- Is PRT “reinventing the automobile?
No! PRT is a public transit system. It cannot replace the automobile, but its service characteristics are such that it can be expected to attract many more people than conventional transit systems, and it can do so using a tiny fraction of the land required for the automobile. While roughly half the population either cannot or should not be driving automobiles, PRT is accessible to everyone. It will be the environmentalist’s dream because of its markedly improved energy efficiency, lack of air pollution, and land savings. Normally the guideways should be spaced not less than a quarter to a half mile apart. They do not replace streets.
- Can small cars move the large numbers of people who would use general mass transit?
Today, automobiles averaging 1.2 people per vehicle carry more than 97 percent of the urban passenger-miles in the United States. Uninterrupted flow is the key to capacity, not vehicle size. As an example, 60-passenger buses coming two minutes apart, a very high flow rate for an American bus system, provide the same number of capacity units per hour as 3-passenger PRT vehicles coming every six seconds. One PRT line can serve more than six times this capacity, more passengers per hour than come into downtown Boston during the morning rush period via a three-lane expressway.
The line capacity is high because of automatic control, an in-vehicle switch, and electromagnetic propulsion and braking. Automatic control is safer and more reliable than human drivers, permitting vehicles to be separated by small distances. In-vehicle switches work faster and more reliably than moving-track switches, again permitting vehicles to be closely spaced on the guideway. Linear electromagnetic braking is reliable in wet and icy weather that forces systems using rotary motors and wheel braking to spread vehicles far apart because of skidding concerns in emergency stops.
- Won’t stations get bogged down with all the small vehicles?Station throughput is determined by the number of station berths, which can be set to meet the demand of any particular station. In general, station throughput is high relative to conventional mass transit because:
Taxi 2000 station inside a building.
Only vehicles that actually need to stop at a station will enter the station. All other vehicles pass by, thus reducing station traffic relative to conventional systems where all vehicles stop in each station regardless of where passengers are going.
PRT stations are closely spaced, often within a quarter mile of each other. This is convenient to patrons, who walk only short distances, and results in smaller, less crowded stations.
The loading time for each vehicle is relatively short, usually completed in a few seconds. As people become accustomed to PRT systems, they will enter and exit vehicles as quickly as cars, increasing station throughput and minimizing trip time.
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- Won’t the problems of reliability make the operation of a large fleet of small vehicles undependable? Actually, because a PRT system will have a large number of small vehicles, rather than a relatively small number of large vehicles, the chance of an individual becoming involved in a failure will reduce in proportion to vehicle size if the reliability of each vehicle is the same. But, because of the use of checked redundancy and advanced failure-management strategies possible within the confines of a PRT system, and the benign environment within the guideway, the reliability of it will be substantially higher than a conventional transit system. It has been shown that the requirements for dependability in a PRT system are independent of system size.
- What happens if a PRT vehicle stops on an elevated guideway between stations? Questions of reliability, safety, evacuation and rescue are fundamental to the design of any elevated transit system including PRT. Each vehicle has two motors and two controllers, modern failure-monitoring systems, fault-tolerance and fail-safe features. The system has alternative power sources so that a power failure will not leave passengers stranded.
There are over 70 elevated automated transit systems operating in the world today that prove that a vehicle stopping when not intended is a very rare event. If a vehicle does stop between stations, Central Control will talk with the passengers through an intercom system and guide the rescue operation. The vehicle behind will soft engage and push the disabled vehicle to the nearest station. In the very unlikely event that the vehicle can’t be moved, a rescue team will come with a ladder and help the passengers out of the vehicle.
- What if there is a power failure? The vehicles receive their power from DC power rails located inside the guideway. There will always be an alternative power source. One way is to power the system from two different utilities, so that if one fails the other is immediately available to take over. A second way is to power the system from gas turbine-generator sets and to use utility power as emergency backup. A third way, appropriate for maximum energy and peak-power conservation, is to power the vehicles from wayside batteries, which can be charged at night when the power rate is low. During a municipal power failure, vehicles would still receive battery power, so they would simply slow down to conserve energy, finish their trips, and strand no passengers. A fourth way is to use large flywheels as back-up power sources.
- What if a truck hits a post? If the guideway runs down the center of an arterial street or on the edge of such a street, highway barriers can be placed to protect the posts or they can be placed on concrete pedestals so that it is not possible for vehicles to hit the posts. The posts are, however, substantial enough so that it would take a high-speed collision of a large truck to shear off a post. If a post were sheared, the guideway will remain intact and the vehicles will remain in the guideway.
- Will the visual impact of PRT be acceptable? Visual impact is important in all transit systems. Many rail transit systems are placed underground because a ground-level system requires destruction of too much existing property and an elevated system is too massive and noisy. A PRT guideway has less than five percent of the cross sectional area of a rapid rail system, will generate almost no noise, and has an external appearance that can be varied to suit any specific community. According to one famous sculptor, PRT adds excitement and grandeur to the urban scene, both for what it is and what it does.
People accept elevated structures if they see them as a practical means to a desired end. In the early 1970s, when conventional heavy rail systems were being promoted, officials argued that elevated structures were acceptable. The People Movers proposed in the late 1970s had massive structures (witness the Detroit and Miami People Movers) but local authorities considered them acceptable because they were believed to fulfill a need. Taxi 2000 will have much smaller visual impact and will provide much better service at lower cost.
- How does a person use PRT? At each station, there will be several conveniently located ticket machines and a map of the system. The patron, or small group of patrons who want to ride together, determine their destination number from the map and go to the ticket machine to punch in the destination. The machine verifies the destination and displays the fare, which may be paid by cash or by prepaid ticket and is per vehicle rather than per person. The machine then dispenses a magnetically coded ticket.
At the ticketing machine.
At the ticketing machine.The patron takes the ticket to a stanchion in front of the first empty vehicle in a line of vehicles and inserts it into a slot. This act transfers the memory of the destination to a microprocessor aboard the vehicle, causes the door to open, and assures the patron that he or she is getting on a vehicle headed to his or her station.
The patron or patrons walk into the vehicle, sit down and press a go-button, whereupon the door closes automatically, the control system waits for an opening in the traffic bypassing the station and commands the vehicle to accelerate to line speed. When the vehicle reaches the destination station, it pulls into a berth and opens the door automatically. The patron(s) exit the vehicle and leave the station.
If the patron is regularly going between a certain station pair, he or she can purchase a pass in advance, bypass the ticket machine and go directly to the stanchion in front of the first empty vehicle. PRT doesn’t need turnstiles since a valid ticket is necessary to gain access to a vehicle.
- Is it possible to stop before the end of the ordered trip? Yes. A stop button will be mounted on the control panel of each vehicle, which if pressed stops the vehicle at the next station.
Vehicles are ADA compliant.11. What about access for the handicapped? PRT will be fully accessible to handicapped patrons and will comply with the Americans With Disabilities Act. Elevators will be provided in elevated stations and the ticket machines and stanchions will include intercoms and Braille plaques to insure ease of use by all patrons. The vehicle accommodates wheelchairs riding sideways, with a jump seat for one traveling companion. The platform is level with the vehicle floor to prevent wheelchair bumps and is textured at the edge to assist the blind.
Taxi 2000 is designed to be easily accessible to all people, whether handicapped, young, old, carrying heavy bags, traveling with a bicycle, or have any other special need. It has been praised and promoted by groups representing the needs of the handicapped.
- How much time does a person have to board a PRT vehicle? As much time as is necessary. The vehicle will not move until the passengers have entered and the door is closed and locked. Loading or unloading time is a statistical variable, which varies from a minimum of about two seconds to a maximum of 10 to 15 seconds.
- Will you have to ride with strangers? No! Each vehicle is occupied by passengers riding alone or together by choice. If someone tries to force his way into a vehicle, a button can be pushed inside the vehicle to alert the police.
- Can a PRT vehicle be entered from either side? Yes. It is not in general practical to design a PRT network in such a way that all stations are on one side of the guideway. Therefore the cars are designed with doors on both sides, but only the door on the station side opens when the vehicle stops.
- How serious a problem is vandalism? Vandalism is minimized in the following ways:
By Surveillance. The stations will be television monitored with two-way voice communication. They are small areas that can be surveyed easily, and infrared detectors will be used to detect the presence of people so that the operator, in slack times, need not constantly view the screen.
By Identification. A means will be provided to permit a boarding passenger to reject a vandalized vehicle. An alarm signal will then be sent to the nearest control room where a human operator is alerted to roll back a video memory unit and make a permanent record of the last passenger to egress from the vandalized vehicle, and to command the vehicle to the nearest maintenance shop. Normal police methods will then be used to apprehend the vandal. Experience at the Morgantown automated people mover system has shown that knowledge of such a procedure, not possible in conventional transit, will by itself deter most vandalism.
By Psychology. In public places, vandalism has been greatly reduced by the application of human psychology (see Psychology Today, September 1982). Plain walls that look like writing tablets invite being written on. Textured walls and walls with diagonal lines or protrusions markedly reduce graffiti. Appropriate colors, music, architectural design, and plants reduce vandalism. Frequently cleaned public places are not as subject to vandalism as dirty ones.
By use of Attendants. In large stations or in stations unusually prone to vandalism it is not unreasonable economically to use attendants, and they may be used if other methods fail.
- Won’t Personal Security be a serious problem? Personal security is less of a problem than in conventional mass transit, and even sometimes less than in automobiles, for the following reasons:
The ride is nonstop, direct to the destination, and alone or with one or two other people of choice. One never rides with strangers.
Computer simulations have shown that in a well-designed system in the rush period about 50 percent of the passengers will wait less than 30 to 40 seconds and 97 percent less than three minutes. During off-peak periods there is no waiting at all. Thus there is little time for commission of acts of aggression.
Television monitors and two-way voice-communication systems will be placed in the stations to survey the platform, stairways and vehicles. To insure that the screens will be watched, infrared sensors will be placed in the stations to alert the monitoring personal of activity in each station.
The station platform is typically no longer that 20 to 40 feet and about 12 feet wide, and is easy to watch—much easier than a large, multi-story parking structure. Care in station design will eliminate areas in which a potential assailant can hide.
A stop button in the vehicle permits the passenger to order the vehicle to stop at the next station for any reason.
A voice communications system will be installed in each vehicle to be used to call for help in any emergency.
- Won’t the issues of safety make it difficult to insure a PRT system? The insurance rate for the first operational Taxi 2000 system will be based on the insurer’s estimate of the frequency and severity of bodily injury sustained while riding, attending to, or being in proximity of the system. In today’s litigious society, it would not do to rush such a system to completion and to permit the public to ride before it was thoroughly tested. Every reasonable practical precaution must be taken in the design of a new PRT system to assure safety, and there will be an adequate period of testing before opening the system for public use.
An extensive series of design features are incorporated into Taxi 2000 both to minimize the probability of failures that may cause injury, and to minimize the consequences of any failure. A remarkable characteristic of PRT is that, because the vehicles are small and light, it is practical to design to assure that no combination of failures can cause injury. The developers of Taxi 2000 are convinced that its system will provide a substantial improvement in both safety and personal security.
Obtaining a reasonable insurance rate for a PRT system depends not only on the design features but also on the program of development and testing undertaken before the public can ride. Before building a demonstration for public use, a half-mile oval test system with one off-line station and four prototype vehicles will be tested. Based on the results of the test program, the first real people-moving demonstration will be constructed, tested, and certified for public use before the public will be permitted to ride. Potential insurers will be invited to monitor the test program in sufficient detail to establish the insurance rate.
- Isn’t there an economy of scale in transit systems, i. e., to carry a given traffic level, won’t a system of many small vehicles cost more than a system of a few large vehicles? The basic features of PRT follow logically as features that minimize the total cost per passenger-mile. These features permit true minimization of guideway cost, vehicle-fleet cost, and operating cost while maximizing service.
Vehicle Cost per Unit Capacity
Data shows that transit vehicles cost about the same per unit of capacity no matter how large or small they are. Contrary to intuition, there is no economy of scale. By using nonstop trips, possible with off-line stations, the average trip time of a PRT system is two to three times less than in a conventional transit system, which means that the fleet capacity (number of vehicles x capacity per vehicle) and therefore fleet cost needed to serve a given number of trips is less by the same factor.
Vehicles of the size required to hold up to three or four seated adults have a much smaller cross section and weigh substantially less per unit of length than large standing-passenger vehicles, and, because of much lower dynamic loading, lead to lower guideway weight (15 times lower) and lower cost.
To compare operating and maintenance (O&M) costs, we define a quantity called a “place-mile.” The number of place-miles of travel in a transit system consisting of vehicles or trains of any size is the number of vehicle-miles of travel multiplied by vehicle capacity. A vehicle-mile is one vehicle traveling one mile. Because PRT vehicles move only when service is demanded, the total number of place-miles per day required to serve a given level of passenger demand is only about a third as much as in a conventional scheduled transit system. Examination of data on O&M costs shows that the O&M cost per place-mile is nearly the same regardless of the type of transit system. Thus the O&M cost of a transit system that carries a given number of people per day is proportional to the number of place-miles per day of travel.
The remarkable result of this kind of systems-economic analysis is a transit system in which the features required to minimize both capital and operating costs are exactly those that provide maximum service, i. e., on-demand, alone or with one or two friends, in seated comfort, any time of day or night, at a predictable average speed two to three times that possible with conventional transit. The only reason for using large vehicles in urban transportation is to amortize the wages of drivers over as many fare-paying riders as possible. Automation permits relaxation of system characteristics toward a true optimum.
- How much will a ride cost? Transit fares are normally set as a matter of public policy, and are as high as the public will bear without significantly reducing ridership. In most conventional systems, the fare covers only about 30% of the operating cost and capital cost is never recovered. Thus, present transit systems require large state and federal subsidies.
Because of its low capital and operating costs, PRT systems will be able to charge fares that are comparable to conventional mass transit, yet will require little or no subsidy. This will permit systems to be installed in communities that need transit but don’t have access to large state and federal subsidies.
- How Was the Vehicle Size Selected? On a strictly economic basis, one-person vehicles minimize capital cost, but they do not serve obvious social needs. Two-person vehicles are too small for a small family, for taking luggage, or for a wheelchair plus attendant. Also, if a party of three wants to travel together, one of them would have to ride alone if the vehicles hold only two persons, which may be socially awkward. So the vehicle should have room for at least three seats side-by side. If the vehicle were to have to accommodate a wheelchair which can rotate to face forward, the floor area is such that there can be two forward-facing seats in the back and two backward-facing seats in the front, which could normally fold up to accommodate the wheelchair. This provides a socially pleasant configuration for occasional use by two couples, but requires the vehicle to be longer, which increases vehicle weight and cost, and station length and cost. (An alternative is to provide special vehicles for wheelchairs, which would be on call by cellular telephone on short notice.)
Interior of 3-Passenger CabFrom the viewpoint of ultimate safety, a passenger can be protected in a sudden stop if he or she is behind a padded dashboard. The above four-passenger configuration has a much longer throw distance than a side-by-side seating configuration and therefore greater probability of injury in the unlikely event of a sudden stop. Since each vehicle will be controlled by a pair of checked-redundant fault-tolerant computers, the probability of a sudden stop will be low, yet such a stop must be considered in the design.
Since more than 95 percent of the trips in an urban area are taken by one, two or three persons travelling together, the more people an individual vehicle is required to accommodate, the more vehicle dead weight there is per person carried and higher capital and energy cost. By charging a fare per vehicle rather than per person, it is possible that the average vehicle occupancy can be increased over that experienced with automobiles.
We see that the factors that must be considered in picking vehicle capacity are not the same in PRT as in a family automobile. A PRT trip is generally quite short and a group larger than can fit into one vehicle can take two or more vehicles, which leave the origin station seconds apart and arrive at the destination seconds apart. Taxi 2000 is designed to carry a total load of 650 lbs.
- Why are the vehicles mounted above the guideway rather than below? A variety of issues must be considered in making this tradeoff:
Switching. The running surfaces for supported vehicles are continuous through switch sections, whereas with hanging vehicles, there must be a slot in the guideway through which an arm that supports the vehicle passes. Thus, in a switch section the support element inside the guideway must pass across the slot, so the supporting load must be transferred to another member. In the Monocab PRT system this required an extra set of wheels that engaged slots in the top inside of the guideway. With maglev the load would probably be transferred to a set of electromagnets. This extra load-transfer means adds extra weight and cost, and reduces reliability. The bottom line is that the fundamental requirement of switching is much easier with supported vehicles.
Visual Impact. A group at the University of Minnesota worked in the PRT field for 13 years before deciding it was necessary to initiate the design of a new PRT system. During this time, we conducted three international conferences on PRT and were deeply involved in a fourth conference held by the Advanced Transit Association in Indianapolis in 1978. In addition, we gave hundreds of presentations on PRT in many places in the United States and abroad and were involved in a number of PRT planning studies. Moreover, our group was funded by UMTA to study the visual impact of AGT systems, and worked with people at the Volpe National Transportation Systems Center on this subject. Out of this experience, we were extremely sensitized to the issue of visual impact of overhead structures. Both the supported and hanging systems must have the same clearance, so the guideway of the hanging system is six to eight feet higher and must be supported by posts with right angle arms at the top to reach out and support the guideway in such a way that the vehicle can pass underneath. The guideway of the supported-vehicle system is lower and the posts are smaller, so, all else being equal, the visual impact is substantially less.
Cost of Posts and Foundations. Because the guideway of the hanging-vehicle system is higher from the ground, the bending moment at the base of the posts due to the maximum crosswind for which the system is designed is higher than in the case of the supported-vehicle system. Moreover, in the hanging-vehicle system the weight of the guideway and the vehicles adds an additional bending moment at the base of the posts. The maximum load condition is fully loaded vehicles nose-to-tail along the guideway. With this load condition and the maximum lateral wind load, we found that the bending moment at the foundation is about twice as much with the hanging-vehicle system as with the supported-vehicle system. So both the posts and the foundations will be correspondingly larger. Yet to avoid utilities and to fit the foundations in, the smaller foundation is preferred.
Torsion in Curves. If the vehicle hangs from the guideway and is provided with a critically damped swivel joint, as it passes through a curve it will swing out as in a coordinated turn of an airplane. Thus the passenger is not subject to much lateral acceleration. If the vehicle is supported above the guideway, the guideway must be superelevated or banked to reduce the lateral acceleration on the passenger. With the hanging vehicle in curves, the torques due to centrifugal force and gravity subtract, whereas with the supported vehicle they add. This seems like a deciding factor, however, the wall thickness of the guideway required to resist a given torque increases as only the cube root of the torque, and from detailed analysis we found that the wall thickness to support a given torque with the supported vehicle is only 13% greater than with the hanging vehicle. This consideration will, therefore, not be an over riding factor, and other factors must be considered.
System Natural Frequency. With the vehicles on top of the guideway, the guideway can be clamped to the posts as first suggested by Dr. Jack Irving in Fundamentals of PRT. From basic structures theory, the maximum deflection of a beam clamped at both ends is only 20% of the maximum deflection of a simply supported beam and the natural frequency is 2.26 times higher. If the vehicles hang from the guideway, the post must be along side of the guideway and a horizontal element must be added over the top of the guideway to support it. It is not practical to make this longer element as stiff as the clamped support, which means that to have the same deflection and natural frequency as the guideway of a system with supported vehicles, the guideway of the hanging-vehicle system must be heavier. This fact more than counteracts the increased thickness of the supported guideway discussed in point #4.
Vehicle Weight. The weight of a supported vehicle and its load is supported at the bottom of the chassis so that the maximum load on the cabin is due only to wind and passenger loads. The weight of a hanging vehicle and its load must be supported by the sidewalls of the cabin, which, all else being equal, would require that they be heavier.
Underground System. If the system is placed underground, the guideway of the supported vehicle can be laid on the ground whereas with the hanging vehicle, the guideway must still support the vehicle. This means that there will be a cost savings in using supported vehicles underground.
Public Preference. On its test track, the Cabintaxi PRT guideway was designed and built to support vehicles both above and below the guideway. Many people rode the system and their reactions were recorded and reported in an assessment report developed jointly by the U. S. DOT and a German consulting firm. The result was that somewhat more people preferred riding above the guideway than below.
Passing Through Buildings. If the guideway is below the vehicle, it blocks any cross traffic due to pedestrians or carts. If the guideway is above the vehicle and there is adequate clearance below, the system would not interfere with cross traffic. Therefore this factor favors the hanging vehicle provided the ceiling is high enough.
Snow, Ice, Debris. A major argument for the hanging-vehicle system is that there need be no worry about interference with snow, ice, or debris. For the supported-vehicle PRT system to be successful, this situation must be addressed very carefully. In the Taxi 2000 PRT system, the guideway is a truss structure with a cover over it that provides eight benefits, one of which is to keep out ice, snow and other debris. We found that the chassis, which is constrained within a U-shaped guideway need only be four inches wide and the slot through which it passes need be only five inches wide, giving a nominal half-inch clearance on each side. The main support tires, which are cushion synthetic-rubber tires, are supported on eight inch by six inch by half inch steel angles, and there is a slot eight inches wide between them. We have designed a plow that will pick up any foreign substance on the running surfaces and toss it down in the eight-inch gap. The plow has been tested in winter conditions and has been found to be completely satisfactory.
- Why are the moving switch parts in the vehicle rather than in the guideway? There are five reasons an in-vehicle switch is superior to a moving-track switch.
Reliability. The simplicity of the in-vehicle switch makes it inherently more reliable than the in-track switch, and an in-vehicle switch can easily be made bi-stable by means of a spring. The worst that can happen with a well-designed in-vehicle switch is that one small vehicle will be misdirected, whereas if an in-track switch fails, it ties up a whole line of traffic, thus delaying many people. Also, the in-track switch requires an electric or hydraulic actuator mounted in the guideway, which upon failure shuts down the line. The result is that the required reliability cannot realistically be attained if the switch is in the track, but is easily attained if the switch is in the vehicle.
Capacity. Because of the time required 1) to move an in-track switch, 2) to verify that it is locked in position, and 3) to be able to stop before the vehicle reaches the switch if verification is not obtained, the minimum time headway will be too long to be of use in a PRT system. An in-vehicle switch completely removes this barrier to high capacity.
Ride Comfort. In-track switches often consist of a series of articulated straight pieces of guideway that swing back and forth. Passengers will feel such a strong lateral jerk each time the vehicle passes one of the joints that the vehicle will have to slow down for every passage, and there can be four or five straight pieces in each switch. An in-vehicle switch permits guideway branch points to be made with simple smooth curves, maximizing passenger comfort and minimizing jerk loads on the undercarriage of the vehicle.
Visual Impact. Articulated in-track switches with their actuators attached greatly increase the visual impact of a switch section. Beyond simply being much larger than a simple branch section, in-track switches have a track leading into empty space, which is a discomforting view for passengers as well as passers by.
Cost. An in-vehicle switch has very few moving parts and is very simple and inexpensive to build. An in-track switch is much larger, often consisting of several articulated track sections that move back and forth, and contains many large parts which increase both capital and maintenance cost significantly.
- Are one-way guideways practical? Because of the very small guideway and in-vehicle switching, one-way guideways are an option, but not a necessity. If the guideways are one-way, for a given investment, twice as much land area can be placed within walking distance of stations as with two-way systems. If planners want two-way systems, they are easily provided. We have analyzed the problem of extra trip circuitry with one-way guideways and find that, with a reasonable layout, the extra travel time going nonstop from origin to destination is so small that the cost per passenger-mile is generally less with a one-way system.
- Will magnetic levitation help PRT? Not at urban speeds. Comparisons of systems levitated by magnetic fields, air cushions, and wheels shows that, by using low-rolling-resistance tires, there is no advantage of either magnetic or air suspension over wheels at urban speeds, and indeed serious disadvantages.
- Can PRT be complimentary to other transit systems? Absolutely! This is a major advantage of PRT.
- Where are vehicles stored when not in use? In an n-berth station, n vehicles can be stored when there are no demands for service. During the night when demand is low or zero, the bulk of the vehicles will be stored at special storage barns strategically located in the network, usually at the same locations as cleaning and routine maintenance facilities. Because it is not necessary to get a specific vehicle out of storage before the others, the volume of storage facilities per mile of guideway is usually not more than would be required to store about four or five automobiles in a multistory parking structure.
- What will be the cruising speed? The first Taxi 2000 systems will be designed to operate at line speeds between 20 and 40 mph, and speeds up to at least 80 mph are practical for later applications.
- How is ride comfort assured? Taxi 2000 differs from other automated guideway transit systems in that the running surfaces are adjustable with respect to the basic guideway frame, which is built to normal structural tolerances. Before service is started, ride comfort is tested, adjustments are made and the running surfaces are firmly bolted in place. (In other systems, it is virtually impossible to correct any misalignment once the guideway is installed.) The vehicles run on smooth synthetic rubber tires of stiffness needed to meet ride-comfort criteria. Since the running surfaces are smooth and adjustable, secondary suspension is not needed.
- How is snow, ice or debris kept from interfering with operations? The Taxi 2000 guideway is a truss structure with covers over the sides and part of the top and bottom. There is a six-inch-wide slot at the top for the vehicle’s vertical chassis to pass through, and an eight-inch-wide slot at the bottom to permit ice, snow, rain, or debris to fall through. A pair of 7.5-inch-wide running surfaces (angle sections) inside the guideway near the bottom support the main wheels and are spaced six inches apart to permit anything that may drop in the top to pass through.
Running vehicles continuously during snow or ice storms will usually be sufficient to clear the running surfaces; but we have designed and tested a plow that, if necessary, will be installed on vehicles to deflect anything that lands on one of the running surfaces down into the slot between. A maintenance vehicle will occasionally inspect the interior of the guideway with a television camera and will be equipped to remove any foreign material.
- What about energy use? Because of frequent stopping and starting, about two thirds of the operating energy used by today’s transit vehicles or automobiles in an urban area is kinetic energy lost in heat as the vehicle is braked to a stop. Therefore, elimination of the intermediate stops by itself almost triples energy efficiency.
Careful attention to vehicle-weight minimization, streamlining, lowering of road resistance by careful selection of tire parameters, and use of electric propulsion that eliminates idling energy add to efficiency, putting PRT in a class by itself in terms of energy efficiency. The electrical energy use will be less than 200 watt-hours per vehicle-mile. The power will peak at about 20 kw per vehicle and will average about 4 kW per vehicle.
- What about air pollution? Taxi 2000vehicles run on 600 volt DC electricity that can be supplied from wayside batteries or flywheels that can be charged by any electrical energy source including renewable energy such as wind, solar, or biomass. The system produces air pollution only in the processes of manufacture and at the power plant, both of which can be closely controlled.
- How quiet is the operation of a PRT vehicle? Movement of the vehicles will be much quieter than automobiles. They are propelled and braked through linear electric motors, which are driven by variable-frequency drives. Such drives may produce a humming sound, which is minimized by careful design and by sound insulation. Since there is no braking or traction through the wheels, the tires are smooth and they run on smooth surfaces, so the tire noise will be substantially less than produced by an automobile. There are no other noise-producing elements.
- Will the use of electric and magnetic components adversely affect the health of riders? Because the vehicles weigh a small fraction of conventional rail transit vehicles, the electric current required is correspondingly less and any magnetic field in the cabin will be proportional to the current. While there are at present no generally accepted safety standards that limit human exposure to magnetic fields, care has been exercised in the design of the vehicles to minimize the exposure of passengers to magnetic fields.
The motors are designed to constrain the magnetic fields to their immediate vicinity and are located remotely from the passenger compartment, which also desirably lowers the center of gravity of the vehicle. Residents living or working near a PRT guideway will not be exposed to any significant increase in magnetic fields since the power to the vehicles is provided from 600-volt DC power rails inside a shielded guideway. Harmful effects of transmission lines have been reported only when the lines carry several hundred thousand volts.
- Will PRT guideways withstand earthquakes and high winds? The elevated guideway is designed to the local code for maximum accelerations during earthquakes and the maximum expected wind load. The guideway is a small, light-weight, flexible, steel structure with thermal expansion joints in every span, a configuration well suited to surviving earthquakes and high winds. The possibility of aeroelastic coupling such as caused the collapse of the Tacoma Narrows bridge has been studied, and it was found that features that prevent such catastrophes are exactly those selected for the Taxi 2000 guideway for other reasons.
- Can a bicycle be taken aboard a Taxi 2000 PRT vehicle?There are two ways to load a bicycle on a PRT vehicle: either on a rack outside, or inside. With our 50-inch interior width and room for a wheelchair, most bikes with one person would fit inside, and that would be the fastest and safest way to load. We have definitely included the need to accommodate a bicycle in our specifications for the PRT vehicle cabin. We have always seen compatibility between bikers and PRT, both because a fraction of the population would like to bike to and from PRT stations, and because bikeways and walkways can be placed on the ground under a PRT line.
- How can we be so sure of the characteristics of Taxi 2000 in advance of testing? Because the design has been reviewed so thoroughly and because it was found that achievement of the characteristics described is well within the state-of-art.
- Why has it taken so long to get true PRT into operation? The PRT concept germinated in the early 1950s and received enough attention by the mid 1960s to be the subject of government-funded analysis. By the early 1970s, there were many competing ideas on how to design automated transit systems, but there was no theory of PRT and there were insufficient funds to explore the dozens of alternative design features. This “Tower of Babel” discouraged decision makers, caused government funding to dry up, and left the continued search for an optimum configuration up to a few people.
A major reason it was possible in the 1980s to carry PRT research and development far enough to regain the attention of major transit decision makers was the emergence of the personal computer and associated software. Finding the optimum transit configuration and proving it required sophisticated and data-intensive engineering and economic calculations, detailed simulations of control and vehicle dynamics, and a great deal of data processing, which during the 1970s was much slower and required large resources, generally funded only by governments. The PC enabled engineers of ordinary means to purchase enough computer power to develop the optimum system and element designs. In parallel, the development of powerful fault-tolerant microprocessors and software elements have placed the control requirements of PRT well within the current state-of-art.
While many new ideas have emerged from institutional research during this century, new ideas in previous centuries generally emerged only when the individuals who discovered and developed them could do so without anyone else’s approval. Development of PRT required understanding of engineering sciences and sophisticated technology of the 20th century melded with the individual initiative of earlier centuries, a marriage made possible by the low-cost, high-performance personal computer.
The PC and the microcomputer, coupled with the development of the necessary transit systems theory, test and operational experience with a wide variety of automated transit systems, the realization that conventional rail transit systems cannot solve the problems of congestion in cities, and the steady worsening of congestion and air pollution have made it possible for the idea of PRT to reemerge.
Careful research over decades has shown no flaw that will or should stop the development of PRT, but rather that PRT is a badly needed solution to a variety of transit problems. It is a new configuration of now very ordinary parts well within the current state-of-art.
Development of new concepts in public transportation differs from development of many other emerging concepts in that the resources needed to prove a concept are large, many people are involved in deciding to take a positive step, the level of credibility must be unusually high, and the “fear factor” that drove military programs is not present. In such circumstances, it is not surprising that several decades have been required to bring the concept of PRT to maturity.
A relevant quote from Machiavelli’s masterpiece “The Prince”:
“It must be realized that there is nothing more difficult to plan, more uncertain of success, or more dangerous to manage than the establishment of a new order of government [or a new system (J. Edward Anderson)]; for he who introduces it makes enemies of all those who derived advantage from the old order and finds but lukewarm defenders among those who stand to gain from the new one. Such a lukewarm attitude grows partly out of fear of the adversaries, who have the law on their side, and partly from the incredulity of men in general, who actually have no faith in new things until they have been proved by experience. Hence it happens that whenever those in the enemy camp have a chance to attack, they do so with partisan fervor, while the others defend themselves rather passively, so that both they and the prince are endangered.”
- When? This has been the one real concern about PRT. There is active interest in more and more cities and countries. The possibilities are exciting. Many people have given of their free time because they have seen in PRT a means for a profound improvement in the functioning of urban areas. Those willing to listen, study and compare are seeing that in greater and greater numbers. A critical mass of interest is developing.