An ULTra PRT vehicle on a test track.

A vehicle on WVU's Morgantown Personal Rapid Transit.

Personal rapid transit (PRT), also called podcar, is a public transportation 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.

In PRT designs, 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 approach 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).

PRT was a major area of study in the 1960s and 1970s. In 1975, Morgantown PRT, an experimental automated system which exhibits some (but not all) features of PRT, was opened to the public after significant construction cost overruns. Morgantown PRT remains in use today, and there have been discussions on expanding it.^[2]

A PRT system (by 2getthere) went into operation in Masdar City in the UAE in November 2010. The system has 10 passenger and 3 freight vehicles serving 2 passenger and 3 freight stations connected by 1.2 kilometers of one-way track. The system is in operation 18 hours a day, seven days a week serving the Masdar Institute of Science and Technology. Trips take about 2 and a half minutes (i.e., an average speed of roughly 12mph / 20km/h) and are presently free of charge. Average wait times are expected to be about 30 seconds.

The award winning ULTra PRT system began passenger trials at London Heathrow Airport, Terminal 5, in October 2010^[3] and opened for full passenger service 22 hours a day, 7 days a week, in May 2011. It runs between the Terminal 5 and its business parking lot, with 2.4 miles of track and 21 vehicles. Operational statistics in May 2012 demonstrate >99% reliability and an average passenger wait time over the one year period of 10 seconds. Ultra has achieved a number of awards from the London Transport Awards and the British Parking Awards.^[4][5]

A system using a system supplied by Vectus is in construction in Suncheon, South Korea. There is a commitment to a large (100,000 people per day) City PRT system using Ultra PRT technology in Amritsar, India.^{[citation needed]}

Overview[link]

Most mass transit systems move people in groups over scheduled routes. This has inherent inefficiencies.^[6] 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.^[6] 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.^[6]

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.^[6] 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.^[6] The smaller structures translate into lower construction cost, smaller easements, and less visually obtrusive infrastructure.^[6]

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.^[6]

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:^[7]

• 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 will not commit to building PRT because of the risks associated with being the first public implementation.^[7][8]

The PRT acronym was introduced formally in 1978 by J. Edward Anderson.^[9] 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.

Existing and planned networks[link]

Currently, two PRT networks and one quasi-PRT network are operational, and several more are in the planning stage.

Designs[link]

The following table summarizes several well-known PRT designs.

• Morgantown PRT is known as a quasi-PRT system, because it lacks some PRT features such as 100% on-demand service.
  

History[link]

Origins[link]

Modern PRT concepts began around 1953 when Donn Fichter, a city transportation planner, began research on PRT and alternative transportation methods. In 1964, Fichter published a book,^[32] which proposed an automated public transit system for areas of medium to low population density. One of the key points made in the book was Fichter's belief that people would not leave their cars in favor of public transit unless the system offered flexibility and end-to-end transit times that were much better than existing systems – flexibility and performance he felt only a PRT system could provide. Several other urban and transit planners also wrote on the topic and some early experimentation followed, but PRT remained relatively unknown.

Around the same time, Edward Haltom was studying monorail systems. Haltom noticed that the time to start and stop a conventional large monorail train, like those of the Wuppertal Schwebebahn, meant that a single line could only support between 20 and 40 vehicles an hour. In order to get reasonable passenger movements on such a system, the trains had to be large enough to carry hundreds of passengers (see headway for a general discussion). This, in turn, demanded large guideways that could support the weight of these large vehicles, driving up capital costs to the point where he considered them unattractive.^[33]

Haltom turned his attention to developing a system that could operate with shorter timings, thereby allowing the individual cars to be smaller while preserving the same overall route capacity. Smaller cars would mean less weight at any given point, which meant smaller and less expensive guideways. To eliminate the backup at stations, the system used "offline" stations that allowed the mainline traffic to bypass the stopped vehicles. He designed the Monocab system using six-passenger cars suspended on wheels from an overhead guideway. Like most suspended systems, it suffered from the problem of difficult switching arrangements. Since the car rode on a rail, switching from one path to another required the rail to be moved; a slow process that limited the possible headways.^[33]

UMTA is formed[link]

By the late 1950s the problems with urban sprawl were becoming evident in the US. When cities improved roads and the transit times were lowered, suburbs developed at ever increasing distances from the city cores, and people moved out of the downtown areas. Lacking pollution control systems, the rapid rise in car ownership and the longer trips to and from work was causing significant air quality problems. Additionally, movement to the suburbs led to a flight of capital from the downtown areas, one cause of the rapid urban decay seen in the US.

Mass transit systems were one way to combat these problems. Yet during this period, the US federal government was feeding the problems by funding the development of the Interstate Highway System, while at the same time funding for mass transit was being rapidly scaled back. Public transit ridership in most cities plummeted.^[34]

In 1962, President John F. Kennedy charged the United States Congress with the task of addressing these problems. These plans came to fruition in 1964, when President Lyndon B. Johnson signed the Urban Mass Transportation Act of 1964 into law, thereby forming the Urban Mass Transportation Administration.^[35] The UMTA was set up to fund mass transit developments in the same fashion that the earlier Federal Aid Highway Act of 1956 had helped create in the Interstate Highways. That is, the UMTA would help cover the capital costs of building out new infrastructure.

PRT research starts[link]

However, planners who were aware of the PRT concept were worried that building more systems based on existing technologies would not help the problem, as Fitcher had earlier noted. Proponents suggested that systems would have to offer the flexibility of a car:

The reason for the sad state of public transit is a very basic one - the transit systems just do not offer a service which will attract people away from their automobiles. Consequently, their patronage comes very largely from those who cannot drive, either because they are too young, too old, or because they are too poor to own and operate an automobile. Look at it from the standpoint of a commuter who lives in a suburb and is trying to get to work in the central business district (CBD). If he is going to go by transit, a typical scenario might be the following: he must first walk to the closest bus stop, let us say a five or ten minute walk, and then he may have to wait up to another ten minutes, possibly in inclement weather, for the bus to arrive. When it arrives, he may have to stand unless he is lucky enough to find a seat. The bus will be caught up in street congestion and move slowly, and it will make many stops completely unrelated to his trip objective. The bus may then let him off at a terminal to a suburban train. Again he must wait, and, after boarding the train, again experience a number of stops on the way to the CBD, and possibly again he may have to stand in the aisle. He will get off at the station most convenient to his destination and possibly have to transfer again onto a distribution system. It is no wonder that in those cities where ample inexpensive parking is available, most of those who can drive do drive.^[36]

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,^[37] 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 the US 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. In 1969, members of the study team published the first widely publicized description of PRT in Scientific American.^[38] In 1978 the team also published a book.^[39]

In 1967, aerospace giant Matra started the Aramis project in Paris. After spending about 500 million francs, the project was canceled 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.^[40]

The oil crisis of 1973 made vehicle fuels more expensive, which naturally interested people in alternative transportation.

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 kilometres per hour (37.3 mph) on a 4.8-kilometre (3.0 mi) 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 1.6-kilometre (1.0 mi) 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."^[41] However, this HCPRT program was diverted into a modest technology program.^{[citation needed]} 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.^[42]

In 1975, the Morgantown Personal Rapid Transit project was completed. It has five off-line stations that enable non-stop, individually programmed trips. This is a crucial characteristic of PRT. However, it is not considered a PRT system because its vehicles are too heavy and carry too many people. When it carries many people, it does not operate in a point-to-point fashion, instead running like an automated people mover from one end of the line to the other. Morgantown PRT is still in continuous operation at West Virginia University in Morgantown, West Virginia with about 15,000 riders per day (as of 2003^[update]). It successfully demonstrates automated control, but was not sold to other sites because the steam-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 that was considered fully developed by the German Government and its safety authorities. The system was to have been installed in Hamburg, but budget cuts stopped the proposed project before the start of construction. With no other potential projects 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 PRT 2000, 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 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.^[43][44]

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-metre (1,969 ft) long (one-way) and featured only two stations. The six-month operations were intended to research the public acceptance of PRT-like systems. The CyberCab as designed for the exhibition was very open. It was comparable to a Neighborhood electric vehicle, except it steered itself using magnetic guidance points embedded in the pavement.

Ford Research proposed a dual-mode system called PRISM.^[45] It would use public guideways with privately purchased but certified dual-mode vehicles. The vehicles would weigh less than 600 kg (1,323 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 in Cardiff, Wales, was certified to carry passengers by the UK Railway Inspectorate on a 1 km (0.6 mi) test track. ULTra was selected in October 2005 by BAA plc for London's Heathrow Airport.^[46] As of September 2011, a pilot system of the Heathrow PRT is fully operational, transporting passengers from a remote parking lot to the central terminal area. Further plans call for expansion throughout the airport and the surrounding region, pending the results of the pilot phase.^[19]

In June 2006, a Korean/Swedish consortium, Vectus Ltd, started constructing a 400-metre (1,312 ft) test track in Uppsala, Sweden.^[47] This test system was presented at the 2007 PodCar City conference in Uppsala, Sweden.^[48]

The Vectus project was based on The Fornebu/Oslo PRT Project. At the time, the urban development area around Telenor's new headquarter (at the Fornebu area near Oslo) was subject to intense debates as to various more or less innovative public transport systems. The idea of a PRT came up as a possible local solution as well as a business opportunity. In 2000, The Fornebu/Oslo PRT Project started as a part of an internal educational exercise within ICT strategy innovation within Telenor ASA, a major ICT corporation. As the poster shows, the student project was later transformed into a fast working concept, technology and business development project with various industry partners and a project group of around 10. The Korean steel company POSCO joined in, and developed the project further in Uppsala, Sweden, in part through new partners, but with all essential elements from the Fornebu/Oslo PRT Project, as further industrial or governmental support found in the Oslo area vanished. The poster describes the consortium and main results from the Oslo PRT project period. Key persons in this concept development phase were - as to technology and operational features development - Ingmar Andreasson, Göteborg, Sweden, Jan Orsten, indep. traffic planner, Oslo, Alan Forster, Force Ltd, GB, and Andrew Howard, HWG Ltd, GB. Beyond the general conceptual description, the ICT systems were developed by Noventus AB and others at later stages.

In 2007, the Polish PRT system MISTER was prototyped, and permission was given to install it in two Polish cities. MISTER is a typical overhead PRT system engineered for economical aerial reuse of streets' right of ways, that still gives ground-level access to wheelchairs and freight.

System design[link]

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

Vehicle design[link]

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. 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,^[49] and most industrialized countries commonly average below two people; not having to share a vehicle with strangers is a key advantage of private transport. 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 use an auto for a model, and choose larger vehicles, making it possible to accommodate families with small children, riders with bicycles, disabled passengers with wheelchairs, or a pallet or two of freight.

Propulsion[link]

All current designs (except for the human-powered Shweeb) 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. Most such systems retain a small on-board battery to reach the next stop after a power-failure.

ULTra uses on-board batteries, recharged at stops. This increases the safety, and reduces the complexity, cost and maintenance of the guideway. As a result, a grade-level ULTRa guideway resembles a sidewalk with curbs and is very inexpensive to construct. ULTRa resembles a small automated electric car, and uses similar components.

Switching[link]

Most designers avoid track switching, instead advocating vehicle-mounted switches or conventional steering. Those designers say that vehicle-switching permits faster switching, so vehicles can be closer together. It also 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. Other designers point out that track-switching simplifies the vehicles, reducing the number of small moving parts in each car. Track-switching replaces in-vehicle mechanisms with larger track-moving components, that can be designed for durability with little regard for weight or size.

Track switching 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.

Mechanical vehicle switching minimizes inter-vehicle spacing or headway distance, but it also increases the minimum distances between consecutive junctions. A mechanically switching vehicle, maneuvering between two adjacent junctions with different switch settings cannot proceed from one junction to the next. The vehicle must adopt a new switch position, and then wait for the in-vehicle switch's locking mechanism to be verified. If the vehicle switching is faulty, that vehicle must be able to stop before reaching the next switch, and all vehicles approaching the failed vehicle would be affected.

Conventional steering allows a simpler 'track' consisting only of a road surface with some form of reference for the vehicle's steering sensors. Switching would be accomplished by the vehicle following the appropriate reference line- maintaining a set distance from the left roadway edge would cause the vehicle to diverge left at a junction, for example.

Infrastructure design[link]

Simplified depiction of a possible PRT network. The blue rectangles indicate stations. The enlarged portion illustrates a station off-ramp.

Guideways[link]

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 easing ground-level installation. An overhead track is necessarily higher, but may also be narrower.^[6] 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^{[citation needed]}, so most proposals plan to resist snow and ice in ways that should be less expensive.

Stations[link]

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, without facilities such as rest rooms. For elevated stations, an elevator may be required for accessibility.

At least one system, MISTER provides wheelchair and freight access by using a cogway in the track, so that the vehicle itself can go from a grade-level stop to an overhead track.

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.

When user demand is low, surplus vehicles could be configured to stop at empty stations at strategically placed points around the network. This enables an empty vehicle to quickly be despatched to wherever it is required, with minimal waiting time for the passenger.

Operational characteristics[link]

Headway distance[link]

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.^[50] 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.^[51]

Capacity[link]

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[link]

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 will not generally be filled to capacity, due to the point-to-point nature of PRT. At a more typical average vehicle occupancy of 1.5 persons per vehicle, the maximum capacity is 2,700 passengers per hour. Some researchers have suggested that rush hour capacity can be improved if operating policies support ridesharing.^[52]

Capacity is inversely proportional to headway. Therefore, moving from two-second headways to one-second headways would double PRT capacity. Half-second headways would quadruple capacity. Theoretical minimum PRT headways would be based on the mechanical time to engage brakes, and these are much less than a half second. 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.^[53] Using the above figures, capacities above 10,000 passengers per hour seem in reach.

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.

Grade separated light rail systems can move 15,000 passengers per hour on a fixed route, but these are usually fully grade separated systems. Street level systems typically move up to 7,500 passengers per hour. Heavy rail subways can move 50,000 passengers per hour. As with PRT, these estimates depend on having enough trains. Neither light nor heavy rail scales well for off-peak operation.

Networked PRT capacity[link]

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^[54] 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 also consider the cost per line.

PRT systems should 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.

Travel speed[link]

For a given peak speed, nonstop journeys are about three times as fast as those with intermediate stops. This is not just because of the time for starting and stopping. Scheduled vehicles are also slowed by boardings and exits for multiple destinations.

Therefore, a given PRT seat transports about three times as many passenger miles per day as a seat performing scheduled stops. So PRT should also reduce the number of needed seats threefold for a given number of passenger miles.

While a few PRT designs have operating speeds of 100 km/h (60 mph), and one as high as 241 km/h (150 mph),^[55] 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 is reduced about threefold by scheduled stops and passenger transfers.

Ridership attraction[link]

If PRT designs deliver the claimed benefit of being substantially faster than cars in areas with heavy traffic, simulations suggest that PRT could attract many more automobile drivers than other public transit systems. Standard mass transit simulations accurately predict that 2% of auto drivers will switch to trains. These same methods predict that 25% to 60% of auto drivers would switch to PRT.^{[citation needed]}

Control algorithms[link]

The typical 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. Traffic jams are prevented by placing north/south vehicles in even slots, and east/west vehicles in odd slots. At intersections, the traffic in these systems can interpenetrate without slowing.

On-board computers maintain their position by using a negative feedback loop to stay near the center of the commanded slot. Early PRT vehicles measured their position by adding up the distance using odometers, with periodic check points to compensate for cumulative errors.^[39] Next-generation GPS and radio location could measure positions as well.

Another style of control assigns a path and speed to a vehicle, after verifying that the path does not violate the safety margins of other vehicles. This permits system speeds and safety margins to be adjusted to design or operating conditions, and may use slightly less energy.^[56] 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[link]

Computer control eliminates errors from human drivers, so PRT designs in a controlled environment should be much safer than private motoring on roads. 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 company ULTra PRT reported that AGT systems have a better safety than more conventional, non-automated modes.^{[citation needed]}

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

Energy efficiency[link]

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.^[57]

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 (i.e. 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 do not 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.^[58]

ULTra PRT estimates its system will consume 839 BTU per passenger mile (0.55 MJ per passenger km).^[59][60] By comparison, automobiles consume 3,496 BTU, and personal trucks consume 4,329 BTU per passenger mile.^[61]

Due to PRT's efficiency, some proponents say solar becomes a viable power source.^[62] PRT elevated structures provide a ready platform for solar collectors, therefore some proposed designs include solar power as a characteristic of their networks.

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.^[61]

Cost characteristics[link]

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. Proponents' 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.^[63][64] These costs may not include the purchase of rights of way or system infrastructure, such as storage and maintenance yards and control centers, and reflect unidirectional travel along one guideway, the standard form of service in current PRT proposals. Bidirectional service is normally provided by moving vehicles around the block. To reach capacities of competing systems, a system requires thousands of vehicles. Some PRT proposals incorporate these costs in their per-mile estimates.

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

A design with many modular components, mass production, driverless operation and redundant systems should in theory result in low operating costs and high reliability. Predictions of low operating cost generally depend on low operations and maintenance costs. Whether these assumptions are valid will not be known until full scale operations are commenced since reliability cannot 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.

Opposition and controversy[link]

Opponents to PRT schemes have expressed a number of concerns:

Technical feasibility debate[link]

The Ohio, Kentucky, Indiana (OKI) Central Loop Report^[65] 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.^[51] 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,^[66] the most reliable service levels, the highest level of frequency of service and geography coverage, and was most able to maintain schedule.^[67] 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.^[68]

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."^[69]

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."^[69]

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.

Lessons from baggage handling[link]

There is a history of PRT style systems constructed at airports for use in moving bags from check in to x-ray and loading points for various aircraft. Some of these systems failed or had problems and this has led to conclusions that PRT systems would fail in a similar way.^[70]

More recently however there have also been some significant success stories in this area. The Bagtrax system provided by Vanderlande Industries is successfully installed at 6 airports now. These include major airports such as Schiphol Airport in Amsterdam and Terminal 5 at Heathrow Airport, London. The performance of these modern systems exceed conventional conveyor systems by far. Also the "Telebag System" is operational in 5 airports including London's Luton Airport, which handles significant volumes of traffic. The "Autover System"^[71] is another that is also used at a number of airports. At Dubai Airport for example, 31 vehicles carry 1200 pieces of luggage per hour (one piece of luggage per vehicle) from 6 origins to 24 destinations along 618 m of track making 9 loops with 16 junctions.

Regulatory concerns[link]

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.^[72]

Also, several PRT systems have been proposed for California,^[73][74] but the California Public Utilities Commission (CPUC) states that its rail regulations apply to PRT, and these require railway-sized headways.^[75][76] 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.^[77]

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. ^[78]

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[link]

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.^[79]

New urbanist opinion[link]

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^[80][81] 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".^[82]

Group rapid transit[link]

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.^[83] 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.^[83] 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.^[83][84] Such a system may constitute an "optimal" surface transportation routing solution in terms of balancing trip time and convenience with resource efficiency.^[83]

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.^[83][84] 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.^[84]

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

See also[link]

• Duke University Medical Center Patient Rapid Transit, a permanently discontinued personal rapid transit system.
• Slope car

References[link]

Specific references:

External links[link]

• "Will You Commute via PRT?" (CNN) -retrieved 31 March 2011
• Systems Analysis of Urban Transportation Systems, Scientific American, 1969, 221:19–27
• advancedtransit.org—A history of PRT.
• " Personal Rapid Transit" - Book containing papers from the proceedings of the 1973 International Conference on Personal Rapid Transit (published by the University of Minnesota)

The New York City Subway is the world's largest rapid transit system by length of routes and by number of stations.

Clapham Common tube station on the London Underground

Interior of a Helsinki Metro train

Inside an underground tunnel on the Taipei Metro

A rapid transit, underground, subway, elevated railway, metro or metropolitan railway system is an electric passenger railway in an urban area with a high capacity and frequency, and grade separation from other traffic.^[1][2] Rapid transit systems are typically located either in underground tunnels or on elevated rails above street level. Outside urban centers, rapid transit lines may run on grade separated ground level tracks.

Service on rapid transit systems is provided on designated lines between stations using electric multiple units on rail tracks, although some systems use guided rubber tyres, magnetic levitation, or monorail. They are typically integrated with other public transport and often operated by the same public transport authorities. Rapid transit is faster and has a higher capacity than trams or light rail (but does not exclude a fully grade separated LRT), but is not as fast or as far-reaching as commuter rail. It is unchallenged in its ability to transport large amounts of people quickly over short distances with little land use. Variations of rapid transit include people movers, small-scale light metro and the commuter rail hybrid S-Bahn.

The first rapid transit system was the London Underground, which opened in 1863. The technology quickly spread to other cities in Europe, and then to the United States where a number of elevated systems were built. At first these systems used steam locomotives, with the term later coming to entirely mean electric systems. More recently the largest growth has been in Asia and with driverless systems. More than 160 cities have rapid transit systems, totaling more than 8,000 km (5,000 mi) of track and 7,000 stations. Twenty-five cities have new systems under construction.^{[citation needed]}

The biggest metro system in the world by length of routes (including non-revenue track) and by number of stations is the New York City Subway; by length of passenger lines, the largest are the Shanghai Metro^[3] and London Underground. The busiest metro systems in the world by daily and annual ridership are the Tokyo subway, Moscow Metro and Seoul Metropolitan Subway.

Terminology[link]

The Paris Metro style entrance to Square-Victoria Station of the Montreal Metro

Meiji-jingumae station Fukutoshin Line platform in the Tokyo Metro

An engraving from the Illustrated London News showing the initial construction stages of London's Metropolitan Railway at King's Cross St. Pancras in 1861

Metro is the most common term for underground rapid transit systems.^[4] Rapid transit systems may be named after the medium through which their busier inner-city sections travel: use of tunnels inspires names such as subway,^[5] underground,^[6] Untergrundbahn (U-Bahn) in German,^[7] or Tunnelbana (T-bana) in Swedish;^[8] use of viaducts inspires names such as elevated (el or L), skytrain,^[9] overhead or overground. One of these terms may apply to an entire system, even if a large part of the network (for example, in outer suburbs) runs on ground level.

In English English a subway is a pedestrian underpass, so the expressions underground and tube may be preferred. In Scotland, Glasgow's underground rapid transit system is called the Glasgow Subway.

History[link]

Rapid transit evolved from steam railways during the late 19th century. In 1890 the City & South London Railway in London was the first electric rapid transit railway.^[10] The electric railway eventually was merged into London Underground. The technology swiftly spread to other cities in Europe, as in Budapest, Hungary in 1896, and then to the United States. A number of elevated systems were built, starting with the 1893 100% designed electric Liverpool Overhead Railway. The elevated railways in Chicago and New York were converted to electric from steam propulsion.^[11]

By 1940, there were 19 systems, and by 1984, there were 66. This included smaller cities like Oslo and Marseille which opened extensive systems in the 1960s. More recently the growth of new systems has been concentrated in Southeast Asia and Latin America.^[7] Western Europe and North America have instead seen a revival of the tram, with light rail systems supplementing full scale urban railways, and less focus on building rapid transit.^[12] At the same time, technological improvements have allowed new driverless lines and systems. Hybrid solutions have also evolved, such as tram-train and premetro, which have some of the features of rapid transit systems.^[10]

Operation[link]

Inside a tunnel on the Turin Metro

Rapid transit is used in cities, agglomerations, and metropolitan areas to transport large numbers of people at high frequency. The extent of the rapid transit system varies greatly between cities, with several transport strategies: in larger metropolitan areas the underground system may extend only to the limits of the inner city, or to its inner ring of suburbs with trains making frequent station stops. The outer suburbs may then be reached by a separate commuter rail network, where more widely spaced stations allow higher speeds. These trains are often more expensive and less frequent, and in some cities operate only during rush hour periods. They may or may not satisfy the criteria for an urban rapid transit system, and in some cases the differences between urban and suburban rapid transit systems are not clear. See also the Variations and Comparison sections below.^[2]

Ximen Station on the Blue Line of the Taipei Metro. Rapid transit allows for high passenger throughput.

Rapid transit systems are often supplemented by other systems, either buses, trams or commuter rail. This combination of transit modes serves to offset certain limitations of rapid transit, such as limited stops and long walking distances within station infrastructure between outside access points and the vehicle itself, (sometimes involving long vertical distances if the station is in a deep-level tunnel or on a high overpass). Given these limitations, combined with the high density nature of many urban environments, short trips are often more easily performed on trams or buses. Many cities have chosen to operate a tram system in the city core with the metro expanding beyond it,^[8] although many cities (especially outside of Europe) later converted their surface systems from trams to buses in the 1950s and 60s. Another common strategy is to use a bus or tram feeder system to transport people to rapid transit stops, freeing the suburban or feeder system from the requirement to drive all the way to the city centre.^[13] Surface route terminals connected directly to rapid transit stations, provided especially in newer systems with single, consolidated transit agencies turn rapid transit stations into neighbourhood centres - nodes of activity that can blossom into satellite business districts of their own. This scheme promotes transit ridership in lower-density outlying neighbourhoods that do not have the population density or sufficient attractions to support their own rapid transit lines by offering a connection to business districts that saves users from driving in big city traffic conditions, and reduces congestion in urban centres by consolidating passengers into dedicated high-frequency, high-capacity corridors, instead of being reliant on city centre streets, which are often among the oldest in their respective cities and were often never designed to accommodate modern traffic volumes.

In Toronto, over 50% of its rapid transit stations have bus and streetcar terminals within the fare-paid zone, providing a connection without requiring proof-of-payment, and speeding boarding by allowing passengers to board through all doors. Several subway stations, especially current or former termini for their respective lines, have developed significant satellite commercial districts of their own.

Rapid transit systems have high fixed costs. Most systems are publicly owned, by either local governments, transit authorities or national governments. Investments are often financed by taxation, rather than by passengers, but must often compete with funding for roads. The systems may be operated by the owner or by a private company through a public service obligation. The owners of the systems often also own the connecting bus or rail systems, or are members of the local transport association, allowing for free transfers between modes. Almost all systems operate at a deficit, requiring fare revenue, advertising and subsidies to cover costs. The farebox recovery ratio, a ratio of ticket income to operating costs, is often used to assess operational profitability, with some systems including Hong Kong's MTR Corporation,^[14] and Taipei^[15] achieving recovery ratios of well over 100%. This ignores both heavy capital costs incurred in building the system, which are often subsidized with soft loans^[16] and whose servicing is excluded from calculations of profitability, as well as ancillary revenue such as income from real estate portfolios.^[14] Some metros, including Hong Kong, are even financed by the sale of land whose value has been increased by the building of the system.^[17]

Lines[link]

R46 stock of the New York City Subway remains in service in 2010 since its 1975 launch.

A Bangalore Metro train running on the purple line

Each rapid transit system consists of one or more lines. Each line is serviced by at least one specific route with trains stopping at all or some of the line's stations. Most systems operate several routes, and distinguish them by colors, names, numbering, or a combination thereof. Some lines may share track with each other for a portion of their route, or operate solely on their own right-of-way. Often a line running through the city center forks into two or more branches in the suburbs, allowing a higher service frequency in the center. This arrangement is used by many systems, such as the Copenhagen Metro.^[18] Alternatively there might be a single central terminal (often shared with the central railway station), or multiple interchange stations between lines in the city centre, for instance in the Prague Metro.^[19] The London Underground^[20] and Paris Métro^[21] are densely built systems with a matrix of crisscrossing lines throughout the cities. Other systems, such as the New York City Subway ^[22] are also densely built and their layout facilitate trips to a central area, such as Manhattan, the city's business and cultural center. The Chicago 'L' has most of its lines converging on The Loop, the main business, financial, and cultural section. Some systems have a circle line around the city center connecting the radially arranged outward lines, such as the Moscow Metro's Koltsevaya Line and Tokyo's Yamanote Line.

The capacity of a line is obtained by multiplying together the car capacity, train length and service frequency. Heavy rapid transit trains might have six to twelve cars, while lighter systems may use only four or fewer cars. Cars have a capacity of 100 to 150, varying with the seated to standing ratio—more standing gives higher capacity. Bilevel cars, used mostly on German S-Bahn type systems, have more space, allowing the higher seated capacity needed on longer journeys. The minimum time interval between trains is shorter for rapid transit than for mainline railways owing to the use of block signaling: the minimum headway might be 90 seconds, which might be limited to 120 seconds to allow for recovery from delays. Typical capacity lines allow 1200 people per train, giving 36,000 people per hour. The highest attained capacity is 80,000 people per hour by the MTR Corporation in Hong Kong.^[23]

A fictional schematic diagram representing a section of a transit map

Passenger information[link]

Information panel showing the current location and upcoming stops of an East-West Singapore MRT train.

Rapid transit operators often have built up strong brands; in particular the use of a single letter as a station sign has become widespread, with systems identified by the letters L, M, S, T and U, among others.^[24] Branding has focused on easy recognition—to allow quick identification even in the vast array of signage found in large cities—combined with the need to communicate speed, safety and authority.^[25] In many cities, there is a single corporate image for the entire transit authority, but the rapid transit uses its own logo that fits into the profile.

A transit map is a topological map or schematic diagram used to show the routes and stations in a public transport system. The main components are color-coded lines to indicate each line or service, with named icons to indicate stations. Maps may show only the rapid transit, or also include other modes of public transport.^[26]

Transit maps can be found in the transit vehicles, on the platforms, elsewhere in stations and in printed timetables. Their primary function is to help users of the system: for instance they show the interchange stations where passengers can transfer between lines. Unlike conventional maps, transit maps are usually not geographically accurate: instead they use straight lines and fixed angles, and often a fixed distance between stations, to simplify the display of the system. Often this has the effect of compressing stations in the outer area of the system and expanding those close to the center.^[26] Timetables are mostly only published if the service frequency is so low that passengers can profitably time their arrival at the station; if the service is frequent enough (say 6 or more trains an hour) passengers will never have to wait long, and will not need a timetable.^[27]

Safety and security[link]

Northward view of Chicago 'L' from Adams/Wabash in Chicago Loop

Rapid transits are a public space, and may suffer from security problems: petty crimes such as pickpocketing and baggage theft, and more serious crimes such as violence. Security measures include video surveillance, security guards and conductors. In some countries a transit police may be established. These security measures are normally integrated with measures to protect revenue by checking that passengers are not traveling without paying.^[28] Rapid transits have been subject to terrorism with many casualties, such as the 1995 Tokyo subway sarin gas attack.^[29]

Compared to other modes of transport, rapid transit has a good safety record, with few accidents. Rail transport is subject to strict safety regulations, with requirements for procedure and maintenance to minimize risk. Head-on collisions are rare due to use of double track, and low operating speeds reduce the occurrence and severity of rear-end collisions and derailments. Fire is more of a danger underground, and systems are built to allow evacuation of trains at many places throughout the system.^[30][31]

Infrastructure[link]

The Paris Metro runs mostly underground.

Getafe Central in Line 12 of Madrid Metro, with a Cercanías transfer: new stations are built deliberately ample, with several, cross-visible levels and elevators for disabled people.

Most rapid transit trains are electric multiple units with lengths from three to beyond ten cars.^[32] Power is commonly delivered by a third rail or by overhead wires. The whole London Underground network uses fourth rail and others use the linear motor for propulsion.^[33] Most run on conventional steel railway tracks, although some use rubber tires such as the Montreal Metro and Mexico City Metro. Rubber tires allow steeper gradients and a softer ride, but have higher maintenance costs and are less energy efficient. They also lose friction when weather conditions are wet or icy, preventing above ground use of the Montréal Metro but not rubber-tired systems in other cities.^[34] Crew sizes have decreased throughout history with some modern systems now running completely unstaffed trains.^[35] Other trains continue to have drivers, even if their only role in normal operation is to open and close the doors of the trains at stations.

Variations[link]

5-Line of Metro de Santiago (Chile) combines rubber tire traction with elevated right-of-way.

Underground tunnels move traffic away from street level, leaving more land available for buildings and other uses. In areas of high land prices and dense land use, tunnels may be the only economic route for mass transportation. Cut-and-cover tunnels are constructed by digging up city streets, which are then rebuilt over the tunnel; alternatively tunnel-boring machines can be used to dig deep-bore tunnels that lie further down in bedrock.^[10]

Street level railways are used only outside dense areas, since they create a physical barrier that hinders the flow of people and vehicles across their path. This method of construction is the cheapest, as long as land values are low. It is often used for new systems in areas that are planned to fill up with buildings after the line is built.^[17]

Elevated railways are a cheap and easy way to build an exclusive right-of-way without digging expensive tunnels or creating barriers. In addition to street level railways they may also be the only other feasible alternative due to considerations such as a high water table close to the city surface that prohibits, or makes even more costly, underground railways (e.g. Miami). They were popular around the beginning of the 20th century, but fell out of favor; they came back into fashion in the last quarter of the century—often in combination with driverless systems, for instance Vancouver's SkyTrain, London's Docklands Light Railway^[36], the Miami Metrorail, and the Bangkok Skytrain.^[37]

Chicago Transit Authority control tower 18 guides elevated Chicago 'L' north and southbound Purple and Brown lines intersecting with east and westbound Pink and Green lines and the looping Orange line above the Wells and Lake street intersection in the loop.