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.