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 [1]. 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 [2]. 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 [3], 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[4]. 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 [5].
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 [6].
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" [7]. 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.
References:
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.
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Last modified: December 16, 1999