Your own personal monorail?

Imagine a tiny red vehicle traveling 50 miles an hour on a guideway 3 feet wide that winds 20 feet above ground. Another vehicle hums quietly in front.

A half mile later, the car exits the guideway, eases into a station, and stops. There's no driver - everything is automated. The hatch opens, and the passenger walks two blocks to the office.

This is the vision of inventors of personal rapid transit, or PRT, systems. PRT arrives on demand and isn't tied to a schedule. Like taxis, the cars are private and the trips are nonstop.

Proponents say that PRT could change the face of mass transit in the world's cities within a few decades by combining the convenience of a taxi with the automated, traffic-free flow of a subway system. However, as with any new technology, there are concerns about the costs and logistics of building such a system as well as its visual impact on a skyline.

Two systems are being tested on guideways in Cardiff, Wales, and Fridley, Minn. Minneapolis city councilman Dean Zimmerman is proposing a system for his city. Lawmakers in Duluth, Minn., also are exploring whether to implement PRT, says Dennis Jensen, CEO of the Duluth Transit Authority (DTA).

"I see it as a solution to some of the transportation issues we have with connecting the parks on the lakeshore with downtown," says Mr. Jensen. "But the city council sees it also as an economic development opportunity. If we build the test track here, we have a good chance of getting the manufacturing jobs that will likely come with it."

Jensen argues that PRT would augment rather than replace transit options.

Passenger and safety tests of PRT systems in Britain and other European countries were completed in August, and transit officials there are considering whether to build them. The Welsh government and the European Union's City of Tomorrow Programme, an urban development group, have spent millions of euros evaluating systems for Cardiff as well as for Almelo, Netherlands; Huddinge, Sweden; and Ciampino, Italy.

Since the cars are small, their elevated guideways can be erected on modest-sized towers or mounted on the sides of buildings. Each car has a computer on board linked to a central-command network. Empty cars would be routed to the station with the fewest cars, say the designers, so there would be little or no wait. Passengers swipe prepaid cards to board.

"We think this year will be the tipping point," says J. Edward Anderson, CEO of PRT developer Taxi 2000, based in Fridley, Minn. He waits in his office for a group of Minnesota legislators to ride a 60-foot test guideway in his factory. "There are six bills on PRT in the legislature...."

Taxi 2000 and Advanced Transport Systems, based in Cardiff, are among a handful of developers working on the technology.

Support for the systems is not unanimous, however.

In the US and Europe there are questions surrounding where to build a large outdoor test track and how to come up with the money to fully implement PRT. And in Minnesota and other states, there has been debate about PRT's merits.

"I'm skeptical because we already have a potentially quite functional bus system that has been grossly underfunded for a long time," says Betsy Barnum, a Green Party activist. "If it were fully funded it would be a good transit system. I think it should be fully utilized before starting up another whole new system."

Ms. Barnum adds she is concerned about the potential visual impact of the overhead guideways.

"Visual impact is probably the most important problem we have to cross," Taxi 2000's Anderson says. "If it's going to be practical and safe it has to be out of the streets. Underground is too expensive so elevated is the only place you can put it. So we have to make the smallest, most attractive guideway we can."

The technology was initially proposed more than 30 years ago. Richard Nixon promoted PRT in the early 1970s during a time of increasing dissatisfaction with urban congestion.

His administration spearheaded a model project in Morgantown, W. Va., but what politicians ended up with was a sort of automated bus system, with huge cost overruns, that continues to serve Morgantown but tainted PRT for nearly 20 years.

Now because of growing challenges with urban transportation, the technology is back on the drawing board - and promising greater efficiency.

PRT's infrastructure makes it less expensive to build than other types of mass transit, proponents say.

"[It] can be installed for a cost between one-half and one-third of the cost of light rail," says Martin Lowson of Advanced Transport Systems, maker of the ULTra PRT system. Lowson estimates that it would cost one-tenth the expense of building a roadway to install PRT. His company has found people would be willing to pay $3 per fare.

In February, 71 percent of Welsh passengers who used public transportation said they'd pay more for PRT than buses, and 11 percent said they'd pay as much as they'd pay for a taxi.

Mr. Zimmerman, the Minneapolis councilman, says a test track in his city would cost $61 million; a 68-station, 31-mile system like he's proposing, $600 million. Construction is fast and operating costs are low because the system is automated, according to Skyweb.

[Clarification: However, engineers and transit experts have raised concerns about the costs of PRT. For example, a 2001 engineering study for the Cincinnati area projected costs six times what PRT proponents had estimated, in part because of the system's complexity and the need for greater physical support of the structure. Independent transit experts note other hurdles, including the high price of acquiring right of way.

But PRT advocates see a market for the technology.]

These systems appeal to people who already use, or would like to use, public transit, because they won't have to wait for vehicles and there won't be stops, says Tom Miler, president of the nonprofit group Citizens for PRT in Minneapolis.

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