The PRT articles I read said it's not suitable for very large cities like Chicago. The large numbers of people would overwhelm it.
I think PRT people undersell it, sometimes for dumb reasons. They still want the mass transit people to like PRT. The die-hard mass transit people can't stand PRT it seems. They love the idea of mass transit. The biggest critics of PRT are mass transit people; they are tedious but incredibly consistent. Hell, I hear Ken Avidor is even trolling Seattle discussions now (he's been a long time Minneapolis troll).
I was wondering who Ken Avidor is, but he seems to have answered it himself (below).
I'm not against PRT. It's just that we just finished a 10-year debate on light rail vs monorail vs PRT vs more buses, and the worst choice won. A light rail that has traffic crossings in two sections, and whose first phase is not where the bulk of population is. But I learned that in order to get anything done, you have to get everybody to agree, or at least enough of the movers and shakers. That's the problem with PRT. Not that it's intrinsically bad, just that it gets laughed off the table. Nobody wants to commit millions of dollars to be the guinea pig. We got the same complaints about monorail but were able to point to working systems in Japan, Malaysia, and Las Vegas. I would like to see PRT tried citywide somewhere, and Joe who suggested Phoenix may be onto something. Nobody has been able to find a better solution for post-WWII automobile cities, so PRT could be it. But it would take a very forward-looking city to consider it, with much risk.
The capacity argument was specifically directed at London and Los Angeles, saying that PRT could not handle hundreds of thousands of people going downtown all at once. I assumed Chicago was the same size and dynamic, but you know more about it than I do. PRT is like an elevator system or supermarket check-out line. It works excellent when under capacity, but deteriorates rapidly when it reaches capacity. So PRT complements a trunk system rather than replacing it, and would work in individual neighborhoods or districts or medium/small cities (including most of the US). PRT is good if there are five or ten people at a stop, but not for clearing out a baseball game.
I agree that traditional transit activists have had an effect on the way that PRT is sold. PRT proponents now tend to shy away from pushing PRT as a replacement for traditional transit.
The problem, it seems, is that traditional rail activists fought back hard, and dirty. A lot of them (Avidor in particular) have resorted to outright lies and scare tactics in their propaganda campaign. So I think that caused PRT proponents to back off the assertion that PRT could replace most rail systems.
I certainly don't think it's a technical limitation. From what I've seen, a mature PRT system could easily serve a large city like Chicago, assuming of course it was extensive enough to cover the whole city. Don't forget, when you look at cities like Chicago and New York, you are comparing PRT to existing systems that would probably cost trillions of dollars to construct in today's network. With that kind of money, a monstrous PRT network could be built, and well-designed PRT scales better than any other form of transit because of the network effect.
So I think the stated PRT limitations are motivated more by political concerns than technological ones.