Advanced Transport Systems' ULTra podcar on its test track in Cardiff, Wales, in 2002.

ATS Ltd/BWP Media/Getty Images

PRT Test Sites

For about as long as the idea's been around, personal rapid transit (PRT) has had its critics and proponents. Naysayers call PRT proponents dreamers at best and unscrupulous hucksters at worst. And criticism of PRT has come from some surprising sources. One episode of the cartoon sitcom "The Simpsons" spoofed the Broadway musical "The Music Man" while simultaneously leveling what appears to be subtle condemnation against PRT. In the episode, a too-good-to-be-true monorail proponent comes to town and fleeces the people of Springfield out of tax dollars for a rail system they don't need and that doesn't work.

It's difficult not to draw parallels between the fiction Springfield of "The Simpsons" and some real urban areas. Cities like Minneapolis, Minn., and Seattle, Wash., have seriously entertained installing PRTs before ultimately abandoning their plans. It should be said, however, that none of these systems had the likes of Homer Simpson as conductor of the rail lines. After all, PRTs are run by computers. But opponents point out that these computers can crash. 

This is perhaps the biggest drawback of the PRT concept: the logistics of the worst-case scenario in unmanned PRT systems. One big problem that faces proposed systems on a large scale is capacity [source: TreeHugger]. If PRT replaces subways, buses and taxis in the future, it has to be able to accommodate commuters at rush hour at least as well as current methods of public transportation can.

And what happens when one car breaks down? Under the current vision of most PRT systems, cars will tailgate one another at constant speeds. To accommodate passengers, the rail systems must be packed with podcars. If one stops suddenly, cars behind it will follow suit, leaving possibly hundreds or thousands of angry commuters stranded. Anyone who's had the bad fortune of being stuck on a rollercoaster can commiserate with that picture.

PRT engineers and designers have taken a step back from attempting to implement their systems on a large scale (like in Minneapolis and Seattle) and instead have begun testing on small tracks and in confined applications. In Sweden in 2005, podcar company Vectus constructed a test track and is currently seeking permission with the Swedish Railway Authority to implement a PRT system in the city of Uppsala [source: Vectus]. A system designed by the British company Advanced Transportation Systems is in use in Terminal 5 in London's Heathrow Airport. The mile-long track transports Heathrow customers between the terminal and the parking lot in the company's ULTra podcars [source: BBC].

By far, the largest testing ground for a PRT system is found in the United Arab Emirates. Masdar City, a development under construction outside Abu Dhabi, is looking to become the world's first carbon neutral city. A big part of that goal will be the implementation of the development's zero-emission, solar-powered electric PRT system with 1,500 stops scattered throughout the city [source: NPR].

With the introduction of more capital and an increasing willingness among developers and urban planners to adopt PRT systems, the forecast for the concept has never looked sunnier. Ultimately, a future with PRT rests in the public perception of the reliability of the transportation systems.

For more information on transportation and other related topics, visit the next page.