PRT tries to combine the best aspects of private automobiles and public transportation. It uses car-sized pods that each carry just a few people at a time. Passengers arrive at a station and, rather than wait for a bus or train headed their way, they hail a pod-car. The pod-car then carries them to their destination via an elevated monorail track. Theoretically the PRT pods do this without stopping at intermediate stations.
PRT advocates say it provides point-to-point transit, meaning it goes everywhere a car can go, since, theoretically, a city could build elevated lanes atop any street.
It’s a compelling theory, which is why it’s been kicking around the periphery of the transit planning industry since at least the early 1970s.
Unfortunately it doesn’t work very well. It turns out that combining the convenience of personal cars with the efficiency of mass transit results in something that’s neither convenient nor efficient.
PRT is less convenient than cars
The basic idea of PRT is to build an elevated lane for special pod-cars that can only drive on that lane. The pod-cars could run on wheels or tracks, but the key characteristic is a small vehicle on its own elevated lane.
Meanwhile, the great thing about personal automobiles is that you can drive one anywhere there’s a road. PRT and transit both compromise that convenience by only offering service along routes, or PRT tracks.
In theory, PRT compensates for that inherent inconvenience by offering a dedicated elevated lane that can whiz pod-cars by faster than shared road lanes. But dedicated, elevated lanes are inherently expensive to build. Much more expensive than a normal surface street.
That means that unless we are willing to build an expensive duplicate infrastructure of new elevated lanes over every street in Arlington, by definition PRT cannot go everywhere roads can go. In turn, that means we have to prioritize the most important streets and design routes that carry people along only the most important paths.
In other words, we have to build a transit network.
And since elevated tracks are expensive, we can only build PRT on the most important, high-ridership corridors. Which brings up another problem:
It’s all about transit capacity
What happens when the new pod-car lane fills up?
PRT advocates like to show pretty renderings of little pod-cars whizzing over highway traffic without a care in the world. Those pictures rely on an assumption of low ridership. If more people rode the pod-cars, there would have to be either more pod-cars (leading to pod-car traffic jams), or much bigger pod-cars (ie buses for the pod-car lane).Capital Bikeshare. Georgetown gondola elicited eye-rolls at first, but people seem to be warming to it as Georgetown’s unique set of issues come into focus. Even CaBi was a bit of a risk, but the program has worked because it fills a gap in demand for transportation around town for certain types of trips. But the issues on Columbia Pike are not the same issues that PRT or CaBi can solve. Just being an out of the box idea doesn’t automatically make something smart. Often, such ideas have failed to take hold specifically because they’re impractical. Moreover, proposals like this can muddle conversations that have already been plagued with confusion and misinformation that led to the demise of the streetcar program in the first place. That’s the case with PRT.