Personal rapid transit
From Wikipedia, the free encyclopedia.
Personal rapid transit, or 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 somewhere between a moped and bicycle (US$0.10-0.03/rider-mile).
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. Proponents claim that PRT can provide full speed, nonstop point-to-point travel even at rush hour. A well-designed system is said to require waits of less than a minute, 95% of the time. This is said to require no new technology, just good execution of known techniques on a large scale. If proponents are correct, PRT could solve cities' transportation problems.
PRT systems are designed to be used by children and disabled adults, as well as working-age adults. These are the same population served by busses and trains. PRT is like a car in that one does not normally need to wait for a vehicle to arrive, and the service is nonstop from the pick-up to the drop-off chosen by the passenger. In contrast, 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.
Travel on PRT systems is expected to be ten thousand to one million times safer than in cars, due to computer control and grade separated guideways. This means that PRT can not run into pedestrians or other vehicles. Most systems also have the vehicle running gear fully enclosed by the guideway which makes derailments essentially impossible. Vehicles usually have dual redundant motors and electronics, and in the event of a break down, can be pushed to the repair facility by a following vehicle.
PRT systems are usually powered by electricity. Energy use is claimed to be roughly 25% of autos and need not come from oil. Solid state passive magnetic levitation is now (2003) possible, permitting normal travel at 100mph, and intercity PRTs to travel in a vacuum tube at several thousand miles per hour. (See Unimodal project)
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, 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 complete before the U.S. presidential election.
Morgantown's West Virginia University PRT, remains in operation (2003) built by Boeing, which has been in operation since 1975, with about 15,000 riders per day. 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 Morgantown campus using 5 stations (Walnut, Beechurst, Engineering, Towers, Medical) and a 4 mile track. The vehicles are rubber-tired and powered by electrified rails. Steam heating keeps the elevated "guide-way" free of snow and ice. It is sufficiently reliable and low-cost that 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, France was a large scheme, documented by Bruno Latour in Aramis: or the Love of Technology. It started out as a true point to point robot taxi system but, as the years went by it, became just another automated people mover.
In Germany, the Cabinentaxi project built a test track on which vehicles traveled both on and under the track, doubling 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 another, currently under study by Chicago.
The Unimodal project proposes to use magnetic levitation in solid-state vehicles that achieve speeds of 100mph.
In 2003, Ford Research proposed a system called PRISM. PRISM would use public guideways with privately-purchased but certified dual-mode vehicles. The vehicles are less than 600kg (1200lb), permitting 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 safety 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 has met all cost and performance goals.
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 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 they do not pollute. Most systems plan multiply-redundant power supplies, from track-side batteries or natural-gas-powered generators. Stationary pwoer 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.
Some systems plan to group vehicles into platoons to serve a group of passengers and reduce aerodynamic drag. Platoons could share an intercom.
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 1.2 persons per vehicle in the U.S.) Some systems have special vehicles for wheel-chair users and bicyclists and light cargo 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."
Engineering Economics
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.
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. That can only decelerate at 1/2 gravity before their tires lose traction.
However, restrained sitting passengers can tolerate emergency stops at 6 gravities, which is a deceleration similar to the more exciting roller coaster rides at amusement parks. At 6Gs, 70mph (115kph) vehicles stop in 0.52 seconds, about 27 feet (8m). With seat belts, people easily tolerate emergency stops of 16 Gs. With torso restraints, people tolerate 32G emergency stops, permitting 0.1 second stops and 11 feet (3.2m) 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 2-3 meters.
This (to a light-rail planner) "absurdly short" inter-vehicle distance raises right-of-way utilization to very high levels, even with the smaller numbers of 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 therefore 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 PRT witht he same type of track theoretically has two to three times as many trips per seat as a bus or train. So PRT should utilizes its average seat 50 to 300 percent more efficiently. This is contested, of course.
Because of route utilization, most PRT plans start with a loop downtown. If simulations are right, PRT could substitute for a train or high-capacity bus route in a transit corridor. This would allow PRT to be used in a multimodal 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 (including bus and light rail), the rails, road surface and right of way are depreciated by a route's total passenger traffic. However the vehicles are depreciated on a different 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 routes at most times, vehicles are 85% to 95% empty, and only rush-hour trips on important central routes approach vehicle (and route) capacities. Ridership in bus and train systems 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 1.2 persons per automobile in commuter areas, 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, a PRT system's vehicular 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. This is hotly contested.
Costs of rights-of-way- trading technology for less land-use
Planners also dispute the cost-estimates of 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 (30-100m) 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 (10m) 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.
Note that PRT rights of ways may 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.
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. Topside 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. Single mode vehicles operate only on guideways. Dual mode systems can operate on streets as well, and may require a driver to do so.
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 overhead guideway is a rail suspended from a cable (See the aerobus). The fastest (theoretical) system would use magnetic levitation, which has recently (2002) had breakthroughs. 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.
The discussion can be difficult. Some guideways are monorail beams, other are dual rail or guide beams and others still are just cables embedded in an asphalt or concrete roadway.
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.
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 already has fully-realized train, freeway, and bus plans. These have failed, and the city is now (2003) said to be investigating PRT.
PRT systems are proven, at least in the Ultra system at Cardiff, Wales and the systems at SeaTac and Morgantown, West Virginia. Ultra now has demonstrated cost figures.
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 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 in gridlock or traffic jams. When combined with nonstop routing, 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.
PRT systems offer 2x to 15x faster transportation (depending on assumptions) compared to 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.
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.
Crime should be prevented because criminals would not know the destination, and most designs include a panic button that takes the unit to a police station. Transit police are not required.
Per passenger-mile, the above traits let proponents cost-out PRT systems at 3%e 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 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 in gridlock or traffic jams. When combined with nonstop routing, 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.
PRT systems offer 2x to 15x faster transportation (depending on assumptions) compared to autos, buses or trains. They provide on-demand (no waiting!) nonstop, private tranle arrives.
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 as a line 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
External links
More information
- ATRA, The Advanced Transit Association, a professional group
- Innovative Transportation Technologies web site by Jerry Schneider
- Open Directory: Personal Rapid Transit
Working Hardware
- Ultra, Cardiff Wales, UK
- SkyWebExpress, Minneapolis Minnesota, US
- MicroRail
- Postech, Pohang University, Korea
- Cabinlift, from the Cabinentaxi Project, Germany
- WVU's Morgantown PRT, Morgantown Virginia, US (Boeing)
Proposals
- UniModal, Maglev 180kph (100mph), California, US; New Delhi, India
- Unimodal's Former web Site/Skytran, Maglev 180kph (100mph), California, US
- RUF, Dual-mode, Denmark
Advocacy
- Personal Rapid Transit (PRT) or Personal Automated Transport (PAT) Quicklinks
- Get There Fast, Seattle, WA group
- CPRT, Citizens for Personal Rapid Transit, US national group
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