165> Tips for Designing PRT

Recently I have been corresponding with a new PRT developer, and I was struck by how similar his logic is to the many who have come before.  He initiated his communications by saying that the first step to a successful solution should be a fresh look at the problem.  He then stated his list of design criteria, tying back each decision to the problems that it would solve.  Before very long, he had narrowed his design to something very similar to the SMART platform, yet quite different in many important ways, and I found myself defending the specifics of my approach.  I thought I would share.

Design differences don’t just come from differing views on the practicality of one mechanical solution over another, but also from the original assessment of the problem itself.  PRT is the would-be solution to a host of different grievances.  To some, it is primarily an energy/climate saver.  To others the main emphasis is downtown mobility, while some would try to solve the problem of gridlock on our highways.  The various PRT solutions that are out there, it seems to me, are all pretty well crafted for their intended purposes.  What worries me is they stick to their respective missions a little too tightly. 

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.

Above is a “heat map” of 5 layouts, representing increasing numbers of loops and stations.  The warmer colors represent greater traffic.  These are not representations of city blocks nor would it ever be possible to have such evenly spaced stations.  Passengers are only where you find them, and then only some of the time.  What is shown, though, is how stations are interdependent for passenger traffic.  Each station is both source of new passengers and a destination for others who would not otherwise ride. In a full-sized system, (not shown) the vast majority would be hot yellows and oranges, and only the outer perimeter would be the unprofitable purple or blue.  PRT clearly can’t start too small.

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.
 

When it comes to priorities for designing the perfect PRT system, this business of needing a whole network from the start is a whopper.  It is the elephant in the room.  Yet with proper design I believe a formula for growth can be found.  There’s more.  Next time you are somewhere that you think would benefit from a PRT system, try identifying the next closest station sites, and estimate how many passengers each such station would attract.  If your experience is anything like mine, you will soon see that good sites are not at all easy to come by, and those with lots of foot traffic are usually on private land or are squeezed by roads that have been widened as far as limited easements will allow. At best they are in patches, separated by areas that don’t need PRT at all.  I pity the poor city planner tasked with finding station sites every half-mile that have enough pedestrian traffic to warrant an elevator equipped station.

The mobility problems faced by the urban and suburban populations differ greatly, and often PRT designers are biased by their own experience.  There are differences due to geography (such as climate) and even city-to-city topographical differences, such as water frontage or hills.  Political differences are important because they effect funding and attitudes toward public works in general. There are even historical differences.  One that I am often reminded of is (then) President Eisenhower’s “cold war” strategy of initiating of the US interstate highway system, resulting in urban neighborhood segmentation and the suburbanization of an entire nation.  Now, 1 out of 4 miles driven in the US is on an interstate, often at maddeningly slow speeds! The point is that these are among the factors that individualize a city’s needs, even as the economics of starting a PRT company push toward a more “cookie cutter” approach, and therein lies the rub.

This is where “SMART” comes in.  “Suspended Multi-axis Automated Rail Transport” is not a PRT system per se, but a development platform.  Any system that can move vehicles in 3D can also work in 2D.  Any system that can travel at 90 mph can also travel at 30.  If it can turn in a 6’ radius, it can certainly manage any street corner.  The idea to avoid “baking in” needless limitations.  The SMART vehicles are very capable and full featured because someday they might be expected to be that way. After all, people will eventually want, from PRT, the same qualities they would expect from a manually driven taxi service, and investors, equally, will prefer a fast, efficient and comfortable fleet to maximize returns.  In the meantime, there are tough choices in store.  SMART, with all of its “bells and whistles” is, admittedly, an extremely (if not prohibitively) expensive way to start.  Yet its basic architecture, especially the track, addresses the above problems squarely by being uniquely versatile.   

With PRT requiring a whole network to operate profitably, and with the vast differences between different cityscapes, the problems that predicate a PRT designer’s job are immense and varied.  Some examples include how to put dirt-cheap stations everywhere and anywhere; how to take the shortest route between lucrative sites instead of being detoured by worried landowners; how to piggy-back other services, like carrying utilities or street lighting; How to give local planners more and better options; How to get property owners to allow stations on their parking lots and in their lease spaces; How to attract the most riders without cutting fares, how to encourage night traffic and freight, how to minimize operating costs, and so forth.  

Why PRT has not entered the mainstream is not simply a matter of suspended vs. supported design, linear vs. rotary motors, or third rail vs. battery power.  It is the big structural issues regarding network size, versatility, and the business model, and translating these overarching issues that into physical form should be “job one” for the PRT designer.