by J. Edward Anderson, Ph.D., P. E.
Professor Vuchic seems to assume that workers in PRT base their findings on theoretical analyses of parts of a network. This was undoubtedly true thirty years ago; however, since then many investigators have developed detailed and accurate simulation models of network PRT systems of a variety of sizes in which questions such as: "What happens if a train unloads a large group of people at a PRT station?" can be fully answered. One of the requirements of the Phase I study of PRT for the Chicago RTA required us to show, based on a network simulation, the performance of a PRT system subject to a sizable pulse of demand at any station. The result was fully satisfactory.
A number of comprehensive simulations of PRT systems were developed in the 1970s. The ones I am most familiar with are those developed by The Aerospace Corporation (networks with up to 1000 stations and 60,000 vehicles), the German joint venture DEMAG+MBB (networks covering medium sized cities), IBM Corporation (networks up to a size needed for Manhattan Island), and the University of Minnesota (networks for the Twin Cities and Duluth). In the 1980's, I developed a simulation in which I can simulate a PRT network of any configuration, and have done so most recently for applications at two universities. Dr. Ingmar Andreasson, Gothenburg, Sweden, has developed a simulation in which he has been able to study the performance of PRT networks covering entire metropolitan areas. He has applied his program with good results to a PRT system covering the city of Gothenburg as well as to other Swedish cities. The Raytheon Company, in its work with the Chicago RTA, has developed simulation capability in which any network can be simulated, and many have been. Use of such simulations, rather than unaided intuition, is the way to analyze correctly the operations of PRT systems. Working with them has led to increased enthusiasm for the potential of PRT. Enough work has been done on failure modes, effects, response strategies, and operations to show how to develop extremely reliable and economical PRT systems.
One important result of these simulation studies and the associated cost analysis is that the need to start with a large PRT network is a myth. We have found many applications in which small PRT systems having only a few stations are more economical than large-vehicle transit systems, the main reason being the much lower cost and visual impact of the guideway. Such a system can be expanded incrementally.
With today's technological capability, as travel density increases, headway can and should be decreased while keeping the vehicles small enough to accommodate one individual or a very small group traveling together by choice. Today, this is practical and much more cost effective than increasing vehicle size. This is because 1) the smallest vehicles result in the lowest-cost guideway, 2) by using off-line stations, each trip can be nonstop, which substantially increases the average travel speed, and 3) more stations can be used, which increases accessibility to the community. In very dense cities, untypical of the vast majority of American cities, subways are practical and their effectiveness can be improved by using PRT systems as feeders.
In his response, Professor Vuchic refers to "efficiency of rail transit." What kind of efficiency is implied? If he were to mean high travel speed, rail transit can compete with the automobile in most cities only if the stops are miles apart, which reduces accessibility. If he were to mean high daily average load factor, i.e., high utilization, light rail transit (LRT) is not the system to select. Federal data given in the Section 15 Data report shows daily average load factors of streetcars (i.e. LRT) in the range of ten to twenty percent. If he were to mean energy efficiency, LRT would be efficient if the vehicles were always full, but with the load factors that can be practically attained, the energy efficiency of LRT is typically equivalent to an automobile system in which the cars average less than 12 to 15 miles per gallon.
If he were to mean safety, in the surface-level streetcars, promoted as a way to reduced costs, the number of accidents per vehicle-mile is about 60 times the figure for exclusive guideway systems, for which the costs are now upwards of $100M per mile. If he were to mean land efficiency, surface-level streetcars are better for a given flow of people per unit of time than automobiles, but they still take a 38-foot strip of valuable surface street, now clogged with automobiles, whereas an optimally designed PRT system requires a tiny fraction of that land. If he were to mean total cost per passenger-mile, that number, which conventional rail people consistently avoid mentioning, for systems installed in the United States over the past two decades is outrageously high, about 5 to 10 times that of bus systems. As to passenger carrying capability, there is no streetcar system in the United States reported in the FTA Section 15 Report that actually carries in the same corridor and with the same length more people per hour than could be carried by an optimally designed PRT system.
The Swedish Transportation Research Board has funded city-wide planning studies that compare PRT with conventional modes. A study for the city of Ume showed that PRT was the only system that produced a benefit-cost ratio greater than one. In a paper on this study presented at the International Conference on PRT and Other Emerging Transportation Systems held in Minneapolis, November 18-20, 1996, a conclusion was: "a PRT system provides such a broad range of desired qualities that it should be given the highest priority in research, development, testing and demonstration for implementation in the urban environment." At the same conference, a British study concluded: "Advanced PRT systems of the type proposed in this paper provide a cost-effective and environmentally advantageous solution to the problems of transport in the 21st century." A Korean study concluded: "... the advent of PRT technology offers the way to achieve all of these objectives while at the same time offering enhanced urban form and improved lifestyle."
Professor Vuchic commented on various automated transit system programs that have terminated. In detailed lectures that I give, I discuss dozens of such systems and show why they failed, but also what alternative characteristics are needed to make them succeed. There is no question that there are many wrong ways to design PRT. I lament them as much as anyone. Our very active efforts today are devoted to the ways we have found to produce success - by learning from experience and by applying state-of-the-art technology. Our confidence has been steadily increasing.
We live in an era of great change. Because of increasing pressure on resources, there will be even greater change in the decades ahead. Sooner or later the rail transit industry will find that its interest lies in becoming involved in the changes, however great they may be, rather than in fighting them - in this particular case by fighting new transit systems while trying to convince policy makers to install systems that were effective a century ago when the competition was a horse cart on a mud road rather than an automobile on a freeway.
J. Edward Anderson is President and CEO of the TAXI 2000 Corporation.
Last modified: 13 August, 2002