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 time.

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 system.

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 rail.

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 cases.

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 ramp.

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.

PRJ response:

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 triangular rail.

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 rail.

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 darker.

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 car).

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 bigger.

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 nearby.

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.

PRJ response:

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 before parking.

c. The retrieval time for a private RUF will probably be larger than for a public PRT, but the same as for a public RUF.

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 system.

The advantage that DMS needs no stations, comes as a disadvantage for people without a drivers license.

PRJ response:

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 rail.

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 very close.

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 moderate.

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.

PRJ response:

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 RUFs also.

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 solution.

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 Electric Vehicles

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.

PRJ response:

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.

13) Conclusion

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 alternative.

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 .

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Last modified: April 14, 1998