Sunday, May 10, 2009

32> I Just Can’t Let This Stand Unchallenged

I have just finally gotten a few days to devote to PRT and have spent some of it studying PRT International. I am glad to see that a lot has changed since the days of Taxi 2000. One of my major peeves, the huge track, has been replaced with a stronger slimmer skinned truss. The control system apparently will allow for more and variable speed. The one problem I have with it is the top riding design. In the site there is a point-by-point comparison by Ed Anderson. The arguments are pretty thin, I’d say. I would invite my inquisitive readers to open it up in a separate window so as to get both sides.

It starts with the fact that it is harder to do switch. I’ll certainly admit that. That’s point (1) Then in (2) it says that vehicles on top look better, and that they have so much experience we should believe them. The gondola design hangs 8 feet lower; the theory goes, so it is closer and more visually apparent. (Of course the part that is always there is 8 ft closer but anyway, to that I say, “OK, if it’s really a problem, let’s raise the rail 10 or twenty feet.” Oops! There’s a problem. If all your stations need elevators, I guess you don’t want to do that, do you? Especially if your system isn’t designed for slopes. The next point (3) deals with the costs and size of foundations and supports. Please. What about the costs and foundations for elevator-equipped stations? He says the weight is off balanced, doubling the stress at the ground. First that isn’t much of a problem, Second, it doesn’t take an aerospace engineer to see that you could make a “?” shaped top to the supports and balance the load if it was a big deal. As for (4) and (5) he makes some good points that took some pretty fancy math to figure out. I’ve studied the equations as best I could and will not quibble about the conclusion of the study which says that, in effect, if all else is equal, then hanging vehicles have essentially no structural advantage. Fine. But all else isn’t equal. I have seen studies where municipalities have voiced concern over corner “clipping,” (that being where right-of-way is needed over valuable corner property) to put in a proposed turn of PRT track. Now I don’t pretend to know exactly how sharp a turn his vehicles can make but I would bet they would lose a cornering speed contest.. In point (5) he talks about “natural frequency.” I really don’t think that will be a problem if the turn is essentially a right angle, because that will always involve two closely spaced supports anyway. It will also require slowing for the turn and quick acceleration out of it. By the way, about banking the track- What speed to you bank it for in his system? Gondolas self-bank to the proper amount for any speed, a fact left out in his comparison. Anyway, by his own figures hanging wins point 4 and I say nothing in point 5 really tilts that balance. As for (6) I really think he is just looking for another point to make. He implies that hanging vehicles need more beef. Note the qualifier, “all else being equal” and the actual wording “the sidewalls will be heavier.” I suppose that implies that if one looped a couple of steel bands around one of his vehicles and lifted it, the sidewalls would collapse. He should have claimed overall weight advantage, and I think there probably is one, because of the added functionality of the hanging design, so I’ll stop and just give him that one. I would maintain, however that it is a minor point. Number (7) ..Huh? If it runs underground ? Geeze. O.K, How ‘bout this. When it floods his track will have to be pumped out. I would be really interested to know just how long a run it is to get his system back up to altitude..
Number (8).Cabintaxi? CABINTAXI? “Somewhat more people preferred riding above the guideway than below” in giant square boxes on giant concrete roadways in the 70s? Sheesh. Number (9) He gives that one (not having the track in the way) to the hanging vehicle, but only as an advantage in buildings. He never mentions that the same applies everywhere else as well. Finally number (10) mentions that he has a successful plow and a ditch to push stuff (that falls into his track) into. I think he could at least say that he gives this one to the hanging camp.

Let me add a number (11) to the list. If you are in a hilly city, like San Francisco, and you are going down a hill and there is a turn at the bottom, the non-hanging vehicle will throw the passengers right out of their seats, unless it goes very, very slowly. In fact it has not been demonstrated, (to me at least) that bottom-mounted vehicles are capable of serving hilly cities at all. Not once is the self-leveling quality of hanging vehicles mentioned. Although I touched on it before, I think it deserves a number (12) to note that track banking is speed specific, and therefore inexact. Not so self-banking, hanging vehicles. Also previously mentioned but worthy of it’s own number (13) is the need for expensive raised stations with elevators, Too few stations take away a huge advantage of PRT, that is the “point-to-point” aspect. They cannot come down to earth because if they did they would either tip people out of their seats or block a huge swath of real estate on a gradual descent. That would open up the possibility of vandals climbing, painting, or putting stuff in the slot. Heck, if he would add a number just to make a point about what would happen to PRT underground, we should be able to call this number(14). Stuff in the crack. If the track isn’t above the trees, then leaves, seeds, etc will fall in the crack. Add a little rain and time and it will be a planter full of soil and rust. If it ever is at ground level, there’s a lot more than leaves to worry about. There’s garbage, and that a five inch crack is big enough to fall into up to the thigh. That’s point (15),

I guess my main gripe is that he never mentions the major drawbacks to his system (no slopes, major problems at ground level, and consequently expensive (and therefore less numerous) raised, elevator equipped stations. He keeps using the term “all else being equal.” All else is not equal. Raised stations may not be a big deal downtown, but they render the whole system impossible to scale outward into the suburbs, where station traffic would be less but the benefit of car miles eliminated would be more. There is already a system out there that can move people around downtown but is too expensive to scale. It’s called light rail. In all fairness, however, he’s locked in. Once money is raised, it’s pretty hard to tell your investors that it would be better to start from scratch. It is also a fact that, from a business point of view, you don’t want to shellshock your customers with too many new ideas. I just hope he makes his track easily upgradeable.


timote said...

I appreciate you challenging the status quo (such as it is). To me, the best argument you made (early in the blog) was the concept of very cheap guideways to reduce installation costs and therefore penetrate the market more easily.

All of the supported structure systems I've seen have pretty heavy support structures and never come down to street level. In comparison, your vision is for lightweight support structures and a high number of stops (stops are cheap if basically all you're doing is a loop down instead of a whole elevator and second-story structure).

So my question is - does it have to be this way? The stresses and space requirements seem like they should be the same for the above and below cases, am I missing something here? Why the two different visions?

Peter Muller said...

I tend to agree that, in many situations, at grade stations are desireable. They have the advantage of being easily seen and requiring no alternative vertical circulation means.

Captive bogey systems like Skyweb Express tend to avoid at grade stations probably partly because burying the guideway becomes undesirable. Suspended systems tend to deal with at grade stations a little better. Open guideway systems are quite conducive to at grade stations as evidenced by the PRT system at Heathrow Airport where all the stations are at grade but most of the guideway is elevated.

Learn more about PRT at

Dan said...

Timote , I thought you had left us forever.. Thanks for stopping by. To give credit where credit is due, Ed Anderson got the message loud and clear about the Raytheon/Taxi 2000 track I complained about early on. In his latest venture, PRT International, he has come around in a big way and now boasts a very long slender covered truss. I have also heard him criticizing the Vectus system, which has a similar track style. I wish I could remember the actual dimensions, but I recall them as similar to my educated guess as to what is possible, maybe a bit under 3’x4’ for a 90’ span. That seems big, but so is the span. Lesser spans, lesser dimensions. The track size coming into stations could be much smaller. I also read somewhere that the Americans for Disabilities people would not budge on demands for a fairly big turning radius in the PRT vehicle itself, forcing a re-design of the original, smaller unit, which required wheelchairs to ride sideways. This, of course, causes a redesign of the track as well, or increased headway..
I actually think hanging vehicles are heavier, on balance, because they contain some extra hardware, but I’ll finish that thought after welcoming …

Peter, it is good to have you aboard. I was just about to get into the hanging vs. supported thing… I just have to say (as I actually look at city and suburban streets) that I, for the most part, believe raised stations are simply unacceptable. It just won’t happen- at least not with stations within walking distance of each other, or stations that only handle a dozen passengers per hour at best, (like the convenience of a bus stop, but where the “bus” goes non-stop to your destination) To me those types of limitations (like high turnover stations) undercut the promise (and premise) of PRT.
I maintain that in spite of increased technical and business difficulties inherent in a hanging system, it is the ONLY method of exploiting the “network effect” where new track is so desirable that neighborhoods demand it, and it is considered an option to widening roads, not just as an option to light rail… Where it is a method of commuting, not just moving around downtown. The Ultra system, while being better at ground level, is likely to be world changing only in that it demonstrates the PRT concept. It is a niche system, that niche being airports and campuses, i.e. situations where the customer owns the land. At least that is my gut feeling. I’m afraid there is no real business model for working the bugs out of hanging PRT, at the moment, other than to make it clear that it is the ultimate destination, and we better get on with it. Again, just a gut feeling…
PS nice site.. Good luck. I realize that I am advocating “PRT2” while criticizing something that is already meeting resistance (PRT1) but we are essentially on the same team, so let me know if I can help with your efforts.

ItsEric said...

The truly main difference is that suspended designs need smaller tracks for the same capacity. Smaller tracks means smaller vertical supports which means everything is much cheaper to build and maintain. Side benefits are it's much more difficult for dumb folks to get on top of them and walk on them. This reduces liabilities and system slowdowns when these dummies are encountered.