On Making the Transition to a Dualmode System: Pallets, AHS and RUF Compared


Palle R. Jensen

Much reflection has been given to the transition process from our present system to a Dualmode system.

Should it be solved by constructing the Dualmode system as a roadway system for ordinary cars which have been modified in order to be able to drive automatically on a special roadway. This is the Automated Highway System (AHS) approach.

Another possibility is to design a pallet system which is able to carry ordinary cars along a guideway.

The RUF system solves the problem by using completely new vehicles on a special guideway.

Which approach is the best?

It seems natural to make modifications to cars (AHS) or to modify train cars to be used as pallets to carry unmodified cars.

However, when the situation is carefully analysed, one finds a large range of problems which cannot be solved by these methods. The analysis points to a completely new configuration which is optimised for the purpose of being an ideal Dualmode system.


It must be extremely safe to use the system.

AHS claims to be more safe than a car driven by a human driver. This is probably true under normal circumstances. The interesting question is, what happens when something goes wrong? When the system fails, the consequences are catastrophic because a lot of cars are driving at high speed and very close. Both lateral guidance and distance keeping are critical factors in the AHS. In a RUF system, the vehicles are always "riding" very stable when they drive fast and the distance is zero which is much safer than 1.5 m. Preliminary computer simulations have shown that RUF on the rail is stable to at least 200 km/h.

A pallet system can be made safe if the cars are securely fastened. If ordinary cars are used, the securing mechanics will take time. This time will severely limit the flow in the system. The RUF can drive directly from road to rail without waiting time.

Emergency braking in a pallet system based upon traditional rail-car technology will be limited by the friction between the wheels and the rail. This friction is defined by the materials (steel against steel) and the pressure as defined by weight and force of gravity. In a train system this friction is low. Accordingly, the safe distance between trains has to be long (often more than 1000 m). This puts severe limits on system capacity. In a train system, this is solved by using large vehicles with many passengers. In a Dualmode system with individual vehicles, the capacity will become too small if the braking distance is too long. In the RUF system, the special railbrake ensures that emergency braking can be actuvated very fast under all circumstances.

Pallet systems have the advantage that the rail-car does not leave the rail system. On the other hand, this principle creates a problem with empty vehicles just as in traditional PRT systems. In the AHS and the RUF system, the vehicles have to be tested before being allowed to enter the guideway/roadway. This is especially critical in the AHS where a damaged vehicle can cause a serious accident.

Testing of RUF's takes place at the access ramp. Normally there will be several access ramps at one entry point. If a RUF fails the test, it will not be allowed access to the main rail, so it will not block the flow. This should be able to prevent most problems. The interface between RUF and rail is made very simple in order to minimize the number of potential problems. The transition from "driving" to "riding" comes naturally because of the triangular shape of the rail.


In the AHS, cars drive in a platoon with a separation of 1.5 m. This will reduce the air resistance so the rate of pollution will decline. In a RUF system this effect is much stronger since the vehicles are designed for close coupling. Rolling resistance in the AHS is the same as in the ordinary road system because the road wheels are used for braking. In the RUF system, special smooth rail wheels ensure a low rolling resistance. The RUF is driven by special wheels pressing on the top of the rail. This pressure can be increased if traction is needed to climb a hill or reduced when driving at constant speed so that rolling resistance is minimized.

The pallet system will increase the air resistance because the cross section of pallet + car is much larger than that of a car. Since the rail part of the trip is supposed to be the long and fast part of the trip, this problem is a major problem for the pallet concept. The RUF system is minimising air resistance because the RUFs are created to form trains.

The weight that has to be moved in order to move one passenger will increase because the pallet itself has to be strong and heavy in order to carry a car. In the RUF system, only the weight of the simple rail wheels is added. On the other hand, the RUF has to carry them all the time. The pallet will drive empty some of the time, which will add to the problem.

Noise from the AHS will be a major problem since the number of vehicles per lane will increase while the noise from each car will remain the same.

A pallet system based upon traditional rail technology will make the same noises as a train. If the pallet is using RUF technology, the noise will be minimised because part of the wheel noise will be absorbed in the gap between the rail and the RUF.

Noise in a RUF system will be negligible because of the special wheel configuration. Smooth wheels are rolling on a smooth rail. The points of contact are partly covered by the RUF chassis.


Doubling or tripling of lane capacity is said to be the goal of the AHS. Since the distance between cars in a "platoon" is expected to be 1.5-2 m, the capacity of a RUF-rail will be 30-50% higher, since the RUFs are running closely coupled.

Capacity is a serious constraint of the pallet system. It takes time to load and unload the pallets. The loading process involves many steps:

a) the pallet must be ready

b) the access ramp must be in place (like when you enter a ferry)

c) the driver must enter the pallet and be sure to hit the right spot

d) the driver must be sure that the car is securely fastened

e) the driver must connect the power supply in order to charge the batteries while driving if the car is electric

This sequence of actions will take too much time to insure a high capacity. If it takes 10 sec. to load (an optimistic assumption), 10 loading berths are needed to get a line capacity of 3,600 vehicles/hour. AHS and RUF can change mode while driving, so a lot higher capacity can be obtained.

In a pallet system, waiting time will be equivalent to a the waiting time in a PRT system. The arriving car cannot assume that there always is a pallet ready for it. The car may have to wait as long as a passenger waiting for a PRT vehicle. A few minutes waiting time could easily be expected.

A RUF mode change involves a shift from one set of wheels to another. Is that a big problem? It is not comparable to a jumbo-jet landing at 300 km/h. The RUF mode change takes place at 30 km/h and the vehicles are precisely controlled during the entire transition. In the most frequent case where a RUF is leaving one rail and shortly after (2 sec.) entering another rail, the free running rails wheels will still be rotating almost at the correct speed. After a long rail ride, the road wheels will stand still, so they will have to be brought into rotation before the mode shift. This can be done actively or passively. The driving wheels can easily be preset to the correct speed. The other wheels can be moved by passive means. If they hit some soft material before hitting the ground, the wear will be on the system part, so it can be looked after by the operating company. The same principle applies when the rail wheels hit the rail for the first time.

The dynamic forces involved in moving a car on top of a pallet are critical. Sidewind will try to make the pallet tilt. The center of gravity is placed high above the rail. When the pallet drives around a corner it has to slow down in order to keep the pallet on track (except if the pallet is very heavy). The RUF has its center of gravity placed below the top of the triangular rail. Sidewind and other dynamic forces will not create problems because the drive wheels will prevent the RUF from tilting.


Cars are popular because they represent a high degree of flexibility (plus a lot of other factors). AHS and RUF have the same degree of flexibility as the car. A pallet system will always have a weak point in the interface between the pallet and the car. Only if it can be insured that a pallet is always available and that the loading can be performed while the car moves, the same flexibility can be obtained.

The switch is another issue of great importance for the flexibility of a system.

The AHS has in principle a very flexible switch working at high speed. The complexity involved is huge and I think it is too risky. Very different vehicles have to be safely controlled at high speed during the switching and with very short separation between the cars.

The RUF switch uses the Dualmode principle at 30 km/h. It means that the vehicles have to slow down, but on the other hand it is much safer than the AHS switch.

Switches in a system with steel wheels against steel rail are normally very slow (10-20 sec). A pallet based upon this technology will run into serious trouble if many switches are involved (network).


AHS will have to be implemented as a stand alone system since it is unthinkable that people will accept lane conversions to AHS in a situation where very few have invested in cars that are AHS capable. If lanes are converted, it will create increased congestion and frustration in a long transition period. A mixed mode with normal and AHS vehicles running on the same lane have been proposed. This will create huge problems with safety, and the system complexity will increase dramatically.

A stand-alone AHS infrastructure will require new lanes exclusively made for AHS. Since the cars are running on normal wheels all the time, the lanes have to be at least 2 m wide. They will probably need to be elevated in order to find space for them. The cost of such a system will be very high.

A pallet system can make use of existing railroad tracks, but if the system is going to make an important contribution to solution of the congestion problems, many more rails are needed. In a modern city, this will become extremely expensive.

The RUF system uses a very slender guideway (85 cm wide). This means that both cost and visual impact is minimized. It is possible to cover a city like Los Angeles with guideways at reasonable cost. The system needs fewer guideways than a PRT system because of the Dualmode principle. The guideways can often be placed along existing right-of-ways.

Instead of building more Light Rail systems, the same money can be used to create the first guideways and use them for public transport based upon the MAXI-RUF vehicles. This form of public transport will attract more riders than Light Rail since it is much more flexible and is able to offer door-to-door service.

Once the system of guideways is established, people can start buying private individual RUFs and use the same guideway to commute fast, safely with little pollution.


Making the transition from the present system to a Dualmode system is not an easy task. Each one of the two modes, road and rail, have been evolved to a high level within its own paradigm. On the other hand, these paradigms have been defined many years ago when the traffic situation and city form was very different from the present situation. Attempts to stick to the old systems will make a Dualmode system look less attractive.

The RUF concept has been defined from the start as a Dualmode system. By using a special vehicle and a special rail it has been demonstrated that a lot of attractive qualities can be obtained: High capacity, high safety, low energy consumption, low noise, low pollution (ZEV) and low stress. The concept is based upon known technology combined in a new way.

RUF has been endorsed by the CALMODE organisation. A test track is under development in Denmark (April 98)

Palle R. Jensen , RUF International, Roedovre Centrum 155, 2610 Roedovre, Denmark Phone (+45) 3670 8895; Fax (+45) 3670 8866; E-mail PRJ@RUF.DK Further information is available on www. ruf.dk

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Last modified: August 18, 2002