow and expand quickly enough?
Will dual mode be able to muster the financing and weather the time frame necessary to
carry it to the fleet build up stage?
Some final thoughts - It is just a kind of an artistic/aesthetic reasoning, but dual
mode vehicles such as road/rail, air/road, or boat/road just haven't ever seemed to cut
it. The most successful, i.e. air/boat, remains a fairly modest transportation form. The
reason seems to be that dual modalism compromises the vehicles so that they cannot take
full advantage of either mode. I believe much of that exists in considering PRT trade
The best design for a PRT vehicle to take full advantage of automation is considerably
different than an automobile. It is a lot of things, like windows and doors, power trains,
braking, seating, communications devices, vehicle weight, safety devices, fuel, windshield
wipers, bumpers, lights, etc. The list is long. Perhaps most important in the long run is
in performance. Attainable single mode speeds would surely be far higher than dual mode.
Having said that I believe dual mode has a far more personal aspect - owning ones own
PRT vehicle. Single mode as most commonly conceived is "Public Transit". Unless
single mode can offer private ownership, which seems a difficult hurdle, dual mode could
prove to be more socially adoptable.
Dual vs. single mode is an important and fascinating issue, without a clear cut
winner. It has huge ramifications in ultimate land uses. I hope more people will weigh in
on the discussion.
Dennis Manning has been a transportation engineer
with CalTrans for many years. He has recently retired.
Last modified: September 11, 2002