The Online Encyclopedia and Dictionary






Personal rapid transit

Older children can use personal rapid transit without adult help.
Older children can use personal rapid transit without adult help.

Personal rapid transit (PRT) is a transport method that offers on-demand non-stop transportation between any two points on a network of specially built guideways.

PRT has been reinvented many times because it optimizes standard transit-planning math. PRT developers and advocates therefore claim that it will provide more convenient service than cars, with the social advantages of public transport, and excellent environmental and economic benefits. Several PRT systems are in development, and rapid transit using technologies very like PRT is in regular operation. Several PRT designs have been safety-certified by government authorities. Some authorities claim that these developments prove feasibility.

Several PRT proposals have failed dramatically and publicly when their projected costs exceeded their budgets. Other PRT projects have failed technically, some with large monetary losses, often when political needs, schedules or budgets interfered with a technical requirement. Some authorities claim that as of 2005, there are no true PRT systems in operation, and that the lack of systems proves infeasibility.



PRT can be best understood by comparing it 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. There is no requirement to ride with strangers.
  • 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 16 mph (26 kph) when intermediate stops are included.
  • PRT vehicles are available on an on-demand, around-the-clock basis.
  • Some PRT developers and advocates claim 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.
  • It is far less polluting than cars, since it is typically electrically powered.
  • Vehicles are not privately owned (except in the case of dual-mode PRT systems, discussed below).
  • Passengers get rides at discrete locations similar to bus stops or taxi stands. Most systems locate these about a quarter-mile (400 meters) apart.
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 thru traffic to continue 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.
  • PRT's nonstop travel reduces energy use compared to buses, trains and automobiles. 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.
  • If PRT records people's identity, it could provide police and other investigators with detailed information about individual travel. This reduces privacy but also increases security and provides automatic alibis with relatively little inconvenience.

PRT's advocates claim that with properties like these, PRT should solve urban transportation problems. They point to ridership simulations suggesting that PRT systems could absorb between 15% and 60% of vehicular traffic. However, PRT's detractors claim that many of these properties are illusory or exaggerated, and point to the continuing lack of any operational PRT systems as evidence of their claims.


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".

In 1974, Boeing began construction of the first major PRT project in Morgantown, West Virginia, designed for West Virginia University. WVU's original campus is located in the valley of the Monongahela River. It proved impossible to build nearby in the narrow valley. WVU expanded to a separate parcel above the valley.

The Morgantown Personal Rapid Transit project was started on a too-tight development schedule by a now-defunct research department of the U.S. Department of Transportation. Some observers believe the project was poorly designed because it was rushed to completion before the U.S. presidential election.

The WVU PRT has been in continuous operation since 1975, with about 15,000 riders per day (as of 2003). The system uses about 70 vehicles, with an advertised capacity of 20 people each (although the real number is more like 15). The system connects the university's disjointed campus using 5 stations (Walnut, Beechurst, Engineering, Towers, Medical) and a 4 mile (6 km) track. The vehicles are rubber-tired and powered by electrified rails. Steam heating keeps the elevated guideway free of snow and ice. Most 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. It uses rubber tires for braking, so that intervehicle spacing is large, and therefore route utilization is also low compared to true PRT. Morgantown vehicles weigh several tons and run on the ground for the most part, with higher land costs than true PRT.

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. The failing system had custom-designed motors, sensors, controls, digital electronics, software and a major installation (the "CET") in southern Paris. The technology demonstration in 1970 worked. Point-to-point travel for passengers, an essential PRT feature, was removed from the specifications around 1973 because of the extra cost of the turn outs. Aramis was documented by Bruno Latour in Aramis: or the Love of Technology.

In Germany, the Cabinentaxi project, a joint venture from Mannesmann Demag and MBB, created the most extensive PRT development in history. The system was considered fully developed by the German Government and their safety authorities, and deemed capable of installation in urban areas for the carrying of passengers. The extensive development created three PRT systems in one, an over running version, an under running version, and the combintation of both where the vehicles traveled both over and under the track, doubling route capacity. Besides the PRT aspects of the technology, the system had the ability to form married pairs of two 12 passenger vehicle (24 passengers) and two 18 passenger vehicles (36 passengers,) to give the system added flexibility in the early stages where PRT networks were not yet mature, but higher capacity routes were desired. The system also had standing passenger versions of both the upper and lower running systems, giving the technology the most adaptible technology of any urban transport technology yet developed.

Cabintaxi was considered one of the leading contenders for the US Downtown People Mover Program, and was widely recognized as the favorite system to win the Detroit People Mover Project. For the Detroit project, the system's over-and-under beam was a major advantage over competitors as the City of Detroit specified a single beam system, and the Cabintaxi system was the only installation ready technology in the world capable of bi-directional operation on a single beam. Unfortunately for the technology, the system was planned to be installed in Hamburg during the same time, and the schedule for the US People Mover Program and the Hamburg application appeared to conflict. The Cabintaxi suppliers chose to withdraw from the US competition and concentrate their efforts on Hamburg. This highly aggravated the German Government, the funding source, as the system had been developed as an export product. When the American Government requested increased defense spending by the NATO allies, it resulted in a mandatory funding cut to all departments of the German Government. The ministry of research and technology that was to fund the Hamburg project withdrew funding with a statement that among other things, the failure to pursue the export market - specificly Detroit, and the mandated budget cuts, led to the decision to stop the project. The developing firms found themselves without a market opportunity in Europe or the United States, and withdrew from the public transit field. The United States firm of Cabintaxi Corporation obtained the technology shortly after the development team withdrew from the field, and continues to pursue private sector transportation applications based on this technology.

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 Taxi2000 system remains under study by Chicago, and is pictured.

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

In 2003, Ford Research proposed a 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. That is, traffic jams are more likely than with other PRT.

in January 2003 the prototype ULTRA system in Cardiff, Wales (ULTRA) was certified to carry passengers by the UK Rail Inspectorate on its 1km test track and undertook very successful passenger trials. ULTra has met all project milestones to time and cost and is currently awaiting its first full application contract.

In 2004 the British Airports Authority requested proposals for a PRT system to be implemented at London's Heathrow Airport. This system is planned to transport some 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 allow the capacity of the existing tunnels to be increased without enlargement. BAA plans to have the initial system operating by the end of 2007 and to expand it in 2009.

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 a 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.

Embarkation stations are on turnouts so other vehicles can move at full speed. Systems can embark passengers as fast as busses 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.

Designers prefer solid-state electromagnetic line switching built into vehicles rather than the track, so that tracks stay in service. A track failure drastically degrades many systems' capacity. This also allows closer spacing of vehicles as no time delay is needed to allow the track to switch.

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

Most 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.

Engineering economics

Many PRT advocates claim that it will have a per-passenger trip costs between $0.03 and $0.10/mile ($0.02 and $0.06/km) -- somewhat cheaper to operate than a moped. However many transportation planners disbelieve the "ridiculously low" cost estimates of proponents, especially when cast in terms of cost per rider-mile. 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.

A track should not accumulate snow or rainwater, and should not need to be heated. Systems where the vehicles ride atop the track must use wheels and tracks designed not to collect precipitation or dust. Weather is better handled by overhead tracks. Note that in this area, PRT systems can save substantial money over conventional streets and vehicles.

As for fuel, PRT systems are usually powered from the track, and purchase power from the cheapest electric utility. Unlike trains and electric busses, PRTs only accelerate and stop once per passenger, saving substantial energy. Ordinary electric motors are 98% efficient, and non-polluting.

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).

Route capacity- strongly affected by superior braking

The carrying capacity of a route is disputed, and also critical, because guideways are the major initial expense. Many transportation planners dismiss as absurd the short inter-vehicle distances designed into PRT systems.

Light rail must decelerate at a maximum of 1/8 of a gravity (1.2 m/s²), so standing passengers will not be harmed. Therefore, legally-required intertrain stopping distances must be 1285 ft (391 m) for a 70 mi/h (116 km/h) train. Sitting, unrestrained passengers can also only tolerate about 0.6G without upset. Cars and buses can only decelerate at about 0.5Gs because at higher decelerations their wheels lose traction.

Since PRTs have sitting, perhaps belted passengers, and can have automated emergency braking against steel guideways, many designers plan shorter emergency stops than trains. Their goal is to get more cars on the guideway, and increase its utilization.

For example, some conservative PRT systems run on rubber tires and have braking systems similar to those in automobiles. One such system has permission from the British Rail Authority to carry the public at a speed of 25mph and a headway (time between vehicles) of three seconds. This requires a very modest deceleration, about 0.2 gravities. This conservative approach provides 4,800 seats per hour per guideway for this 4-seat system. At an occupancy of 1.5 per vehicle the capacity is 1,800 people per hour per guideway - similar to that of a freeway lane.

Some authorities propose to charge people for the expense of the guideway that is wasted when they do not use their seat belts. These systems would use long headways on cars containing unrestrained passengers. Sitting, restrained passengers can tolerate emergency stops at 6 gravities (59 m/s²), a deceleration like a more exciting roller coaster. At 6 G (59 m/s²), 70 mph (115 km/h) vehicles stop in 0.52 seconds, about 27 feet (8 m).

Such an aggressive 27 foot (8 m) inter-vehicle distance raises right-of-way utilization to very high levels, even with only one passenger per vehicle.

Some controversial designers have even proposed emergency stops with the same passenger decelerations as automobiles' crumple zones: With torso restraints, healthy young people tolerate 32 G (314 m/s²) emergency stops with only minor injuries, permitting 0.1 second stops and 11 foot (3.2 m) safe inter-vehicle distances. Many designers consider such a violent deceleration irresponsible and unsafe, even though it is broadly accepted in other vehicles.

Therefore, when PRT systems do not brake by wheels, a PRT guideway can replace one to four lanes of automotive traffic, depending on assumptions. Braking against a linear motor or steel rails for emergency stops decreases the safe inter-vehicle spacing, which raises the right-of-way utilization, and therefore lowers the cost per passenger-mile of a route.

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 percent 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.

Capacity utilization- 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 ball-games, about 1/3 of vehicles on the guideway need to be empty to get the best response time.

Capacity utilization- 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.

Capacity utilization- 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) claim 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. Some question the viability of systems with only two seats since 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. The public's worst-case needs are shown by its choice of automobiles, 85% of which have four seats plus or minus one. Some PRT vendors therefore have chosen vehicles accommodating three or four passengers with luggage.

Capacity utilization- 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 riderships between 35% and 60% of automobile users. In contrast, new light rail systems and bus lines normally attract about 2% of automobile users.

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, usually spaced every thirty feet (10 m) on a street, PRT designers hope to use land very economically. This is far less than a conventional elevated train, because small PRT vehicles with passengers weigh under 1,000 pounds (450 kg), while even one train car weighs many tons.

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 claim that if auto- and bus-based transit systems include the costs of the roadways needed for buses and automobiles, PRT systems are substantially cheaper than bus and automobile systems.

A surprising expense in many PRT systems is the extra track to decelerate and accelerate from the numerous stops. 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 options can reduce this cost. Control algorithms can 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 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.

About 1/3 of the vehicles can be stored at stations, waiting for passengers. Storage facilities need very little space, because the vehicles are all the same size and they can be available in any sequence.

Guideway choices and cost

The debate continues over the best guideway for PRT systems. Most systems' guideways are 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 subtle. 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. 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. One system eliminated vehicle suspensions by making running surfaces adjustable. 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.

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.

The dual mode concept permits a long-term migration toward PRT-like traffic systems, without large initial sacrifices or expense. For example, Ford's PRISM proposal would certify very small cars to permit PRT-like electric power, spacing and automation on a guideway. The same small cars could still operate on conventional roadways.

A notable disadvantage is that any dual mode system's performance is limited by its compatibility with existing infrastructure. This is most important in the power source and braking.

A system like Taxi 2000 is single mode because the vehicles are always used on the guideways, within the system, in a completely automatic mode. The Danish RUF system is dual mode because the vehicles can operate on guideways in an automatic mode, or leave the guideways and operate on city streets, with drivers controlling them. British Ultra is now single mode, but its promoters envision the possibility of making a dual mode version in the future.

Many of the disadvantages and/or advantages listed below apply to single mode systems but not dual mode systems, and vice versa.


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.

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 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.

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 vehicles motors to actively damp vibrations in the guideway.


Since PRT systems are designed to be safer than automobiles, widespread use of them could prevent the death and maiming of thousands of people per year just in North America.

Proponents say that 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 two to fifteen times faster transportation (depending on assumptions) than autos, buses or trains.

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 are proven, at least in the Germain Cabintaxi, the Ultra system at Cardiff, Wales and the system at Morgantown, West Virginia. Ultra now has demonstrated cost figures.

PRT proponents claim 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.

Per unit of passenger-distance, the following traits let proponents cost-out PRT systems at 3-10% of autos.

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.

Proponents therefore claim that PRTs' lower costs can be completely offset by fares, eliminating government subsidies.

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 one-mile grid, 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.

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 can't 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.


Most planners say that no economically successful PRT system has been demonstrated, and there have been too many failures for a prudent person to spend public funds.

Transit planners normally evaluate a new transport method as part of an intermodal network. In these cases, a PRT line may compete against a rail or bus line. When operated in an intermodal transit network, PRT may not fully realize the travel time reductions advanced by proponents, because connections to other mass-transit modes are only possible when the other vehicle arrives; a disadvantage where infrequent transit can be the weakest link in an intermodal system. Timed connections between conventional mass-transit modes, though rare, can be more efficient than PRT intermodal use.

The claims made by proponents depend on certain reasonable but nonstandard design features (see above). Many planners argue that if conservative ridership, operating expense ratios and inter-vehicle lead distances (for bus and train systems) are used, PRT systems are less attractive than bus and train systems.

In transit planning with standard ratios, if PRT were built in an existing high density corridor, it would be less efficient than trains. Only if additional capacity were required in a low density corridor, would it be more efficient than a bus line or automobile, since the capital costs of streets are already sunk.

Because of network effects, PRT is not fully useful until it is widespread. In this view, a small PRT system will not attract demand because it does not go to many destinations. Many people say that only a large PRT can attract sufficient demand to be self-sustaining. How it could grow from a niche to a local or metropolitan network is unclear to these persons. Growth to a national network is thought especially unlikely.

Skeptics say that PRT just idles entire vehicles, which is true. The effects of vehicular recycling at rush hours are also disputed by some transit planners, because they are simulations. Some skeptics have said that since gross capacities have to be comparable (because the same number of people are being transported in the same time), no advantage can occur. However, comparing capacity (people per hour), and capacity utilization (money per person per hour) is a fallacy.

Some experienced advocates claim that the chief problem is that PRT threatens existing livelihoods associated with cars, busses, trains and related services. Since the market in rapid transit has a limited (government) budget in each city, and existing options are the best-funded, existing options and organizations tend to win political battles. As of 2001, this may be changing, because existing options have been unable to solve traffic problems.

The claimed very high vehicle utilizations (vehicles are usually carrying passengers at full speed, rather than parked), means that there might be less need for, and investment in private vehicles, and auxiliary private services such as repair and insurance. Although these are social advantages, they directly threaten the livelihoods of many persons.

PRT systems may be as unattractive as other public transit. People cannot customize them to their tastes, and therefore rarely have anything approaching the enthusiasm shown for a new car. At Morgantown, most students use, but casually despise the transportation system, and recount stories of its failures. Some jokingly claim the term "PRT" is said to stand for "Pretty Retarded Train."

Some call PRT a prime example of a federally funded "pork barrel" project, one of many located in West Virginia due to the influence of Senator Robert Byrd.

A PRT system is said to have lower costs and automated operations. These could lead to simpler organizations and smaller staff at governmental transportation offices. This directly reduces the responsibility and authority of government officials, which in most civil service systems, reduces their pay. It does not offer much incentive to administrators to adopt it.

Many authorities say that the cost of constructing and operating the system is unlikely to be as low as claimed. Some systems (such as Morgantown) have had much higher costs than planned (Morgantown has to use steam heat to keep its tracks free of snow). Any new technology has to climb a learning curve, and for every new system, promoters must make speculative claims when asserting low construction and operating costs. Historically, costs are underestimated on transit projects and demand overestimated. Further, methods of recovering unplanned cost overruns can cause political and public strife.

The neighbors of such a system could oppose unsightly towers holding an elevated rail system, as well as the guideway itself. New infrastructure is hard to build, particularly without the support of the community.


  • "Transit Systems Theory", J.E. Anderson, 2000
  • "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

External links

More information

Working hardware


  • 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
  • PRISMProposal 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.


PRT Skepticism

Last updated: 05-17-2005 04:18:28