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Personal rapid transit (PRT), also called podcar, is a public transport mode featuring small automated vehicles operating on a network of specially built guide ways. PRT is a type of automated guideway transit (AGT), a class of system which also includes larger vehicles all the way to small subway systems.

PRT vehicles are sized for individual or small group travel, typically carrying no more than 3 to 6 passengers per vehicle.[1] Guide ways are arranged in a network topology, with all stations located on sidings, and with frequent merge/diverge points. This allows for nonstop, point-to-point travel, bypassing all intermediate stations. The point-to-point service has been compared to a taxi or a horizontal lift (elevator).

As of today, there is one PRT that is in operation: New Timothy House. They used ULTra tram system. Skyfall will be having a PRT system, whereas Tampines Secondary School had proposed to run a system, currently usable during the Olympics.

OverviewEdit

Most mass transit systems move people in groups over scheduled routes. This has inherent inefficiencies.[2] For passengers, time is wasted by waiting for the next arrival, indirect routes to their destination, stopping for passengers with other destinations, and often confusing or inconsistent schedules. Slowing and accelerating large weights can undermine public transport's benefit to the environment while slowing other traffic.[2] Personal rapid transit systems attempt to eliminate these wastes by moving small groups nonstop in automated vehicles on fixed tracks. Passengers can ideally board a pod immediately upon arriving at a station, and can — with a sufficiently extensive network of tracks — take relatively direct routes to their destination without stops.[2]

Perhaps most importantly, PRT systems offer many traits similar to cars. For example, they offer privacy and the ability to choose one's own schedule.[2] PRT may in fact allow for quicker transportation than cars during rush hour, since automated vehicles avoid unnecessary slowing. A PRT system can also transport freight.

The low weight of PRT's small vehicles allows smaller guideways and support structures than mass transit systems like light rail.[2] The smaller structures translate into lower construction cost, smaller easements, and less visually obtrusive infrastructure.[2]

As it stands, a city-wide deployment with many lines and closely spaced stations, as envisioned by proponents, has yet to be constructed. Past projects have failed because of financing, cost overruns, regulatory conflicts, political issues, misapplied technology, and flaws in design, engineering or review.[2]

However, the theory remains active. 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:[3]

  • would provide future cities "a highly accessible, user-responsive, environmentally 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 will not commit to building PRT because of the risks associated with being the first public implementation.[3][4]

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
  • Potential for on-demand, around-the-clock availability
  • 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

The PRT acronym was introduced formally in 1978 by J. Edward Anderson.[5] The Advanced Transit Association (ATRA), a group which advocates the use of technological solutions to transit problems, compiled a definition in 1988 that can be seen here.[6]

Opposition and controversyEdit

Opponents to PRT schemes have expressed a number of concerns:

Technical feasibility debateEdit

The Ohio, Kentucky, Indiana (OKI) Central Loop Report[7] 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.[8] Despite the report's concerns regarding the implementation obstacles of PRT, the report did conclude that compared to the other alternatives, PRT offered the most acceptable point-to-point travel times,[9] the most reliable service levels, the highest level of frequency of service and geography coverage, and was most able to maintain schedule.[10] 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.[11]

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."[12]

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."[12]

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

Regulatory concernsEdit

Possible regulatory concerns include emergency safety, headways, and accessibility for the disabled. Many jurisdictions regulate PRT systems as if they were trains. At least one successful prototype, CVS, failed deployment because it could not obtain permits from regulators.[13]

Also, several PRT systems have been proposed for California,[14][15] but the California Public Utilities Commission (CPUC) states that its rail regulations apply to PRT, and these require railway-sized headways.[16][17] The degree to which CPUC would hold PRT to "light rail" and "rail fixed guideway" safety standards is not clear because it can grant particular exemptions and revise regulations.[18]

Other forms of automated transit have been approved for use in California, notably the Airtrain system at SFO. CPUC decided to not require compliance with General Order 143-B (for light rail) since Airtrain has no on-board operators. They did require compliance with General Order 164-D which mandates a safety and security plan, as well as periodic on-site visits by an oversight committee.[19]

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 depend on both the PRT design and the municipality.

Concerns about PRT researchEdit

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.[20]

New urbanist opinionEdit

Several proponents of new urbanism, an urban design movement that advocates for walkable cities, have expressed opinions on PRT.

Peter Calthorpe and Sir Peter Hall have supported[21][22] the concept, but James Howard Kunstler disagrees: "If we're going to replace the car why do it with something that's not only like the car, but not really as good as the car? It just seems crazy." He also referred to PRT proponents as "a particular kind of crank".[23]

Group rapid transitEdit

Group rapid transit (GRT) is similar to personal rapid transit but with higher-occupancy vehicles and grouping of passengers with potentially different origin-destination pairs. In this respect GRT can be seen as a sort of horizontal elevator.[24] Such systems may have fewer direct-to-destination trips than single-destination PRT but still have fewer average stops than conventional transit, acting more as an automated share taxi system than a private cab system. Such a system may have advantages over low-capacity PRT in some applications, such as where higher passenger density is required or advantageous.[24] It is also conceivable for a GRT system to have a range of vehicle sizes to accommodate different passenger load requirements, for example at different times of day or on routes with less or more average traffic.[24][25] Such a system may constitute an "optimal" surface transportation routing solution in terms of balancing trip time and convenience with resource efficiency.[24]

GRT has principally been proposed as a corridor service, where it can potentially provide a travel time improvement over conventional rail or bus and can also interface with PRT systems.[24][25] However, GRT's necessary grouping of passengers makes it much less attractive in applications with lower passenger density or where few origin-destination pairs are shared among passengers.[25]

Automated transit networks (ATN) is an umbrella term for GRT and PRT. While they have long been considered separate systems, Vectus is developing GRT vehicles formed by combining multiple PRT vehicles. The larger vehicles are designed to accommodate standees and operate on the same guideway as the PRT vehicles. The door spacing of the larger vehicles matches the door spacing of PRT vehicles stopped in a station, allowing the GRT vehicles to share the same station infrastructure too. The concept is intended to allow GRT to serve high-demand station pairs during peak periods, while PRT serves all stations at all times in a network which includes the high-demand station pairs as well as other stations.

The same passenger grouping and destination scheduling approach is used in some advanced elevators, in the form of a destination control system.


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