Response to Anderson's Dualmode Critique
Palle R Jensen , RUF International, Denmark
The following is a direct response to the
Dualmode Critique by Dr. J. E. Andersen (JEA). The document
created by JEA is divided into 13 parts. In the following, I
(PRJ) will summarize each part before responding.
1) Dual Mode Operation
In this section JEA lists most of the attractive
features of DMS, but states that this does not come without
disadvantages which must be overcome if it is to be
practical. He suggests an exchange of ideas with the purpose
of obtaining optimum system characteristics.
PRJ response: I agree completely, and I find this
attitude very constructive.
2) Variable Vehicle Condition
In this section, JEA points out that DMS requires an
organization to ensure that DM vehicles entering the
automated guideway are functioning properly, since the
vehicles may have been away from the system any period of
PRJ response: I agree, but since I consider the
RUF system as a
PRT -system, it already has an organization to manage
the PRT/GRT part (public RUFs and
MAXI-RUFs ) it is no big issue in the RUF system as it
would be in a pure Dualmode system.
3) Inspection at Entry Point
This point is similar to the previous, but points out
that the inspection must be placed immediately before entry.
Otherwise, the DM vehicle could deteriorate before access and
cause trouble. Such a facility must be manned, which will add
to an increase in the cost of the system.
PRJ response: The issue of inspection is
important, but not as important in a RUF system as in
alternative DM systems.
First of all, the potential problems in a RUF system are
of limited consequences:
a. If the drive system fails, other RUFs can push and
pull the RUF in a safe way to the next egress point where it
can leave the system (be removed by the staff). Blocking of
the wheels will be extremely rare, since the rail wheels are
very simple wheels, supported in both sides (unlike a car
wheel, and even car wheels are very rarely blocked!).
b. If the power supply system (rail power) fails, the
batteries can do the job and drive the vehicle to a service
stop. This is not normally possible in a pure PRT
c. The steering on the RUF rail is passive, unlike the
kind of DMS, where the road wheels are also used on the
guideway. Such a DMS can have catastrophic failures on the
guideway. The RUF system is completely stable because the
vehicles are "riding" on top of the triangular
d. Steering during switching is critical in a RUF
system. It must be checked immediately before entering the
switching area. This can easily be done automatically using
sensors along the rail to see if the wheels move correctly
before entering the road.
e. Steering before entering the first rail is also
critical, but again this is easily checked automatically, so
that a vehicle can be repelled if it fails the test. There
will be a piece of road before the access ramp where the RUF
drives automatically. If a problem appears, it will be
redirected, or the brake will be activated. Braking distance
at 30 km/h is very low. Since it has been able to drive
manually to the rail, it can also be removed manually in most
f. In case of a severe problem, the other RUFs are not
blocked, since they can use the road network to reach (with
some delay) the next access ramp. Most RUFs will have
sufficient battery power to drive 5 km extra (typically
distance between access ramps). Often there will be more
feeder-ramps at one access point. This enables a local
redirection of RUFs to use one of the other feeder ramps
(within 500 m typically) in case of a problem at one feeder
PRJ conclusion: Inspection is important, but
since most of it can be performed automatically (self test
and test immediately before access) and since RUF is a PRT++
system where the switching points (stations) are manned, the
problems are not critical.
4) Reduced Station Throughput
a. As a consequence of the previous mentioned problems,
JEA points out that the throughput will be limited if the
inspection time is long. He points out that more entry points
may be necessary.
b. He also suggests that for given area coverage and
given ridership, the required station capacity is inversely
proportional to the number of stations.
4a) More entry points may be necessary in order to
obtain high capacity, but this may be an advantage, since the
access points to feeder-rails can be placed intelligently in
the surrounding street area, in order to optimize the flow to
and from the system.
If for instance, feeder rails are placed to cover an
area where large events take place (a station, an exhibition
hall, etc.) this could give the system a very large peak
capacity. The feeder rails should be long enough (>400 m)
to build long trains before they all merge to become one high
capacity rail (theoretical maximum flow = 45.000 seated
passengers s/h if all vehicles are 100% full MAXI-RUFs and
the trains are 300 m long).
I don't think that many feeder rails are required in
most cases, since the inspection will mostly be performed
automatically and fast.
4b) The purpose of the RUF system is primarily to offer
an alternative to the cars using the congested and polluted
freeway system. The mature RUF system consists of both
high-speed rails (>20 km between access ramps and > 150
km/h) and medium speed rails (5 km between access and max.
100 km/h). The rail system is fed via feeder ramps. The
access frequency is limited either by the inspection time, or
the safety distance between vehicles during access.
Switching and access speed is limited to 30 km/h
(8m/sec), so if a safe separation between vehicles at 30 km/h
under system control is 5 m and the RUF is 3 m long, then 1
RUF can access the ramp per second. If 3 feeder ramps are
used to build trains of a maximum length of 300 m, then a
train of 100 RUFs can be merged to the rail every 33rd
second. This corresponds to 3 RUFs/sec. A train with the
length of 300 m plus 20 m separation, can pass a certain
point in 40 sec. This means that 3 feeder rails are enough to
fill a rail to its maximum capacity. If 4 or more feeder
rails are used, the access time is increased (more time for
inspection and longer separation during access).
5) Complex, Expensive Stations
JEA points out that in order to have a PRT ++ system
with both PRT features and Dual-Mode features, the stations
will become very complex.
PRJ response: If the PRT is a Dual-Mode PRT, then
it is quite simple to let the DM vehicles merge with the PRT
vehicles, since the switching of all vehicles is performed
using the Dual-Mode principle. The station design in the RUF
system is based upon the DM principle and has high capacity,
large redundancy (a problem in one berth will not affect the
others) and short loading/unloading times.
Another aspect is that the cost of a station should be
seen in relation to how many passengers it can handle. A high
cost can be justified if the capacity is large.
Station size and configuration is flexible. In suburban
areas, a station may not be relevant. An access ramp is
enough. In dense areas, access ramps may be impossible, so
only simple off-line stations are used.
6) Wide, Expensive Guideway
JEA points out that DMS can choose between two evils:
either a) use an elevated roadway, or b) use a separate
suspension system for the rail.
a. will be visually intrusive.
b. will be more complex than a true PRT system.
PRJ response: I agree that solution a) is
unacceptable, not only because of the visual intrusion, but
for many other reasons: Poor stability, complex steering or
low speed, poor braking performance, high noise level,
sensitive to side wind, etc.
RUF uses solution b) but:
The complexity is low, since the extra wheels are simple
wheels. They are not used for steering and they don't need
inflated tires and brake linings.
The transfer from rail to road is very simple and
natural, because of the way it "rides" on the
The added complexity can be justified by the large
number of RUFs which can be produced because of the scope of
a RUF system is to substitute the car in many cases where the
PRT is not relevant (suburban areas).
Another very important aspect of the principle of
"riding" is that the visual impact is the lowest
possible, since the RUF vehicle actually hides the rail while
it is riding on it. Unlike the
PRT 2000 system, where the vehicle runs on top of the
guideway. The PRT 2000 vehicle adds to the visual impact of
The PRT 2000 rail (178 cm x 170 cm) is also far more
massive than a RUF rail (100 cm baseline of triangle and 100
cm height). The sides of the triangular RUF rail will reflect
the sky so it will have similar color and illumination as the
sky. The sides of the PRT 2000 rail will reflect high
building and part of the street so it will tend to be
7) Reduced Line Throughput
JEA compares PRT with the elevated roadway DMS and
concludes that PRT has a higher throughput than DMS.
PRJ response: RUF does not suffer from these
disadvantages. As a matter of fact, it has a higher
throughput than PRT because of its train building
capabilities and its rail brake.
JEA argues that a Linear Induction Motor (LIM) brake is
better than wheels on a roadway, because of the problems with
a wet guideway. In the RUF system, the brake is even better
than a LIM brake, and it is not affected by water, since the
active surface is approx. vertical. Snow and ice will not be
a problem either, since it will not stick to both sides at
the same time. The rail brake will always have at least one
friction surface, which is enough, since it can be put under
high pressure (unlike gravity limited braking as in a
The energy efficiency of a LIM is poor, so the energy
consumption in a LIM based system is higher than RUF. The
close coupling of RUFs makes this difference even
8) Downtown Congestion Beyond System Control
JEA points out that DMS cannot control congestion at
street level in the Central Business District.
PRJ response: I agree, but this problem can be
solved in different ways in a RUF system:
a. Commuters are encouraged to use public RUFs if they
are working in the CBD. This way, they can leave the RUF just
as a PRT.
b. Automatic parking of private RUFs can be done via the
rail system. The owner leaves the RUF at a PRT-station and
the RUF is automatically guided to a parking facility
c. The egress rails can be placed in a way that
congestion is minimized.
9) Downtown Vehicle Storage and Retrieval
JEA discusses the storage problem and argues that DMS
has the same need of space as a car whereas a PRT vehicle can
be parked more compactly.
a. A public RUF can be parked just as compactly
(probably better) than a PRT.
b. A private RUF is equipped with the same electronic
guidance system as all RUFs (lateral control via alternating
magnetic fields in the road). This system can be used to park
the RUFs quite close, since the driver can leave the vehicle
c. The retrieval time for a private RUF will probably be
larger than for a public PRT, but the same as for a public
10) Vehicle Usage and Amortization
JEA points out that the number of necessary PRTs is 6-10
times lower than for DM vehicles which he considers as being
privately owned. This will affect the overall cost of the
The advantage that DMS needs no stations, comes as a
disadvantage for people without a drivers license.
a. I agree that PRT and public RUFs are more economical
overall than private RUFs. On the other hand, this is not a
system problem, but an individual problem. If many users
prefer to own their RUF's, the system can live with it
without problems. The cost is paid for by the individual user
which will pay only for electricity, when using the
b. PRT vehicles are passive when the demand is low,
whereas a public RUF can be used for small errands near the
user's residence during evenings and weekends. This improves
the attractiveness of the RUF compared to a PRT. You have to
walk a few hundred meters to the PRT unless you are living
very close to a PRT station, and most people will not live
c. People without a drivers license can be served well
in a RUF system. The MAXI-RUF can drive as a telebus
("Dial-a-RUF) and collect users in their area and drive
them to an access ramp (station). Here the chauffeur leaves
the MAXI-RUF, but the passengers remain seated and continue
using the MAXI-RUF as an Automated People Mover. If they have
to transfer from one MR to another, the waiting time will be
low, and walking distances will be short. Since the chauffeur
is used only for the road part of the journey, and the trip
can be optimized via computer, the cost of this service is
11) Elitist Solution
JEA points at the very important question of how to get
users to a rail before the rail network is large enough to be
attractive. Only the rich will be able to buy the more
expensive vehicles, and all others will have to pay taxes to
create the rail system.
a. I imagine that a RUF system is started as a PRT/GRT
system using public RUFs and MAXI-RUFs. This system will be
an attractive public transportation system and in many
respects better than a PRT system. Once the rail system is
there, it can be used for private/public individual Dualmode
b. Many cities, especially in the USA, are currently
considering LRT systems which are extremely expensive to
build in an existing city. They have a much larger problem,
since these systems cost more, and will not be able to
attract as many users as a Dualmode RUF system. To build a
RUF system instead, will be an economically sound
c. I consider a RUF system as being a PRT++system, so if
PRT can be implemented in a city, so can RUF.
12) The Alternative: Captive-Vehicle PRT + Small
JEA describes an alternative where the core is a PRT
system, but where it is supplemented by a fleet of small
Electrical Vehicles at the stations.
JEA suggests that the user drives in his car to the PRT
system, transfers to the PRT system and finally transfers
from PRT to the EV at the station.
This solution is not very pleasant to the user who has
to change twice and carry any luggage in any kind of weather.
It also involves a waiting time from car/EV to PRT. It may be
short, but waiting time is always a problem.
The RUF system offers a transfer-free travel if it
starts within a distance of 25 km from the rail system and
ends within 25 km from the rail system. There is no waiting
time in the system.
If you have to start a longer distance from the rail
system, the RUF system includes a very attractive Park and
Ride facility. The walking distance is extremely short
(<10m) and there is no waiting.
JEA concludes that in his opinion, a Dualmode System is
appealing, but has some fundamental problems which cannot be
solved technically. In his opinion, it is not a practical
PRJ response: Dualmode Systems may be a major
technical challenge, but they are justified by the much more
attractive features which they can offer.
PRT can only be attractive in very dense areas, where
walking distances are short. Since most modern cities are
much more widespread, the Dualmode System is much better
adapted to the real world.
The RUF system can be considered as a PRT ++ system,
since it is both a Personal Rapid Transit system in the dense
areas, a Group Rapid Transit system between suburban city
centers, and a Dualmode system in the suburban areas. It can
be gradually expanded to eventually become a substitute for
the car, in most cases, within large cities.
Palle R. Jensen can be reached at RUF International in
Copenhagen, Denmark. More details are available at the
RUF website .
Last modified: April 14, 1998