Personal Rapid Transit tries for a comeback.

Personal Rapid Transit is probably best described as a hybrid between the private car and public transit, with some more familiar elements of the taxi and elevator thrown in. Picture, in short, a pod car. Engineers and researchers (even Google!) have been fantasizing for several decades now about the concept, which would personalize public transit in small vehicles – perhaps running on or hanging from an elevated track – that would transport you straight to your destination without any of the stops and delays of a bus route, or without the cost of a taxi ride.

In concept, an automated PRT vehicle would hold four people or fewer, mimicking the private, quiet ride of a car. Relative to all of our existing alternatives, there’d be very little emissions, no traffic congestion, no loud teenagers or offensive odors. It’s the kind of public transit – again, in theory – that holdouts in private single-occupancy cars on the highway might actually be willing to ride.

Researchers first began to advocate the concept in the 1960s (around the time of related monorail enthusiasm). But the expense always overwhelmed any serious efforts to build these things. West Virginia University in Morgantown has had a related system since the 1970s. O'Hare International Airport in Chicago floated one two decades later to no avail. "That was a big setback for PRT folks," says Wayne Cottrell, an associate faculty associate professor of engineering at National University in San Diego.

But the idea is still kicking around today with some renewed momentum, in part because PRT might be built more cheaply today than it could have been in the 1960s, and because it meets our more modern demand for energy efficiency. Futuristic pod cars might also be the solution that destigmatizes public transit for drivers who fear its unreliability. Advocates also, finally, have a few actual examples of its application: Heathrow Airport in the U.K. opened a PRT system in the summer of 2011, and the built-from-scratch supposedly net-zero community of Masdar City in the United Arab Emirates is planned around one as well.

A Heathrow pod is seen in this undated photograph. (Reuters)

"The concept was originally germinated as an alternative to the auto," Cottrell says. "Public transit was seen as being big and heavy and losing money, and unreliable. And we wanted something that would be more like the car, but that would be automated and safer."

On a PRT system, you’d hop into your own pod, with one arriving every minute or even less at a station on a designated track. You could share one as you would a taxi, with someone heading to the same destination. Or, you’d ride it alone like a car. You’d then direct the vehicle to a specific destination within the system, to which it would travel without making any stops in between (and without fighting normal congestion). It would be almost like stepping into an elevator and pressing a button to the sixth floor. “But even an elevator isn’t necessarily personalized,” Cottrell says. So picture an elevator that makes no stops to collect anyone else on your way.

Everything about this scheme mirrors the personalized experience of a private car, on common public infrastructure like a set of train tracks.

"That was the original idea," Cottrell says. "Advocates have stuck to that over the decades: This is a great idea, we just need to convince somebody with money to invest in this."

Relative to building a new Bus Rapid Transit system, or simply staging more buses on an existing road, PRT would clearly be more expensive. But compare it to alternatives like the subway, and it looks a little less like a stretch. PRT could also be more efficient than driving four riders around in a 50-person bus.

"In really large cities, PRT probably won’t be successful because the demand is going to be too high and PRT won’t be able to handle moving a lot of people the same way a metro or subway can," Cottrell says. "But in smaller to medium cities, or in neighborhoods, connecting to the El [in Chicago], that’s where a PRT is going to come in most useful."

Simulations of the idea in European cities suggest that a sizable share of drivers – sometimes 10 percent of more – might shift to the mode, if it existed. Perhaps this is what we need to do to take more cars off the road: build more public transit that tricks people into feeling like they're in cars.

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