Personal rapid transit

Personal rapid transit (PRT), also called personal automated transport (PAT), is a category of proposed public transportation systems designed to offer automated on-demand non-stop transportation, usually targeted at urban use, on a network of specially-built guideways.

Although generally promoted as a potential wide-scale urban transport system, to date only pilot programmes have been constructed and as of 2007, the two most advanced projects are small-scale schemes: ULTra at Heathrow Airport in London, which is scheduled to be the first public PRT system to open, and one at the Dubai International Financial Centre in Dubai. Both are scheduled to come into operation in 2008.

PRT is one of a number of schemes being proposed to solve urban traffic problems, with competitors including light rail, monorail, modern trams, bus rapid transit, guided busways and conceptual systems such as "driverless car" systems and auxiliary wire guidance for private motor vehicles.

The difficulties faced by PRT in achieving wide-scale implementation have been described as "formidable, but not insurmountable". Barriers to wider deployment include lack of existing systems, proprietary technology, technically unproven, evacuation concerns.

PRT is a political football in the Twin Cities area, where proponents have worked hard to persuade the local authority to fund a system. This bid failed in 2006 but looks to be revived in 2007, possibly as a stalking horse against light rail.

Overview
PRT is a system of small vehicles under independent or semi-independent automatic control, running on fixed guideways. The idea attempts to address a number of perceived weaknesses of public mass transit including fixed timetabling, limited routes, and sharing travel space with unrelated travellers (see comparison below).

In 1988, The Advanced Transit Association (ATRA), a group which advocates the use of technological solutions to transit problems, published a definition for PRT as follows :
 * Fully automated vehicles capable of operation without human drivers.
 * Vehicles captive to a reserved guideway.
 * Small vehicles available for exclusive use by an individual or a small group, typically 1 to 6 passengers, traveling together by choice and available 24 hours a day.
 * Small guideways that can be located aboveground, at ground level or underground.
 * Vehicles able to use all guideways and stations on a fully coupled PRT network.
 * Direct origin to destination service, without a necessity to transfer or stop at intervening stations.
 * Service available on demand rather than on fixed schedules.

The definition does not specify a particular technology, such as electric motors, linear motors, magnetic levitation, or rubber wheels. It does not specify whether vehicles are to be supported on the guideway or suspended from the guideway. Instead, it is derived from analysis of the functionality, efficiency, scalability, and service provided by the total engineering and design of the system.

Proponents say that the low weight of small vehicles has the important benefit of allowing smaller guideways and support structures compared to other mass transit systems like light rail, translating into lower construction cost, smaller easements, and less visually obtrusive infrastructure.

Sceptics note that many of the easements for traditional urban transit modes run along existing roadways and can be negligibly visually intrusive (e.g. tram tracks).

The literature
The body of PRT literature is small, typically favorable toward the concept, and "might be improved by greater introspection and criticism".

The concept has been independently reinvented many times since the 1960s. It is considered controversial, and the city-wide deployment with many closely-spaced stations envisaged by proponents has yet to be constructed. Past projects have failed due to lack of financing, cost overruns, regulatory conflicts, political issues and flaws in engineering or design. Currently, a small number of proponents dominate the literature.

From 2002–2005, the EDICT project, sponsored by the European Union, conducted a study on the feasibility of PRT in four European cities. The study involved 12 research organizations, and concluded that PRT: The report also concluded that, despite these advantages, public authorities will not commit to building PRT because of the risks associated with being the first public implementation.
 * would provide future cities "a highly accessible, user-responsive, environmental friendly transport system which offers a sustainable and economic solution."
 * could "cover its operating costs, and provide a return which could pay for most, if not all, of its capital costs."
 * would provide "a level of service which is superior to that available from conventional public transport"
 * would be "well received by the public, both public transport and car users."

History
Some of the key concepts of PRT has been toyed with since before the 1900s, but the full concept of PRT really began around 1953 when Donn Fichter, a city transportation planner, began research on PRT and alternative transportation methods. In 1964, Fichter published a book entitled "Individualized Automated Transit in the City", which proposed an automated public transit system for areas of medium to low population density. In 1966, the United States Department of Housing and Urban Development was asked to "undertake a project to study … new systems of urban transportation that will carry people and goods … speedily, safely, without polluting the air, and in a manner that will contribute to sound city planning". The resulting report, entitled "Tomorrow's Transportation: New Systems for the Urban Future," was published in 1968, and proposed the development of PRT, as well as other systems such as dial-a-bus and high-speed intraurban links.

In the late 1960s, the Aerospace Corporation, an independent non-profit corporation set up by Congress, spent substantial time and money on PRT, and performed much of the early theoretical and systems analysis. However, this corporation is not allowed to sell to non-federal government customers. Members of the study team published in Scientific American in 1969, the first wide-spread publication of the concept. The team subsequently published a text entitled Fundamentals of Personal Rapid Transit.

In 1967, aerospace company Matra started the Aramis project in Paris. After spending about 500 million francs, the project was cancelled when it failed its qualification trials in November 1987. The designers tried to make Aramis work like a "virtual train," but control software issues caused cars to bump unacceptably. The project ultimately failed. It is described in the book Aramis, or the Love of Technology by Bruno Latour.

Between 1970 and 1978, Japan operated a project called Computer-controlled Vehicle System (CVS). In a full scale test facility, 84 vehicles operated at speeds up to 60 km/h on a 4.8 km guideway; one-second headways were achieved during tests. Another version of CVS was in public operation for six months from 1975–1976. This system had 12 single-mode vehicles and four dual-mode vehicles on a one-mile track with five stations. This version carried over 800,000 passengers. CVS was cancelled when Japan's Ministry of Land, Infrastructure and Transport declared it unsafe under existing rail safety regulations, specifically in respect of braking and headway distances.

In 1972, President Nixon announced a federal PRT development program, saying "If we can send three men to the moon 200,000 miles away, we should be able to move 200,000 people to work three miles away."

On March 23, 1973, U.S. Urban Mass Transportation Administration (UMTA) administrator Frank Herringer testified before Congress: "A DOT program leading to the development of a short, one-half to one-second headway, high-capacity PRT (HCPRT) system will be initiated in fiscal year 1974." However, this HCPRT program was diverted into a modest technology program. According to PRT supporter J. Edward Anderson, this was "because of heavy lobbying from interests fearful of becoming irrelevant if a genuine PRT program became visible". From that time forward people interested in HCPRT were unable to obtain UMTA research funding.

During the last oil crisis, siting "A severe recession or actual depression" on September 10, 1974 the United States Senate Committee on Appropriations requested a study of long term solutions to the oil crisis. In June 1975 US DOT responded with a 375 page study "Automated Guideway Transit: An Assessment of PRT and Other New Systems". The report accurately identified past innovation failures and the future of PRT for the next 37 years, "UMTA's R&D programs [US DOT Urban Mass Transit Administration] … have neglected near-term … simpler approaches to correct transit problems."

In 1975, the Morgantown Personal Rapid Transit project was completed. Despite its name and fact that it has five off-line stations that enable non-stop, individually programmed trips that are characteristic of PRT, this is not considered a PRT system by authorities because its vehicles are too heavy and carry too many people, and because most of the time it does not operate in a point-to-point fashion, running instead like an automated people mover from one end of the line to the other. The PRT system is still in continuous operation at West Virginia University in Morgantown, West Virginia with about 15,000 riders per day (as of 2003). It successfully demonstrates automated control, but was not sold to other sites because the heated track has proven too expensive.

In the 1970s and 1980s, Mannesmann Demag and MBB cooperated to build the Cabinentaxi project in Germany. They created an extensive PRT development which was considered fully developed by the German Government and its safety authorities. This project was canceled when a disagreement over the site for the initial implementation coincided with non-defense budget cuts by the German government.

In the 1990s, Raytheon invested heavily in a system called PRT2000 that was based on technology developed by J. Edward Anderson at the University of Minnesota. Raytheon failed to install a contracted system in Rosemont, Illinois, near Chicago, when estimated costs escalated to US$50 million per mile, allegedly due to design changes that increased the weight and cost of the Raytheon system relative to Anderson's original design. In 2000, rights to the technology reverted to the University of Minnesota, and were subsequently purchased by Taxi2000.

In the late 1990s, Douglas Malewicki started the SkyTran project, later renamed UniModal. His proposal calls for vehicles with relatively few moving parts and features such as speech recognition. By using Inductrack passive magnetic levitation, expected vehicle speeds are 100 mph (160 km/h); assumptions of capacities are based on these speeds and on half-second headways.

In 2002 2getthere operated 25 4-passenger "CyberCabs" at Holland's 2002 Floriade horticultural exhibition. These transported passengers along a track spiraling up to the summit of Big Spotters Hill. The track was approximately 600 meters long (one-way) and featured only two stations. The 6-month operations were mainly intended to research the public acceptance of PRT-like systems. The CyberCab as designed for the exhibition, being very open, is comparable to a Neighborhood Electric Vehicle, except it steers itself using magnet guidance points embedded in the lane.

In 2003, Ford Research proposed a dual-mode system called PRISM. It would use public guideways with privately-purchased but certified dual-mode vehicles. The vehicles would weigh less than 600 kg (1200 lb), allowing small elevated guideways that could use centralized computer controls and power.

In January 2003, the prototype ULTra ("Urban Light Transport") system from Advanced Transport Systems Ltd. in Cardiff, Wales, was certified to carry passengers by the UK Railway Inspectorate on a 1 km test track. It had successful passenger trials and has met all project milestones for time and cost to date.

In October 2005, ULTra was selected by BAA plc for London's Heathrow Airport. This system is planned to transport 11,000 passengers per day from remote parking lots to the central terminal area. PRT is favored because of zero on-site emissions from the electrically powered vehicles. PRT will also increase the capacity of existing tunnels without enlargement. BAA plans to begin operation by the summer of 2008 and to expand the system in 2009.

In June 2006, a Korean/Swedish consortium, Vectus Ltd, started constructing a 400 metre test track in Uppsala, Sweden.

System design
Among the handful of prototype systems (and the larger number that exist on paper) there is a substantial diversity of design approaches, some of which are controversial.

Vehicle design
Vehicle weight influences the size and cost of a system's guideways, which are in turn a major part of the capital cost of the system. Larger vehicles are more expensive to produce, require larger and more expensive guideways, and use more energy to start and stop. If vehicles are too large, point-to-point routing also becomes less economically feasible (for example, when the system at West Virginia University moved from six-passenger to 20-passenger vehicles, point-to-point operations were largely abandoned). Against this, smaller vehicles have more surface area per passenger (thus have higher total air resistance which dominates the energy cost of keeping vehicles moving at speed) and larger motors are generally more efficient than smaller ones.

The number of riders who will share a vehicle is a key unknown. In the U.S., the average private automobile carries 1.16 persons, and most industrialized countries commonly average below two people. Based on these figures, some have suggested that two passengers per vehicle (such as with UniModal), or even a single passenger per vehicle is optimum. Other designs choose larger vehicles, making it possible to accommodate families with small children, riders with bicycles, and disabled passengers with wheelchairs. As of 2006, all systems known to be under active development use four-passenger vehicles.

Propulsion
All current designs are powered by electricity. In order to reduce vehicle weight, power is generally transmitted via lineside conductors rather than using on-board batteries. According to the designer of Skyweb/Taxi2000, J. Edward Anderson, the lightest system is a linear induction motor (LIM) on the car, with a stationary conductive rail for both propulsion and braking. LIMs are used in a small number of rapid transit applications, but most designs use rotary motors.

Switching
Most designers avoid track switching, instead advocating vehicle-mounted switches or conventional steering. Designers say that vehicle-switching simplifies the guideway, makes junctions less visually obtrusive and reduces the impact of malfunctions, because a failed switch on one vehicle is less likely to affect other vehicles.

Track switching also greatly increases headway distance. A vehicle must wait for the previous vehicle to clear the track, for the track to switch and for the switch to be verified. If the track switching is faulty, vehicles must be able to stop before reaching the switch, and all vehicles approaching the failed junction would be affected.

Guideways


There is some debate over the best type of guideway. Among the proposals are beams similar to monorails, bridge-like trusses supporting internal tracks, and cables embedded in a roadway. Most designs put the vehicle on top of the track, which reduces visual intrusion and cost as well as facilitating ground-level installation. An overhead track is necessarily higher, but may also be narrower. Most designs use the guideway to distribute power and data communications, including to the vehicles. Addressing some issues with prototypes, many proposals also aim to be self-clearing in bad weather.

Stations
Proposals usually have stations close together, and located on side tracks so that through traffic can bypass vehicles picking up or dropping off passengers. Each station might have multiple berths, with perhaps one-third of the vehicles in a system being stored at stations waiting for passengers. Stations are envisioned to be minimalistic, and not include facilities such as rest rooms. For elevated stations, an elevator may be required for accessibility.

Some designs have included substantial extra expense for the track needed to decelerate to and accelerate from stations. In at least one system, Aramis, this nearly doubled the width and cost of the required right-of-way and caused the nonstop passenger delivery concept to be abandoned. Other designs have schemes to reduce this cost, for example merging vertically to reduce the footprint.

Headway distance
"Headway distance" can mean "distance/time between vehicles (front to back)" or "distance/time between the fronts of vehicles (front to front)". Usually the latter is referred to when talking about capacity and vehicle frequency.''

Spacing of vehicles on the guideway influences the maximum passenger capacity of a track, so designers prefer smaller headway distances. Computerized control theoretically permits closer spacing than the two-second headways recommended for cars at speed, since multiple vehicles can be braked simultaneously. There are also prototypes for automatic guidance of private cars based on similar principles.

Very short headways are controversial. Some regulators (e.g. the UK Railway Inspectorate, regulating ULTra) are willing to accept two-second headways. In other jurisdictions, existing rail regulations apply to PRT systems (see CVS, above); these typically calculate headways in terms of absolute stopping distances, which would restrict capacity and make PRT systems unfeasible. No regulatory agency has yet endorsed headways as short as one second, although proponents believe that regulators may be willing to reduce headways as operational experience increases.

Capacity
PRT is usually proposed as an alternative to rail systems, so comparisons tend to be with rail. PRT vehicles seat fewer passengers than trains and buses, and must offset this by higher average speeds and/or shorter headways. Proponents assert that equivalent or higher overall capacity could be achieved by these means. Since there are no full-scale installations, capacity calculations are based on simulation and modeling and are disputed.

With two-second headways and four-person vehicles, PRT can achieve theoretical maximum capacity of 7,200 passengers per hour. However, most estimates assume that vehicles will not generally be filled to capacity, due to the point-to-point nature of PRT. At a more typical average vehicle occupancy of 1.5 persons per vehicle, the maximum capacity is 2,700 passengers per hour. Some researchers have suggested that rush hour capacity can be improved if operating policies support ridesharing.

Capacity is inversely proportional to headway. Therefore, as compared to two-second headways, one-second headways would double the capacity, and half-second headways would quadruple capacity. Although no regulatory agency has as yet (June 2006) approved headways shorter than two seconds, researchers suggest that high capacity PRT (HCPRT) designs could operate safely at half-second headways.

In simulations of rush hour or high-traffic events, about one-third of vehicles on the guideway need to travel empty to resupply stations with vehicles in order to minimize response time. This is analogous to trains and buses travelling nearly empty on the return trip to pick up more rush hour passengers.

Light rail systems can achieve capacities over 7,500 passengers per hour under normal operations. Heavy rail subway systems regularly transport 12,000 passengers per hour or more. As with PRT, these estimates are dependent on having enough trains available. Neither light nor heavy rail scales well for off-peak operation.

The above discussion compares line or corridor capacity and may therefore not be entirely relevant for a networked PRT system. In addition, it has been estimated (see Muller et al TRB) that while PRT may need more than one guideway to match the capacity of a conventional system, the capital cost of the multiple guideways may still be less than that of the single guideway conventional system. Thus comparisons of line capacity should include a consideration of per line costs. In addition, PRT systems would require much less horizonal space than existing metro systems, with individual cars being typically around 50% as wide for side-by-side seating configurations, and less than 33% as wide for single-file configurations. This is an important factor in densely-populated, high-traffic areas. A triple-guideway system using cars with single-file seating would have a capacity of over 21,600—almost twice the capacity of existing metro systems—partly because of the reduced transit times for individual passengers.

Travel speed
For a given peak speed, point-to-point journeys are quicker than scheduled stopping services. While a few PRT designs have operating speeds of 100 km/h (60 mph), most are in the region of 40–70 km/h (25–45 mph). Rail systems generally have higher maximum speeds, typically 90–130 km/h (55–80 mph) and sometimes well in excess of 160 km/h (100 mph), but average travel speed may be reduced by stopping at additional stations, and by passengers transferring.

Ridership attraction
If PRT designs deliver the claimed benefit of being substantially faster than cars in areas with heavy traffic, simulations suggest that PRT might attract significantly higher than the predicted mode switch from private motoring than is the case for other proposed public transit systems (figures between 25% and 60% have been discussed). The basis for the claimed mode switch is, however, untestable in the absence of any real-world systems.

Control algorithms
One possible control algorithm places vehicles in imaginary moving "slots" that go around the loops of track. Real vehicles are allocated a slot by track-side controllers. On-board computers maintain their position by using a negative feedback loop to stay near the center of the commanded slot. One-way vehicles can keep track of their position is by integrating the input from speedometers, using periodic check points to compensate for cumulative errors. Next-generation GPS and radio location can also be used for accurate positioning.

Another style of algorithm assigns a trajectory to a vehicle, after verifying that the trajectory does not violate the safety margins of other vehicles. This permits system parameters to be adjusted to design or operating conditions, and may use slightly less energy.

The maker of the ULTra PRT system reports that testing of its control system shows lateral (side-to-side) accuracy of 1 cm, and docking accuracy better than 2 cm.

Safety
Computer control is considered more reliable than drivers, and PRT designs should, like all public transit, be much safer than private motoring. Most designs enclose the running gear in the guideway to prevent derailments. Grade-separated guideways would prevent conflict with pedestrians or manually-controlled vehicles. Other public transit safety engineering approaches, such as redundancy and self-diagnosis of critical systems, are also included in designs.

The Morgantown system, more correctly described as an Automated Guideway Transit system (AGT), has completed 110 million passenger-miles without serious injury. According to the U.S. Department of Transportation, AGT systems as a group have higher injury rates than any other form of rail-based transit (subway, metro, light rail, or commuter rail) though still much better than ordinary buses or automobiles. More recent research by the British PRT company ATS indicates that AGT systems have a better safety than more conventional, non-automated modes.

As with many current transit systems, passenger safety concerns are likely to be addressed through CCTV monitoring, and communication with a central command center from which engineering or other assistance may be dispatched.

Energy efficiency
The energy efficiency claimed by PRT proponents is based on two operational characteristics: low vehicle weight; elimination of unnecessary starting and stopping through direct origin to destination service.


 * Parastic Energy Consumption. Parastic Energy Consumption is a complicated way of defining the weight to be moved that is not people (or cargo). In order to achieve the lowest consumption of energy per passenger, it makes sense that the lighter the vehicle, the less energy consumed. As an example, for a PRT vehicle with a capacity of four persons and weighing 882 lbs. (400 kg), the parasitic weight is 220.5 lbs. per seat.  By comparison, a Siemens SD600A light rail car of the type used by Portland's TriMet agency has a weight of 109,000 lbs.; with 72 seats and a standing capacity of 261 passengers, the parasitic weight is 1,513 lbs. per seat or 417 lbs. per person, respectively. An additional claimed efficiency is that a four-seat pod would only move with at least 25% occupancy (one rider), or moving empty to position itself in anticipation of same.


 * Elimination of unnecessary starting and stopping. It takes less energy to maintain a steady speed, as evidenced by hybrid vehicles that recapture at time of deceleration the energy consumed by acceleration.

The combination of parasitic weight and starting and stopping means that on average rail transit consumes 2,750 BTU per passenger mile, whereas ATS Ltd. estimates its ULTra PRT will consume 839 BTU per passenger mile (0.55 MJ per passenger km).

By comparison, buses consume 4,318 BTU, cars consume 3,496 BTU, and personal trucks consume 4,329 BTU.

Solar
Due to PRT's efficiency, some proponents say solar becomes a viable power source. PRT elevated structures provide a ready platform for solar collectors, therefore some proposed designs include solar power as a characteristic of their networks.

Cost characteristics
Estimates of guideway cost range from US$0.8 million (for MicroRail) to $22 million per mile, with most estimates falling in the $10m to $15m range. These costs may not include the purchase of rights of way or system infrastructure, such as storage and maintenance yards and control centers, and reflect unidirectional travel along one guideway, the standard form of service in current PRT proposals. Bidirectional service is normally provided by moving vehicles around the block. To reach capacities of competing systems, a system requires thousands of vehicles. Some PRT proposals incorporate these costs in their per-mile estimates.

PRT designs generally assume dual-use rights of way, for example by mounting the transit system on narrow poles on an existing street. If dedicated rights of way were required for an application, costs could be considerably higher. If tunneled, small vehicle size can reduce tunnel volume compared with that required for an automated people mover (APM). Dual mode systems would use existing roads, as well as special-purpose PRT guideways. In some designs the guideway is just a cable buried in the street (a technology proven in industrial automation). Similar technology could equally be applied to private automobiles.

A design with many modular components, mass production, driverless operation and redundant systems should in theory result in low operating costs and high reliability. Predictions of low operating cost generally depend on low operations and maintenance costs. Whether these assumptions are valid will not be known until full scale operations are commenced since assumptions regarding reliability cannot be proven by prototype systems. Low operating cost projections are also derived from the relatively high capacity utilization (for a public transport system) of the on-demand service characteristic.

Some planners dispute the cost-estimates of PRT when compared to light rail systems, whose costs vary widely with non-grade-separated streetcars being relatively low cost and systems involving elevated track or tunnels costing up to US$200 million per mile. Systems such as buses and streetcars, which run over the road network, require no further rights of way. This can represent a substantial cost saving over those requiring construction of dedicated routes, but may also result in increased congestion on existing roads.

Ridership and operating cost
For scheduled mass transit such as buses or trains, there is a fundamental tradeoff between service and operating cost. This is due to the fact that buses and trains must run on a predefined schedule, even during non-peak times when demand is low and vehicles run nearly empty. For this reason, transportation planners typically control operating costs by attempting to predict periods of low demand, running on reduced schedules and/or with smaller vehicles at these times. This, however, increases wait times for passengers. In many cities, trains and buses do not run at all at night or on weekends, because the low demand does not justify the expense. PRT vehicles, in contrast, would only run in response to demand, allowing 24-hour service without many of the cost implications of scheduled mass transit.

Proposals
ULTra ("Urban Light Transport") is a system from Advanced Transport Systems Ltd. in Cardiff, Wales. The ULTra system differs from many other systems in its focus on using off-the-shelf technology and rubber tires running on an open guideway. This approach has resulted in a system that is more economical than designs requiring custom technology. An ULTra system to connect Terminal 5 at London's Heathrow Airport with a car parking area will begin operation near the end of 2008.

Cabinentaxi was a German urban transit development project, undertaken by the joint venture of Mannesmann Demag and MBB under a program of the German BMFT (German Ministry of Research and Development).

SkyTran (from UniModal Transport Solutions Inc.) is a project by Douglas Malewicki for a 160km/h (100mph) personal rapid transit system that would use electric linear propulsion and a form of passive magnetic levitation called Inductrack. No prototype exists. The beginnings of a prototype are underway. It has a U.S. Department of Transportation grant at the University of Montana and is working with NASA's National Center for Advanced Manufacturing in New Orleans to develop the vehicles.

Opposition and controversy
Opposition to PRT schemes has been expressed based on a number of concerns:

Technical feasibility debate
The Ohio, Kentucky, Indiana (OKI) Central Loop Report compared the Taxi 2000 PRT concept proposed by the Skyloop Committee to other transportation modes (bus, light rail and vintage trolley). In the Taxi 2000 PRT system, the Loop Study Advisory Committee identified "significant environmental, technical and potential fire and life safety concerns…" and the PRT system was "…still an unproven technology with significant questions about cost and feasibility of implementation." Skyloop contested this conclusion, arguing that Parsons Brinckerhoff changed several aspects of the system design without consulting with Taxi 2000, then rejected this modified design. Despite the report's concerns regarding the implementation obstacles of PRT, the report did conclude that compared to the other alternatives, PRT offered the most acceptable point-to-point travel times, the most reliable service levels, the highest level of frequency of service and geography coverage, and was most able to maintain schedule. The report further concluded that, compared to the other alternatives, PRT would have over 3 times the ridership of the next closest alternative, including new transit riders over 9 times higher than the next closest alternative.

Vukan R. Vuchic, Professor of Transportation Engineering at the University of Pennsylvania and a proponent of light rail, has stated his belief that the combination of small vehicles and expensive guideway makes it highly impractical in both cities (not enough capacity) and suburbs (guideway too expensive). PRT supporters claim that Vuchic's conclusions are based on flawed assumptions.

The manufacturers of ULTra acknowledge that current forms of their system would provide insufficient capacity in high density areas such as central London, and that the investment costs for the tracks and stations are comparable to building new roads, making the current version of ULTra more suitable for suburbs and other moderate capacity applications, or as a supplementary system in larger cities.

Regulatory concerns
Possible regulatory concerns include emergency safety, headways, and accessibility for the disabled. If safety or access considerations require the addition of walkways, ladders, platforms or other emergency/disabled access to or egress from PRT guideways, the size of the guideway is substantially increased. Because minimizing guideway size is important to the PRT concept and costs these concerns may be significant barriers to PRT adoption. The U.S. and Europe both have legislation mandating disabled accessibility for public transport systems.

For example, the California Public Utilities Commission states that its "Safety Rules and Regulations Governing Light Rail Transit" (General Order 143-B) and "Rules and Regulations Governing State Safety Oversight of Rail Fixed Guideway Systems" (General Order 164-C) are applicable to PRT. Both documents are available online. The degree to which CPUC would hold PRT to "light rail" and "rail fixed guideway" safety standards as a condition for safety certification is not clear.

Other concerns
Concerns have been expressed about the visual impact of elevated guideways and stations. The 2001 OKI Report stated that Skyloop's elevated guideways would create visual barriers, loss of privacy, and be inconsistent with the character of historic neighborhoods. Some in the business community in Cincinnati who were opposed believed elevated guideway would remove potential customers from the street level where their shops are advertised.

Some have also objected to PRT promotion on the grounds that it is a distraction from other, more established transit solutions. Objectors claim that advocacy for PRT has reduced support for other alternatives to private motoring. There is, however, no evidence that light rail has ever been rejected due to the existence of a competing PRT proposal.

As with other modes of public transit, there are also concerns about policing against terrorism and vandalism, although the impact of such terrorism might be minimized by the lack of large concentrations of people.

Additional references

 * Transit Systems Theory, J.E. Anderson, 1978
 * Control of Personal Rapid Transit Systems, J.E. Anderson, 2003 (PDF)
 * Systems Analysis of Urban Transportation Systems, Scientific American, 1969, 221:19–27
 * Individualized Automated Transit in the City, Donn Fichter, 1964
 * advancedtransit.org—A history of PRT.