Personal Rapid Transit in New Zealand - Future Transit Now

A variety of issues must be considered in making this tradeoff:

1. Switching. The running surfaces for supported vehicles are continuous through switch sections, whereas with hanging vehicles, there must be a slot in the guideway through which an arm that supports the vehicle passes. Thus, in a switch section the support element inside the guideway must pass across the slot, so the supporting load must be transferred to another member. In the Monocab PRT system this required an extra set of wheels that engaged slots in the top inside of the guideway. With maglev the load would probably be transferred to a set of electromagnets. This extra load-transfer means adds extra weight and cost, and reduces reliability. The bottom line is that the fundamental requirement of switching is much easier with supported vehicles.

2. Visual Impact. A group at the University of Minnesota worked in the PRT field for 13 years before deciding it was necessary to initiate the design of a new PRT system. During this time, we conducted three international conferences on PRT and were deeply involved in a fourth conference held by the Advanced Transit Association in Indianapolis in 1978. In addition, we gave hundreds of presentations on PRT in many places in the United States and abroad and were involved in a number of PRT planning studies. Moreover, our group was funded by UMTA to study the visual impact of AGT systems, and worked with people at the Volpe National Transportation Systems Center on this subject. Out of this experience, we were extremely sensitized to the issue of visual impact of overhead structures. Both the supported and hanging systems must have the same clearance, so the guideway of the hanging system is six to eight feet higher and must be supported by posts with right angle arms at the top to reach out and support the guideway in such a way that the vehicle can pass underneath. The guideway of the supported-vehicle system is lower and the posts are smaller, so, all else being equal, the visual impact is substantially less.

3. Cost of Posts and Foundations. Because the guideway of the hanging-vehicle system is higher from the ground, the bending moment at the base of the posts due to the maximum crosswind for which the system is designed is higher than in the case of the supported-vehicle system. Moreover, in the hanging-vehicle system the weight of the guideway and the vehicles adds an additional bending moment at the base of the posts. The maximum load condition is fully loaded vehicles nose-to-tail along the guideway. With this load condition and the maximum lateral wind load, we found that the bending moment at the foundation is about twice as much with the hanging-vehicle system as with the supported-vehicle system. So both the posts and the foundations will be correspondingly larger. Yet to avoid utilities and to fit the foundations in, the smaller foundation is preferred.
4. Torsion in Curves. If the vehicle hangs from the guideway and is provided with a critically damped swivel joint, as it passes through a curve it will swing out as in a coordinated turn of an airplane. Thus the passenger is not subject to much lateral acceleration. If the vehicle is supported above the guideway, the guideway must be superelevated or banked to reduce the lateral acceleration on the passenger. With the hanging vehicle in curves, the torques due to centrifugal force and gravity subtract, whereas with the supported vehicle they add. This seems like a deciding factor, however, the wall thickness of the guideway required to resist a given torque increases as only the cube root of the torque, and from detailed analysis we found that the wall thickness to support a given torque with the supported vehicle is only 13% greater than with the hanging vehicle. This consideration will, therefore, not be an over riding factor, and other factors must be considered.

5. System Natural Frequency. With the vehicles on top of the guideway, the guideway can be clamped to the posts as first suggested by Dr. Jack Irving in Fundamentals of PRT. From basic structures theory, the maximum deflection of a beam clamped at both ends is only 20% of the maximum deflection of a simply supported beam and the natural frequency is 2.26 times higher. If the vehicles hang from the guideway, the post must be along side of the guideway and a horizontal element must be added over the top of the guideway to support it. It is not practical to make this longer element as stiff as the clamped support, which means that to have the same deflection and natural frequency as the guideway of a system with supported vehicles, the guideway of the hanging-vehicle system must be heavier. This fact more than counteracts the increased thickness of the supported guideway discussed in point #4.

6. Vehicle Weight. The weight of a supported vehicle and its load is supported at the bottom of the chassis so that the maximum load on the cabin is due only to wind and passenger loads. The weight of a hanging vehicle and its load must be supported by the sidewalls of the cabin, which, all else being equal, would require that they be heavier.

7. Underground System. If the system is placed underground, the guideway of the supported vehicle can be laid on the ground whereas with the hanging vehicle, the guideway must still support the vehicle. This means that there will be a cost savings in using supported vehicles underground.

8. Public Preference. On its test track, the Cabintaxi PRT guideway was designed and built to support vehicles both above and below the guideway. Many people rode the system and their reactions were recorded and reported in an assessment report developed jointly by the U. S. DOT and a German consulting firm. The result was that somewhat more people preferred riding above the guideway than below.

9. Passing Through Buildings. If the guideway is below the vehicle, it blocks any cross traffic due to pedestrians or carts. If the guideway is above the vehicle and there is adequate clearance below, the system would not interfere with cross traffic. Therefore this factor favors the hanging vehicle provided the ceiling is high enough.

10. Snow, Ice, Debris. A major argument for the hanging-vehicle system is that there need be no worry about interference with snow, ice, or debris. For the supported-vehicle PRT system to be successful, this situation must be addressed very carefully. In the Taxi 2000 PRT system, the guideway is a truss structure with a cover over it that provides eight benefits, one of which is to keep out ice, snow and other debris. We found that the chassis, which is constrained within a U-shaped guideway need only be four inches wide and the slot through which it passes need be only five inches wide, giving a nominal half-inch clearance on each side. The main support tires, which are cushion synthetic-rubber tires, are supported on eight inch by six inch by half inch steel angles, and there is a slot eight inches wide between them. We have designed a plow that will pick up any foreign substance on the running surfaces and toss it down in the eight-inch gap. The plow has been tested in winter conditions and has been found to be completely satisfactory.