Response to Comments by Roxanne Warren to Comments by J. E. Anderson on the Appendix to her book The Urban Oasis.


J. Edward Anderson , President, Taxi2000 Corporation

January, 2000

The basic discussion in this exchange is about urban form. I will discuss that and its relation to transit first, and then will respond point-by-point to various technical issues Warren has raised.

Urban Form

Should we attempt to improve transit for urban areas as they exist, should we work to develop relatively car free zones in which population density is increased, or is there a way that both can be done? This is the whole debate about New Urbanism.

I am a mechanical engineer, not an urban planner, so in the 30 some years I have been involved with questions of transit I have consulted urban planners, architects, landscape architects, transportation geographers, transportation economists, political scientists, sociologists, and others interested in these issues. In the early 1970s at the University of Minnesota I coordinated a Task Force on New Concepts in Urban Transportation consisting of 15 professors in almost as many fields, plus transit commissioners and members of the League of Women Voters. In addition to working on, lecturing on, and teaching courses in transit systems analysis and design for many years in 1970 I organized, coordinated, and lectured for 15 years in a large interdisciplinary course called "Ecology, Technology, and Society," in which usually over 20 professors lectured on a wide variety of issues related to the environment and the implications of technology on society. This involvement made me much more aware of the problems of the automobile in modern society than I had been as a mere user, although I had watched one spot after another of grass, bushes and trees became parking lots, I had been involved in debates over urban freeways, and after freeways were built noted the noise, air pollution and lead poisoning they generated. So I agree very much with Warren's position that the auto has been a major factor in the deterioration of urban environments. The challenge has been to devise ways to do something about it within a democratic society.

I mention this background because in the Preface to Warren's book there is the sentence: "Far-reaching decisions are very often made by technical specialists who, no matter how gifted and erudite they may be in their own fields, approach the problem with a kind of tunnel vision, basing their proposed solutions only on impeccable mathematical formulae, and purposely avoiding a universalist approach."(italics mine). Having observed that many fundamental problems of society fell in the cracks between disciplines, I went out of my way to prevent that from happening in my work, yet I have not neglected the necessary engineering science, which involves a good deal of mathematics.

During the past few years, the terms "Smart Growth" and "New Urbanism" have become more and more frequently heard. I am much aware of problems of urban sprawl, but also aware of how difficult it will be in a free society to slow it down, particularly in the present information age where many people can live and work wherever they wish. I quote here from an issue of Ken Orski's Innovation Briefs for Jan/Feb 2000.

"Smart Growth" – The Cure May Be Worse Than The Malady

Commenting on the "smart growth" movement in our Sept/Oct 1999 Brief, "In Search of Livability," we quoted a local Montgomery County, MD official as saying "…the issue of sprawl does not energize the electorate. Quite the contrary, it's the prospect of higher densities that brings out citizen opposition… If people conclude that 'smart growth' means infill development, higher densities and more crowding, the public will turn against it..." His words were prophetic. Two recent events illustrate the strength of the emerging anti-densification backlash.

He went on to report that a Berkeley Planning Commission rejected a proposed smart-growth land-use plan that contained the latest in smart-growth concepts: high-density, mixed-use developments with height limits that would have tripled or quadrupled existing height limits. He then reports a similar rejection in Montgomery County, Maryland. The reader is urged to read and ponder the entire brief as well as similar articles that can be found on the Internet.

It is too late to worry now about how autos help or hurt cities. The auto genie is out of the bottle. Except for cases in which whole new cities or distant suburban lands are being developed, no land-use policy or code in the current cities and inner suburbs could affect more than a small margin of future auto use in such places. The real challenge is to develop transit alternatives to the auto that could provide some of the advantages of the auto and reduce some of its disadvantages. The current developed areas that were the sprawl areas of the 1960's-1990's are traffic-congested because the people who live and/or work in these places don't have transit that is spread around enough to enable them to get to where they need to go. So, those who have autos use them. Those who don't have autos or can't drive on account of age or disability are too often marooned and isolated.

Plans for developments such as Warren's Urban Oases confront political realities. My circumstances have been such that I lived in apartments and condos for almost 30 years so I appreciate the need for such facilities and am not opposed to them. Moreover, if I were to have sold my house and moved to a high-density unit in order to be near a transit line, someone who wanted a low-density living arrangement would have bought my house and used his auto. History shows that there are at least as many, and probably more people wanting low-density arrangements than the other way around. No densification policy or land use code is going to roll back or repeal the existing suburban areas. So, residents of these areas will need to use their autos until we provide high service transit modes low enough in cost to be widely diffused to link the main activity centers and residential areas of such suburban areas.

The scale of developments such as Warren advocates reminds me of the disastrous results of densification produced by Great Society programs in the 1960s. Perhaps it is possible to avoid past errors, but caution is needed and concentration only on high-density nodes neglects the transit needs in vast portions of urban areas of single-family housing and it may neglect the need for suitable affordable housing for low-income people. When public investment is urged, as Warren does on page 159 of her book, there is a very real concern about the bureaucracies that would be put in place to carry out such programs. Why can we expect them to behave any differently than they have?

A concern I have is expressed in the following sentence at the bottom of page 31 of Warren's book: "It has therefore long been an axiom of transportation planning that, as demands for new housing and job facilities arise, they should be built, to the greatest extent possible, at densities sufficiently high to justify the operation of associated mass transit services."(italics mine)  Is this the core of the argument? Do transportation planners urge high-density development because only then will conventional rail-transit systems be cost effective? Do transportation planners approach their problem with "a universalist approach" or are they simply looking for a way to justify the rail-transit systems they understand?

Here, a digression into some simple arithmetic is worthwhile: Consider a freeway lane. Based on experience, it can carry about 2000 automobiles per hour. The average auto occupancy in the rush period in the U. S. is no more than about 1.1 people per vehicle, so a freeway lane can handle about 2200 people per hour. Wendell Cox (praise be to him for clarifying a great deal of transit data) at his website , under the heading REFUTING THE 6-LANE MYTH shows (in data obtained from federal sources) that the most heavily used light-rail line in the United States actually carries per day on the average about one third the capacity of one freeway lane. (I know that Warren advocates much smaller vehicles than conventional light rail at headways down to about one minute, but I need to start here.) To find the peak-hour flow, it is necessary to multiply the daily flow by the ratio of peak-hour to daily flow. The later ratio is about 0.1 for automobiles on freeways and about 0.16 for rail systems. Using these figures, the peak-hour flow on the most heavily used light-rail line in the U. S. would be about (2200/3)(1.6) or about 1200 people per hour. The average station flow on such a line will be less by the ratio of station spacing to average trip length, but of course downtown stations will carry more. Assuming the station spacing is one mile and the average trip length is a typical five miles, the average station flow would be 240 people per hour.

Now a question that must be asked is this: Since automobiles averaging 1.1 people each can carry 2200 people per hour per freeway lane, why do light-rail trains have to be large to carry a fraction of the capacity of a freeway lane? The answer is that they must stop on line, whereas automobiles exit the freeway before stopping. By stopping on line and also operating on surface streets, the common spacing between light rail trains for reasons of safety is usually no closer than about 6 minutes, which gives 10 trains per hour per direction. So to carry 1200 people per hour, each train must carry on the average 120 people. Typically, modern light-rail vehicles have a capacity of 180 persons, weight 80,000 lb, and for a two-way system require a right-of-way of about 28 feet. Since a freeway lane is typically 12 feet wide, the land taken for the light-rail system could carry by auto about twice as many people per hour. The expense of these heavy systems (notwithstanding the name "light rail") is such that urban density must be increased to make them cost-effective.

Why must these trains stop on line? Because they can't switch rapidly and reliably enough to do otherwise. Why? Because that is the nature of the in-track switch, a technology worked out in the 1880s and still the basis of modern rail systems. The irony is to have watched the enormous advances during the 20th Century in aviation, automobiles, ships, computers, telecommunications, etc., and yet to see transit still clinging to 19th Century concepts as we enter the 21st.

The simple expedient is to put the switches in the vehicles with no moving parts in the track, to use modern dual redundant computer systems and state-of-the-art electronic sensing, and to do the math needed to make it all work optimally. Then with stations placed on by-pass guideways off the main line, the vehicles can be very small, corresponding to the way people actually prefer to travel as witnessed by rush-period auto occupancy, and by use of linear electric drives they can operate with a very high level of safety at headways substantially smaller than practical with manually driven automobiles. With very small vehicles, the guideway can be of minimum size and cost, and the system can be cost effective, even profitable at today's population densities. This is the logic of personal rapid transit, and the reason there is growing interest in it. With the smallest vehicles, the trips can be nonstop and considerable analysis has shown that nonstop travel requires that one person or a small group traveling together by choice occupy each vehicle. With nonstop travel, the average trip speeds can equal and even exceed those of autos in the denser portions of cities, so ridership, according to recent Swedish studies, markedly increases.

So the feature of "no riding with strangers" is a result of a cost-effectiveness analysis, not an initial assumption. Is this a good thing? More than 97% of urban trips are made that way now, and it permits people to travel more quickly, from a wide variety of locations, when they need to, where they need to go, and where socialization takes place. It does not take much experience in riding large-vehicle transit to note how deliberately most people avoid eye contact. We have found from much comment, particularly among women, that not knowing who may board when one is alone in the late hours is a strong deterrent to use of an automated horizontal transit system, notwithstanding that people do exactly this in elevators. On city buses at late hours assault is a common occurrence. I have seen the data. Our objective is to attract riders to transit, not to drive them away.

So the bottom-line question of this essay is: Should attempts be made to densify cities to make conventional transit work, or should we use our ingenuity to develop transit systems that will work economically in cities as they are? Without being at all "anti-urbanist," having no problem with higher-density developments, and after consulting a wide variety of professionals, I have opted for the latter. Vast already-settled portions of urban areas are virtually unserved by conventional transit and the only transit we see that can genuinely provide an alternative to auto travel is PRT, not any PRT, but carefully optimized PRT, which requires intensive application of engineering science and the mathematics associated with it. Such systems have several times the capacity of one freeway lane, which from the above analysis indicates that they could handle 6 to 8 times the flow actually attracted to light rail.

So high-capacity PRT could be used wherever modern light-rail systems have been used and will, because of their service characteristics, attract many more riders [2].  The need for profitability is an important objective, and it guides our work. The argument that PRT would not have sufficient capacity to be applied in denser portions of most cities is not valid. The APM systems with relatively small vehicles that Warren advocates have less capacity than light rail and retain the feature of on-line stopping. PRT will and can start out in small applications and slowly enough to permit ample time for understanding of its characteristics. It can be expanded only if it is found to meet expectations better than alternatives, and, in my wish, only if it can be expanded profitably as a private enterprise. Our objective now is to prove an optimized PRT system so that it can be included as one of the alternatives available to transportation planners.

Responses to some technical points raised in Warren's response

Dwell Time. The dwell times we use are derived from human-factors experiments with a wide variety of people. With one seat per vehicle 50 inches wide, to provide room for a wheelchair riding sideways, and a statistical distribution of occupancy that gives an average of about 1.5 people per vehicle, we have been using a normal distribution of dwell times with a conservative mean of 6 sec, a standard deviation of 2.5 sec, and we permit a maximum dwell up to 20 sec. In addition, we assume a door opening plus closing time of 4 sec. A paper by Anspach  [3] states that in analysis of the Raytheon PRT 2000 dwell times of 20 seconds were used in the system simulations. By simply taking a watch and observing how long it takes for one, two, or three people to walk through a door three or four feet away and sit down, it may be seen that a dwell time of 20 seconds is much too long for an average person or small group. This is an important factor in the predictions of PRT station capacity.

Ridership. I agree that with more convenient transfers in shuttle-loop systems, the transfer should be less onerous than in conventional bus systems, but how much better I don't know. Careful studies need to be made. Transferring from one 100-meter APM to another, as stated on page 145 of Warren's book, may be acceptable for a few such transfers, but beyond that I wonder.

Light Rail in Pedestrian Zones. These vehicles are advertised to be quiet—so quiet that there is a growing body of data on people being run over by them and killed. It troubles me that advocates of these systems ignore these events. Exclusive guideway systems such as all types of AGT do not have this problem.

Concrete Spaghetti. Our guideways are steel, not concrete, and designed to be as small as practical—about three feet by three feet in cross section. One leading Chicago sculptor referred to our system as "moving sculpture." All is in the eye of the beholder. When compared to alternatives, I find increasing enthusiasm for elevated PRT, but recognize that it will not work everywhere. Introducing surface-level systems that I know Warren does not advocate cannot reduce congestion, whereas congestion relief is the reason the public wants alternative transit. We are very sensitive in our designs to minimization of the visual impact and we have worked with some leading architects and landscape architects on this question. PRT need not be everywhere in an urban area to be useful, and it must start small.

Station Sizes. We have worked a great deal on this problem. As mentioned in my previous comments, by using only 0.1 sec of on-line deceleration into and acceleration out of a station, we can reduce the off-line length by more than 100 feet. In addition, by designing the transition to the maximum local speed we can reduce its length by over 30%. For a feeling for required station sizes in a rather dense urban area, see the paper "PRT: Matching Capacity to Demand".From many simulations we have done, most stations will require only three or four berths. The length of the vehicle and hence each berth in determining station length and maximum throughput is an important reason to design the vehicle to be as short as practical. With one bench seat the vehicle can be as short as about 8.5 feet, whereas in the examples in Warren's response, the vehicle was 13.5 feet long. In our simulation for Cincinnati, at the Reds Ball Park, we used 14-berth stations to more than match the demand.

Around this stadium we used a line speed of 15 mph and found the off-line length to be 462 ft. The length of the off-line guideway for a typical three-berth station is 238 ft for a line speed of 20 mph, 263 ft for 25 mph, 288 ft for 30 mph, etc. Because of the expense and the problem of direct interfacing with a building, we do not recommend stations with two adjacent off-line guideways, and we have not found applications in which they would be necessary. Disparaging PRT because it uses off-line stations, disparages it for the very reason it is a major breakthrough. As I stated in my previous response to Warren's book, by use of design optimization techniques we typically find that the total off-line guideway length is only about 12% of the total mainline guideway length. When comparing the scale of optimized PRT with a highway, the addition of the off-line guideway is relatively quite small.

Guideway Sizes. I am aware that the small APM systems Warren mentions have much smaller guideways than conventional rail systems, and that is a good thing. The comparison, however, involves many more factors. In the third paragraph of my last communication, I pointed out how PRT won handsomely over on-line-station APM systems in studies in Sweden and at SeaTac International Airport.

Guideway Radii. For 15 mph Warren gives a curve radius of 151 ft, and for 40 mph you 1680 ft. From mechanics the horizontal acceleration in a curve is V2/R, where V is the speed and R is the radius. For 15 mph, a 151-ft radius gives a horizontal acceleration of 0.10 g; and for 40 mph, a 1680-ft radius gives 0.064 g, so Warren's results are not consistent. Moreover, for seated passengers, the comfort lateral acceleration is 0.25 g. (A value of 0.1 g is low for standing-passenger vehicles.) But the curve can be superelevated, i. e., banked up to 12 deg., in which case a horizontal acceleration of at least 0.47 g meets standard ride-comfort criteria. Hence, for 15 mph, the radius of a superelevated curve is 32 ft, and for 40 mph 228 ft.


To meet the basic environmental objectives stated in Warren's book, with which I agree, PRT is needed. It is a tool to accomplish many needs of the modern urban environment. There is nothing about it that is "flawed." Fighting PRT will guarantee exactly the conditions that Warren wishes our cities to avoid [4]. Our intention is to bring our system to maturity using private funds. Others have similar intentions. The state of our design is such that we can do that in less than two years. We appreciate Warren's comments and examine carefully all opposing views. They continue to strengthen our case. We live in a time of rapidly changing technology. Such changes need to be applied to urban transportation.


1.  J. E. Anderson. "Technology, Society and the Future," Futurist, 3:3 (1979) 259-275.

2.  On page 876 of Hearings before a Subcommittee of the Committee on Appropriations, House of Representatives, 93rd Congress, March 1973, Frank C. Herringer, then Administrator of the Urban Mass Transportation Administration, said: "This means that a high-capacity PRT could carry as many passengers as a rapid rail system for about one quarter the capital cost."

3.  Gretel Anspach, "PRT 2000 Operational Characteristics and Modeling," Proceedings of Automated People Movers VI, Las Vegas, Nevada, April 9-12, 1997, ASCE, 1801 Alexander Bell Drive, Reston, VA 20191.

4.  John Pucher and Christian Lefèvre, The Urban Transport Crisis in Europe and North America, MacMillan Press Ltd., Houndmills, Basingstoke, Hampshire RG21 6XS, conclude their study of the performance of conventional transit with the dismal statement: "The future looks bleak both for urban transport and for our cities: more traffic jams, more pollution and reduced accessibility."


Last modified: January 15, 2000