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Personal rapid transit (PRT) is a category of proposed public transport modes designed to offer automated on-demand non-stop transportation between any two points on a network of specially built guideways. The concept dates back to the mid 1960s, and has been independantly reinvented many times since then. Although elements of PRT design have influenced the design of some existing people mover systems, as of 2006 no PRT project has yet progressed beyond the prototype stage.

Although the design concepts and engineering challenges of PRT are well understood, questions remain about its actual production and operation costs, its safety, aesthetics, and acceptance in a public installation. Past failures have been caused by: lack of financing; concerns about cost overruns; conflicts with regulatory agencies; political interference in the design requirements; and flaws in design and/or engineering. There is also opposition from backers of other transport modes.

Because there has never been a real world installation, PRT is a controversial concept. PRT proponents claim that it could provide service that matches the speed and convenience of cars and freeways, surpasses the social and environmental benefits of conventional mass transit, and costs less than either. This has yet to be proven in a real world setting, however, and is hotly contested by some. Two projects currently under development may begin to resolve this debate: one at Heathrow Airport in London , scheduled to come into operation in 2007; and another is planned at Dubai International Financial Center in Dubai scheduled to be operational in 2008.

Overview

PRT has similarities to and differences from other forms of transport. To compare the proposed features:

History

The concept originated with Don Fichter, a city transportation planner, and author of a 1964 book entitled "Individualized Automated Transit in the City".

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 on PRT entitled Fundamentals of Personal Rapid Transit .

The Morgantown Personal Rapid Transit project has been in continuous operation at West Virginia University in Morgantown, West Virginia since 1975, with about 15,000 riders per day (as of 2003). The vehicles are rubber-tired and powered by electrified rails. Steam heating keeps the elevated guideway free of snow and ice. Most WVU students habitually use it. This system was not sold to other sites because the heated track has proven too expensive. The Morgantown system demonstrates automated control, but authorities no longer consider it a true PRT system. Its vehicles are too heavy and carry too many people, making it more similar to light rail schemes. Most of the time it does not operate in a point to point fashion for individuals or small groups, running instead like an automated people mover or elevator from one end of the line to the other. It therefore has reduced capacity utilization compared to true PRT. Morgantown vehicles also weigh several tons and run on the ground for the most part, with higher land costs than other systems.

The Aramis project in Paris, by aerospace giant Matra, started in 1967, spent about 500 million francs, and 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.

A project called Computer-controlled Vehicle System (CVS) operated in Japan from 1970 to c.1978. In a full scale test facility, 84 vehicles operated at speeds up to 60 km/h on a 4.8 km guideway; 1 second headways were achieved during tests. Another version of CVS was in public operation for six months during 1975-76. This system had 12 single-mode vehicles and 4 dual-mode vehicles on a one mile track with five stations. This version had over 800,000 passengers. CVS was cancelled when Japan's Ministry of Land, Infrastructure and Transport adjudged it unsafe under existing rail safety regulations, specifically in respect of braking and headway distances.

In Germany, the Cabinentaxi project, a joint venture from Mannesmann Demag and MBB, created an extensive PRT development in the 1970-80s 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.

Raytheon invested heavily in a system called PRT2000 in the 1990s, and failed to install a contracted system in Rosemont, near Chicago, when its estimated costs exceeded $50,000,000 per mile. This system may be available for sale by York PRT. In 2000, rights to the technology reverted to the University of Minnesota, and were purchased by Taxi2000.

The UniModal project proposes using magnetic levitation in solid-state vehicles to achieve speeds of 100 mph (161 km/h).

In 2002, 2getthere, a consortium of Frog Navigation Systems and Yamaha, operated "CyberCabs" at Holland's 2002 Floriade festival. These transported passengers up to 1.2 km on Big Spotters Hill. CyberCab is like 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 be less than 600 kg (1200 lb), allowing small elevated guideways which could use centralized computer controls and power.

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

ULTra was recently (October, 2005) 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 begin operation by the end of 2007 and to expand the system in 2009.

Vectus Ltd., a Korean/Swedish consortium, is constructing (2006) a test track in Sweden.

PRT System Design

There are currently no agreed-upon standards in PRT system design. Among the handful of PRT systems that are currently developing hardware -- and the many dozens of PRT designs that exist on paper -- there is a tremendous diversity of design approaches. Not only are the designs diverse, they are also in many cases quite contentious. The following sections provide an overview of the primary different design approaches, and highlights the major disputes, where they occur.

Vehicle Design

Capacity

Vehicle size is one of the most important aspects of PRT system design. Larger vehicles are more expensive to produce, require more energy to move, and require bulkier infrastructure to support. Therefore, PRT designers attempt to design for the smallest practical vehicle. This is controversial: critics of PRT claim that smaller vehicles reduce the overall passenger capacity of any transit system. PRT designers respond that this is only true if long headway distances are assumed (see below), and that any reduction in vehicle capacity can be offset by an increase in the number of vehicles, and a decrease in the headway distances between them.

Critical to the question of vehicle size is the average number of passengers that would actually ride in each vehicle. Until a public PRT system is constructed, this cannot be known for certain, and must be estimated. Since PRT uses a fundamentally different system design than other mass transit systems, ridership cannot be extrapolated from fixed-route systems such as buses or trains. The obvious precedent for personal point-to-point travel is the private automobile, which in commuter areas in the U.S. average 1.16 persons per vehicle. This has led some PRT designers to claim that the optimum vehicle size is 2 passengers (or less). Some systems (notably UniModal / Skytran) have been designed this way, resulting in extremely compact and lightweight vehicles and infrastructure. However, other PRT designers believe that larger vehicles are necessary, to accommodate handicapped passengers, as well as families traveling together. As of 2006, all PRT systems which are known to be under active development (ULTra, Vectus, Skyweb, Taxi 2000) use 4-passenger vehicles.

If PRT vehicles are too large, then point-to-point routing becomes uneconomical, as most vehicles would be highly under-utilized. For example, when the PRT system at West Virginia University grew from a 6-passenger to a 20-passenger vehicles, point-to-point operations were largely abandoned.

Propulsion

PRT vehicles are powered by electricity. Most systems plan multiply-redundant power supplies, from track-side batteries or natural-gas-powered generators. Stationary power reduces the vehicles' weight.

According to designer of Skyweb/Taxi2000 J.E. Anderson (below), the lightest-weight system, and therefore the one with the lowest system cost, is a linear induction motor (LIM) on the car, thrusting against a stationary conductive rail for both propulsion and braking. Loss of traction due to precipitation, ice or sudden braking is therefore not an issue, since a LIM's magnetic interaction with the rail would be unaffected. This aspect contributes to the feasibility of short headways between PRT vehicles. LIMs also minimize the number of moving parts in the car, reducing maintenance costs, and lowers the relative fabrication expense for the rail. It's also easy for an on-board computer to control. A similar system was proposed by Doug Malewicki for Skytran.

The Raytheon and ULTra systems use off-the-shelf rotary electric motors. Matra used a "variable reluctance motor" in Aramis.

Switching

Most designers eschew track switching (or points) built into the track because failure of an in-track switch would degrade capacity. Vehicle-mounted switches are preferred so that tracks stay in service, and to allow closer spacing of vehicles since no time delay is needed to allow the track to switch. Alternatively, the vehicles may have more conventional steering.

Infrastructure Design

Guideways

There is some debate over the best guideway for PRT systems. No standard has been agreed and guideways in proposed systems may be incompatible with both each other and existing transportation technologies. Some points of agreement exist: guideways should permit fast switching and effective braking, be inexpensive, be capable of being built at ground level or elevated, and not visually intrusive. Ideally they should not need to be cleared of dust or snow. Most systems would also use the guideway to distribute power, data, and routing indications to the vehicles.

Structurally, some guideway designs are similar to monorail beams, several are bridge-like trusses supporting internal tracks, and others are just cables embedded in a conventional or narrow roadway that can be elevated. An elevated track structure scales down dramatically with lower vehicle weights. Therefore, the vehicle's weight budget is critical. The heavier the vehicle, the more costly the track, and the track is the gating system cost. As well, large tracks are visually intrusive, so small vehicles contribute to a more attractive track.

Most designs put the vehicle on top of the track, which reduces visual intrusion and cost, as well as allowing low cost ground-level installation, and allow simpler track switching. Overhead suspended vehicles are said to unload the skins of the vehicle, which can therefore be lighter - many materials are stronger in tension than they are in compression. An overhead track is necessarily higher, and therefore more visible, but also narrower, and therefore creates less shadow, while having a small silhouette.

Fast, reliable switching is a key requirement for PRT that rules out some designs. For example, in most monorails, the rail is so heavy that the switch movement time would increase the time between PRT cars so much that the guideway is no longer competitive with a bus.

Some PRT systems have had substantial extra expenses from the extra track needed to decelerate and accelerate from the numerous stations. In at least one system, Aramis, this nearly doubled the width and expense of the required right-of-way, and caused the nonstop passenger delivery concept to be abandoned. Other systems have schemes to reduce this cost. Control algorithms can space vehicles to reduce siding lengths (see below). Elevated tracks can also "vertically merge" and keep to a narrow right of way.

Stations

Embarkation stations are on sidings so that through traffic can bypass vehicles picking up or dropping off passengers. Each station might have multiple berths, with perhaps 1/3 of the vehicles in a system being stored at stations waiting for passengers.

Embarkation stations can be small, inexpensive, and should not require amenities such as seating or restrooms. Stations may be elevated to guideway level, or be sited inside buildings or at street level.

Operational Characteristics

Headway Distance

The spacing of PRT vehicles on the guideway defines the maximum passenger capacity of the system. Designers therefore attempt to minimize the headway, the distance between vehicles. Some have planned for very short headways. Capacities are stated to be equivalent to or greater than light rail. Computerized control permits closer spacing than the two-second headways recommended for cars without degrading safety, since all cars can be braked simultaneously.

Very short headways are controversial. Some regulators (e.g. the British Rail inspectorate, regulating ULTra) are willing to accept two second headways. In these systems, a PRT guideway carries the same number of passenger-miles as a lane of freeway traffic. Regulators may be willing to reduce headways with increased operational PRT experience, achieving passenger densities perhaps four times that of a freeway lane.

Rail regulations legally apply to PRT systems in some places (See CVS, above); rail regulators may calculate headways in terms of absolute stopping distances, as is traditional in heavy rail. This may make PRT systems uneconomical.

Capacity Utilization

Due to lack of full-scale installations all capacity utilization calculations are based on simulation and modelling, and are therefore contested by those skeptical of PRT.

In theory, if the peak speeds of PRT and a train were the same, the PRT should be two to three times faster, simply because the PRT vehicles do not stop every few hundred yards to let passengers on and off, yielding two to three times as many trips per seat as a bus or train.

PRT systems could also automatically divert vehicles to busy routes, and travel nonstop at maximum speeds. Simulations suggest that at these speeds vehicles can be recycled for new trips as often as several times per hour, even during busy periods and low-density cities. In simulations of rush hour or high-traffic events about 1/3 of vehicles on the guideway need to be empty to minimize response time. It is also stated that higher speeds should allow a smaller fleet to serve the same number of passengers.

Control Algorithms

One algorithm places vehicles in imaginary moving "slots" that go around the loops of track, analogous to Token Ring networking. 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.

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 system permits system parameters to be adjusted to design or operating conditions. may use 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.

Ridership Attraction

Ridership simulations with standard assumptions show that PRT, which should be substantially faster than autos in areas with traffic jams, would attract between 35% and 60% of automobile users. This is significantly larger than typical ridership of other public transit modes, both in reality and in simulation.

Some PRT systems (See Unimodal) plan speeds substantially faster than automobiles achieve on empty expressways. In simulations, these attract even more traffic than slower, conservative PRT designs. If true, the high riderships would substantially decrease the cost per rider of PRT compared to trains and buses.

Safety

Safety engineers at PRT companies assert that travel via PRT systems should be much safer than public roads. Computer control is considered more reliable than drivers. Grade-separated guideways prevent collisions with pedestrians or manually-controlled vehicles. Most PRT systems enclose the running gear in the guideway to prevent derailments. Vehicles usually have computer-diagnosed, dual-redundant motors and electronics.

The Morgantown system, which is more correctly described as an "Automated Guideway Transit" system (AGT) has now completed 110 million passenger-miles without serious injury. However, 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). Still, as a result of total grade-separation, AGT systems are safer than ordinary buses or automobiles.

As with many current transit systems, safety concerns are likely to be addressed through grade separation (elevated tracks and/or dedicated right-of-ways), CCTV monitoring, and communication with a central command center from which engineering or other assistance may be requested.

Cost Characteristics

Estimates of guideway cost range between $0.8 million and $22 million per mile.. These estimates are considered low by sceptics, and do not account for cost overruns common in public projects. Prototype projects have reportedly been built within budget.

Standard transit-planning assumptions concerning overhead per vehicle are said to fail in PRT systems. These assumptions include operator salaries and transit policing. Assumptions regarding capacity utilization (the proportion of theoretical capacity which is actually utilized) are not addressed by prototype systems. The design, with many modular components, mass production, driverless operation and redundant systems, should result in low operating costs and high reliability. There are already some operational driverless transit systems. It has been observed from U.S. federal data that operations and maintenance costs (O&M) are nearly constant per seat for a wide variety of systems: buses, trains, aircraft and private automobiles. Predictions of low operating cost are predicated on weither unusually low O&M costs or increased load factor (O&M/passengers per destination). Whether this assumption is valid will not be known until full scale operations are commenced.

The WVU PRT project failed commercially due to the cost of heating its track to eliminate snow. Some systems in which the vehicles ride atop the track therefore enclose the track to keep precipitation or debris away from the track. Snow and debris clearance is also an issue for conventional transit.

Conversion efficiency of electric vehicles may be in the range 40 to 90%. The typical automobile is 30% efficient; hybrid cars are 30 to 40% efficient. Smaller vehicles tend to be less efficient for a given journey than larger ones.

Planners dispute the cost-estimates of PRT rights-of-way. In modern metropolitan areas, rights-of-way for light rail cost as much as $50 million per mile ($30 million/km). A typical light-rail right-of-way is 100 to 300 feet (30 to 100 m) wide, and necessarily includes the highest-density and most expensive parts of the operational area. Tunneling is much more expensive. PRT rights of way should cost less than a conventional road system, but the road system usually exists already.

Urban Integration

In mass transit with scheduled service, this "ridership" factor is generally calculated for an entire system, then applied to all vehicles. On most trips of most routes, vehicles are 85% to 95% empty, and only rush-hour trips on important central routes approach vehicle (and route) capacities. The low ridership of bus and trains therefore often causes a substantial cash drain through depreciation and the salaries paid for operators and mechanics. Further, the drain cannot be offset by fares.

PRT addresses the fixed cost issues by automated fare collection, driving and only running in response to demand, or in timely expectation of demand. This idling of PRT vehicles that are in-service but not in use should save energy compared to scheduled transit modes compared to buses or trains, which move a large proportion of empty seats during non-peak periods.

The lower estimates of PRT designers depend on dual-use rights of way, for example by mounting the transit system on narrow poles placed on an existing street. PRT's small size can reduce the volume of its tunnel to less than a quarter of that required for an automated people mover (APM). Dual mode systems would use existing roads, as well as special-purpose PRT guideways. In some cases, the guideway is just a cable buried in the street (a technology proven in industrial automation).

Opposition and controversy

Opposition has been expressed to PRT schemes and their proponents 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). Consulting engineers with Parsons Brinckerhoff found the Taxi 2000 PRT system had "...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 disputed this conclusion.

Vukan R. Vuchic, Professor of Transportation Engineering at the University of Pennsylvania described what he believes are problems with the PRT concept

"The PRT concept is imagined to capture the advantages of personal service by private car with the high efficiency of rapid transit. Actually, 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."

This has prompted an ongoing debate between Vuchic and PRT proponents.

Regulatory concerns

Possible regulatory concerns include emergency safety, headways, and accessability 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 US 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 and environmental impact of (especially elevated) guideways. The 2001 OKI Report stated that Skyloop's elevated guideways and elevated stations would be visual pollution that neighborhoods would challenge in an Environmental Impact Statement (EIS). Some in the business community in Cincinnati were opposed to Skyloop's elevated guideway because it would remove potential customers from the street level.

As with other modes of public transit, there are also concerns about policing against terrorism and vandalism, and some public transit advocates have objected to PRT promotion on the grounds that it is a distraction from other, more traditional transit solutions.

References

1. Irving, Jack (1978). "Fundamentals of Personal Rapid Transit". D.C. Heath and Company. {{cite web}}: Unknown parameter |accessyear= ignored (|access-date= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
2. "Personal Automated Transportation: Status and Potential of Personal Rapid Transit, p.89" (PDF). Advanced Transit Association. 2003. Retrieved 25 March. {{cite web}}: Check date values in: |accessdate= and |year= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)CS1 maint: year (link)
3. "Infrastructure cost comparisons" (Microsoft Word). ATS Ltd. Retrieved 25 March. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
4. "Ohio, Kentucky, Indiana (OKI) Central Loop Report". {{cite web}}: Unknown parameter |accessyear= ignored (|access-date= suggested) (help); Unknown parameter |yesr= ignored (help)
5. Vuchic, Vukan R (September/October, 1996). "Personal Rapid Transit: An Unrealistic System". Urban Transport International (Paris), (No. 7, September/October, 1996). {{cite web}}: Check date values in: |date= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)CS1 maint: date and year (link)

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, Don Fichter, 1964

See also

• Morgantown Personal Rapid Transit, 1975-present
• Cabinentaxi (personal rapid transit) - PRT system extensively tested in the 1970s, approved by the West German government for public use.
• People mover
• Light Rail

External links

Pilots and prototypes

• ULTra (Urban Light Transport), Cardiff Wales, UK
• MicroRail, from MegaRail Transportation, Fort Worth, Texas
• Postech, Pohang University, Korea
• SkyWebExpress, Minneapolis, Minnesota, US. 18-meter sample guideway.

Proposals

• UniModal - California & Montana, US; New Delhi, India
• Tritrack - dual-mode system, but its PRT part is neccessary for viability.
• RUF, Dual-mode, Denmark
• Thuma, a system for varying sizes of containers.
• Vectus Ltd. - Has 385 meter test track under construction in Uppsala, Sweden.
• Skycab - A Swedish concept (website and documents in Swedish)
• EcoTaxi - Finnish version of PRT, termed "Automated Goods & People Mover" (APGM).

Advocacy

• ATRA, The Advanced Transit Association, a professional group.
• CPRT, Citizens for Personal Rapid Transit (US)
• Get There Fast, Seattle, WA
• Get On Board Personal Rapid Transit, Seattle, WA PRT news & analysis
• PRT News
• Innovative Transportation Technologies web site by Jerry Schneider.
• Open Directory: Personal Rapid Transit
• PRT Wiki
• Transportationet PRT from engineering and law point of view. Site by Oded Roth, member of Israeli Retzef team.
• Skyloop
• A Review of the State of the Art in PRT Systems, J. E. Anderson; Journal of Advanced Transit 34:1(2000)

PRT Skepticism and Criticism

• Personal Rapid Transit PRT Skeptic Page
• Monorail, PRT, AGT, and "Gadget Transit" Analyses - several Light Rail Now articles skeptical of PRT
• Transit for Livable Communities resolution opposing public funding for PRT.
• Planetizen Article on PRT and "Gadgetbahnen".
• Public Transit gadgetbahn articles.
• The Road Less Traveled: The pros and cons of personal rapid transit." by Troy Pieper

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