Friday, November 5, 2010
Here is omething to think about… As I have previously pointed out, because of the boom in highway projects started in the 50’s, cities in the U.S. tend to be spread out. Instead of one central business district, there are usually many little ones. To get people to leave the car at home is to get people to travel many miles without it, and there is no predictable direction of travel, just the likelihood that where they are going isn’t far from a freeway. Therefore I have tried to accommodate this need with my designs.
Building PRT along freeways follows a different logic than many of the systems that are out there today. It is like creating “worm-holes” between the more traditional routing schemes. Naturally the structural requirements are different for both track and vehicles.
The track can be much cheaper as I will show.
Urban track must not be too visually imposing and must be routed to minimize distance between stations. This generally means one-way. There is also a lot of interference of all kinds in terms of track support placement. This includes driveways, corners, underground utilities, telephone poles, signs, and traffic issues during construction. Freeway following track, on the other hand, can have more frequently placed posts, which means shorter (less expensive) spans. It can have two-way traffic supported by each post. There is less of the double track associated with stations and “Y” interchanges. (With two-way traffic this necessitates up to four tracks, something that would create an unwanted umbrella effect, hence the general adoption of one-way urban designs.)
Freeway following track represents a real chance to highlight one of the advantages of PRT, which is the limited weight of the vehicles. Freeways must support 18 wheel trucks, and this sturdy construction is overkill for commuting purposes. When a highway becomes congested, the call goes out to estimate the cost of another lane. Each new lane is expected to absorb a certain number of cars for a certain number of dollars. PRT vehicles can be substituted for cars in this equation. I recently read a Houston Metro study, which was an evaluation of a HOV lane running parallel to a section of I-10 (the main East-West interstate across the southern U.S.) The project was evaluated as a success primarily on the number of vehicles diverted from the main road. As most of us know, one lane of PRT equals many lanes of ordinary highway. It would be hard to make such a clear-cut case in a typical PRT route of convoluted loops. Of course PRT vehicles are not free to the city, like cars, which are supplied by the commuters. But autos are not revenue producers either, unless it is a toll road. With PRT there is also no increase in traffic congestion on either end of the highway leg. It may be a complicated cost-benefit analysis, but I think it’s less so than most of the other route proposals I have seen. They generally look like nightmares of compromise and contentious litigation.
And so I have devoted some time to the problem and have come up with this. In the picture above, the need for excavating has been eliminated by creating a support structure with a “foot.” It occurred to me that the weight of concrete traffic dividers could be utilized along with a wide base to create a system that could be placed in the breakdown lanes. Such a structure could not be made with just concrete and rebar because the edges must taper to nothing, so that vehicles’ tires can roll onto it freely. Therefore there must be some clever integration of steel plate into the design. In addition to its shear weight, (about a ton per running foot) the base can be further immobilized by pinning it into holes drilled into the concrete.
Such track could be installed cheaply and rapidly by being pre-fabricated and trucked to the job site. Sections of the concrete lane-divider would be interlocking or connected by steel splines, so that individual sections cannot be tipped or moved by impact.
Hopefully I will be able to figure out a realistic cost estimate in the near future, since estimating labor and materials appears to be a pretty straightforward job. I do not think I would go with the previously shown truss design, though, because it is for long spans, not cheap and easy construction. Anyway, my guess is that we will be pleasantly surprised.