Sunday, April 4, 2010
I guess I have come up with a solution, of sorts, to the problem I posed in last week’s post. By introducing a rotating joint where the swing arm attaches to the cab, the cab can be pivoted sideways. This gives the clearance required for vertical travel. It also opens up some interesting station design options, such as the closely spaced front boarding shown in this video for Monic PRT.
The picture above shows the indoor station problem to scale. The rail here is fairly large, (nearly 36” tall) and is the high-speed design shown in post 74. The vehicle is 63” tall at the pivot point. This cannot be reduced much without making the seats too low or sacrificing headroom. (No, the rail is not part of the back wall; the viewing angle just makes it appear that way...)
The ceiling as shown is 10’ 9”. Luckily, most modern buildings have more distance than this between floors. That makes the bottom of the track at 7’9”, barely within reach of the average adult. Because being able to reach it at all is a bit troubling, work continues on trying to find the best way to shave a few inches off of that 36” dimension.
In the second illustration the cab has been turned sideways to the track in preparation for descent and in the last it is shown on a vertical track section. Note that the swing arm must be at least half of the width of the cab, but not so long as to push the overall height of the system higher than is necessary to keep track out of easy reach, lest the system be too tall to fit between the floors of most buildings.
One thing I would like to accomplish with these designs is to create a system architecture that enables a business model that is not as reliant on busy stations. I understand that previous designers have had to keep in mind that “the squeaky wheel gets the grease,” so any initial system is likely to be for a very busy area. Also, in the beginning, limited funding will mean that PRT will have to prove itself with a minimum of both track and stations. Still, PRT’s main strength lies in concept of point-to-point travel, and that means lots of stations. Trying to do otherwise is like having a taxi service that only goes to and from a few locations. We already have that; they’re called shuttles, and are most efficient when transporting larger groups.
Reducing the cost of the stations is a main factor favoring hanging vehicles over bottom-supported designs, which require extensive means to keep people away from the track, like elevators, gates, fences, etc. Cheaper stations will eventually pay off in higher ridership. Can you imagine, for instance, public buses trying to operate with two thirds of the bus stops removed? Who would want to walk that far, both before and after the ride, and presumably on the return trip as well? PRT is no different. It is unavoidable that initially PRT will have to start in an environment where shuttles would be competitive, but it is unwise to create a system architecture that is only economically viable in these situations. This is particularly true in many U.S. cities, where activities like shopping and entertainment are often done very far from the city center. By the way, the larger track size shown is fully compatible with larger group (GRT) vehicles, if it can be shown that they would be more effective in certain routes. The vertical or steep slope travel capability, however, is PRT only, and aimed primarily at situations where a small footprint is needed, yet the ridership isn’t sufficient to justify an elevator equipped station. Sharing space with a bus stop comes to mind.
Finally, I have to acknowledge the differences with European hanging designs, which have no swing-arm at all, although they have similar track heights, because they tend to be taller, walk-in style vehicles. A quick “walking view” tour in Google Maps of various European city centers reveals huge masses of pedestrians compared to the U.S… many, many times more. I can certainly see that a lower speed, inner-city centric approach could make a lot of sense there. While we struggle with “urban renewal” projects here in the U.S., it appears that they’ve “been there, done that,” in Europe, perhaps hundreds of years ago! (Some of the oldest U.S. cities seem to have more vibrant downtowns as well, and being constrained by water seems to help somewhat.) The whole structure of many American cities, especially in the fast growing “Sunbelt”, is about growth along freeways, giving the cities long tentacles of urbanization. The designs I have shown reflect this landscape.
Posted by Dan at 10:34 PM