How’s this for blogging on the cheap? Below is a paper I wrote for English 101 a while back. I’ve reworked it slightly (removed the silly Chicago citations), but it retains the stilted prose and mechanical structure of any good My First Research Paper. In any case, I’m fond of its clarity. Perhaps in some future post, I’ll follow up with a lengthy discussion of the political and aesthetic obstacles keeping PRT’s from being taken seriously.
In 2002 in the United States, approximately forty-three percent of the oil we consume—making sixteen percent of the total energy we consume—is consumed by personal cars and light trucks. In the same year, cars and light trucks accounted for thirty-three percent of the carbon dioxide, fifty-one percent of the carbon monoxide, and thirty-four percent of the nitric oxides emitted in the United States. Were Personal Rapid Transit (PRT) systems built in America’s large- to medium-sized cities, America could greatly reduce its use of energy for personal transport and reduce its air pollution.
In a Personal Rapid Transit system, vehicles for two-to-four persons travel on a fixed track. The vehicles always travel one way down a particular length of track, only changing course at bifurcations in the track which resemble freeway off-ramps; two pieces of track might merge into one, like a freeway on-ramp. In most PRT designs, the track is elevated off the ground to make traffic interference a non-issue.
At many small stations along the track, passengers pay their fare and enter the first empty vehicle waiting in line; the station track is a detour off the main track to allow traffic to by-pass the station without stopping, just like freeway on-ramps and off-ramps allow traffic to flow uninterrupted. The vehicles are controlled by their own individual computers, using a small command vocabulary: ‘go’, ‘stop’, ‘take the left/right track’; each vehicle receives its macro-level directions from a central network, e.g. ‘go to this station’, but the set of micro-level actions necessary to carry out that directive are left to the individual vehicle. The central system is responsible for distributing unused vehicles to stations where they will most likely be needed based upon projected traffic patterns. PRT designers consider any kind of stoppage or slow-down in the system as a failure and not to be tolerated as part of normal operation; a properly implemented PRT system would never have traffic jams except in isolated cases. At worse, an influx of demand should only result in waits to board vehicles. (Here’s a nice graphic of the whole thing.)
How the vehicles are propelled varies greatly among different PRT designs. In the PRT design of prominent PRT proponent and designer, J. Edward Anderson, an engineer and former professor at the University of Minnesota, a linear induction motor in the vehicle is powered from the track. Some propose supplementing the track’s power supply by having the track double as a place to put solar panels.
The first reason a PRT system would save significant amounts of energy compared to cars is that the vehicles are significantly lighter. Small vehicles don’t need the acceleration of larger ones, and when powered from the track, they aren’t further weighed down by fuel. Similarly, the smaller volume and lower traveling speed of the vehicle reduces the comparative amount of drag.
Another reason a PRT trip consumes less energy than the same trip in an automobile is that PRT’s typically travel at lower cruising speeds, thus requiring less energy to maintain speed. For safety reasons, most PRT designs propose a speed between twenty-five and forty miles-per-hour; while safety at greater speeds is most certainly possible, greater speed is not as desirable as you might think because the lower travel speeds are made up by the vehicles never stopping on route between origin and destination.
That PRT vehicles only accelerate up to speed once per trip is the most important factor in why PRT’s consume less energy. In a typical car trip, the many accelerations from a dead stop account for a disproportionate amount of the energy consumed even though they only account for a small fraction of the time and distance traveled.
Because the vehicles are powered from the electrical grid rather than gasoline or diesel, there is zero local pollution. This does not solve the problems of air pollution—energy is only as clean as its total amount of emissions at the point where it is both generated and emitted, not just where it is emitted—but prospects for producing clean energy at electrical plants are much more promising than the prospects for our automobiles.
Unlike other solutions for reducing the pollution and energy consumption of personal transport, PRT systems don’t gamble upon uncertain future advances in technology. Electric-powered, hydrogen-fueled, and bio-fueled cars—the prospects for making these technologies economical (or, in some cases, work at all) are unknown. The problem is that science is not magic: some of these technologies may simply turn out to be dead-ends. Of course, the PRT idea hasn’t yet proven itself either, but the engineering required to make the system work consists wholly of combining well-understood technologies. For instance, probably the greatest area of uncertainty surrounding PRT’s is how such systems will handle heavy traffic loads. Though Anderson and other researchers have performed statistical analysis, this is the sort of thing where the details can only be worked out given a real-world system.
The closest analog to this kind of work is the writing of a computer operating system: an important component of an operating system is its “scheduler”, the code which allots processing time to the many programs running at one time on your computer; if the scheduling algorithm is not well-designed, human users will feel their system is unresponsive, and time-sensitive tasks (interactive games, audio, video, and so forth) will lag and stutter because their programs aren’t getting processor time often and long enough to do their jobs on time. The academic literature produced in the 1970’s and 1980’s on scheduling algorithms, while hardly worthless, turned out largely to be wrong (or at least inadequate) when multi-processing operating systems finally arrived to the masses in the 1990’s. So too will research on PRT traffic patterns be flawed until we have real-world systems to observe and tinker with. Still, that such kinks in the system can be worked out is a better bet than a “Manhattan Project” for energy, where engineers would be working on the edge of our understanding of how the universe works. Just as faster-than-light travel may simply not be possible, sufficiently efficient and green car motors may not be possible, so it would be foolish to stake our future entirely on the prospect of sufficient advances in automobile tech.
The failure of the United States to successfully adopt mass-transit in most of its cities is widely attributed to Americans’ dislike for riding mass transit. The usual objections to mass transit largely don’t apply to a PRT system, as it offers these advantages over other transit systems like buses and light-rail:
- Because PRT track is much smaller than light-rail track and PRT stations are much smaller than light-rail stations, the construction of PRT track and stations is much faster and less obtrusive and requires less man-power.
- PRT stations can economically be placed at short intervals along the track (one at every half-mile in Anderson’s design), meaning a station is much more likely to be within easy walking distance of passengers’ departure points and destinations.
- The system can operate around the clock, and passengers never need worry about train or bus schedules.
- More often than not, an empty vehicle will await passengers when they arrive at their departure station; even in peak hours, waits for vehicles should rarely be longer than a few minutes.
- No driver or other staff need be employed per vehicle, as with buses and trains.
- PRT’s occupy less land than light-rail systems, and consequently, they are less visually intrusive and free up space for other purposes.
- The vehicles travel non-stop between origin and destination, never picking up other passengers or requiring passengers to make a transfer.
- PRT vehicles produce far less noise than cars, let alone buses or trains.
- Passengers in a PRT have a private car to themselves, so they can relax, read, talk on their phones, use their laptops, or do their makeup while in transit without consideration of other passengers.
- Despite the lower cruising speed, transit by PRT would be significantly faster than by other mass transit systems because of its (usually) low waiting times, the non-stop travel, and the fact that origin and destination stations are likely closer to passengers’ points of travel.
Even with all these comparative advantages to current mass transit, PRT’s still must compete with Americans’ fondness for cars. Fortunately, transit by PRT is in many ways even more appealing than travel by personal car.
- Passengers in a PRT don’t have to drive, allowing transit time to be used for entertainment, chatting, or work.
- Passengers don’t have to find parking or pay for parking at their destinations.
- In a city with a PRT system that covers a wide enough area, more people can live without the cost and hassle of owning, licensing, insuring, and maintaining a car; at the very least, families could get by with fewer cars.
- On the whole, PRT’s would be much safer than other forms of transportation, as there are only three plausible safety concerns: malfunctioned vehicles breaking or stopping unexpectedly, vehicles not breaking when entering a station, and collisions at merge points. To mitigate these possibilities, the proposed cruising speed for most PRT’s is below forty miles-per-hour; also, sensors on each car engage the breaks when the car ahead is detected doing something abnormal, removing human delay and error. These factors greatly reduce the frequency and severity of accidents, especially compared to what we tolerate today on our roads.
- A PRT would increase the mobility of those unable to drive: the young, the elderly, and the handicapped (in Anderson’s design, the vehicles provide easy embarking and disembarking of wheelchairs).
- PRT’s offer quieter rides for their passengers and create significantly less street noise.
- Transit by PRT would be faster than car trips for routes with many traffic lights and stop signs along the way.
Aside from direct benefits for passengers, a successful PRT that reduces car traffic would thereby increase quality of life by reducing the amount of noise and air pollution and allow the diversion of resources from road maintenance. After a number of years, cities may even begin reducing the width of existing roads and the size of parking lots, reclaiming space for sidewalks and other purposes. Asphalt surfaces capture heat in hot weather, leading to heavier use of air conditioning; reducing the amount of paved surface in our cities would lead to a surprisingly significant reduction in energy consumption.
Other hidden costs of our roads include traffic cops, traffic courts, traffic accidents, traffic reports, traffic schools, traffic lights, traffic engineers, traffic signage. In particular, there are often over-looked societal and economic costs just associated with parking: parking structures and the extra parking lanes on the sides of streets not only must be built and maintained, they take up prime real-estate. Perhaps least obvious are the psychological tolls, such as the aggravation of finding parking or the danger of cars (even when we’re not in a car ourselves, most of us must be wary of being hit by a car just twenty steps from our doorstep).
Of course, operation of a PRT will introduce many such hidden costs of its own, not all now foreseen, but on the whole, they seem relatively modest. For instance, emergency crews and equipment will be needed to occasionally clear malfunctioned cars and rescue stranded passengers. The track and vehicles themselves, of course, will need upkeep. And so forth.
In the last thirty years, several attempts at constructing PRT systems failed, mostly due to lack of funding and political support and inadequacies of the day’s computer and electric-engine technology. As of yet, there is no PRT success story, and until there is, municipalities and governments won’t take the idea seriously. At the moment, two PRT’s are scheduled to start operation in 2008: the “ULTRa” system at Heathrow Airport in London and another system at the International Finance Center in Dubai. By proving itself in a localized capacity, hopefully the PRT idea will begin to see adoption as a city-wide transit system.