In my last post I ended by wondering out loud about whether anyone significant was actually endorsing the vehicle-mounted linear motors. After I thought about it, I remembered a slide in the PRT International streaming lecture, and so I have just listened again to Ed Anderson’s explanation of the various design trade-offs from his perspective.
Let me be clear, first of all, about one thing. I would love to see any PRT, including the design embraced by PRT international, implemented by some city, somewhere. There’s a lot to skepticism to disprove and much to learn. I also have great respect for Dr. Anderson and his work, and sympathize with the position he is presumably in, having investors to answer to. He has to, for their benefit, defend the exact design choices that moved forward, even as technologies change over time. Some “sound-byte” logic and “glossing-over” is to be expected anytime the politics of business is in play. That being said, it drives me nuts, and so here I am again. The last time I went off on this poor guy was only a few months ago. Of course, with 30% new readers each week, and an average viewing of only about two minutes, most readers won’t know the difference, and so, if you are new to the site, you’ll find some pretty smart people have weighed in on this general topic in the past, if you are interested…
First though, I think it is worth noting that there is a fundamental difference in vision that leads to the design differences. There is no need for PRT International, or any other PRT vendor, to have a product that will work in every city. Indeed there is strong incentive to create the simplest design that will work for the most profitable situations. Even if the design is useless for 95% of a city’s transit needs, it is only the lucrative 5% that is of interest to the PRT vendor anyway. If that seems cynical, I would point out that I think those numbers are very high compared to light rail. My environmentalist friends, however, should brace themselves for disappointment at the limited scope of change that PRT will bring, at least short term.
I, on the other hand, believe in the network effect, and see any limitations, especially budgetary, as potentially lethal. The spread of the railroad, for example, was transformational in a way that is similar to the internet. Both would have been economically viable on some scale, even if the build-out costs were, say, ten times as much. But both designs proved versatile and cheap enough for a massive, world-changing implementation, wherein the existence of the network itself created whole new businesses. That’s what I like to think about. PRT 2. All that having been said, here’s what I don’t like about the PRT International approach.
1. First there is inability to go up and down steeply or sharply. This has spin-off disadvantages galore. It’s not just the cost of raising the stations, but the political cost of lowering the track so the stations don’t have to be so high. Ed Anderson points out that, for a given track height, a hanging system is more visually intrusive. When track has to be kept low to keep station costs down, that is a concern indeed. He can never really contemplate raising the track way up, which is probably the best way to address local opposition to a proposed track segment. Also, although it is never ruled out, it is pretty obvious that bringing one of his pods to ground level is not very likely to happen. In other words, access to the system from a bus stop or private parking lot is pretty well abandoned. Again these limitations are not particularly important in the urban configurations he is designing for.
2. Second is the linear motor. I just believe “wheel” or “hub” motors are better, and that if they had it to do over, they would use them instead. The Linear Induction Motors (LIMs) have major efficiency drops with anything but very close “near contact” with the track, which must be outfitted, for it’s entire length, with a “reactor” plate. How much does that drive up the cost of the track? Also keeping the LIM close creates limitations on track design, specifically on sharp turns up or down. (As in, say, dropping down to a station from a higher track level) This is, again, is not much of a limitation if you are only talking about getting around downtown, and your cost basis is already less than the competition.
3. Then there is the issue of needing to bank turns. This includes the track on either side of the turn itself. In a truss system this has got to be expensive, especially on tight turns constricted by landowners not willing to cede right-of-way. Again, in an urban environment, competing with massively expensive systems like light rail, this is not such a big issue. If it can’t go fast or make sharp turns, so what? Neither can the competition. It’s an issue if you want the fixed part of the system “dirt cheap” so it can be massively expanded, however. Or if you see your ultimate competition as the automobile, not light rail, or if your ultimate objective is green prosperity through mechanical efficiency rather than moving on to the next big contract.
4. Finally there is the issue of stuff getting into the track. If on the ground there are serious potential problems ranging from flooding to peoples feet getting stuck. Even raised there is the potential for freezing rain being blown in and accumulating, or the more exotic sandstorm. In the PRT International presentation, the position of the LIM facing downward indicates that the reactor plate sits directly under the track’s slot, where debris would enter. I will be the first to say, I am certain that this is not news to them, and that a remedy has been engineered. But it is one other advantage to a hanging system, with the slot on the bottom.
In conclusion, I believe that the PRT International/Taxi 2000 type design has limitations that would need to be overcome before it would find wide acceptance in anything other than inner city use. This is largely because what seems to be the simplest, most straightforward design has the unintended consequence of simplifying the cars at the expense of the track and stations. Therefore the design has less chance of achieving the “network-effect,” one of PRT’s primary advantages over other forms of mass transit. The design has very little utility for the delivery of freight, so this is one potential loss of revenue, especially at night, when the track would be largely empty anyway. (limited routing precludes freight anyway) The use of LIMs doesn’t seem to solve anything worthy of modifying the whole track for, because there is no reduction in moving parts. I believe wheel motors to be more efficient both as propulsion and brakes, and, unlike the LIMs, they are sealed.
However, as anyone who has studied my designs well knows, a truly flexible, very inexpensive system involves many other tradeoffs as well. In particular, the tradeoffs for cheap track and stations involve some pretty sophisticated engineering on the vehicle side. In practice the daunting challenge of any PRT vendor will be to become a bridge builder, vehicle manufacturer, and public works contractor, all at once. Starting with a very simple design is, in practice, essential.
In the US at least, transportation projects involving road “improvements” follow a totally different path than mass transit, being drawn up years in advance and being triggered by road usage reports. There is currently no business model for entering that market with PRT. Mass transit involves a more holistic approach, cracking the door for more “out-of-the-box” thinking, giving the opportunity to PRT vendors to air proposals. I do not want any of my criticisms of current designs to give pause to any transit authority official or city planner anywhere. Period. I believe they will work, as promised and engineered, and probably well beyond expectations, for the stated purpose. I just hope companies like PRT International, if they start making money, put some of it into developing a product for commuters.
