PRT Frequently Asked Questions

Adapted from original FAQ written by Dr. Edward Anderson.

  1. Is PRT "reinventing the automobile?"

    No! PRT is a public transit system. It cannot replace the automobile, but its service characteristics are such that it can be expected to attract many more people than conventional transit systems, and it can do so using a tiny fraction of the land required for the automobile. While roughly half the population either cannot or should not be driving automobiles, PRT should be accessible to everyone. It will be the environmentalist's dream because of its markedly improved energy efficiency, lack of air pollution, and land savings. Many PRT studies recommend guideways spaced not less than a quarter to a half mile apart. They do not replace streets.

    The lead article of the July 1969 issues of Scientific American is still a most useful introduction to PRT and its profound differences from today's means of urban transportation.

  2. Won't stations get bogged down with all the small vehicles?

    Station throughput is determined by the number of station berths, which can be set to meet the demand of any particular station. In general, station throughput is high relative to conventional mass transit because:

  3. Won't the problems of reliability make the operation of a large fleet of small vehicles undependable?

    Actually, because a PRT system will have a large number of small vehicles, rather than a relatively small number of large vehicles, your chances of becoming involved in a failure will reduce in proportion to vehicle size if the reliability of each vehicle is the same. But, because of the use of checked redundancy and advanced failure-management strategies possible within the confines of a PRT system, and the benign environment within a vehicle, the PRT system will be substantially more reliable than a conventional transit system. It has been shown that the requirements for dependability in a PRT system are independent of system size.

  4. Will the visual impact of PRT be acceptable?

    Visual impact is important in all transit systems. Many rail transit systems are placed underground because a ground-level system requires destruction of too much existing property and an elevated system is too massive and noisy. Typical PRT guideway designs exhibit cross sectional areas an order of magnitude less than that required for a rapid rail system, will generate very little noise, and have an external appearance that can be varied to suit a specific community.

    People accept elevated structures if they see them as a practical means to a desired end. In the early 1970s, when conventional heavy rail systems were being promoted, officials argued that elevated structures were acceptable. The Downtown People Movers proposed in the late 1970s had massive structures (witness the Detroit and Miami People Movers) but local authorities considered them acceptable because they were believed to fulfill a need. PRT systems will have much smaller visual impact and will provide much better service at lower cost.

  5. Won't personal security be a serious problem in PRT?

    Personal security is less of a problem than in conventional mass transit, and even sometimes less than in automobiles, for the following reasons:

  6. Isn't there an economy of scale in transit systems; i.e., to carry a given traffic level, won't a system of many small vehicles cost more than a system of a few large vehicles?

    The basic features of PRT follow logically as features that minimize the total cost per passenger-mile. These features permit true minimization of guideway cost, vehicle-fleet cost, and operating cost while maximizing service.

    Data shows that transit vehicles cost about the same per unit of capacity no matter how large or small they are. Contrary to intuition, there is no economy of scale. By using nonstop trips, possible with off-line stations, the average trip time of a PRT system is two to three times less than in a conventional transit system, which means that the fleet capacity (number of vehicles x capacity per vehicle), and therefore fleet cost needed to serve a given number of trips, is less by the same factor.

    Vehicles of the size required to hold up to three seated adults have a much smaller cross section and weigh substantially less per unit of length than large standing passenger vehicles, and, because of much lower dynamic loading, lead to lower guideway weight (15 times lower) and lower cost.

    To compare operating and maintenance (O&M) costs, we define a quantity called a "place-mile". The number of place-miles of travel in a transit system consisting of vehicles or trains of any size is the number of vehicle- miles of travel multiplied by vehicle capacity. A vehicle-mile is one vehicle traveling one mile. Because PRT vehicles move only when service is demanded, the total number of place-miles per day required to serve a given level of passenger demand is only about a third as much as in a conventional scheduled transit system. Examination of data on O&M costs shows that the O&M cost per place-mile is nearly the same regardless of the type of transit system. Thus the O&M cost of a transit system that carries a given number of people per day is proportional to the number of place-miles per day of travel.

    The remarkable result of this kind of systems-economic analysis is a transit system in which the features required to minimize both capital and operating costs are exactly those that provide maximum service; i.e., on-demand, alone or with one or two friends, in seated comfort, any time of day or night, at a predictable average speed two to three times that possible with conventional transit. The only reason for using large vehicles in urban transportation is to amortize the wages of drivers over as many fare-paying riders as possible. Automation permits relaxation of system characteristics toward a true optimum.

  7. Will magnetic levitation help PRT?

    Not at urban speeds. Comparisons of systems levitated by magnetic fields, air cushions, and wheels show that, by using low rolling resistance tires, there is no advantage of either magnetic or air suspension over wheels at urban speeds, and indeed several serious disadvantages.

  8. Where are vehicles stored when not in use?

    In an n-berth station, n vehicles can be stored when there is no demand for service. During the night when demand is low or zero, the bulk of the vehicles will be stored at special storage barns strategically located in the network, usually at the same locations as cleaning and routine maintenance facilities. Because it is not necessary to get a specific vehicle out of storage before the others, the volume of storage facilities per mile of guideway is usually not more than would be required to store about four or five automobiles in a multi-story parking structure.

Return to PRT Home Page