this weekSomething in the Air

December 18, 2004

Urban Monorail Systems

The rise of Personal Rapid Transit

I’ve never regretted the decision I made a few years ago to live without a car. After all, if I walk down the hill a few blocks from my home, I can catch a subway, streetcar, or bus to take me nearly anywhere in San Francisco I may want to go. But every now and then, that “nearly” part causes me grief. There are certain spots in the city I can reach via public transit only by taking a subway, a streetcar, and two buses—and then walking for 20 minutes. The prospect of all that waiting and transferring, especially on weekends or when buses are running late, tempts me to take a taxi (which gets quite expensive) or rent a car (forcing me to worry about parking and traffic). Even in a compact city such as this one, getting from place to place quickly, inexpensively, and safely can be difficult. Owning a car can help in some ways, but for many of us, it would be more trouble and expense than it’s worth.

It’s a Bird, It’s a Train, It’s a…Taxi?
Several articles here on Interesting Thing of the Day have mentioned ways of addressing the urban transportation problem: car sharing programs, carfree cities (including Arcosanti), and personal flying machines, for example. A while back, a reader suggested I check out an innovative urban transportation system called SkyTran. Later, another reader wrote to tell me about a different urban mass-transit solution called the RUF (Rapid Urban Flexible) system. Although the two differ significantly, they are both monorail transit systems designed for cities. As I began reading about these, I discovered that they are just two among many similar proposed designs. Clearly, this was a meme worth investigating.

The beauty of elevated monorail-based systems is the relative ease with which they can be retrofitted into an existing urban environment. Unlike subways, they require an absolute minimum of disruptive street closures (and no digging). Unlike streetcars, monorails don’t have to compete with cars and pedestrians for space on the roads. And unlike conventional elevated light-rail train tracks, monorails can be constructed quickly and inexpensively. Seattle already has a (very short) monorail line, as do some other cities. But some proposals currently being advanced call for much more elaborate and pervasive systems—with some interesting innovations that could make them much more efficient than buses or trains. These systems are known generically as Personal Rapid Transit (PRT). Unlike conventional mass transit, PRT replaces large vehicles with small cars that hold only two to six people—and therefore use very small and inexpensive tracks as well.

Packet-Switching Meets Mass Transit
The SkyTran is one such PRT system. Its designer proposes to install a network of tracks that can take riders within a few blocks of any location in a city, using a flexible point-to-point scheme rather than a fixed route. The cars can travel much more rapidly than a train or bus, and a sophisticated computer system prevents collisions and congestion. In theory, there would always be at least one car available at each stop; after you board, the car zips from the station’s bypass track onto the main track, where it picks up speed and takes you directly, without stopping, to your destination. This seems to combine all the advantages of a taxi with the advantages of a subway—and then some. Similar designs include the SkyWay Express, the ATN (Automated Transportation Network), and the ULTra (Urban Light Transport) system, which is being developed in the U.K.

If you want to avoid any walking—or be able to travel outside the immediate area served by the transit system—you might prefer another variant of PRT. The RUF (Rapid Urban Flexible) system being developed in Denmark is an example of a hybrid that uses specially modified electric cars that can operate automatically when riding on the tracks or manually on the road. Use your car around town as usual, but when you want to travel farther or faster, drive into a station where your car is guided onto a special monorail track. From there, allow the computer to drive you to your destination stop, where you drive off the track and resume manual control. RUF requires much less track than SkyTran and provides passengers with greater independence; on the other hand, the cars themselves are much more complex and expensive.

Proponents of PRT systems invariably point out that the cost of installing a citywide monorail system of this sort would be comparable to the cost of installing a traditional light-rail line; the additional efficiency should make it so cost-effective that it’s a no-brainer. Municipal governments are understandably hesitant to sink tens or hundreds of millions of dollars into unproven technology—but this is a chicken-and-egg problem; until one of these designs is actually put into large-scale use somewhere, there’s no telling how well it will live up to its promises. There’s another source of hesitation too: however flawlessly such a system may work, the question remains whether the teeming masses will like it and trust it enough to give up their cars. But as more urban dwellers go carless anyway (out of choice or necessity), PRT systems look increasingly appealing. —Joe Kissell

More Information about Urban Monorail Systems…

For an overview of PRT systems, read Urban Transport: Present Problems and A Possible Solution by Coridon Henshaw.

To read more about particular PRT implentations, see:

The Seattle Monorail was the first commercial urban monorail in the United States.