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.)
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