Personal Rapid Transit in New Zealand - Future Transit Now!

Synthesis of cost-effective transit alternatives using automated vehicles requires consideration of a wide range of factors that are moot in determination of the optimum size of manually driven vehicles where the need to amortize driver wages dominates the economics. Discussions of many of these factors have appeared in previous papers. This article brings them together into consideration of one specific system characteristic: the optimum automated-transit-vehicle size.

Fundamental to the design and planning of PRT systems is knowledge of the practical throughput of the off-line stations.

The throughput has been studied by means of computer simulations and by means of simple analytical formulae.

While simulation is essential to quantify the flow in specific cases, a theoretical understanding of the factors that determine maximum station throughput is needed to try to determine the most effective ways to increase it.

In a synchronously controlled PRT system a vehicle waits at an origin station until the path through all merges to the destination is clear. The system then reserves this path for the specific vehicle in question, which then proceeds without any maneuvers directly to the destination. Such a scheme was proposed in the late 1960s for the automated transit system deployed in Morgantown, West Virginia. Because of its inflexibility in face of failure of any vehicle to maintain its path, interest declined, but the clear-path idea has emerged again on the Transit Alternatives mailing list on Internet. It is thus worthwhile to show why, by estimating the wait time for an origin-to-destination reservation, that fully synchronous systems are not practical except in very small networks.

One of the most difficult tradeoffs in the design of PRT systems is the choice between use of supported or hanging vehicles, i.e., Supported-Vehicle Systems (SVS) or Hanging-Vehicle Systems (HVS).  The DEMAG+MBB group solved this problem by developing a guideway that permits one set of vehicles to ride above the guideway and another set that ride below.   In my textbook I examined the tradeoff between systems using one-way guideways and two-way, above-below guideways, and found that the cost per passenger-mile of the one-way system was somewhat lower.  The two-way system reduces circuity while riding the system, but to make the use of larger, two-way guideways economical the lines must be spread farther apart, which results in longer walking distances, which add more to the trip time than the two-way system reduces it.  Moreover, walking is weighted in ridership studies much greater than riding. The two-way guideway had about twice the bulk of the one-way guideway, which increases visual impact and cost.  I thus concluded that it is better to concentrate on one-way-guideway systems and then compare SVS vs. HVS in every way we can. 

The paper begins with a review of the rational for development of personal rapid transit, the reasons it has taken so long to develop, and the process needed to develop it. Next I show how the PRT concept can be derived from a system-significant equation for life-cycle cost per passenger-mile as the system that minimizes this quantity. In the bulk of the paper I discuss the state-of-the-art of a series of technical issues that had to be resolved during the development of an optimum PRT design. These include capacity,switching, the issue of hanging vs. supported vehicles, guideways, vehicles, control, station operations, system operations, reliability, availability, dependability, safety, the calculation of curved guideways, operational simulation, power and energy. The paper concludes with a listing of the implications for a city that deploys an optimized PRT system.