An artist's rendering of the Vectus PRT system.

Photo courtesy Vectus

Personal Rapid Transit (PRT)

The transportation infrastructure of major cities would all but collapse without fleets of taxis. For example, in New York City in 2003 only about 25 percent of residents had a license to drive [source: Washington Post]. Public transit, like buses and subways, is also extremely important for getting people from point A to point B in urban areas. In Hong Kong, 80 percent of residents in 2005 used public transportation for their commutes [source: Taipei Times].

Both taxis and public transit have their drawbacks, however. Taxis are cars, after all, and belch their fair share of carbon dioxide into the air. And subways have prescribed stops, in contrast to taxis that'll take you anywhere you want to go. It's enough to make you wonder if there's any relief to the dilemmas presented by these two horrors of modern urban life.

One proposed solution is personal rapid transit (PRT). The idea for PRT goes as far back as 1953, when an American urban planner began hammering out the logistics of combining the privacy of taxicabs with the subway's ability to move massive amounts of people [source: The Guardian]. While the details of PRT systems have evolved alongside technology (such as the idea of using solar power to generate electricity for the systems), the basic premise has remained the same. 

Essentially, PRTs are systems of established rail lines on which small, electric-powered vehicles (often referred to as podcars) ride. They work like this: You go to the nearest station, press a button that calls the closest available car and wait. In short order -- a matter of only 12 seconds in the case of Advanced Transport Systems' ULTra pods -- a car will arrive [source: The Guardian]. In other conceptions of PRT, enough cars will be available at any time so you won't have to even wait at the station. Under either design, you get into the car, tell the computer where you want to go, and you're off.

PRTs are unique in that they're exactly what the name suggests -- personal. The car that arrives when you call it is yours (not yours and some stranger's). So PRT will be just what the doctor ordered for clinically xenophobic commuters. "Personal" also means that the cars don't stop until its passengers are delivered to their specified destination. They're also driverless, electric vehicles, and since they don't require humans to operate, PRT systems will run 24 hours a day [source: CPRT].

There are several competing visions for PRT. Some designs include elevated monorail lines with cars that run on rails or dangle from an overhead rail. Other engineers envision PRT systems as a network of narrow, ground-level highways adjacent to streets that carry cars that move on wheels. Some proposed systems are electric, juiced up from solar power. This is perhaps the optimal design, since 50 percent of all electricity in the United States is currently produced by burning coal [source: PBS].

If all of this sounds a bit far-fetched, don't worry: You're not alone. PRT has its detractors. But the concept of PRT also has plenty of proponents, and they're gaining traction. Around the globe, urban areas are testing PRT systems. Find out on the next page about some of the cons of PRT and how the kinks are being worked out in places like Abu Dhabi and Sweden.