48> PRT Stations...Continued

Almost immediately after posting last week I realized that loading cars one at a time was probably not what the designers of “taxi stand” (serial) stations had in mind for crowded situations. By loading a group of PRT vehicles at once, they may depart as quickly as the leading vehicle is ready, a fact that was quickly pointed out in the comments.

In particular, alert reader Akauppi added a link to a fairly comprehensive study of the subject by Peter Muller of PRT Consulting, who pointed out that numbers of simultaneously loaded vehicles could depart in “platoons”. The study concentrated mostly, however, on what he termed “open guideway systems” such as Ultra, because of the many ways they may be maneuvered. (These systems are essentially automatic cars, and may be parked and boarded in any way a car can.) In a statement reminiscent of criticisms that I have also made, Muller states, “Captive bogey PRT systems, such as those being developed by Vectus and Skyweb Express, show little variation in station design. The stations are always off-line, and the bays are always arranged in line with each other. This lack of variation probably results from the intended relative high capacity of these systems and their inability to accommodate tight radii.”

The obvious countervailing argument is that those “open guideway systems” fail to address one of the obvious benefits of most PRT systems, that being reduction of roads. We are rapidly coming to a time when we will have electric cars. Does separating precious real estate for less nimble automated ones really solve anything? Isn’t a separate road less efficient than just widening the one that’s already there? Well that’s a different subject, so I will return to what he terms a “captive bogey” system. (Pod on top of a rail.)

I did have one idea on this subject, which I have not seen, and that is this. The necessarily elevated stations, such as is required by the PRT International and Skyweb Express systems, must include an elevator (for the disabled) a feature I have criticized in the past as not being repeatable throughout the suburbs, for cost reasons. I think it noteworthy, however, that such a system, in the inner city, may have a very small footprint at sidewalk elevation. The sheltered area below the station might actually be an asset, as it could house news or food stands, or other sidewalk vending, at a natural spot for pedestrians to congregate. (No, that’s not the idea.) The idea is this. The “offline track” can be further split in two with the elevator in the middle, its structure supporting both tracks and a common boarding area, which needn’t cross any track. It would look something like this.

In this picture the large E is the elevator, which has two doors, ED1 (elevator door 1) and ED2. This is how it would work. The elevator, at street level, would open ED2 and passengers would be lifted to the boarding area, and ED2 would reopen, letting them board. ED2 would close and ED1 would open to let arriving passengers into the elevator, which would descend to street level and open (ED1) to let arriving passengers leave. ED1 would then close and ED2 would open, starting the process anew. Back on the boarding level, green and red lights, or even gates would be indicating the available vehicles, starting at the front of the line. The vehicles could move out as a platoon or the front vehicle could start out when ready. Once passengers have disembarked the vehicles on one of the tracks, the now empty vehicles in the arrival area would move up (to the boarding area) as a group.

In a hanging (gondola) system, a similar design can be used at street level, without the elevator. Very highly trafficked areas could have an elevated level as well, so that the station would, by the berth count illustrated, be capable of 12 simultaneous loadings and 16 unloadings.

Finally, here is a depiction of dual docking track described above, but with the “saw tooth” boarding scheme favored by Mister and Ultra. (fig. 1) Note that passengers may either board or disembark from any berth. The saw tooth design, being inherently parallel, gains less from splitting the track into two than does a straight line (serial) arrangement.

Actually the long, narrow footprint of a single row of berths probably fits better into the available space of most sidewalk environments. (fig. 2) Each vehicle may leave immediately after boarding, but both departing and docking vehicles may have to wait for each other because this system requires backing up into potential traffic to leave the station. A good way to control traffic would be alternate between groups of vehicles entering the station and groups leaving, (like the previous example) rather than a pure “first come, first serve” basis. In Muller’s study, he suggests that all vehicles can pull out together, ignoring the issue of vehicles not being ready at the same time. I would suggest perhaps three at a time. (In fairness, he showed a three-berth station) I should also note that with a gondola design, with no slots in the floor to step into, open-air stations could be extremely cheap, and therefore positioned with much more frequency than other station designs I have considered, and therefore could be made with fewer berths. (I am a big fan of cheap stations) Going back to the original, hypothetical three-station loop, however, it would seem that “platooning” is increasingly advantageous as traffic increases, but is a drawback if the traffic is minimal in “saw tooth” stations. Note that in this system, the berths are not exactly equal. Vehicles in the front berth, for example, would have little problem leaving but when empty the front berths could have slower vehicle replacement times. The opposite would be true for berths in back. Optimal traffic management for very busy stations would seem to be relatively complex. It is not immediately apparent to me if the saw tooth design would have better throughput than the dual track strategy outlined above, but I think it would out-perform the single line “serial” designs (PRT International, Vectus) with ease. I would also point to the variation of the saw tooth design I pictured in the last post, would not require platooning but requires rapid elevation changes in the track to avoid pedestrian/vehicle interference. It would seem to call for a comparatively large station, if the number of berths was, say, less than eight, but would be a very compact and efficient way to arrange a dozen or more berths.

In conclusion, I would reiterate the observation of the last post, that in order for PRT to be a valuable transportation alternative in initial limited and trial situations, high capacity stations are a must. In these posts I have explored the ideas of boarding and departing in groups and splitting the offline track as a means of increasing station throughput. I have not exhaustively studied every possible way to achieve simultaneous parallel “processing” of passengers, but it seems that PRT can have passenger turnover rates approaching group transit alternatives.

I also believe that a mix of both high capacity and very inexpensive suburban stations or stops is required to have a system capable of reducing commuter traffic, and that most current PRT designs do not meet this test. I hope that consideration of these station alternatives will prompt a reexamination of PRT vehicle design, especially with regard to both vertical and horizontal turning radii.

Alert read cmfseattle sent us this link to an article written by Bill James, (Jpods) a fellow who has come up with some designs (track and bogey), which are so much like mine it’s spooky. (And he seems to have done it first, but who is counting?) What is very cool is his trademarked term for PRT, the “Physical Internet.” Very descriptive, don’t you think? Let’s agree on something just that versatile and unstoppable.

47> Serial vs. Parallel

In my last post I brought up the idea of GRT as a way to get the first PRT track built because one of the problems with PRT is that it becomes less compelling with fewer stations. I think it is legitimate to ask, “At what point (in terms of scale) is PRT cost effective and profitable?” Is it worth building a mile of track with just a couple of stations?

This type of question got me to thinking about station design. Below (left) is a drawing showing the typical station usually associated with PRT. The vehicles wait in line for passengers, and then leave. In my example, they get off at one area and enter further down the line, cutting “turnaround” time. The problem with this design is that the whole station throughput is dependent on how fast the passengers enter the pods and seat themselves. In a very minimal system, this could limit the number of paying customers on the track.

On the right is a station with parallel berths for boarding, allowing the throughput of the station to be much higher. My original question was how minimal a system could be, and I will demonstrate my points with the following formula, where H is the headway between vehicles, T is the turnaround time, or the time I takes to enter a vehicle, get seated and get up to speed, and N is the number of stations.

Let’s consider the case, for example, of a system that could financially “break even” at 6 cars a minute, on (24 hr) average (10 second headway), but that means, for example, that the headways must be far closer during peak times, say, 2 seconds. Now let’s assume that the city wanted to start with only small triangle of track between 3 stations. If the stations were configured as in the left hand illustration, and the turnaround time (T) was, say, a minute, then it can be seen that H = 60secs divided by three, which is 20 seconds. Therefore the minimum headway is 20 seconds, or ten times as much as is required to break even. It can be shown that with the taxi-line style stations, it would take 30 stations to achieve the desired traffic throughput.

The second (right hand) illustration shows how parallel docking areas can be configured to allow multiple boardings at a time. In such a configuration the berths may be considered as separate stations, as far as the formula is concerned, but even with the additional berths the number of stations needed to break even is over 7. In a situation where the customer really only wants a 3 station loop, the stations had better be able to accommodate 10 berths each and the foot traffic had better be there to require them, or the project will lose money.

I think that there are important design implications to consider here, first and foremost being the importance of rapid boarding.

I, off-hand, do not recall seeing parallel boarding schemes in any illustrations other than the Mister System. Because of the slots that would be in the floor on most bottom track designs, I wonder how parallel boarding would even work, although two berths would be easy, because the elevator could be positioned between the tracks. In any case, designers should consider ways of speeding the boarding process as much as possible for limited system designs.

46> Many Vehicles, One Track

Here’s something to think about. In this picture, the vertical blue lines represent payload capacity, something like 50 to 75 kg per line. I originally started drawing this picture to explore weight distribution of possible track compliant vehicles, but then I realized that I had better share a bit of thinking first.

The baskets (A) represent something that could be used in manufacturing or for baggage handling. It’s not PRT, but it is a possible avenue to a proving ground and test track. A successful system would be a great confidence builder for the hardware, software, and the company that installed it. Having a demonstration project (that you get paid for) seems like something worth considering. (Currently the “state-of-the-art” technology involves conveyor belts or other floor space intensive systems, with laughable forking and destination sorting technologies.)

Cargo containers (B) can be similarly used, including outdoors or across town, if necessary. Cargo can be loaded and scheduled to depart automatically, so it can travel in the middle of the night, eliminating the cost of drivers working nights or being stuck in traffic in the day and, naturally, saving gas and payroll. Cargo containers could also be programmed to take the “long way” to their destinations to reduce network congestion, (for a reduced rate, of course). These would be great for moving mail between substations, for example.

The small “pods” (C) represent what would be best for 95% of rides taken, being designed for two or less passengers. These small vehicles could be pretty fast, passenger profile permitting, and would therefore be competitive with cars for longer commutes time-wise. Although current models generally assume identical velocity, these models also assume very limited networks. It should be noted that no system will be full of vehicles 24 hours a day, and that in a more comprehensive network, routes can be dedicated to faster or slower traffic dynamically.

The full size “pods” (D) can be used for families, people with bikes, baby carriages, the disabled, people with luggage, etc. These would have the “standard,” more gentle and slower ride, and employ greater headway between vehicles. Because there are societal benefits to traveling with a bike, or traveling in groups, or enabling the disabled, etc., I think that this class of vehicle should be subsidized and be more numerous within the network than ridership statistics alone might dictate.

The last, labeled (E) is a GRT or “Group Rapid Transit” vehicle and is designed for downtown or shuttle environments. It cannot ascend or descend steep slopes, and is designed for more expensive, high capacity stations. It designed to run profitably in simple loop or “back and forth” configurations such as between airport terminals, and, most importantly, could get the first PRT compliant track built. GRT can skip stations without passengers, like a city bus, but it can also have the intelligence to coordinate with other such vehicles to match passengers and destinations. This, and their small size, compared to buses or trains, means that no passengers have to sit through more than a couple of quick stops. After more loops are completed PRT vehicles can be added as needed, and the GRT vehicles retired or converted to night delivery use. (Or perhaps they will be found to have continuing utility within the system. I am not aware of any studies involving GRT and PRT sharing track, and really have no opinion, at this time, one way or the other.)

Most people are linear thinkers. The linear thinkers among us will see all of these vehicles as a hopeless distraction. I, on the other hand, usually look forward and imagine an optimal embodiment and environment, and then backtrack from there. This enables a viewing of possible pathways to a desirable outcome that cannot be had otherwise. It’s like cheating at solving a maze by starting at the end, so if I seem to fluctuate, sometimes, between reality and sci-fi, that is the madness behind the method…I mean, “the method behind the madness.”

45> A Critique of PRT International’s Design Approach

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.