Response to Comments by J.E. Anderson on the Appendix to the
Book Entitled The Urban Oasis
The following is my response to comments by J. Edward Anderson on the PRT-related portions of my book, The Urban Oasis, which have been posted on this PRT debate page.
Not being an engineer, I will not attempt to argue with Anderson about the minimum operating headways of one-half second that are technically possible for PRT according to computer simulations. However, it is not only the headways between vehicles moving on the guideway, but, in particular, the dwell times in the stations that may be the factor limiting capacity. Do PRT's assumed dwell times take into consideration the typically unpredictable behavior of human passengers?
Neither will I argue with Anderson about energy consumption, except to say that the applications of APMs of medium capacity that I was advocating would be serving zones that are either completely pedestrianized or at least traffic-calmed, and that higher levels of ridership and higher capacity requirements could therefore be predicted.
In fact, it seems that Anderson has completely missed the basic intent of my book. It's subject is one of urban space planning, rather than of pure transportation planning. The book stresses a need for pockets of environmental relief within our vast and ever-expanding metropolitan areas, which should be designed at a human scale, convenient and friendly for pedestrians, rather than for cars -and richly landscaped. Because of the enormous size of automobiles and their paved parking areas, this can only be accomplished by creating zones that are (largely) free of cars. Not that cars would be entirely and ruthlessly exiled, but they would be discouraged through street design that incorporates traffic calming, and long-term parking would be kept to the periphery. The reliably frequent service of medium-capacity APMs would be ideal as a functional framework for such an application.
Anderson's comments about my advocacy of surface-level light rail are based on a misreading of the text. I stated that light rail has worked well in the pedestrian zones of existing cities, since when cars are eliminated, the problems of accidents between light rail and cars are eliminated. It also cannot be denied that in existing cities the introduction of the elevated guideways required by APMs is often spatially and economically problematic. Light rail has been used in pedestrian zones in the centers of cities throughout Europe, with very positive results for both retail merchants and real estate values. Because the vehicles are reliably confined to their tracks (unlike buses), pedestrians know exactly where to expect them, so they do not conflict with the relaxed atmosphere of the zones. Amsterdam and Zurich are prime examples, but there are many others.
Nor do my objections to PRT have anything to do with the fact that it is not fully proven. I am a staunch advocate of unproven ideas that may offer hope of improving the human condition (as should have been obvious from the image of urban oases that I depict in my book). My objections to PRT stem, rather, from the belief that it is a basically flawed concept.
While I certainly admire the engineering virtuosity that has gone into the development of PRT, and have never tried to claim (as Anderson implies) that it is an impossible or intimidating technology, there are several reasons why I believe that its deployment would be misguided:
1. The most basic of these is that we should not be attempting to serve relatively low-density development by building more "concrete spaghetti" in the sky. This type of development can be served less expensively and less obtrusively by small, energy-efficient private cars and municipal systems of small, rentable urban "E-cars"- which, in operating on the existing network of roads, can provide literally door-to-door transportation. Our government regulations can, and certainly should, be re-tailored to discourage inefficient and oversized vehicles, such as SUVs, which are dangerous to small cars that share the same roads, and to more effectively encourage the further development and widespread dissemination of a marvelous and promising assortment of new E-cars for this purpose.
2. More specifically, as an architect, I take strong exception to PRT because of the space required for its stations, relative to capacity. To support my objections, I refer to an article written in 1997 by three PRT planners who truly believe in the concept . Rather than supporting the validity of PRT, I believe this article rather strongly confirms its basic spatial problems.
Because PRT is a small-vehicle, taxi-type system with individualized service, its operations at stations would be no more compact than those of conventional taxis operating at the street level. A series of zones would be needed for the off-line guideways at each station: a demerging zone, deceleration zone, berthing zone, departure queue, acceleration zone, and a merging zone, resulting in very long and often broad stations and extensive ramps.
In order to achieve even modest capacities of only 565 vehicles per hour per direction, a station would need 16 berths, which for 4 m (13.12 ft) long vehicles would require a berthing zone 68 m (223 ft) long, allowing a short distance between vehicles when docked . For a cruising speed of 48 km/hr (30 mph) a 34 m (112 ft) ramp would be required for deceleration and a 70 m (230 ft) ramp for acceleration, in addition to merging zones of 7.5 m (25 ft) on each end , comprising a total length of 187 m (614 ft). In addition, a departure queue would ideally follow the berthing zone, whose length might, typically, be long enough to hold one "cycle" of departing vehicles. Anderson's projected speeds of 15.5 km/hr (25 mph) would, of course, shorten the lengths of the deceleration and acceleration ramps and demerging and merging zones. But the entire length of the station and its ramps would be furnished with, in essence, a double guideway.
The length of the berthing zone itself could be cut in half if the station were 2-sided. A 2-sided platform with 6 berths on each side would increase the capacity to (a still very modest) 1382 vehicles per hour per direction .
However, this would require two parallel off-line guideways, which when added to the mainline, would mean a total of three parallel guideways.
It would certainly be difficult to justify the insertion of such a bulky, yet low-capacity station into an urban environment.
One of PRT's unique features is that each station could be sized for a different capacity, according to anticipated demand. However, a potential problem lies in the likelihood that, if an embryonic network with relatively small stations were expanded into a larger, area-wide system, ridership could be expected to grow in proportion to the increased pairs of origins and destinations on the system - and in this case stations that had been modestly sized initially would become inadequate. Therefore, authors Daly, Komerska and Anspach recommend not only phased growth plans for key stations, but the consideration of two-sided stations even at sites where single-sided stations could meet the initial anticipated demand .
There are several basic assumptions that underlie the PRT concept that I would explicitly challenge:
1. Anderson contends in his paper, "Personal Rapid Transit: Matching Capacity to Demand" - that the higher densities of development that can be stimulated and made logistically functional by higher-capacity transit are, in themselves, "socially undesirable" . This assumption is contradicted by the evidence of thousands of years of history, during which cities with ten times the densities of our automobile-generated development have performed as the cradles of human culture. It has only been with the advent, and planned dominance, of the automobile, which has exploded our per capita spatial needs and paved our landscapes, that high densities have become undesirable.
Therefore, I would turn this assumption around, to say that we need, rather, to re-define dense development, by designing it for humans rather than cars. Transit policies to alleviate congestion in our urban areas should not be based on anti-urbanism.
2. The PRT concept also assumes that the need to share transit vehicles with strangers currently serves as a significant dissuasion to the use of transit in the U.S. (hence, PRT's small "personal" vehicles). It may be observed, however, that every day a very large portion of the public take elevators with complete strangers as a matter of routine and with equanimity. Rather, the primary reason for transit's unpopularity in the U.S. is that the dispersal of our development has made it extremely inconvenient for the majority of the public to use, and therefore expensive to provide for the few who do use it. At the turn of the last century, the reverse was the case - development was clustered around all kinds of rail stations, which afforded a remarkable level of highly affordable mobility to the general public. Transit vehicles were not less popular because they were shared.
3. An additional premise in support of PRT is that, with any other mode, the need to transfer will be a major deterrent to transit patronage, and that a transit system that does not provide non-stop service is bound to fail in attracting riders. But this assumption has been based upon observations of modal split with the feeder bus - a notoriously slow and erratically available mode, since it operates in mixed traffic - and not with simple shuttle and loop AGTs. So it is not a conclusive comparison.
4. Advocates claiming that PRT would comprise the most compact of available transit options compare the sizes of its vehicles and guideways not with medium-capacity APMs, but with those of either conventional rail transit or of the large AGT systems deployed at airports, where large crowds disembarking from aircraft in large surges must be accommodated [8, 9]. So neither is this a conclusive comparison.
Two more relatively minor points:
1. Anderson suggests in his comments that the APM system at Las Colinas initially failed because it was a scheduled APM system. But the failure was due in large part because of a downturn in the local economy occasioned by the "bust" in the oil market, and because of incompatible planning. That is, not only was it serving largely vacant buildings, but a lot of parking garages were provided, which made private motoring more competitive. Nor was there a regional transit system to tie into the APM.
2. Anderson states in his comments that the "...ability of a guideway designed for low speed to handle higher speeds depends on minimum headway and random vs. equal spacing of vehicles." But it also depends very much upon the radii of horizontal curvature of the track. It is my understanding that, as speeds increase from 24 to 64 km/hr (15 to 40 mph), the required minimum radii of the guideway increase from 46 to 512 m (from 151 to 1680 ft), respectively.
1. Daly, Richard Farrell, Marblehead, MA: Transit Systems Planning, Inc., Richard J. Komerska, Transit Systems Planning, Inc., and Gretel Anspach, Marlborough, MA: Raytheon Electronic Systems, Inc. (April 1997). "Station Capacity: An Issue in PRT Planning". Paper delivered at the 6th International Conference on APMs, Las Vegas, NV. 2. Ibid. 3. Sulkin, Maurice A. (January 1999). "PRT/Deja Vu", Paper presented at the Annual Meeting of the Transportation Research Board, Washington, DC. 4. Daly, Komerska, Anspach, 1997, op. cit. 5. Ibid. 6. Ibid., pages 9 and 10. 7. Anderson, J. Edward (February 1998). "Personal Rapid Transit: Matching Capacity to Demand". Advanced Transit Association, Mammoth Lakes, CA 8. Ibid. 9. Wilde, William A., Spring 1998, "The Simple, Compelling Case for PRT", Journal of Advanced Transportation, vol. 32, no. 1, pp. 45-55.
Last modified: December 16, 1999