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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 independently 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 advocates of other transport modes.

Because there has never been a real world installation, PRT is a controversial concept. 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.

Fully automated driverless rapid transit systems do exist, but these are based on larger vehicles.

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 prototype PRT systems — and the larger number that exist on paper — there is a substantial diversity of design approaches, some of which are contentious.

Vehicle Design

The vehicle's weight determines the size and expense of the system's guideway. A larger vehicle requires a larger and more expensive guideway, and the guideway is the major up-front expense of a PRT system. Larger vehicles are also more expensive to produce, and require more energy to start and stop. Against this, smaller vehicles are generally more affected by air resistance: air-resistance dominates the energy cost of keeping vehicles moving at speed. Larger motors are also generally more efficient than smaller ones.

The number of riders who will share a vehicle is therefore a key variable. Until a public PRT system is operational this can only be estimated; ridership cannot be extrapolated from fixed-route systems such as buses or trains. One possible prototype for personal point-to-point travel is the private automobile: the U.S. averages 1.16 persons per vehicle and an average of below 2 is common in most industrialized countries. Thus, some PRT designers claim that the optimum vehicle size is 2 passengers (or less) and some systems (notably UniModal / Skytran) have been designed this way. However, other designers believe that larger vehicles are necessary, to accommodate disabled passengers, as well as families traveling together. As of 2006, all PRT systems known to be under active development use 4-passenger vehicles.

If PRT vehicles are too large point-to-point routing becomes uneconomical as most vehicles would be 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. Designers attempt to design for the smallest practical vehicle, aiming to offset the reduction in seating capacity per vehicle through reduced headways (see regulatory concerns).

Propulsion

All current PRT vehicle designs are powered by electricity, generally powered from lineside conductrors rather than batteries to reduce vehicle weight. According to designer of Skyweb/Taxi2000, J.E. Anderson, the lightest-weight system, and therefore the one with the lowest system cost, is a linear induction motor (LIM) on the car, with a stationary conductive rail for both propulsion and braking. Such systems are also less vulnerable to weather and other contamination. LIMs are used in a small number of rapit transit applications.

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

Switching

Most designers avoid track switching, preferring vehicle-mounted switches or conventional steering to allow closer spacing of vehicles without the time delays inherent in track switching. This also reduces vulnerability to track switch failures.

Infrastructure Design

Guideways

There is some debate over the best type of guideway. No standard has been agreed and guideways in proposed systems may be incompatible with both each other and existing transportation technologies. All designs share common goals of fast switching and effective braking, and most should be capable of being built at ground level or elevated. Most designs use the guideway to distribute power, data, and for data communications with the vehicles. Following some issues with prototypes many also aim to be self clearing in bad weather.

Structurally, guideway designs encompass beams similar to monorails, bridge-like trusses supporting internal tracks, and cables embedded in a roadway. Most put the vehicle on top of the track, which reduces visual intrusion and cost as well as allowing low cost ground-level installation. Overhead suspended vehicles are said to unload the skins of the vehicle, which can therefore be lighter since 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.

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, for example merging vertically to reduce footprint.

Stations

Embarkation stations are designed to be on side tracks 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 are not envisaged to include facilities such as rest rooms. For elevated stations, an elevator may be required for accessibility.

Operational Characteristics

Headway Distance

The spacing of PRT vehicles on the guideway defines the maximum passenger capacity of a track. Designers therefore attempt to minimize the headway, the distance between vehicles. Some have planned for very short headways, with capacities said to be equal to or greater than light rail. Computerized control theoretically permits closer spacing than the two-second headways recommended for cars at speed, since PRT vehicles 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 other jurisdictions rail regulations apply to PRT systems (See CVS, above); these typically calculate headways in terms of absolute stopping distances and may make PRT systems uneconomical. Regulators may be willing to reduce headways with increased operational PRT experience.

Capacity Utilization

Since there are no full-scale installations, capacity utilization calculations are based on simulation and modelling, and are therefore contested by skeptics PRT is usually proposed as an alternative to proposed rail systems, so comparisons tend to be with rail.

In theory, if the peak speeds of PRT and trains were the same, PRT would be faster, simply because the PRT vehicles do not stop every half mile to let passengers on and off. While a few PRT system designs have operating speeds of 60 mph, most PRT system proposals are designed with maximum speeds of 25-45 mph. Rail systems, by comparison, have maximum speeds between 55 and 80 mph.

PRT vehicles seat fewer passengers than trains and buses, but proponents state that higher average speeds shorter headway distances could result in equivalent or higher overall capacity. Most systems are designed for four or six passenger vehicles, and planners admit that most travel will be single- or double-occupancy commuter travel. With two-second headways, an average of 1.5 persons per vehicle implies a maximum capacity per guideway route of 2700 passengers per hour. Regulatory approval to allow one second headways could increase this to 5400 passengers per hour, given sufficient vehicles. For comparison, 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.

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 travel empty to resupply stations with vehicles - in order to minimize response time.

Ridership Attraction

If PRT systems can deliver the claimed benefits of being substantially faster than cars in areas with heavy traffic, simulations show that PRT could attract between 35% and 60% of automobile users. This is significantly more than other forms of public transit. The higher ridership spreads the cost of the PRT system over more passengers per day, also causing greater utilization of the PRT system's capacity.

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.

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.

Safety

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

The Morgantown system, 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) though still much better than ordinary buses or automobiles.

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 requested.

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 reflect unidirectional travel along a single guideway, the standard form of service in PRT. Bidirectional service is normally simulated by moving vehicles around the block. Most PRT systems are designed to operate in a network. Traffic is automatically diverted by central controls, so heavily trafficked corridors do not exist, and therefore require no additional construction. In high-traffic single-point destinations, such as stadiums, larger stations would be necessary, increasing costs at those points. These estimates are considered low by sceptics, who claim they do not account for cost overruns common in public projects and new technologies.

The lowest cost estimates of PRT designers depend on dual-use rights of way, for example by mounting the transit system on narrow poles on an existing street. If a PRT system requires dedicated rights of way, acquisition could be considerable. If tunnelled, PRT's small size can reduce tunnel volume 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).

A PRT 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, mostly at airports and tourist attractions.

U.S. federal data shows 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 PRT's low operating cost depend on either unusually low O&M costs or increased load factor (O&M/passengers per destination). Whether these assumptions are valid will not be known until full scale operations are commenced since assumptions regarding reliability and capacity utilization (in particular) cannot be proven by prototype systems.

The WVU PRT project failed commercially, in part due to the cost of heating its track to eliminate snow. Other transit systems are also affected by weather and debris clearance (see the wrong kind of snow).

Almost all PRT designs plan to use electric vehicles. Thermodynamic efficiency of electric vehicles may be in the range 40% to 90%, while the typical automobile is 30% efficient and hybrid cars are 30% to 40% efficient.

Some planners dispute the cost-estimates of PRT when compared to light rail systems. See light rail for a discussion of its costs, which range from nearly zero (for non-grade-separated streetcars) to US$ 65 million per mile (for elevated heavy trains in high density city centers). PRT rights of way may cost less than a conventional road system, but the road system usually already exists, and thus requires no further investment.

Ridership and cost

For scheduled mass transit such as buses or trains, there is a fundamental tradeoff between service and 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 costs by attempting to predict periods of low demand, and running on reduced schedules 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 cost.

PRT vehicles, in contrast, only run in response to demand, and have only two to four seats. Therefore, for PRT designs, the vehicle load factor is always high, even during times of very low demand. This would allow PRT to provide 24-hour, on-demand service, without the cost implications of scheduled mass transit.

PRT also aims to reduce cost by automated fare collection and driverless operation.

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 stated

"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. Israeli proponents say that PRT may protect against terrorism by reducing the scope of vehicle that can be attacked.

Some have also objected to PRT promotion on the grounds that it is a distraction from other, more proven transit solutions. There is some evidence that advocacy for PRT has reduced support for other alternatives to private motoring, with the result that neither alternative has been implemented.

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
• Skytran/Unimodal
• 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 necessary 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
• 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

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

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