A novel kind of transit system, in which cars are replaced by a network of automated electric vehicles, is about to get its first large-scale testing and deployment. Two of these Personal Rapid Transit (PRT) systems are being installed this year, one at Heathrow International Airport, near London, and one in the United Arab Emirates, where it will be the primary source of transportation in Masdar City, a development that will eventually accommodate 50,000 people and 1,500 businesses and is designed to emit no carbon dioxide.

PRT systems are supposed to combine the convenience and privacy of automobiles with the environmental benefits of mass transit. Automated electric vehicles, or pods, each designed to carry from four to six people, wait at stations throughout a city or development, like taxis waiting at taxi stands. A person or group gets in a pod and selects a destination and the vehicle drives there directly.

The concept isn't new--the basic idea goes back at least to the 1950s. But it hasn't caught on for a variety of reasons, including the cost of the initial systems and the difficulty of integrating them into existing cities. A number of small test systems have been installed, and one system that is similar to a PRT has been in operation in Morgantown, WV, since the 1970s. But the systems at Heathrow and in the UAE will be the first real-world demonstrations of a true PRT.

Although PRT systems vary, the basic design involves a network of stations connected by a track that loops past all of the stations in a system. Large networks can include many interconnected loops. When a vehicle leaves a station, it travels along an on-ramp until it merges with the main loop. When it reaches the destination station, it exits this central loop via an off-ramp. The ramps allow individual pods to stop at a station while others pods continue to travel at top speed along the main track. As a result, it can be faster than buses, which have to stop frequently. Simulations suggest that the systems could run with as little as half a second between each vehicle, but the initial systems, such as the one in Masdar City, will keep the vehicles three to four seconds apart--enough to stop a pod should the one in front of it suddenly break down. A central computer controls the traffic.

At both Heathrow and Masdar City, the vehicles will be battery-powered, driverless cars. The system at Heathrow--built by Advanced Transport Systems, based in Bristol, UK--uses cars powered by lead-acid batteries along a concrete track and guided by laser range finders, says Steve Raney, a consultant for the company. For Masdar City, a Dutch company called 2getthere has developed cars powered by more-advanced batteries made of lithium iron phosphate. The pods travel on pavement equipped with embedded magnets placed every five meters, which the vehicle uses, along with information about wheel angles and speed, to determine its location, says Robert Lohmann, the marketing manager at 2getthere. When a person selects a destination, a central computer designates a path for the vehicle, and an on-board computer makes sure the car sticks to the path. (The system is being used now to control vehicles that transport cargo in warehouses.)

The closest thing to a large-scale, real-world PRT system is the project in Morgantown, WV. However, the vehicles are bigger than those in a PRT system: each one can carry about 20 people. During peak hours they run on a schedule, like a conventional transit system. That system, which was expensive and suffered from many problems, especially at first, may have helped give PRT systems a bad name, says Jerry Schneider, a professor emeritus of urban planning and civil engineering at the University of Washington in Seattle and a long-time advocate of PRTs. "People would get on the vehicles and they wouldn't stop. It got ridiculed roundly in the press. At one point they talked about dynamiting it, tearing it down."

After the initial problems, however, he says that the system has run well--it still transports students at the University of West Virginia. What's more, he says that technology has improved since then--for example, small computers are now more powerful than the large mainframes used to control the Morgantown system. Several new PRT vehicles have been developed and tested on small tracks. But there haven't been adequate demonstrations to convince local governments to approve the new designs and convince investors to take the risk. "The simulations run fine," says Schneider, "But until you put people in the cars and run them out in the open air, you can't really be sure what's going to happen."

While Heathrow and Masdar could provide the demonstrations necessary to convince other cities to adopt PRT, they are special cases with controlled environments, says Luca Guala, a transportation planner at Systematica, a company planning the layout of the PRT system at Masdar. In both cases, cars are banned, so there's no competition. What's more, at Masdar, the organization of buildings within the city has been modified to accommodate the system. Indeed, the city will be constructed so that the main level is several meters above the ground, primarily to make room for the PRT. It will be more challenging to incorporate PRT systems into existing cities. However, he says that the projects at Heathrow and Masdar will help drive down costs, and that could make them feasible elsewhere.

Copyright Technology Review 2009.