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Personal rapid transit

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File:SkyWebKids.jpg
Promotional photo of SkyWeb Express system (previously known as Taxi 2000)

Personal rapid transit (PRT) is an emerging transport mode that would offer on-demand non-stop transportation between any two points on a network of specially built guideways. PRT has been "reinvented" many times, in different parts of the world, by people seeking to optimize mass transit so as to meet a wider range of travel needs.

PRT developers and advocates say that it can provide transit service that combines the convenience of cars with the social and environmental advantages of public transport. Some advocates estimate PRT's per-mile costs as ranging between $0.10/mile (the cost of a moped) to $0.01/mile (a bicycle is $0.03/mi). Several PRT systems are in development, and several PRT designs have been safety-certified by government authorities (this is the last stage of engineering development), notably Cabinentaxi and ULTra. Transit using similar automated technologies is in regular operation, with some systems dating back to 1974. Systems include West Virginia Universityand Schiphol Airport. These are said to prove the viability of small-scale projects in dense, high traffic applications such as universities and airports. Some proponents say that these developments prove technical feasibility.

Several PRT proposals failed when their final projected costs exceeded initial expectations. Other PRT projects have failed technically, some with large monetary losses, often when political needs, schedules or budgets interfered with a technical requirement (notably Aramis). One proposed implementation, Skyloop, was rejected after a technical evaluation, although its validity is contested by advocates. As of 2006, there are no true PRT systems in operation as public carriers, which some detractors believe implies infeasibility.

Overview

PRT is unfamiliar, but can be compared to existing transportation methods. The following describes the intentions of developers; criticisms of these claims will be discussed below.

Comparison of Personal Rapid Transit (PRT) to existing transit systems
Similar to automobiles
  • PRT vehicles are small -- typically 1 to 6 passengers.
  • PRT vehicles are individually hired, like taxis, and only shared with the passengers of one's choosing. Also like taxis, there is no requirement to ride with strangers. The on-demand and non-stop nature of PRT service makes such a requirement impractical or unnecessary in most cases.
  • Vehicles travel along a network of guideways, much like a network of streets. Routing is point-to-point, with no intermediate stops or transfers. This makes it substantially faster than tram or bus systems, which in urban areas have maximum speeds under 26 km/h (16 mi/h) when intermediate stops are included.
  • PRT vehicles are available on an on-demand, around-the-clock basis.
  • Some PRT developers and advocates say that PRT would be cost-effective enough to operate commercially, without public subsidy.
Similar to trams, buses, and monorails
  • It is a public amenity, designed for use by commuters, children, and disabled persons.
  • Similar systems have been extremely safe compared to automobiles.
  • It causes far less local pollution than cars, since it is typically electrically powered.
  • Vehicles are not privately owned (except in the case of mixed dual-mode/PRT systems, discussed below).
  • Passengers get rides at discrete locations ("stations") analogous to bus stops or taxi stands. Most systems locate these about 800 meters (1/2 mi) apart, so that typical walking-distance access is half that, 400 meters (1/4 mile).
Unique attributes
  • PRT is highly automated: computers collect the fares, drive the vehicles, and constantly optimize the routing of the network.
  • PRT is generally elevated above street-level, on a very lightweight guideway. This can be much lighter-weight than for other transport systems, because the vehicles themselves are much lighter.
  • PRT stops are along sidings, allowing through-traffic to by-pass the station unimpeded. Combined with the absence of cross-traffic along the route, this allows PRT vehicles in urban situations to be considerably faster than even automobiles.
  • Because of the rapid computer-controlled reaction times, headway distance (the separation between vehicles) can be very short -- 2 seconds or less. In some systems, PRT vehicles even form dynamically-recombining "trains" of vehicles, separated by a few inches, to reduce drag and increase speed and energy efficiency.
  • Because of the short headway distances, a single PRT line has a large carrying capacity -- from one to three freeway lanes, depending on the design.
  • A PRT network achieves high capacity by each vehicle quickly serving a passenger or group of passengers, one after another. For example, simple arithmetic shows that a 35mph vehicle can make more than 10 five-mile trips in an hour. Thus, a 1,000-vehicle fleet could make more than 10,000 such trips in the same period.
  • PRT's nonstop travel reduces energy use compared to buses, trains and automobiles (see "Minimized overhead and operating costs," below) which have to stop at bus-stops, train-stations, and traffic lights. Aerodynamic drag is a large part of the energy use of most PRT systems, and some PRT systems' cars utilize low-drag aerodynamics.
  • PRT uses so little land that a network could free much open space for other uses.

PRT's advocates say that with properties like these, PRT should be considered as a tool for dealing with urban transportation problems. They point to ridership simulations suggesting that PRT systems could absorb between 15% and 60% of vehicular traffic. However, PRT detractors claim that many of these claims are unfounded, and point to the continuing lack of any operational PRT systems as evidence.

History

The concept is said to have 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, a civilian arm of the U.S. Air Force, spent substantial time and money on PRT, and performed much of the early theoretical and systems analysis. However this corporation is wholly owned by the U.S. government, and may not sell to non-governmental 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. 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," and incorrect control software caused cars to bump very hard.

In Germany, the Cabinentaxi project, a joint venture from Mannesmann Demag and MBB, created an extensive PRT development considered fully developed by the German Government and its safety authorities. This project was canceled when a scheduling mishap coincided with a mandatory budget cut 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 that 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 "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 are less than 600 kg (1200 lb), allowing small elevated guideways. They could use efficient centralized computer controls and power. The proposed vehicles brake with rubber-tired wheels, reducing guideway capacity by forcing larger inter-vehicle safe braking distances.

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 1km test track. It undertook very successful passenger trials and has met all project milestones to 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 reliable and economical.

ULTra was recently (October, 2005) selected by BAA 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.

Safety and utility

Safety engineers employed by PRT companies say that travel via PRT systems should be ten thousand to one million times safer than via cars because of basic design improvements. Computer control is said to be more reliable than drivers. Grade-separated guideways prevent collisions with pedestrians or manually-controlled vehicles. Most PRT systems enclose the running gear in the guideway to prevent derailments. Vehicles usually have computer-diagnosed, dual-redundant motors and electronics. In the event of a total failure, a car can be pushed to a repair facility by another PRT vehicle.

Tracks and vehicles are timed to "miss" at intersections. Careful engineering at several projects has shown that less-expensive one-way, single-level loops can operate as safely and almost as quickly as systems with far more expensive dual-direction clover-leaf intersections.

The Morgantown PRT system (considered group rapid transit by PRT experts) has now completed 110 million injury-free passenger-miles. By comparison, regular transit injures about a hundred people on average in that many passenger miles.

Embarkation stations are on turnouts so by-passing vehicles can pass by at full speed. Systems can embark passengers as fast as buses or trains, but mass embarkation stations must have a turn-out for each one or two passenger queues.

Theoretically, car-parks (parking lots) can be far smaller for shopping centers, universities, stadiums and convention centers, freeing much valuable land. Roads or rails are required for heavy transport.

All vehicles are powered by electricity, so pollution is much less. Most systems plan multiply-redundant power supplies, from track-side batteries or natural-gas-powered generators. Stationary power reduces vehicle weights.

Most designers eschew line switching built into the track, because failure of an in-track switch would drastically degrade capacity. Vehicle-mounted switches are preferred so that tracks stay in service, and to allow closer spacing of vehicles since no time delay is needed to allow the track to switch. Vehicle-mounted switches may be mechanical or solid-state electromagnetic. Some systems like 2getthere and ULTra do not need switches since the automated steering system merely chooses which path to follow.

Some systems plan to group vehicles to carry large groups. This also can reduce aerodynamic drag. Groups (called "platoons" or "trains") could share an intercom and destination.

Some systems plan multiple types of vehicles. The smallest vehicles seat two, the largest six. Two has the lowest-per-mile track cost, and handles most trips (average ridership in cars is 1.16 persons per vehicle in the U.S.) Most systems provide for wheel-chair users, bicyclists and light cargo vehicles, sometimes with special vehicles. One study found that light cargo could enable feasibility in a port city.

Most systems have buttons in a vehicle, such as "let me talk to the operator," "take me to the nearest stop," "take me to the hospital," "take me to the police for help," and "this vehicle is too filthy to use."

Vandalism could be investigated from video of the car, reviewed when the button "this vehicle is too filthy to use." is pressed. The Morgantown System reports very little vandalism accredited to the short wait times and cctv monitoring of the stations.

Engineering economics

Many PRT advocates claim that it will have a per-passenger trip costs between $0.01 and $0.10/mile ($0.006 and $0.06/km) -- somewhat cheaper to operate than a moped. However many transportation planners disbelieve these unprecedented low cost projections. How capital costs are incorporated is a critical element in cost estimates, since PRT systems are capital-intensive with low operating costs compared to other technologies.

In all transit systems, vehicles are depreciated on a schedule that accounts for the average number of empty seats per vehicle, and the number of trips per day. This becomes a number called "capacity utilization." When it is higher, fares cover more of the costs of the transit equipment and operators.

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

In PRT, the cost of capacity is less because fare collection, driving and security are automated. Also, PRT idles not seats, but whole vehicles. Idle vehicles should use less energy, and wear and so depreciate more slowly than active but empty vehicles.

Minimized overhead and operating costs

Standard transit-planning assumptions concerning overhead per vehicle are said to fail in PRT systems. One major operating expense of bus and light rail systems is the operators' and mechanics' salaries. Additionally, some systems require transit police as well.

PRT systems eliminate driver salaries by automating guidance and fare-collection. Repairs are far less per vehicle because PRTs have electric motors, with one moving part (on most the only moving parts are wheels and the door), versus hundreds for an internal combustion engine.

Transit police are not required because riders are not forced to share a cabin, and criminals cannot easily predict where vehicles will go, and so cannot wait for commuters.

The WVU PRT project failed commercially (though succeeding technically) because its track had to be heated to eliminate snow. Systems where the vehicles ride atop the track therefore try not to collect precipitation or dust. Weather is better handled by overhead tracks. Note that in this area, well-designed PRT systems can save money over conventional streets and vehicles.

As for fuel, PRT systems can be powered from the track, and purchase power from the cheapest electric utility. Unlike trains and electric buses, PRTs only accelerate and stop once per passenger, saving substantial energy. Ordinary electric motors are 98% efficient, and non-polluting. Some PRT designers have reported or projected very low energy usage, such as ULTra's 0.55 megajoules per passenger km, or less than 900 BTU per passenger mile. This compares favorably to energy usage of over 3,200 BTU per passenger mile for rail transit (U.S. Dept. of Energy, Table 2.11).

Still, it is well-known from U.S. federal data that operations and maintenance costs (O&M) are nearly constant per seat for a wide variety of systems: buses, trains, aircraft and private automobiles, which of course lack paid operators.

Some authorities say that even if PRT has the same costs, the increased load factor (O&M/passengers per destination) of PRT (about 0.33) should reduce costs per passenger mile compared to those of other public transit (which are about 0.15).

Capacity utilization:

Affected by - nonstop passenger travel

Another dispute concerns capacity utilization, which directly affects a transit-system's return on investment.

If the peak speeds of PRT and a train are the same, a well-designed PRT is two to three times as fast for a passenger as a well-designed bus or train route, just because the PRT vehicles do not stop every few hundred yards to let passengers on and off.

Therefore for the same maximum speed, PRT theoretically has two to three times as many trips per seat as a bus or train. So PRT should utilize its average seat 50 to 300 % more efficiently. This is contested, of course.

Such high route utilizations would let PRT replace a train or high-capacity bus route. If true, PRT could be used in an intermodal transport system, and then expand from a proof-of-concept project into a network.

Affected by - trips per day

PRT automatically diverts vehicles to busy routes and travels nonstop at maximum speeds. Simulations with standard assumptions show that at these high speeds, vehicles can be recycled for new trips as much as several times per hour, even during busy periods, even in low-density cities. This yields more trips per hour per vehicle, increasing ridership substantially during rush hour. In simulations of rush hour or high-traffic events like professional sports events, about 1/3 of vehicles on the guideway need to be empty to get the best response time.

Minimizes Fleet size

At idle times fast speeds do not increase ridership, because no-one wants to travel. However, the higher ridership during rush hour lets a smaller fleet serve the same number of passengers. The result is therefore to reduce the absolute fleet size, and the number of idled vehicles during idle times.

Affected by - passenger capacity

PRT vehicles carry only two to four passengers in order to reduce weight. However, this also increases ridership per vehicle, because during idle times every operating vehicle will have a higher ridership (25-50%) than a mass-transit vehicle such as a bus or train (as low as 2% after midnight, 15% during non-rush hours).

Since the U.S. averages 1.16 persons per automobile in commuter areas, many authorities say that the optimum vehicle size in the U.S. for PRT is either 1 or 2 passengers. Some systems (UniModal, Ford Research's PRISM) have found that the weight and cost difference between these sizes of vehicles is so low that two seats is optimum, with tandem seating and a low drag shape.

Other authorities question the viability of systems with only two seats. The public's worst-case needs are shown by its choice of automobiles, 85% of which have four seats plus or minus one. Groups of three or four commonly travel together. Families with young children may be reluctant to split up. Also a person in a wheelchair with a companion and luggage may not be accommodated. Some PRT vendors therefore have chosen vehicles accommodating three or four passengers with luggage.

Affected by - Braking

The spacing of PRT vehicles on the guideway sets the rate at which the guideway, the major system expense, can be depreciated by traffic. Designers therefore attempt to minimize the headway, the distance between vehicles.

Some PRT designers have planned for very short headways, which can allow a single guideway to carry the same number of passenger miles as four freeway lanes. This dramatically increases the capacity utilization of a heavily-used guideway, and substantially speeds trips through the center of a city, by permitting more use of direct routes. It also permits a PRT guideway to achieve carrying capacities similar to light rail.

Short headways are theoretically possible. PRT vehicles normally operate on unshared guideways, on a separate grade from other traffic. This means that emergency braking for side traffic is not required. Since the front vehicle will also be braking, the minimum safe distance between the vehicles will be set by the reaction time of the following vehicle. In most cases, this consists of the brake's mechanical reaction time, and the reaction time of the electronic control system. If the front vehicle electronically signals the following vehicle, and both vehicles use brakes with the same reaction time, the headway might be cut to the on-board computer's response time, which can easily be less than a fiftieth of a second. Even with large safety margins, this permits much closer spacing than the two-second headways recommended for cars.

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

Some persons calculate headways in terms of absolute stopping distances, using vehicle decelerations taken from rail lines, and then prove that PRT systems are impossibly uneconomic. This method of calculation is traditional in heavy rail systems, because heavy rail normally shares its grade with other traffic and has poor reaction times and very poor brakes compared to vehicle weight. These conditions do not apply to PRT. Some authorities argue that even when used for heavy rail, calculating headways from absolute stopping distance is too conservative.

Affected by - attracted ridership

Simulations with standard assumptions show that PRT, which should be substantially faster than autos in areas with traffic jams, should attract between 35% and 60% of automobile users. In contrast, new light rail systems and bus lines normally attract about 2% of automobile users, both in reality, and in similar simulations.

Some PRT systems (See Unimodal) plan speeds substantially faster than automobiles achieve on empty expressways. In simulations, these attract even more traffic than slower, conservative PRT designs.

The ridership simulations are disparaged, but have been repeated many times. If true, the high riderships would substantially decrease the cost per rider of PRT compared to trains and buses.

Costs of rights-of-way- trading technology for less land-use

Planners dispute the cost-estimates of PRT rights-of-way. In modern metropolitan areas, rights-of-way for light rail cost as much as $50 million per mile ($30 million/km). However, a typical light-rail right-of-way is 100 to 300 feet (30 to 100 m) wide, and (naturally) goes through the highest-density, most valuable part of the city. When the railway tunnels to conserve the surface, it becomes even more costly.

The surprisingly cheap, less than $1 million per mile estimates (2002, Orange County, California) of PRT designers depend on dual-use rights of way. By mounting the transit system on narrow poles, placed on an existing street, PRT designers hope to use land very economically. Small PRT vehicles with passengers can weigh as little as 1,000 pounds (450 kg), while conventional rail systems with many passengers often weigh tens or hundreds of thousands of pounds.

In some circumstances, such as at airports, PRT's small size can reduce the volume of its tunnel to less than a quarter of that required for an automated people mover (APM). Even when account is taken of the need for two PRT guideways to match the capacity of one APM guideway, the tunnel volume (hence cost) will be less than half.

PRT rights of way may even cost less than a conventional road system. Proponents say that if auto- and bus-based transit systems include the costs of the roadways needed for buses and automobiles (US $10x10^6 per mile, or $6x10^6 per km), PRT systems are substantially cheaper than bus and automobile systems.

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

Since systems have minimal waiting times, embarkation stations are very small (inexpensive) and lack amenities such as seating or restrooms. Usually there's only a fare vending machine, a gate or two, a line of vehicles and a security camera. The stations are usually mounted on poles with the track, but may also be inside buildings or at street level. In the U.S., systems must provide service to disabled persons. Some advocates say that a bus system to provide free disabled service is cheaper than elevators at each embarkation station, and this meets legal requirements, but this is an untested legal theory.

About 1/3 of the vehicles can be stored at stations, waiting for passengers. Storage facilities need very little space, because the vehicles are automated and interchangeable, so less space is needed for access lanes to pick out particular vehicles or to hold vehicles of different sizes.

Guideway choices and cost

The debate continues over the best guideway for PRT systems. Most systems' guideways would be incompatible with both each other and existing transportation technologies. No technology has been acknowledged by all authorities as clearly superior.

Structurally, some guideway designs are monorail beams, several are bridge-like trusses supporting internal tracks, and others are just cables embedded in a conventional or narrow roadway that can be elevated.

Some points of agreement exist: it should permit fast switching and good braking, be inexpensive, be capable of being elevated, and pleasant to look-at. Ideally, it should not need to be cleared of dust or snow, and able to be built at ground level. Most systems also use the guideway to distribute power, data, and routing indications to the vehicles.

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

Designing a power rail for all weather conditions is difficult. For example, glare ice can almost insulate a rail from a vehicle's brushes.

An elevated track structure scales down dramatically with lower vehicle weights. Therefore, the vehicle's weight budget is critical. The heavier the vehicle, the more costly the track, and the track is the gating system cost. As well, large tracks are visually intrusive, so small vehicles contribute to a more attractive track.

The vehicle weight is so critical to capital costs and visual appearance that exotic aerospace techniques can usefully reduce the cost and size of both the vehicle and track.

Most designs put the vehicle on top of the track, because people prefer it. This also makes the poles shorter, with a smaller silhouette. They are said to be stronger and less expensive. Top mounted vehicles are said to unload the skins of the vehicle, which can therefore be lighter. Vehicles on top of tracks also have simpler line-switching, and in low density areas, can be inexpensively mounted on the ground without poles.

Design teams have used similar justifications for cars suspended (dangling) from an overhead track. Cars are said to be stressed in tension, "making a lighter vehicle structure" because many materials are stronger in tension. An overhead track is necessarily higher, and therefore more visible, but also narrower, and therefore creates less shadow, while having a small silhouette.

The least expensive real systems have used wheels with linear electric motors for drive and braking. To save money, the controls and electromagnets are mounted in the vehicles. Tight tolerance requirements in such systems can offset the structural cost savings. Taxi 2000 eliminated vehicle suspensions by making running surfaces adjustable. The least expensive structure for an overhead guideway is a rail suspended from a cable (See the aerobus). The fastest (theoretical) system would use magnetic levitation, which had some breakthroughs in 2000. The lowest-energy real PRT vehicles have used air-cushion suspension and drive. Controlled vehicle speeds can avoid vibrations in the structures. Combinations seem possible.

Routing indicators are often bar codes laser-cut from steel plates, and read by the vehicles with non-contact magnetic sensors. This system is unaffected by dust or wear and gives high precision positions.

Motors

According to J.E. Anderson (below), designer of Skyweb/Taxi2000, the lightest-weight system, and therefore the one with the lowest system cost, is a linear induction motor on the car, thrusting against a stationary conductive rail. This also minimizes the number of moving parts in the car, reducing maintenance costs, and has a relatively low fabrication expense for the rail. It's also easy for an on-board computer to control. A similar system was proposed by Doug Maliewicki for Skytran.

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

Comparable vehicle costs

The larger number of vehicles does not increase costs. Costs of transit vehicles are relatively constant per passenger. While larger vehicles enclose more space, they are nearly hand-built. A fleet of smaller vehicles can be mass-produced, as the auto industry shows.

Dual mode versus single mode systems

Dual mode systems utilize an existing traffic network, as well as special-purpose PRT guideways. The particular advantage of dual mode systems is that they use existing roads to provide a large initial network, thereby circumventing the initial downside of the network effects. A particular advantage is that dual mode operation can reduce the initial expense of the guideway network. In some cases, the guideway is just a cable buried in the street.

Aesthetics

There are several concerns about the appearance of a PRT system.

People near the guideway are most affected by its shadows. In this view, more sunlight is better, because the sunlight falling on the guideway is useless to people. So, guideways should have minimal horizontal structure.

Another view says that the guideway's visibility is most apparent in long sight lines. In this view, the silhouette of the guideway should be minimized.

Most planners assume that a competent industrial design will provide an attractive appearance for the PRT vehicle.

Tube-enclosed systems can be enclosed in special "bio lung" greenery, with zero visibility of moving vehicles.

Control algorithms

One successful algorithm places vehicles in imaginary moving "slots" that go around the loops of track. Real vehicles are allocated a slot by track-side controllers. The on-board computers maintain their position by using a negative feedback loop to stay near the center of the commanded slot. The vehicles keep track of their position in the slot with on-board speedometers. These have slight measurement errors (about 1%), so to keep the vehicles from bumping, vehicles' position and speed estimates are adjusted as they pass control points on the tracks. The track-side controllers have to keep synchronized with each other, also. The controllers assure that every two moving slots have one vehicle. At intersections "merge" logic manages the four possible combinations.

A slight variation places vehicles on North-South tracks in odd-numbered slots, while East-West vehicles use even-numbered slots. This permits rapid automatic merges and crossing of traffic at intersections. On the straight-aways, adjacent vehicles spread-out, or close-up to reestablish the every-other-slot relation. The alternating slots double the stopping distance in most situations, increasing safety.

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. This has succeeded in full-scale simulations and small test tracks, and uses slightly less energy. (algorithms are from J.E. Anderson's article, below)

The turn-outs to slow down or speed up for stops can almost double the length of track. Designers often increase the distance between vehicles to trade off lower guideway capacity for shorter, cheaper turnouts. Another trick to reduce turn-out lengths (and expense) is to keep vehicles in bunches (sometimes called "platoons"), and then widen the gap behind a slowing vehicle, and speed up (from a stop) into the end of a bunch.

Vibrations in the guideway can add unnecessary mechanical stress, increasing the cost. Most real systems use vehicle speeds that minimize vibrations in the guideway. Some theoretical designs have explored the use of vehicles' motors to actively damp vibrations in the guideway.

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.

Arguments for and against PRT

Advantages

  • By design and definition PRT includes:
  • Non-stop rides from origin to destination
  • On demand transportation, meaning no schedules or waiting
  • Travel is alone or in self-selected groups
  • Grade separation
  • Stations offline on sidings
  • PRT systems have numerous design features to prevent accidents: grade-separated guideways, wheels captured by the track to prevent derailment, automated control (decentralized in most modern designs), redundant safety-critical parts, central power with backups, periodic, often automatic inspections of safety equipment. So, widespread use of them could prevent most accidents caused from cars, and provide more reliable service.
  • In theory, PRT systems will not delay commuters with gridlock or traffic jams. This should make them more attractive than automobiles. Methods vary, but most designs plan to move at or near the maximum system speed more than 95% of the time, including at "rush hour." PRT systems offer transportation two to fifteen times faster than autos, buses or trains (depending on assumptions).
  • Per unit of passenger-distance, the traits of PRT allow proponents cost-out PRT systems at 3-10% of automobiles.
  • With reasonable assumptions, PRT systems are said to have better capital use than other systems. Compared to light rail, a single PRT line integrated into an existing multimodal transit system (not a PRT network) is said to have a comparable passenger capacity to a train or freeway, fifty-fold lower cost of rights of way, 60% more trips per seat, and as an automated system that does not require rides with strangers, substantially lower costs of ownership because it does not need drivers or transit police. If PRT captures more riders, uses semi-automated track-assembly or expands into a network, these effects multiply.
  • Parking costs, and space are not required, because the vehicles remain in use. They also eliminate a need for a driver's license, gas, insurance or sobriety. Of course, temporary storage of vehicles requires some space (and thus cost), because at low ridership hours, not all vehicles would be in use. In any case, much less space is needed than in other transportation systems.
  • In theory, PRTs' lower costs can be completely offset by fares, eliminating government subsidies and interesting private companies who would in turn compete to provide even better systems.
  • Since most PRT ideas include automated vehicles, passengers can relax and do other things while riding.
  • PRT could eliminate much of the world's urgent dependence on oil. Liquid fuels could be reserved for heavy transport. If the need for oil causes wars, this could save more lives and money than any other feature.
  • PRT systems usually are projected to be built much faster than conventional transportation systems, in months rather than years.
  • PRT systems usually operate from the electrical grid, and are therefore far less polluting and less expensive than even fuel-cell automobiles. Because it is electrically powered, pollution occurs at a power plant that can be more easily monitored or improved than automobiles.
  • Transit police are not required. Criminals cannot wait for a vehicle to arrive, because they would not know the car's destination. Most designs include a panic button that takes the unit to a police station. Stops and (in some systems) vehicles would have video cameras.


Pros

  • PRT proponents say that the system offers hope for solving transportation problems that conventional transit options cannot. Chicago is a low-density city with fully-realized train, freeway, and bus plans. These have failed, and the city is now (as of 2003) said to be investigating PRT.
  • Using PRT could let an impoverished yet technical country leap-frog past many more-developed countries' congestion, safety and pollution problems.
  • Simulations show that PRT squeezes the transportation of up to four-lanes of limited-access highway into the ground-space of poles spaced thirty feet apart. Laid in a grid with 1 mile or less of separation between parallel guideways and stations spaced three-quarters of a mile or less apart, it should solve most cities' traffic problems, enabling growth from the low densities at which autos are practical into the densities at which trains become practical.
  • Should estimates of low PRT capital cost be realized, there would be less investment needing to be recouped. Therefore it is conceivable that PRT could be cost effective for even lower density areas, such as urban single-family residential zones. The implication for urban planners is that all areas within the PRT coverage area, of all densities, could be served by PRT in a cost effective way. In effect, this scattered distribution of transit service would be analogous to the distribution of water or electricity by public utilities. A result is that policy decisions regarding land use and economic development/redevelopment could be made purely according to desired land uses and economic outcomes, irrespective of the operational needs of the transit system.
  • The beginning of the section below (Cons) has a discussion of the OKI report which recommended against PRT. After that report was submitted, Taxi 2000 and the Sky Loop committee wrote an extensive rebuttal that responded to the OKI report point by point.

Cons

The 2001 Ohio, Kentucky, Indiana (OKI) Central Loop Report [3] compared the Taxi 2000 PRT concept proposed by the Skyloop Committee[4], to other transportation modes (bus, light rail and vintage trolley). Consulting engineers with Parsons Brinckerhoff[5] 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." It was this report which prompted the extensive rebuttal mentioned in the previous section (Pros).

A Cincinnati Post editorial (08-21-01) expressed the following opinions about PRT:

  • "Our skepticism is grounded in more than the fact that this is an unproven technology:
  • "We hold with those who argue that it would be a mistake to take any more pedestrian traffic off street level..."
  • "No matter how handsome the design, an elevated system would be inherently ugly - and disruptive at points where it would intersect with downtown's historic buildings..."
  • "It seems doubtful that such a system could do much to move crowds following Reds and Bengals games or other major events."

The OKI committee voted 14-8 to reject Skyloop PRT.

In a Cincinnati Post article, "OKI rejects 'Sky Loop' Elevated Rail System" (9/26/01) [6], John Deatrick, Cincinnati's engineering director, said " The city concluded after many meetings that the sky loop was not the best alternative for downtown and Over-the-Rhine. The city was concerned about drawing people away from street-level businesses and fitting the rail and its supporting pillars on narrow city streets." In the same article Michael Shuster, an architect said, We can't get enough people to fill our businesses on street level. And this will detract aesthetically from every existing building.

Vukan R. Vuchic, Professor of Transportation Engineering at the University of Pennsylvania [7] said this about PRT in his article Personal Rapid Transit: An Unrealistic System, Urban Transport International (Paris), (No. 7, September/October, 1996) [8]: "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."

J. Edward Anderson and Dennis Manning have written responses to Vuchic's paper ([9],[10]).

In his article "Personal Rapid Transit Works in Simulation Only - An Answer to Professor J. Edward Anderson ( in the Urban Transportation Monitor of December 20, 1996.), Professor Vuchic comments on PRT's technical problems [11]: "There is a fascination with short headways PRT could operate. Theoreticians like to analyze "subsecond headways" which, even if technically feasible, would neither be desirable nor achievable for many vehicles in sequence. Off-line stations would, presumably, allow undisturbed passage of mainline traffic while vehicle alighting/boarding is performed. But if, for example, a train in Chicago unloads 75 persons at the PRT station, they will need some 40 PRT vehicles, and that boarding would take a much longer time than boarding two AGT or one light rail vehicle. Capacity of boarding can be increased by building long platforms, but large structures make stations even less acceptable in many urban settings. Simulation may show that station operations are fast, but that depends on how realistic are the assumptions used in the simulation model..."

"...The common gap between theoreticians and practitioners is particularly great with respect to PRT systems: the PRT concept attracts the interest of some vehicle designers and control engineers, and a number of operations researchers find challenges in optimizing individual operational processes. However, this concept has never found the support of persons involved in urban transportation planning, or in the design and operation of real world transit or taxi systems..."

In the same article, Professor Vuchic says this about "dual-mode PRT":

"During the 1970s there were serious proposals for development of "dual-mode" systems with vehicles running on guideways or driven on streets. When GM and others attempted to design such guideways and ramps for a specific city, most of the "system assumptions" had to be modified so much that it became clear the system would not be feasible."

J. Edward Anderson has also responded to this Vuchic article.

Transportation consultants[12], Michael D. Setty and Leroy W. Demery, Jr in an article "Conventional Rail vs. 'Gadgetbahnen' "[13], say the following about PRT: "In our view, it is a big waste of time advocating such "gee-whiz" options, given the severe limits of monorails and similar technologies such as PRT, when U.S. transportation problems are almost always sociopolitical and economic–not technical–in nature."

Denver International Airport's baggage handling system, an automated system with conceptual similarities to PRT, failed to work properly due to inadequate planning[14]. After $186 million and years of trying to fix the system, the task is now handled by humans: 'It wasn't the technology per se - it was a misplaced faith in it,' said Richard de Neufville, a professor of civil and environmental engineering and engineering systems at the Massachusetts Institute of Technology. De Neufville said the builders imagined that their great creation would work well even at capacity, leaving no room for the errors and inefficiencies that are inevitable in a complex enterprise."[15]

Transit advocate Ken Avidor said in a guest editorial in the Seattle Post Intelligencer[16], "Basically, PRT is a stalking horse for the highway construction industry. PRT proponents can say things that the highway boosters could never say, such as "People don't like to ride with strangers."[17]. This anti-transit propaganda divides and conquers the opposition to highway projects." Prominent proponents of PRT have campaigned against public funding of light rail transit projects: environmentalist Emory Bundy[18]; former U.S. Department of Transportation railroad official Sheffer Lang (1927-2003)[19] [20]; David Morris of the Minnesota Institute for Local Self-Reliance[21]; Dr. J.E. Anderson [22]. PRT websites carry anti-light rail transit messages [23]. The anti-light rail transit group Coalition for Effective Transportation Alternatives (CETA)[24] has prominent PRT proponents as members including Professor Jerry Schneider[25] and Emory Bundy[26].

References

  • "Transit Systems Theory", J.E. Anderson, 1978
  • "Fundamentals of Personal Rapid Transit", Irving, Bernstein and Buyan
  • The classic reference is "Systems Analysis of Urban Transportation Systems," Scientific American, 1969, 221:19-27
  • The foundational text: "Individualized Automated Transit in the City," Don Fichter, 1964

See also

More information

Working hardware

Proposals

  • UniModal, Maglev 100 mph (161 km/h), California, US; New Delhi, India
  • UniModal's former web site/Skytran, Maglev 100 mph (161 km/h), California, US
  • PRISM Proposal for Individual Sustainable Mobility. Dual-mode, with some of the advantages of single mode.
  • RUF, Dual-mode, Denmark
  • Thuma, a flexible system for varying sizes of containers.
  • Vectus Ltd. - Has 385 meter test track under construction in Uppsala, Sweden. [27] Picture of test track. [28] Vectus is a unit of POSCO, a major South Korean steel manufacturer.
  • Skycab - A Swedish concept (website and documents in Swedish), status as of June 2005 (translated)
  • EcoTaxi - Finnish version of PRT, termed "Automated Goods & People Mover" (APGM).

Advocacy

PRT Skepticism and Criticism

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