Personal rapid transit Totally Explained

Everything about Personal Rapid Transit totally explained

Personal rapid transit (PRT), also called personal automated transport (PAT) or podcar, is a public transportation concept that offers automated on-demand non-stop transportation, on a network of specially-built guideways. Several different designs have been proposed, and as of 2008, at least one is under construction.

Overview

PRT is a system of small vehicles under independent or semi-independent automatic control, running on fixed guideways. The basic systems-theoretic approach is to reduce commute times and expense by disaggregating and automating passenger routing and timing, something like a packet-switching (internet-like) commuter system. The concept has been independently reinvented many times since the 1960s.
In 1988, The Advanced Transit Association (ATRA), a group which advocates the use of technological solutions to transit problems, published a definition for PRT(External Link

).
   The definition doesn't specify a particular technology, such as electric motors, linear motors, magnetic levitation, or rubber wheels. It doesn't specify whether vehicles are to be supported on the guideway or suspended from the guideway. Instead, it's derived from analysis of the functionality, efficiency, scalability, and service provided by the total engineering and design of the system.
   Since these traits are mathematically required, doing one poorly will increase the commute time or expense of a system. Additionally, some of the traits are done very badly by conventional rail systems, and therefore require nonstandard technology.
   Proponents say that the low weight of small vehicles allows 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.
   PRTs' demanding design requirements, need for new technology, and displacement of existing systems have created opposition. It is considered controversial, and a city-wide deployment with many lines and closely-spaced stations, as envisaged by proponents, has yet to be constructed. Past projects have failed because of financing, cost overruns, regulatory conflicts, political issues, and flaws in design, engineering or review.
   However, the theory remains persuasive. For example, 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:

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." The report also concluded that, despite these advantages, public authorities won't commit to building PRT because of the risks associated with being the first public implementation.

Comparison of Personal Rapid Transit with existing transport systems>

Similar to automobiles	Vehicles are small—typically two to six passengers Vehicles are individually hired, like taxis, and shared only with the passengers of one's choosing Vehicles travel along a network of guideways, much like a network of streets. Travel is point-to-point, with no intermediate stops or transfers It can be available on an on-demand, around-the-clock basis Stops are designed to be off the main guideway, allowing through traffic to bypass stations unimpeded
Similar to trams, buses, and monorails	A public amenity (although not necessarily publicly owned), shared by multiple users Reduced local pollution (electric powered) Passengers embark and disembark at discrete stations, analogous to bus stops or taxi stands
Similar to automated people movers	Fully automated, including vehicle control, routing, and collection of fares Usually off-grade—typically elevated—reducing land usage and congestion
Distinct features	Vehicle movements may be coordinated, unlike the autonomous human control of automobiles and bikes Small vehicle size allows infrastructure to be smaller than other transit modes Automated vehicles can travel close together. Possibilities include dynamically combined "trains" of vehicles, separated by a few inches, to reduce drag and increase speed, energy efficiency and passenger density

History

Some of the key concepts of PRT has been toyed with since before the 1900s, but modern 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, 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 was published in 1968, and proposed the development of PRT, as well as other systems such as dial-a-bus and high-speed interurban 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 isn't allowed to sell to non-federal government customers. In 1969, members of the study team published the first widely-publicized description of PRT in Scientific American. In 1978 the team also published a book.
   In 1967, aerospace giant 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.
   The oil crisis of 1973 made vehicle fuels more expensive, stimulating PRT development.
   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 (1.6 km) 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.
   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.
   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 isn't considered a PRT system by authorities because its vehicles are too heavy and carry too many people, and because most of the time it doesn't 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 wasn't sold to other sites because the heated track has proven too expensive.
   From 1969 to 1980, Mannesmann Demag and MBB cooperated to build the Cabinentaxi urban transportation system in Germany. Together the firms formed the Cabintaxi Joint Venture. They created an extensive PRT technology which was considered fully developed by the German Government and its safety authorities. The system was to have been install in Hamburg, but budget cuts stopped the proposed project before the start of construction. With no other project potentials on the horizon, the joint venture disbanded, and the fully developed PRT technology was never installed. Cabintaxi Corporation, a US based company obtained the technology in 1985, and remains active in the private sector market for transportation systems.
   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 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.
   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). Most energy use occurs on highways, so small elevated guideways would inductively power highway use and recharge batteries for off-guideway use. Central computers could do routing.
   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.
   In June 2006, a Korean/Swedish consortium, Vectus Ltd, started constructing a 400 metre test track in Uppsala, Sweden. This test system was presented at the 2007 PodCar City conference in Uppsala, Sweden.

System design

Among the handful of prototype systems (and the larger number that exist on paper) there's 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.

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.

Infrastructure design

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. The Morgantown PRT failed its cost targets because of its steam-heated track, so most proposals plan to resist snow and ice in ways that should be less expensive.

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.

Operational characteristics

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. The UK Railway Inspectorate has evaluated the ULTra design and is willing to accept one-second headways, pending successful completion of initial operational tests at more than 2 seconds. 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 below 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 combining higher average speeds, diverse routes, and 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.
Single line capacity
With two-second headways and four-person vehicles, a single PRT line can achieve theoretical maximum capacity of 7,200 passengers per hour. However, most estimates assume that vehicles won't 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 on a fixed route. Heavy rail subway systems regularly transport 12,000 passengers per hour or more. As with PRT, these estimates depend on having enough trains available. Neither light nor heavy rail scales well for off-peak operation.
Networked PRT capacity
The above discussion compares line or corridor capacity and may therefore not be relevant for a networked PRT system, where several parallel lines (or parallel components of a grid) carry traffic. In addition, Muller 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 horizontal 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, non-stop 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), and one as high as 241 km/h (150 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 doesn't 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 reported 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 advantages claimed by PRT proponents include two basic operational characteristics of PRT: an increased average load factor; and the elimination of intermediate starting and stopping.
   Average load factor, in transit systems, is the ratio of the total number of riders to the total theoretical capacity. A transit vehicle running at full capacity has a 100% load factor, while an empty vehicle has 0% load factor. If a transit vehicle spends half the time running at 100% and half the time running at 0%, the average load factor is 50%. Higher average load factor corresponds to lower energy consumption per passenger, so designers attempt to maximize this metric.
   Scheduled mass transit (for example buses or trains,) trades off service frequency and load factor. Buses and trains must run on a predefined schedule, even during off-peak times when demand is low and vehicles are nearly empty. So to increase load factor, transportation planners try to predict times of low demand, and run reduced schedules or smaller vehicles at these times. This increases passengers' wait times. In many cities, trains and buses don't run at all at night or on weekends. PRT vehicles, in contrast, would only move in response to demand, which places a theoretical lower bound on their average load factor. This allows 24-hour service without many of the costs of scheduled mass transit.
   ATS Ltd. estimates its ULTra PRT will consume 839 BTU per passenger mile (0.55 MJ per passenger km).
    By comparison, automobiles consume 3,496 BTU, and personal trucks consume 4,329 BTU per passenger mile.
   For bus and rail transit, energy usage per passenger-mile is dependent on service frequency and ridership, and can vary significantly from peak to non-peak. Therefore, aggregate statistics are used to calculate overall energy usage passenger-mile. In the US, buses consume an average of 4,318 BTU/passenger-mile, transit rail 2,750 BTU/passenger-mile, and commuter rail 2,569 BTU/passenger-mile. 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
The initial capital costs of PRT are large, but compare favorably with those of other transportation modes. Its system design tries to pay down those costs as quickly as possible, while maximizing the useful lifetime of the project. Proponent's cost estimates in passenger mile range from the cost of a bicycle (US $0.01..0.05/passenger-mile, Unimodal) to the cost of a small motorcycle ($0.20/passenger mile, TAXI 2000), and are strongly disputed by opponents. It's agreed that PRT systems require no individual license, parking or insurance fees, and buy energy in bulk from inexpensive providers.
   Most of the initial investment is in guideways. 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 won't be known until full scale operations are commenced since assumptions regarding reliability can't be proven by prototype systems.
   Transportation systems allocate the cost of their roads by measuring wear. PRT routes are disaggregated, and vehicles only move to carry passengers, so PRT measures wear and energy based on passengers or weight carried, rather than vehicle schedules. This brings large theoretical savings compared to trains, but appears more expensive than buses and streetcars, whose roads are subsidized by sunk, preallocated fuel taxes.
   So, 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.
Proposals and systems under construction
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's more economical than designs requiring custom technology. An ULTra system will connect Terminal 5 at London's Heathrow Airport with a car parking area. It will begin operation in Spring 2009.. ULTra and CyberCab are among the future transport technologies being considered by the town of Daventry in Northamptonshire, England.. 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 160 km/h (100 mph) 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.
MISTER (Metropolitan Individual System of Transit on an Elevated Rail) is a project by Olgierd Mikosza, a Polish engineer who has spent life working all over the world. He invented MISTER while he was working in USA. MISTER website Masdar, a planned zero-emissions city in Abu Dhabi, plans a PRT network to provide public transit and cargo movement within the city. Skyweb Express and 2GetThere have been suggested as the preferred systems for express transit and local transit; cargo and waste service, respectively, but a choice will be done only after a tender is released.
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. 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 traditional forms of transit, 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). According to Vuchic: "...the PRT concept combines two mutually incompatible elements of these two systems: very small vehicles with complicated guideways and stations. Thus, in central cities, where heavy travel volumes could justify investment in guideways, vehicles would be far too small to meet the demand. In suburbs, where small vehicles would be ideal, the extensive infrastructure would be economically unfeasible and environmentally unacceptable."
   PRT supporters claim that Vuchic's conclusions are based on flawed assumptions. PRT proponent J.E. Anderson wrote, in a rebuttal to Vuchic: "I have studied and debated with colleagues and antagonists every objection to PRT, including those presented in papers by Professor Vuchic, and find none of substance. Among those willing to be briefed in detail and to have all of their questions and concerns answered, I find great enthusiasm to see the system built." The degree to which CPUC would hold PRT to "light rail" and "rail fixed guideway" safety standards isn't clear because it can grant particular exemptions and revise regulations.
   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 may be increased. This may impact the feasibility of a PRT system, though the degree of impact would be highly dependent on both the particular design and the municipality involved.
Concerns about PRT Research
Wayne D. Cottrell of the University of Utah conducted a critical review of PRT academic literature since the 1960s. He concluded that there are several issues that would benefit from more research, including: urban integration, risks of PRT investment, bad publicity, technical problems, and competing interests from other transport modes. He suggests that these issues, "while not unsolvable, are formidable," and that the literature might be improved by better introspection and criticism of PRT. He also suggests that more government funding is essential for such research to proceed, especially in the US.
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.
   As with other modes of public transit, there are also concerns about policing against terrorism and vandalism.

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