Personal rapid transit
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.PRT vehicles are generally 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 it, 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 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.
The system developers aim to provide service very similar to that provided by a car, yet with the advantages 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 or more, stop every few hundred yards, and require multiple transfers, with a wait at each transfer.
| Table of contents |
|
2 Plans 3 Engineering Economics 4 Techniques 5 Advantages 6 Disadvantages 7 External links 8 References |
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.
The longest-running operational PRT system is the West Virginia University PRT, 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.
A small system operates at the Seattle international airport SeaTac.
In the United States, the Taxi2000 proposal, developed at the University of Minnesota is another.
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.
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.
The Aerospace Corporation 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.
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.
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.
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 fifty and sixty percent more efficiently than busses and trains. This is hotly contended, of course.
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. It turns out that an elevated track structure scales down dramatically with lower vehicle weights.
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. 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 that cost 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.
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.
Certain techniques have become common in proposed systems.
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.
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.
A track should not accumulate snow or rainwater, and should not need to be heated. A lightweight vehicle enables the track to be placed on poles, lowering the cost of rights of way.
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 mounted on the ground without poles. A significant problem is that topside wheels and tracks must be designed not to collect precipitation or dust, and the track must be wider to balance the vehicle.
Design teams have used similar justifications for cars dangling from an overhead track. Weather is better handled by overhead tracks, and cars are 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.
Automation and redundancy increase safety, open up ridership to nondrivers, and lower costs. Usually a central computer system manages traffic, with each car automatically going to a embarkation station if the central computers or power fail.
Systems place embarkation stations on turnouts, so traffic is not slowed when a vehicle drops off or picks up a passenger. Embarkation systems are usually mounted on poles with the track, but may also be inside buildings or at street level. 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.
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.
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.
The lowest-energy real PRT vehicles have used air-cushion (non-contact)suspension and drive. The cheapest used wheels and linear electric motors. Passive magnetic levitation is now (2003) possible, permitting intercity PRTs to travel in a vacuum tube at several thousand miles per hour.
Inside the vehicles, various systems have some combination of buttons 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."
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.
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 froma proof-of-concept project into a network.
PRT systems are proven, at least in the Ultra system at Cardiff, Wales, SeaTac and the system at Morgantown, West Virginia.
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.
The few existing PRT systems have been 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. Because of computer control,
driver error is eliminated, along with a need for a driver's license, gas, insurance or sobriety. 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 in North America.
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.
PRT offers hope. It is certain that conventional transit options cannot solve most cities' transportation problems. They have failed to do so in Chicago, which has fully-realized train, freeway, and bus plans.
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. This may be the best user evaluation that is possible in the long term.
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.
Operational system(s)
Plans
Engineering Economics
Techniques
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.Advantages
Disadvantages
External links
References
