| 1. |
Heavier beams:
The width of the beam in supported systems is 1.5 - 2 meters (= 5.0 - 6.7 feet), usually the same as for the width of the beam vehicles. The cars weigh about 2 tons and there is a distance between the supporting poles of about 30 meters (= 100 feet). The beam's vertical thickness would be about 0.6 meters (= 2 feet). Such beams will encroach slightly more into the urban environment than the SIPEM and FlyWay systems.
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SIPEM, built by Siemens in Dortmund, Germany, can carry 4 vehicles simultaneously on each beam segment, each vehicle weighing 11 tons + the weight of up to 40 passengers in each vehicle, on a beam which is 0.8 meters wide by 1.1 meters high (= 2.7 * 3.7 feet). In the SIPEM system, the pole supports are 35 meters (= 117 feet) apart (this length constitutes a beam segment). The cross-sectional illustrations to the right (figures 5 and 8) depict the SkyCab system in the USA, which is an example of supported PRT.
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| 2. |
Wider beams:
The width of the beam in supported systems has to be at least as wide as the supported car, in order to catch the car´s sideway pitching, when the car changes travel direction. It is for this same reason that ordinary railway tracks have to be as wide as the bogie cars themselves. This problem never arises with hanging cars. The width of beams for suspended traffic |
does not have to take this sideways pitching into account at all. Granted that the propulsion car could be inside the beam, and thus has a firm grip on it; the beam still has to be quite wide in order to reduce the strain of pitching (figure 6 is of course exaggerated, but illustrates what we are talking about).
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| 3. |
Poor utilization:
The supported PRT as defined above only has 4-passenger vehicles. In most Western countries, the motor car is poorly utilized. Considering the motorcars travelling between home and workplace, 75 % of the vehicles carry only 1 person, and 20 % of the vehicles carry only 2 persons. The remaining 5 % of the vehicles carry 3 or 4 persons. Since the first traveler pays for the whole vehicle, the trip becomes unnecessary expensive,
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since one travels around with mostly empty seats. If an average motorcar can be said to seat 4 people, only 33 % of all seats are thus used, on the average.
In the FLYWAY system, there are no lower limits to how small a vehicle can get. There could be 1-person vehicles under the beams, with the resulting cost reductions in manufacture and operation. |
| 4. |
More expensive:
The unnecessary heavy weight of the PRT 2000 vehicle entails bigger investments in vehicles and in sturdier beams. The beams are often of light rail dimensions. A suspended network 14 kilometers long can be built at the same cost as one kilometer of light rail. Traveling at higher average speeds and at headways of only one second (vs. light rail' two minutes), PRT's capacity in terms of riders per hour is 12.6 times greater. Once these investments have been made, |
the resulting system will no longer be as profitable for low traffic flows. Since operation durin the first few years also has to pay for the investments made, in addition to operational costs. This means that there has to be more potential customers in an area before it really pays to extend the PRT 2000 beam network into this area. The FLYWAY system is more flexible on that point; one could, for instance, build slimmer beams and use smaller vehicles in areas with few potential travelers.
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| 5. |
Bigger and costlier stops:
Since the passenger cabin is atop the beam, the stops will have to be on the same level, high above the ground. They thus have to be equipped with elevators, stairs or escalators. To lower the beam at every stop (i.e. to allow the beam to slope before and after a stop) would be too much of yo-yo-effect on the travelers. It would also be inefficient, energy-wise. The energy required for the car to climb such a slope is not entirely recouped when going down. Suitable for an amusement park, perhaps, but not realistic for commuter transportation. These elevated stops will thus be necessary, and they would be expensive, and another big encroachment into the environment. As noted on other pages; in narrow streets there simply won´t be any place to erect such platforms.
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SwedeTrack´s FLYWAY system uses cabins with elevators. Aren´t they just as energy inefficient as sloping beams? No, not quite. Only the cabin is lowered, not the whole vehicle. The FLYWAY system could use ordinary bus stops, since the beam cars can be lowered to the street level just about anywhere. Also, since platforms above ground will be expensive, there will not be so many of them; they will be far apart. The distance to walk to and from a stop will be longer than for the FLYWAY system. This is also an inflexible system; you don´t move such a platform to another location overnight. Supervision of the supported PRT system stops will also probably be needed. |
| 6. |
Freight handling costlier:
If cargo is to be handled, the elevated platforms require extra lifts to hoist the goods up and down. Those platforms can rarely be placed on the ground.
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Handling of freight will therefore not be profitable in a supported system, other than in exceptional circumstances.
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| 7. |
Banking in curves required:
The PRT 2000 beams are banked in the curves (see figure 7) so that the passengers won't experience any discomfort. This makes for more expensive manufacture of the beams. The beams dominate the total investment costs, which in turn dominate the cost of the whole system.
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This also means that the vehicles have to travel at just the right speed through the curve. Generally speaking, though, one could have unbanked beams, and let the cars bank themselves as is done with the train X2000. |
| 8. |
PRT 2000 has only small vehicles:
The fact that there are only small vehicles in the PRT 2000, and no vehicles with the capacity similar to a bus, limits the beams' capacity further.
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This means that there could be difficulties in handling sudden surges of people at train/subway stations and at big work sites, icehockey rinks and the like.
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| 9. |
PRT 2000 has serial stops:
The PRT 2000 stops are meant for serial stopping, i.e. the vehicles come and go in a line behind each other.
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This lowers the traffic capacity of the stops, such as at situations mentioned under item 7 above. |
| 10. |
Supported systems not so safe:
The supported PRT beam has a slit on the upper side in order to connect the propulsion vehicle inside the beam with the passenger cabin on top. The braking ability can be jeopardized when the from a safety point of view vulnerable convergence/divergence points are subject to snow and ice. This will adversely affect traffic safety.Suspended systems are also safe from derailment. Supported systems can use the cavity in the beams to keep the the vehicle on the track, as shown in figure 6. But this is rarely done, apparently because the beam would then be open to weather and sabotage as shown below. |
As an example, an AirTrain, intended to carry air travelers to and from Kennedy Airport, New York, derailed on a test run in September, 2002, killing its operator (figure 9).
Its speed unknown, the AirTrain slammed into a concrete retaining wall 25 feet above ground. The cause could not be determined, but investigators tought that 8 tons of concrete ballast — put aboard to simulate a load of passengers — had shifted on the gentle curve, leading the front end, and then all three cars, to stray and jump the tracks. The combined weight of the train and ballast was more than 90 tons.
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| 11. |
Supported systems more vulnerable to sabotage:
The slit on the upper side of the supported PRT beam makes the system vulnerable to sabotage by people who throw rocks,
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bottles and other items down the slit. A slit underneath is much better protected. |
| 12. |
Supported systems more vulnerable to adverse weather:
The supported PRT cars are more easily affected by the vagrancies of the weather
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whereas the SIPEM and FLYWAY systems are quite immune to weather conditions.
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| 13. |
Supported systems not safer than ordinary rail systems:
People (mostly teenagers(?)) could easily get hit by the supported PRT vehicles if they venture out on the beams, as kids are wont to do. The danger is about the same with these systems as with ordinary subway systems.
| In suspended systems with elevators, this could only happen if a person on the ground gets hit by a lowering cabin. As shown on other pages, this could quite easily be safeguarded against (but, admittedly, the FLYWAY solution will cost some extra money). |
| 14. |
Supported systems have to safeguard their cars:
The supported PRT vehicles cannot be parked overnight on sidetracks to the beam, since they will then be too accessible to vandals. Iy is easier t enter and to walk on beams for supported cars than the narrow, inaccessible beams for suspended traffic.
| This means that expensive parking garages would have to be built.
This in turn also means that the vehicles will have longer way to travel to reach their starting points. The FLYWAY system could very well leave its' cars near next morning's point of departure.
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| 15. |
Cars in supported systems cannot use elevators:
Generally (as already mentioned above), using suspended beamcars makes it possible to use elevators, which in turn carries with it
other advantages. |
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The Leaves of Autumn
ne would hardly think it could be possible, but autumn leaves that fall on railroad tracks can occasionally cause problems. If a train is braking when some of its brake wheels are positioned on top of leaves, the wheels might lock and the car will slide along until it stops, due to friction on the locked wheels together with the braking power of its unlocked wheels. This friction is quite big, due to the weight of the railroad car, and the locked wheels will be flattened on the exposed surface.
A railroad car thus afflicted has to be taken out of service and the wheels replaced or polished back to their round shape, because using such a car would further damage rail and wheels and be uncomfortable for passengers. This problem came to the fore on the Stockholm underground in November, 2002, when about 90 cars had to be brought out of service for wheel repair, and the operator had to crowd passengers into fewer cars.
Why are we mentioning this? Well simply to give a relevant example of how exposed supported systems are to the environment. Suspended systems would not be affected in the slighest by this sort of thing. |
 Figure 5.
 Figure 6.
 Figure 7.
 Figure 8.
 Figure 9.
Derailment of AirTrain at Kennedy Airport
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he idea behind the PRT system was first voiced in the USA during the 1950-ies. In the 1970-ies preliminary tests were conducted in the USA, Japan och West Germany. Then, in the 1990-ies, several systems were projected and built in the USA and Southeast Asia. Then have so far not been extensive and not profitable for the investors. They have been regarded, rather, as relatively short extensions to other transport systems.
