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Personal rapid transit

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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 from any point on a specially built network to any other point on that network. Developers aim to provide service more convenient than a car, yet with the social advantages of rail transit and trip costs of sending each person somewhere between a moped and bicycle i.e. US$0.10 to 0.03/mile (0.06 to 0.02 $/km).

PRT has been reinvented many times because it optimizes standard transit planning math.

Overview

PRT vehicles are usually electrically powered. The vehicles carry one to six passengers and run on very light-weight tracks, generally elevated above street level. Computers drive, collect fares, and help manage the system.

To use a PRT system, one picks up the vehicle as if at a taxi stand. These pick-up points would be on a grid, about where bus stops are now.

A party as small as a single individual chooses a destination and buys a fare from a vending machine. A waiting automated vehicle opens its door. The vehicle takes the party on the shortest path to the destination, without stopping for traffic or other passengers.

Conventional mass transit systems in low-density cities often have waits of an hour, stop every few hundred yards, and require multiple transfers, with a wait at each transfer. For these reasons, new train and bus lines usually attract less than 2% of the parallel trips that are performed in autos.

In contrast, PRT may be more convenient than a car: Proponents claim that PRT can provide waits of less than a minute, and full speed, nonstop point-to-point travel even at rush hour in low density cities. In standard ridership simulations, PRT usually attracts enough trips to reduce road traffic by 15 to 60%. Similar simulations predict ridership of busses, trains and autos within 5%.

PRT systems are designed to be used by commuters, children, and disabled persons: the same people served by buses and trains.

Proponents 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 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.

Energy use of PRT systems is said to be about 25% of autos and need not come from oil. Solid state passive magnetic levitation is now (2000) possible, permitting normal travel at 100 mph (160 km/h), and intercity PRTs to travel in a vacuum tube at several thousand miles per hour. (See UniModal project)

PRT systems are said to require relatively tame, well-understood technology.

If proponents are correct, PRT could solve cities' transportation problems.

However, many transit planners mistrust how PRT advocates calculate system depreciation, ridership and capacity. When evaluated with standard transit planning assumptions, PRT is less attractive than busses or autos. These assumptions are discussed below.

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".

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 PRT 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 built a test track on which vehicles traveled both on and under the track, doubling route capacity. It was about to be installed in Hamburg when a recession caused its budget to fail.

Raytheon invested heavily in a system called PRT2000 in the 1990s, and won no contracts, despite purchasing a long-running project with a complete set of patents and designs, and completing a technology demonstration.

In the United States, the Taxi2000 proposal, developed at the University of Minnesota is currently under study by Chicago.

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

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.

As of July 2003 the system in Cardiff, Wales (ULTRA) was accepted in second-stage passenger trials on a test loop. In February of 2003, the system was certified to carry passengers by the British Rail Inspectorate. It did meet all cost and performance goals, however, the project was cancelled due to a lack of public interest and increasing costs

Safety and Utility

Safety engineering extrapolations evaluate PRT systems as ten-thousand to one million times safer than automobiles. Existing PRT systems have been safe, because they are automated, periodically-inspected, with self-diagnosing redundant systems. Vehicles are on rails, usually with captured wheels. Computer controls nearly eliminate driver errors and traffic accidents. Cars go to an embarkation station if central computers or power fails.

Automation and redundancy also open ridership to nondrivers, and lower costs.

Systems drive the vehicles so that they do not need to slow or stop while en-route.

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 tack 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 transportation planners disbelieve the "ridiculously low" cost estimates of proponents, especially when cast in terms of cost per rider-mile. Building and operating systems could empirically confirm or disprove critic's views.

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.


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, 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. Busses and automobiles have a similar problem. They can only decelerate at 1/2 gravity before their wheels lose traction.

However, unrestrained (this is not a misprint) sitting passengers can tolerate emergency stops at 6 gravities, a deceleration like a more exciting roller coaster. At 6G, 70 mph (115 km/h) vehicles stop in 0.52 seconds, about 27 feet (8 m). With seat belts, people easily tolerate emergency stops of 16Gs. With torso restraints, people tolerate 32G emergency stops, permitting 0.1 second stops and 11 foot (3.2 m) safe inter-vehicle distances.

Since PRTs have sitting, perhaps belted passengers, and automated emergency braking against steel guideways, PRT designers plan for safe emergency stops as short as 2 to 3 meters.

This (to a light-rail planner) "absurdly short" inter-vehicle distance raises right-of-way utilization to very high levels, even with far fewer passengers per vehicle.

Therefore, the best PRT systems never brake by wheels, because this increases the safe inter-vehicle spacing, lowering the right-of-way utilization, and thus the cost per passenger-mile of a route. Braking is either against a linear motor, or steel rails for emergency stops.

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 vehicle passenger capacity

Capacity utilization is also affected by the number of empty seats per vehicle. In all transit systems, vehicles are depreciated on a schedule that accounts for the average number of empty seats per vehicle.

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 often causes a substantial cash drain through depreciation. Further, the drain cannot be offset by fares.

PRT vehicles intentionally carry only a few passengers. 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.

With two-seat vehicles, proponents claim a PRT system's ridership is at least 50% on all routes at all times. Proponents claim that PRT vehicles' depreciation and operating costs can therefore be completely offset by fares. Skeptics say that PRT just idles entire vehicles. Idle vehicles should wear and so depreciate more slowly than active vehicles. PRT diverts vehicles to busy routes, but at idle times this does not increase ridership.

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 (30-100 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, while even one train car weighs many tons.

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 point-to-point passenger delivery concept to be abandoned. There are other ways. Control algorithms can reduce turn-out lengths (see below). Elevated tracks can "vertically merge" and keep to a narrow right of way.

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, and less visible. 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 less visible for its height

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.

Dual mode versus single mode systems

The debate is intense between proponents of single mode PRT systems and dual mode PRT systems. 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.

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.

A notable disadvantage is that any dual mode system's performance is limited by its compatibility with existing infrastructure.

Guideway choice

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.

Some points of agreement exist: it should permit 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.

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.

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

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.

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, the trackside controllers must update vehicles' position and speed estimates as they pass control points. The track-side controllers have to keep synchronized with each other, also.

A refinement is to place vehicles in alternate slots so that merging can have small delays. On the straight-aways, adjacent vehicles spread-out, or close-up to reestablish the every-other-slot relation.

Another algorithm assigns vehicles a trajectory, after verifying that the trajectory does not violate the safety margins of other vehicles. 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.

Minimized overhead and operating costs

Finally, standard transit-planning assumptions concerning overhead per vehicle fail in PRT systems. The major operating expense of both bus and light rail systems is the operators' and mechanics' salaries.

PRT systems eliminate operator salaries by automating guidance and fare-collection. Repairs are far less per vehicle because PRTs have electric motors, with one moving part, versus hundreds for an internal combustion engine.

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. Ordinary electric motors are 98% efficient, and as polluting as their power source.

Advantages

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.

PRT systems offer two to fifteen times faster transportation (depending on assumptions) than autos, buses or trains. They provide on-demand (no waiting!) nonstop, private transportation from any point of the system to any point of the system. They thus should provide service very similar to that provided by a car, yet with the advantages of a public transit service.

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.

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

PRT would eliminate much of the world's day-to-day dependence on oil. Liquid fuels could be reserved for heavy transport.

Using PRT could let an impoverished yet technical country leap-frog past many more-developed countries' congestion, safety and pollution problems.

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."

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.


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, substantially lower costs of ownership. If PRT captures more riders, uses semi-automated track-assembly or expands into a network, these effects multiply.

Simulations show that PRT squeezes the transportation of a four-lane 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 would not know the car's destination, and most designs include a panic button that takes the unit to a police station. Stops and (in some systems) vehicles would have video cameras.

Per passenger-mile, the above traits let proponents cost-out PRT systems at 3-10% of autos.

Disadvantages

Transit planners normally evaluate a new transport method as part of an intermodal network. In these cases, a PRT line competes against a rail or bus line. When operated in an intermodal transit network, PRT does 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.

The claims made by proponents depend on certain reasonable but nonstandard design features (see above). If standard transit 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 is built in a high density corridor, it is less efficient than trains, and in a low density corridor, it is less efficient than a bus line or automobile, especially 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 much demand because it doesn't go anywhere. 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.

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." This may be the best user evaluation that is possible in the long term.

Some may call the 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 would naturally 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. Additionally, since it is unproven, there is adequate reason to reject it. Therefore, it does not offer as much incentive to administrators to adopt it.

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. New infrastructure is hard to build, particularly without the support of the community.

References

  • "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

More information

Working hardware

Proposals

Advocacy

Anti-PRT

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