Sunday, January 15, 2012
The project in Amritsar has underlined some issues about PRT station design that would seem to merit a second look. The system under construction will have seven stations, and 200 vehicles. That comes to 28.6 vehicles per station. The question, then, is, “Where do the vehicles go when not in use?” The illustration above should give pause. The structure and land use is huge, as are the number of bays, and even this is not enough to house a seventh of the vehicles. Note that there is no parking shown. How do the people get here? More importantly, where do cities get that kind of real estate and how much does it bump up system cost? Here are a few observations:
One of the classic features of PRT design is the off-line station, where empty vehicles wait to take passengers on their way. In the straight-line model favored by Dr. Edward Anderson (now embodied by ITNS, Skyweb Express, Skytran and others) the vehicles wait single-file, similar to a line of taxis. There is none of the inefficiency of backing up, but this advantage must be weighed against the fact that all departing vehicles must wait for the leading vehicle. The greater problem with this serial approach is that it is ill-suited for vehicle storage, as Amritsar exemplifies. For example, if a final occupied vehicle comes into a station that already has 28 empty vehicles in a line, the station would have to be 29 vehicles long to receive it. In the illustration below it can be seen that the tailing vehicle’s passengers cannot get out if there are no outbound passengers.
Two solutions come to mind for such systems. One is to simply have a storage facility. As usage declines vehicles would simply take themselves out of the system. The other is to separate the stations into two halves. There would be arriving and departing sections, separated by enough track to store the vehicles. Then, in the case of the last illustration, this could be the loading station only, with all waiting/stored vehicles already emptied. These are both simple enough solutions, but they involve significant system changes.
The saw-tooth parking approach embraced by ULTra, Mister and others, where the vehicles back out, can, in theory, be quite compact, at least in terms of fitting in normal rectangular building lots. Vehicles that are designed to be front loading can be arranged in a typical perpendicular parking configuration and spaced so close together as to be almost touching. This density would be limited only by turning radius and the dynamics of pedestrian movement in the boarding area.
The preferred embodiment may depend on what shape of space is available. For example, the very long, narrow space required for “straight-line” stations might frequently be readily available within existing easements. If it is not, however, and land needs to be procured, the straight line design might prove highly impractical. To further complicate matters there is the matter of visual impact. It might be that the stations or stored vehicles would be considered obtrusive.
A couple of additional observations: Maintenance and/or recharging might be a factor to consider, as these activities might take vehicles out of service anyway, so some percentage of the fleet would have this additional place to be at night. Also, one interesting idea would be to simply lease office space and store vehicles in a office building or retail space. The space could even double as a private station for tenants. (They probably wouldn’t welcome the extra pedestrians of a public station) Another thought involves open automobile parking. Two of Amritsar’s stations are parking areas, and this may well prove typical. Could automobile parking and “pod” parking be the same? After all, it is after the auto traffic thins out that the PRT vehicles would be most needing spots. In the morning, when the cars roll in, the pods would be there waiting. Alternatively, PRT storage/station could be on the roof over covered parking, something the ULTra illustration would have probably included but for the promotional nature of the rendition.
My own view is that it is important to be as flexible as possible. That means, from a vehicle/track design standpoint, very tight turning radii both vertically and horizontally and very steep slope capability. This enables many configurations for both boarding and storage which are impossible otherwise, such as tightly packed three-dimensional storage arrays. A pod’s door placement is also an extremely important detail that I am still wrestling with. (In post 116 I designed several variations of the in-line type stations which utilize parallel, double-sided boarding, and require left and right side doors, although I have also played with front boarding designs)
My suspicion is that, upon reviewing actual potential routes and station locations, city planners will generally conclude that these are the challenges that are the hardest and most expensive to conquer. It does no good to have a system which, by the mile, sounds cheap, but then requires unexpected purchases of real estate, or requires track layouts that prove unacceptable to the communities involved. These are not details but rather the main challenges - The solutions that get weighed against (and can win against) other transit alternatives. The challenge is to have a practical answer for every case, rather than to force a city to search for alternative routes. If a system won’t work in essentially every instance, it has little chance of ever becoming a pervasive network, and we all know that is where PRT will really shine…if it ever gets that chance.