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 and costs of rail transit. It's like a car in that one does not wait longer than a minute for transit, and the service is nonstop from the point of pick-up to a point 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.
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. Travel would be ten thousand to one million times safer than in cars, at 1/3 to 1/10 the cost. Pollution is minimal, created by electric power plants that power the system. Energy use is roughly 25% of autos. 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 skytran project)
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.
The most criticized assumption of the above theories is that an entire PRT network exists. "Intermodal" transit systems are much less expensive, but also far less attractive, because passengers move at the speed of the slowest mode, and must wait at transfer points. In an intermodal system with conventional transit planning ratios, PRT is less attractive than trains and autos. Proponents argue that the ratios are inaccurate for PRT.
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. Computer systems drive and 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. Careful engineering is said to prevent traffic jams, even during rush hour.
Computers figure fares, direct the traffic, move empty vehicles to busy routes, remove broken vehicles from service, and handle requests for special vehicles.
Vehicles usually have dual redundant motors and electronics, and in the worst case, can be pushed to the repair facility by a following vehicle.
Most systems have some combination of buttons in a vehicle to "let me talk to the operator," "take me to the nearest stop now," "take me to the hospital," "take me to the police for help," and "this vehicle is too filthy to use."
PRT differs from people mover systems in that one person, or small party, selects a destination, while people-movers stay on a fixed route.
Several partial systems, which are not "true" point to point PRT systems all the time, are currently in operation.
The concept is said to have originated with Don Fichter, a city transportation planner, and author of a 1964 book. (see references)
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. (See references)
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 SkyTran project proposes to use magnetic levitation in solid-state vehicles that achieve speeds of 100mph.
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.
A small system operates at the Seattle international airport SeaTac, and is said to have met all goals with regard to speed, cost and reliability.
Standard safety engineering extrapolations evaluate PRT systems as ten-thousand to one million times safer than automobile travel. The few existing PRT systems have been very safe, because they are automated, periodically-inspected, with self-diagnosing redundant systems. Vehicles are on rails, usually with captured wheels. Computer control nearly eliminates driver errors. Usually a central computer system manages traffic, with each car going to a embarkation station if the central computers or power fail.
Automation and redundancy also open ridership to nondrivers, and lower costs.
Systems drive the vehicles in such a way that they do not need to slow or stop while en-route.
Tracks are arranged, and vehicles timed to "miss" at intersections. Careful engineering repeated at several projects has shown that less-expensive one-way, single-level loop systems can operate nearly as quickly and safely as systems with far more expensive dual-direction clover-leafed multilevel intersections.
Embarkation stations are on turnouts so other vehicles remain moving 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. A reduced road grid is still required for heavy transport.
All vehicles are powered by electricity, so the vehicles do not themselves generate pollution. Most systems have dual or triple-redundant power supplies, from track-side batteries or natural-gas-powered electrical generators, and sometimes on-board batteries.
Designers definitely prefer solid-state electromagnetic line switching, and design for it. Line switching is built into vehicles rather than the track, so that the tracks will stay in service. If a track fails, carrying capacity is drastically degraded.
Some systems plan to gang identical vehicles into platoons to serve a group, and reduce drag from moving through the air. The platoons would have a shared intercom. Another system plans to permit the vehicles to operate as a conventional light rail line in a pinch, and have the PRT vehicles double as light electric cars that can go short distances on surface streets.
Most systems plan multiple types of vehicles. The smallest vehicles seat two, the largest six. Two is the best because it has the lowest-per-mile tack cost, and handles most riders (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. Most systems have light cargo vehicles. At least one study indicated that light cargo could make or break the feasibility in a port city.
The most contentious issue in PRT, when evaluated by transportation planners, is the "ridiculously low" cost estimates of proponents, especially when proponents cast these estimates in terms of cost per rider-mile. Ultra now has demonstrated figures. How capital costs are incorporated is a critical element in cost estimates, since PRT systems are capital intensive with relatively low operating costs compared to other technologies.
One very disputed number is the carrying capacity of a route. Professional transportation planners routinely dismiss as absurd the short inter-vehicle distances designed into PRT systems.
The central issue is that light rail must decelerate at a maximum of 1/8 of a gravity, so standing passengers will not be harmed. This means that legally-required intertrain stopping distances have to 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 tires lose traction.
However, unrestrained sitting passengers can tolerate emergency stops at 6 gravities, which is like a less-exciting roller-coaster. So vehicles with sitting passengers can go from 70 mi/h (116 km/h) to stopped in 0.52 seconds, about 27 feet (8 meters). With seat-belts, people tolerate an emergency stop at 16 gravities. With torso restraints, people tolerate 32 gravity emergency stops, resulting in 0.1 second stopping times, and 11 feet (3.2 meter) safe inter-vehicle distances.
Since PRTs have sitting, and sometimes belted passengers, and automated emergency braking against steel guide-ways, PRT designers plan for legally-permissible emergency stops as short as 2-3 meters depending on the speed.
This (to a light-rail transit planner) "absurdly short" inter-vehicle distance raises right-of-way utilizations to very high levels, even with the smaller numbers of passengers per vehicle.
For these reasons, the best systems never brake by wheels, because this causes the safe inter-vehicle spacing to increase, lowering the right-of-way utilization, and therefore the cost per passenger-mile of the track. Braking is either against a linear motor, or steel rails for emergency stops.
Another disputed issue 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 between two and 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.
With comparable assumptions, PRT therefore has two to three times as many trips per seat as a bus or train. Therefore PRT theoretically utilizes its average seat between 50 and 300 percent more efficiently than busses and trains. This is hotly contested, of course.
Most plans start with the use of PRT in a downtown area. If PRT capacity 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.
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 one-hundred to three-hundred 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 expensive.
The absurdly cheap less-than-$1 million-per-mile estimates 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 PRT vehicles weigh just a few hundred pounds, while trains weigh tons.
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 expensive 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 dangling from an overhead track. Cars are said to be stressed in tension, "making a lighter vehicle structure" because 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.
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.
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' 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.
The lowest-energy real PRT vehicles have used air-cushion (non-contact)suspension and drive. The cheapest used wheels and linear electric motors.
The debate is still on as to what constitutes the proper guideway for PRT systems. There are nearly as many guideway technologies as there are PRT proposals and/or prototypes. Each of these guideways is incompatible with each other and with existing transportation technologies. The continuation of this debate over several decades could be viewed as a disadvantage, since nobody knows what should be really built, but it could also be seen as an advantage because rational debate and shared information can lead to a better guideway concept.
The discussion can be difficult because comparing different guideway systems is more arduous than having a contest between the best apple and the best orange. Some guideways are monorail beams, other are dual rail or guide beams and others still rely only on electric signals from cables embedded in an asphalt or concrete roadway.
The debate is even more 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. A system like the Danish RUF is dual mode because while the vehicles can operate on the guideways in a totally automatic mode, they can also leave the guideways and operate on city streets, as normal cars, with drivers controlling them completely. A system like the British Ultra is now single mode, but its promoters envision the possibility of making a dual mode version in the future, complicating further the single versus dual mode debate.
Many of the disadvantages and/or advantages listed below are applicable to single mode systems but not dual mode systems, and vice versa.
In theory, there should be an article for single mode and another for dual mode, but in practice this would leave Ultra and others in the middle.
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, SeaTac and the system at Morgantown, West Virginia.
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.
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% to 25% of the cost of light rail, bus systems or automobiles, with the possibility of displacing more ridership from autos than any other rapid transit system. This is about US $0.03 to $0.10 per passenger mile.
PRT's overhead track mounted on poles preserves neighborhoods and buildings, unlike freeways or railways.
It is certain that PRT systems are more attractive to some users than train or bus systems. Many regions now have PRT advocacy groups, a new political development affecting transit organizations.
If proponents' numbers are right, commuter time savings and improved land-use alone justify PRT systems.
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 the above assumptions, 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). Other systems, such as SeaTac, have met cost projections. 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.
History
Safety and Utility
Engineering Economics
Route capacity- strongly affected by superior braking
Capacity utilization- affected by nonstop passenger travel
Costs of rights-of-way- trading technology for less land-use
Comparable vehicle costs
Minimized overhead and operating costs
Guideway choice
Dual mode versus single mode systems
Advantages
Disadvantages
External links
References
