Encyclopedia > Personal rapid transit

  Article Content

Personal rapid transit

Personal rapid transit, or PRT is a form of transport that tries to combine the benefits of individual transport modes (walking, automobile) with the advantages of rail transit. Like an elevator, people can choose a specific destination when they board. The vehicles carry one to six passengers. The vehicle runs on very light-weight tracks, generally elevated above street level. Computer systems drive and manage the system.

In use, one picks up the vehicle as if at a taxi stand. 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 other passengers. 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.

PRT differs from people mover systems in that one person, or small party, selects a destination, while People Movers stay on a fixed route.

Since PRT systems are ten thousand to one million times safer than automobiles, cheaper, and more convenient, a lack of them is causing the deaths of forty to fifty thousand people per year in North America alone.

Table of contents

Operational system

The best known example of an operational PRT system is the West Virginia University PRT, which has been in operation since 1975, with an average of 15,000 riders per day. The system uses approximately 70 cars, 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 "guideway" free of snow and ice. It is sufficiently reliable and low-cost that most students habitually use it.

Plans

In the United States, the Taxi2000 proposal, developed at the University of Minnesota is another. A small system is planned to be operational in Cardiff, Wales, in 2003. The SkyTran project proposes to use magnetic levitation in solid-state vehicles that achieve speeds of 100mph.

The Aramis project in Paris, France was a large scheme, documented by Bruno Latour[?] in Aramis: or the Love of Technology.

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.

Engineering Economics

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.

The most contentious single 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 gravity, so standing passengers will not be harmed. This means that legally-required intertrain stopping distances have to be 1285ft (391m) for a 70mph (116 kph) train. Busses and automobiles have a similar problem. They can only decelerate at 1/2 gravity before their tires lose traction and they crash.

Unrestrained sitting passengers can tolerate emergency stops at 6 gravities, about what is present on a less-exciting roller-coaster. This means that vehicles with sitting passengers can go from 70 mph to stopped in 0.52 seconds, about 27 feet (8 meters). If passengers wear seat-belts, an emergency stop can decelerate at 16 gravities. With torso retraints, they tolerate 40 gravities.

Since PRTs have sitting passengers, and can brake against steel guideways, PRT designers plan for 6G emergency stops, and can therefore have legal intervehicle distances as short as 8 meters on 116kph systems.

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.

Almost as contentious are the cost-estimates of per-mile 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 wide, and (naturally) goes through the highest-density, most valuable part of the city. When the railway is buried 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, 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. It turns out that an elevated track structure scales down exponentially with the vehicle weight.

Another important issue is that contrary to many persons' intuitions, costs of transit vehicles are relatively constant per passenger. While larger vehicles enclose more space, they are nearly hand-built. 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 salary. PRT systems eliminate that cost by automating guidance and fare-collection.

Techniques

Certain techniques have become common in proposed systems.

Automation and redundancy increase safety, open up ridership to nondrivers, and lower costs by managing the traffic.

Systems place embarcation stations on turnouts, so traffic is not slowed when a vehicle drops off or picks up a passeger. Embarcation systems are usually mounted up on poles with the track, but may also be inside buildings or at street level. Since systems have minimal waiting times, embarcation 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.

Careful engineering, repeated at several projects, has shown that less-expensive single-level "Y" joint loop systems can operate as efficiently as clover-leafed multilevel intersections.

Inside the vehicles, systems have buttons to "take me to the police for help," "this vehicle is too filthy to use" and "take me to the nearest stop now."

Most systems plan multiple types of vehicles. The smallest vehicles seat two, the largest six. Some systems have special vehicles for wheel-chair users and bicyclists. Most systems have light cargo vehicles.

Most systems use a small, lightweight vehicle. The light vehicle enables the track to be placed on poles, lowering the cost of rights of way. The vehicle is normally on top of the track, because people prefer it. This also makes the poles shorter, stronger, less expensive and less visible. Top mounted vehicles also unload the skins of the vehicle, which can be lighter. Topside tracks also have simpler line-switching, and must have wheels and tracks covered to keep out the weather.

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.

All vehicles are powered by electricity. Most systems have dual or triple-redundant power supplies, from track-side batteries or natural-gas-powered electrical generators, and sometimes on-board batteries.

Some systems plan to gang identical vehicles into platoons to serve a group. 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.

Advantages

Proponents usually compare PRT systems to light rail, bus routes and automobiles, as these are the principal competitors for transportation.

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.

PRT systems offer on-demand nonstop 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.

PRT systems should be very safe, because they are automated, periodically-inspected, with self-diagnosing redundant systems. Standard safety engineering extrapolations evaluate PRT systems as ten-thousand to one million times safer than automobile travel. Vehicles are on rails, usually with captured wheels. In the worst case, another vehicle pushes the dead vehicle to the nearest station. Crime is prevented because criminals would not know the destination, and most designs include a panic button that takes the unit to a police station.

PRT systems usually operate from the electrical grid, and are therefore far less polluting and less expensive than even fuel-cell automobiles.

Proponents say that PRT systems will not delay commuters in gridlock or traffic jams. 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.

Proponents claim passengers-per-hour roughly equal to a freeway or train line per PRT route. Although the vehicles are small, they would move without traffic jams or stopping, and continuously recycle vehicles to new passengers.

Per passenger-mile, proponents cost-out PRT systems at 3% to 25% of the cost of light rail, bus systems or automobiles. The lightweight overhead rails may permit PRT systems to operate with rights-of-way costing less than $1 million per mile versus the $20 to $30 million per mile for freeways or light rail. Additionally, since the vehicles are automated, there are no driver salaries, the chief expense of most light rail and bus systems.

Since the vehicle is smaller, it can usually be placed on an overhead track, perhaps even one mounted on poles. This preserves neighborhoods and buildings, unlike freeways or railways.

If proponents' numbers are right, commuter time savings and improved land-use alone justify PRT systems.

Disadvantages

As of 2003, no "classical" PRT system has been built and shown to meet its goals. The most similar system is at West Virginia University at Morgantown, and it is arguably not a PRT system. ALthough it stringly resembles a PRT system, its vehicles carry 15-20 people. While the system is an improvement on the busses it replaced, it is prone to sporadic failure due to its aging hardware and is considered a nuisance and necessary evil by its riders.

Demonstration systems in Rosemont, Illinois and Cardiff, Wales have not been completed. In the 1990s, Raytheon invested heavily in PRT, even completing a technology demonstration, without winning any contracts.

The claims made by proponents depend on certain reasonable but nonstandard design features (see below). If standard 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.

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

A PRT system has lower costs and automated operations. These 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.

A related problem is that it threatens existing livelihoods associated with cars, busses, and related technologies. The very high vehicle utilizations (vehicles are usually carrying passengers, 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.

External link



All Wikipedia text is available under the terms of the GNU Free Documentation License

 
  Search Encyclopedia

Search over one million articles, find something about almost anything!
 
 
  
  Featured Article
Quadratic formula

... common denominator is 4a2. We get <math>\left(x+\frac{b}{2a}\right)^2=\frac{-4ac+b^2}{4a^2}=\frac{b^2-4ac}{4a^2}.</math> Taking square roots of both ...

 
 
 
This page was created in 24.1 ms