Current PRT offerings are not versatile enough to harvest sufficient passengers from the varied, real-world cityscapes to be economically viable until they have reached a critical mass of coverage. The cost to achieve this break-even coverage is simply larger than what cities can afford or, at least, are willing to risk their careers over. Here is a brief synopsis of the problem:
For PRT (or any other public transportation system) to be a serious contender, it must pass two tests. First, it must not lose money. Second, it must perform its function better than alternative choices. When PRT can clearly meet these two criteria, it will be adopted, pure and simple. Starting with the first test, let’s imagine the simplest, cheapest system, a simple loop between two stations. PRT costs money, and that only gets returned through fares. The separate costs of track, stations and vehicles must each be amortized though their respective lives and these numbers (along with operating costs) tell us how many passengers need to use those assets daily to make the system break even. Here’s a hint. It takes lots and lots of passengers to make the payments! With two stations, the only people who will use the system are those who need to go to one of those two destinations. While you can buy fewer vehicles, and you can make smaller stations, you can’t reduce the track costs to where only the occasional pod will pay the bills, so that component (the track) bleeds money from the stations and vehicles, leading to its logical elimination. Now we have another shuttle bus on the streets.
As for the second criteria, such a simple route lends itself better to a larger, scheduled vehicle anyway. Since everyone is going to the same place, why not ride together? Even a few stations between would not represent much added inconvenience.
Needing more closely associated network nodes to be effective is not unique to PRT. The same heat map and lack of profitability at the outer edges can be seen with a bus system or parcel delivery, for example. Yet networks are not necessarily all equal when it comes to the ability to grow “from seed.” The key is making each part of the network viable in its own right. No doubt the builders of the very first stretch of railroad already had a pretty good idea that it would be profitable on its own, and each subsequent branch surely met this test as well.