POTENTIAL ROLE OF PERSONAL RAPID TRANSIT
This is a Panel Report from a study
entitled Automated Guideway Transit: An Assessment of
PRT and Other New Systems, United States Congress,
Office of Technology Assessment, June 1975, pp 12-16, U.S.
Government Printing Office, Washington, D.C.
Since the term "Personal Rapid Transit" first
entered the transit vocabulary in 1968, this highly
innovative concept has fascinated many transportation
planners. PRT offers personalized service with small vehicles
which provide non-stop transportation from origin to
destination at short headways. To date, no systems which can
be classified as PRT are in revenue service or under
construction in the United States, but several test track
installations have been built in Europe and in Japan.
Proponents of PRT view this concept as a reasonable
supplement to the private automobile in high density urban
areas (i.e. greater than 3,000 persons per square mile) and
claim that PRT can provide a very much higher level of
service than other modes of public transportation. Thus, it
is argued that PRT systems would attract a significant
percentage of the rides now being made in private automobiles
and offer obvious benefits:
- less traffic congestion in urban areas
- less land and fewer f acilities used for automobile
- reduced travel time under more comfortable
- less noise and air pollution
- reduction in consumption of petroleum-derived fuels
- reduction in requirements for new arterial roads and
It is also contended that PRT would provide greater
mobility for the transportation disadvantaged, i.e., the
young, the elderly, the poor, and the handicapped.
Proponents admit that the area-wide networks with
closely spaced stations and large numbers of vehicles would
be expensive to build and, perhaps, to operate. The initial
capital cost might equal that of rail rapid transit systems,
but levels of service are envisaged to be much higher than
with conventional modes, except perhaps taxicabs. Proponents
claim the higher service levels will attract significantly
greater patronage than conventional transit. Automation is
expected to allow the high service level to be delivered at a
cost the public is willing to pay.
PRT capacity depends upon short headways: Except in
downtown areas, headways need not be closer than those of GRT
systems (i.e., three seconds). In downtown areas, headways on
the order of 1/2 second would be needed to move 10,000 people
per hour over a single PRT guideway at an average occupancy
of 1.4 people per vehicle. This is roughly equivalent to the
number of people moved on four freeway lanes.
Advocates of PRT have estimated that there are 10,000
square miles of urban areas in the United States where PRT
service might be appropriate. This would require about 20,000
miles of one-way guideways and about three million PRT
Because there are no operating systems, there is no
empirical evidence on PRT. Many of the studies reviewed were
motivated by attempts to sell or reject the concept and were
based upon largely arbitrary assumptions. Therefore, there
are many detailed issues for which objective data are
Technical -- With few exceptions, the engineers
and manufacturers who have made serious studies of the PRT
concept find that there are numerous technical problems that
must be solved before PRT systems can be deployed. Technical
solutions have been proposed but not validated, and a large
program of development testing and demonstration would be
needed to implement a PRT system. Estimates of time and money
required to achieve market-ready systems vary widely.
However, there appears to be general agreement that at least
10 years would be required depending upon the level of
The following technical problems need to be solved:
- Computer control systems must be developed
to exercise command and surveillance over thousands of
vehicles traveling between hundreds of stations at
fractional-second headways. Vehicle management (particularly
the storage of empty vehicles and their redistribution to
satisfy changes in demand) further complicates this
- Advanced control and braking systems must be perfected
to insure that vehicles can be operated safely at very close
- Major improvements in reliability -- far beyond those
levels which have been achieved for any transit equipment in
operation -- are required. Engineering techniques from other
fields may be applicable to this problem.
- Crash survivability should be demonstrated for PRT,
possibly using techniques similar to those required for
automobiles by the National Highway Traffic Safety
Administration. Means for emergency evacuation should be
provided to insure passenger safety in the event of a
- Study of alternative engineering approaches is required
to develop cost-effective systems and components.
Economic -- The economic characteristics of PRT
are so unclear that meaningful analysis is difficult. Several
analyses have been attempted, including one by the Aerospace
Corporation for the Los Angeles area, a general study by the
DOT Transportation Systems Center (TSC), and one of the Twin
Cities area by De Leuw Cather. Cost assumptions vary greatly.
Proponents' estimates for PRT vehicles, for example, are
based upon large production runs, and the estimated cost per
unit presumably goes down with increased production. As
another example, costs are related to solutions to potential
social problems. If passenger security considerations require
the installation of closed circuit T.V. throughout the
system, including vehicles, then the costs would rise
appreciably. Costs for operation and maintenance also vary.
Proponents' estimates assume maintenance levels that are
unrealistically low for transit.
The major economic issue is whether research. without
hardware development and urban demonstration, can answer the
economic questions, or whether hardware development is
necessary to assess the economic characteristics of PRT
Social -- Public acceptance of PRT is open to
question. Despite the many potential advantages of PRT in
comparison to other transit modes, there is a serious
question that the associated proliferation of elevated
guideways and stations would be acceptable to the public,
particularly in residential neighborhoods. Also, the safety
and security aspects of unattended small vehicles require
Advocates contend that PRT should duplicate as closely
as possible the service characteristics of the private
automobile. The wisdom of attempting to provide such a high
level of service at public expense is open to question.
Whether the benefits of such a system would only accrue to
the well-to-do or whether they would also provide for the
needs of the transit disadvantaged is worthy of exploration.
An annual expenditure equal to the debt service on the
capital cost of such a high technology system, when combined
with traffic management systems designed to enhance
conventional transit, might provide better service at lower
cost over a larger service area, than PRT. These observations
may be equally true for other capital intensive systems. Such
tradeoff studies should be undertaken and clear urban transit
goals articulated by UMTA before the agency embarks on new
systems for their own sake.
The major social issues of PRT are summarized as
- What urban objectives will be served by a
- What is the overall social acceptability of PRT, and
what lessons can be learned from less sophisticated AGT
- Can PRT systems offer adequate passenger security,
particularly in numerous unattended stations?
- What environmental impact will guideways and stations
have on the neighborhood?
Institutional -- Groups in Germany, Japan and
France are actively engaged in PRT research and development.
The possibility of cooperative arrangements between United
States firms or the United States Government and their
overseas counterparts thus exists. Such efforts, building
upon United States experience and accomplishments in SLT and
GRT and overseas research in PRT, could lead to stronger and
more cost-effective development programs. On the other hand,
the United States has pioneered much of the work in new
transportation systems and could develop the technology if a
need exists for PRT.
The effect on U.S. balance of payments must be
considered if equipment licenses or royalty payments for the
use of foreign patents are required. Such payments, however,
will be only a small part of the costs for building a system
because most transit system costs are for construction. Thus,
potential foreign exchange savings are too small to justify a
large investment in domestic R & D. PRT poses major
institutional issues as follows:
- Should PRT systems be a substantial part of
UMTA's R&D effort?
- Should other arrangements be considered for PRT
development and deployment?
- To what extent is international cooperation possible and
Before major commitments of funds are made for detailed
simulations or hardware developments, research is required to
resolve the many uncertainties concerning the proper role of
PRT systems, their social acceptability and their economic
feasibility. These preliminary studies may involve
expenditures of $4 to $6 million.
There are possibilities for cooperation with foreign
governments or overseas suppliers in research and development
of PRT. UMTA has recognized these possibilities in starting
negotiations with the West German Government.
Last modified: September 19, 1998