But in the past few years, the idea has started to come back into vogue. At London’s Heathrow airport, a PRT system - with 18 vehicles, 2.4 miles of track, and three stations - is slated to begin running next spring. Masdar, a carbon-neutral city being planned in Abu Dhabi, will include pod cars (albeit, atypically, underground). There is a demonstration track in Uppsala, Sweden, and the Swedish government just announced its intent to build a system in a major city, although the plans are vague at this point. Last month, a Korean steel company indicated that it will assemble a project in Suncheon, South Korea.

“I just don’t believe that it’s a feasible system,” says Vukan Vuchic, an engineering professor at the University of Pennsylvania and a prominent critic of PRT. “The concept of PRT has been discussed now for nearly 40 years. It was promoted very strongly, but it petered out because it did not succeed anywhere.”

Some critics object to the aesthetics of space-agey vehicles in the sky, while others doubt claims that they’d be cheaper and more convenient than more traditional transit such as trains and buses. Many cities are understandably hesitant to sink significant capital into a mode that has not yet proved itself.

The concept of PRT was first developed over 50 years ago. Yet intermittent test projects (notably one near Chicago in the 1990s) fizzled, providing grist to skeptics who deride the idea as unrealistic.

“You can text to your delight while traveling,” says Alain Kornhauser, a Princeton professor and director of Princeton’s Interdepartmental Transportation Research Program. “The idea is to provide a level of mobility that is somewhat comparable to the great mobility that the automobile offers, and to do it in an environmentally friendly way.”

But there’s one system that, according to its proponents, combines the pluses of both options, while largely jettisoning the minuses. Called personal rapid transit, or PRT, it consists of small, light, electric vehicles, known as “pod cars,” which hold just a few passengers and run along a network of elevated tracks. The pod cars are driverless and automated: Passengers select their destination and the vehicle goes directly there, bypassing all other stations. Advocates say these systems could help ease a multitude of problems: global warming, dependence on foreign oil, congestion, and diminishing available land. They would also free commuters to safely engage in the activities they often do anyway while behind the wheel.

When we decide how to get around - how to commute to work, how to schlep to the movie theater across town - most of us (save the intrepid bicyclists) face an imperfect choice. If we drive, we run the risk of accidents and traffic jams, not to mention the environmental and geopolitical downsides of using gas. Taking mass transit, however, often means waiting around, rubbing up against strangers, and walking on either end of the trip - in short, sacrificing the independence, privacy, and convenience Americans prize in their cars.

As far as PRT enthusiasts were concerned, those experiments were flawed, and did nothing to discredit the core concept. In recent years, the designs have been continually refined and computer advances have facilitated automation. At the same time, the problems pod cars are meant to address - congestion, pollution, global warming, land use issues - have only become more pressing.

Over the years, interest has periodically flared anew. In the early 1990s, Raytheon , the Massachusetts-based corporation, embarked on a project outside of Chicago, funded by the Chicago Regional Transportation Authority. But the project was ultimately canceled due to concerns about the cost. In Marlborough, Mass., Raytheon worked on a test track from 1996 to 1999, but this, too, was eventually abandoned.

The idea of PRT first attracted attention in the late 1960s, and the federal government funded several studies. Under the Nixon administration, federal funds were allocated to build a system at West Virginia University in Morgantown. Today, the system is still serving the students on campus, with an excellent safety record. The vehicles, however, carry up to 20 passengers, meaning they are larger than a classic pod car and the tracks are therefore heavier. Some experts say this system is more accurately characterized as “group rapid transit,” and as such, it failed to showcase the true potential of the idea.

But proponents - some of whom verge on evangelical - blame inertia and vested political interests for thwarting plans in the past. Previous projects were poorly executed, they charge, and the technology has matured since then. And they view the idea’s longevity differently: As they see it, its ability to persist despite setbacks shows how compelling it really is. With the current flurry of projects, a verdict may at last arrive - we may soon find out whether pod cars are destined to become a retro-futuristic curiosity or indeed the transportation of tomorrow.

As the debate intensifies, opinions in transit circles are strong and polarized. Partisans on both sides tend to share basic values, convinced of the need to renounce car-centrism. But opponents think PRT is an unworkable distraction from the goal of implementing the reliable mass transit options we already have, such as light rail. In their view, the fact that the idea has existed for so long without successful realization is the best evidence that it’s not viable.

Interest in the United States is also on the rise. A 2007 report for the New Jersey Department of Transit concluded that “PRT has the potential to help the State address certain transportation needs in a cost-effective, environmentally-responsible, traveler-responsive manner.” San Jose recently issued a “request for proposals” with the aim of building a PRT system in the vicinity of the airport. The city council of Mountain View, Calif., where Google’s headquarters are located, is also considering the idea, as are officials in Santa Cruz, Calif., and Ithaca, N.Y. And in the Boston area, a small group of transit advocates is promoting the construction of a PRT system linking local universities.

PRT offers environmental advantages, because it runs on electricity (which could come from renewable sources) and because it maintains a steady speed throughout the nonstop journey - more energy efficient than a stop-start pattern. Because the vehicles are lightweight, proponents say the tracks, called “guideways,” would be cheaper to build than other mass transit systems. The lack of drivers also reduces costs. What’s more, service caters precisely to need - as opposed to trains and buses that are often half-empty or worse. This aspect not only saves money and energy, but also considerably enhances convenience.

Proponents say the trips should be scenic and enjoyable, as passengers glide across the sky at about 30 miles per hour - but, they promise, it would not be scary like a roller coaster. There are a variety of pod car designs, but they tend to look a little like small, sleek smart cars without noses. The ones at Heathrow are white with two dark front windows, which bear a certain resemblance to insect eyes.

The system also entails a network in which vehicles can switch from one track to another in order to smoothly navigate to the rider’s destination. Whereas in a subway system, passengers must transfer from one line to another, this system is more like a grid, in which the vehicles can cruise from any one station to any other. When a passenger arrives at the station, a pod car should either already be waiting or arrive within a few minutes after being summoned - sort of like hailing a taxi in Manhattan, but the subsequent trip would be far smoother and quieter than a New York cab ride. (Of course, keep in mind that such a system has yet to be successfully tried on a large scale.)

A hypothetical PRT system in a metropolitan area would comprise many small stations, which could be integrated into buildings or stand alone on posts. The density of stops would make it highly user friendly; ideally, in the area of service, a station would never be more than five minutes away on foot. The stations would also be “offline,” that is, adjacent to the track. (By way of contrast, a train simply stops on the track, blocking any traffic behind it.) Anderson has called offline stations the “key breakthrough” because they allow a continuous flow of vehicles. In theory, this means that pod cars could be safely spaced only seconds apart to meet peak demand.

“Automobiles are clogging cities. They take a huge amount of the urban land,” says J. Edward Anderson, founder of the company PRT International and one of the preeminent PRT engineers. This new alternative would attract many drivers, he says: “With nonstop service, it’s competitive with the automobile and in many cities it’s faster.”

In recent years, technological advances have made it more feasible to operate such an apparatus safely. Myriad elements - propulsion systems, command-and-control software, position and speed sensors, communication between the vehicles, and centralized computers - have all become increasingly sophisticated.

With small vehicles and comparatively light guideways, PRT is agile and can integrate creatively into urban areas, according to supporters. For the same reasons, they say the erection of infrastructure would be less disruptive than for, say, a subway system or light rail. In response to aesthetic concerns - would it be too weird and Jetsons-esque to have these cars whizzing by overhead? - designers stress the importance of making the vehicles as attractive and unobtrusive as possible.

“It just has a lot more flexibility, it can fit into the city a lot more easily,” says Jerry Schneider, a professor emeritus at the University of Washington. “It can climb hills and go around corners.” As for aesthetics, “When you look at the visual scenery, it’s hard to imagine it would be any worse. We tend to get used to these things.”

While some true believers hope PRT will eventually become a dominant mode of transit, others see it more as a gap-filler. It could serve places like airports, university campuses, and medical centers. As a “distributor,” it could branch out into less dense areas to bring riders to other mass transit hubs. And it could provide a valuable service in “edge cities,” to ferry people from residential areas to shopping areas or office parks - routes that are now taken almost exclusively in automobiles.

“We’re typically looking for niche applications,” says Steve Raney of Advanced Transit Systems, the company that is building the system at Heathrow. Likening PRT to “a shuttle bus on steroids,” he says that although it won’t completely obviate the need for cars, “what previously was a two-car family now becomes a one-car family.”

The Boston area, with its notorious drivers and the congestion endemic to any metropolitan area, could clearly benefit from a system that would lure people out of their cars. Most analysts agree that PRT wouldn’t fly in downtown Boston; it is already well served by the T, and the prospect of elevated tracks in the historic districts would encounter resistance. But in the suburbs, or even in parts of the city such as the Seaport area, a system might make more sense.

Former Massachusetts secretary of transportation Fred Salvucci, who now teaches at MIT, says, “We have a lot of low-density suburbs that are very difficult to serve with traditional rapid transit.” Although he believes the political will and funding will be all but impossible to muster in the near term, he says, “The automobile is totally unsustainable. Personal rapid transit ought to be on the table.”

A small, informal group of transit advocates, mostly former employees of the Boston Redevelopment Authority, is trying to generate interest in a project to connect local universities. A system linking Harvard, MIT, BU, Northeastern, and the Longwood Medical Area, these advocates argue, would increase mobility for students and faculty, and foster collaboration between the different institutions. It could also add an ultramodern look to the area, with pod cars zipping across the Charles River and students hopping in the vehicles to rush to class across campus. The group is planning a weekend-long workshop in January, for students, faculty, and interested citizens to brainstorm specific designs.

It would go “from Harvard Square to MIT in a few minutes,” says Charles Harris, a member of the group and a professor emeritus at Harvard’s Graduate School of Design. “PRT is an ideal system for tying together those institutions.” But the broader aim is to demonstrate on a small scale that the system can work, and then expand to other areas.

Caveats about PRT remain: A big project could hit glitches, and no one can be sure what levels of ridership it would actually attract. To many, the concept will probably continue to seem far-fetched. But, as Alain Kornhauser points out, sometimes transportation trends go in unpredictable directions. If you had said a hundred years ago that the automobile would come to dominate, he notes, “They would have probably thought you were a little loony.”

Rebecca Tuhus-Dubrow is a contributing writer for Ideas. She can be reached at rebecca.tuhusdubrow@gmail.com.

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