Sunday, June 27, 2010

92> Crack in the Track, Jack...

In my quest for the optimal hanging style PRT track I have found yet two more factors that could influence overall profiles and dimensions. But first, let me offer something to new readers who may not be convinced that PRT is a good idea in the first place.

I recently read a critic of PRT espousing the argument that automated cars would bury PRT because they would not require “new” infrastructure. I’m pretty sure the author hasn’t had his large front yard turned into 2 extra lanes and a median like I have, or he would realize that we are putting in thousands of miles of “new” infrastructure every day. Road construction is so “baked in” to the society that we no longer question or even notice it. I would wager that there isn’t a person in a thousand who has any idea of how much of their own personal money goes into road construction annually. When you find out you just might want to join the Tea Party movement.

More automation is coming to personal transportation. That is for sure. But with the current infrastructure that means trying to automate vehicles on icy roads, amongst texting teenagers and lost, darting pets. What will be the automated vehicle’s response to a downed power line? When it wants to go back to manual mode will the driver be available in time? Or caught looking something up?

Honest acknowledgement of the need for infrastructure that is safer for self-driving cars leads to a healthy debate about what it should be. Do we need it all to be big enough for trucks? Do we want to have to salt and plow it in winter? Do we even want it on the ground? Do we want it to enable fast vehicles or should we just slow down and save fuel? Do we want freeway-like non-stop service? If so, how is this best accomplished? Can we leverage what is already out there, in terms of existing roads and bridges? Should we limit the vehicle size since we know that people, given the chance, will choose obscenely oversized ones? What about the larger debate over urban planning and development?

Yes, we need new infrastructure, and yes, some of it will need to be ordinary roads and bridges. But roads are mostly used for single individuals going to particular destinations, and are, frankly, way overbuilt for this purpose. Even without PRT, it is time to re-examine our infrastructure requirements and consider putting down something that is less wasteful of taxpayer’s money. If we want a cheaper, minimum-footprint, longer lasting transportation infrastructure that is designed for fast, appropriately sized, automated vehicles, then we are basically talking about PRT. PRT is a logical outcome of a very logical debate.

Now on to the picture.  Back in Post 83 I showed a track profile with the actual running surface areas in red. This, as you can see, is that track but in the form of a crossing. Clearly there would be some loss of guidance as a bogey passes through, but not enough to be critical in any way. Also note that the wheels must span a two-inch crack in the track. This weighs into the wheel size debate because a smaller, harder wheel, without a special preventive design, would hit the gap hard, whereas a larger inflated rubber tire would glide right over it.

Such a crossing also raises old debates about “brick-wall” stops and the spacing between vehicles, and reminds me to consider the possibility of a failsafe track-based braking system.

It is reasonable to at least consider such a scheme for PRT because it could make merge and crossing collisions impossible by physically keeping two vehicles from entering the same space. So far I have not even considered how such a system would be designed, but something like a tail-hook comes to mind, which should not influence track profile all that much.

Here’s the second factor that could influence track dimensions. When I was first considering these matters, I recalled the ill-fated involvement of Raytheon in the PRT designs pioneered by Dr. J.E. Anderson. Upon their breakup, Dr. Anderson did extensive public polling and redesigned his track accordingly. I had always thought that Raytheon’s track was ugly and my initial reaction was to simply applaud this new, thinner rail.

I have, however, subsequently come to question some of his conclusions. Specifically, he was designing for the very narrow range of applications that were viewed as most advantageous commercially. In these dense urban environments, there are many factors that make setting supports very costly. Hence he opted to go for long spans, even though it meant relatively costly trusses. Contrast this to the approach taken by Higherway Transport Research. 

This illustration shows a how a track profile very similar to mine can be made by roll-forming a few lengths of steel. This can be done on relatively thick stock, by the way. My books have cold-rolled square tubing with up to half-inch wall thickness. Without critiquing this design specifically, I would just say that it illustrates how more frequent supports can greatly simplify the structural requirements of the track. For long runs, along freeways, for instance, there are no buried utilities, no streets to block during construction, no driveways in the way. A higher amount of track noise is acceptable, and there is less worry about the visual impact. I suspect that cheaper alternatives exist to the long spanning truss designs I have shown. 

All of this opens a larger debate about a factor in support spacing and design that I am not well versed in. That is the matter if regional soil types and the best way to anchor to that ground. In some areas, bedrock is very close to the surface; while in others there is sand or clay many meters down. Would Dr. Anderson have gone for the long spanning trusses if setting supports only involved simple drilling, as would be the case following freeways in most gulf coast cities? If miles of track were required in such a situation, what would the optimal span between supports be? Would many lighter supports be, in the end, more practical? After all, this would limit the weight any single support would have to bear. I still believe the truss design to be very important, in any case, since spanning intersections with multiple lanes requires such a design anyway. Still, I really haven’t considered the ideal freeway following configuration, which would be very important in young Sun-Belt cities like Houston, which have urbanized along the freeways, more than from the city center. 

By the way, Speaking of Higherway… designer and founder Tad Winiecki has an interesting suggestion as to the type of tires to use. (Alert readers will remember my search for a small but heavy-duty, high-speed tire for the tilted wheel motor designs.) He suggested small aircraft tires… I’m looking into it.

Anyway, those are my musings for this week… And so, signing off from somewhere near Crack-in-the-Track, Texas… This is Dan the Blogger, wishing you all a good night!


Andrew F said...

Very interesting post.

I'm not sure I understand the rationale for the intersections you show in the first diagram. Cost/space advantages over having the one direction pass above or below the other? The cost in operational complexity and reduction in throughput (some vehicles might even have to stop to allow the other direction to pass unless they are very well choreographed) would seem to be high for the cost savings that could be achieved. I could see this being useful in buildings, however!

Dan said...

Dan The Blogger Responds -
Hey, I never said it was particularly useful..;o)

I guess I just wanted to see if it could be done at all. You're right though, about in buildings, and no, it wouldn't be good for high traffic areas. That would be like replacing a cloverleaf with some yield signs. Might find use in some station designs though...

cmfseattle said...

Dan, where are you getting your info re: Anderson and public polling, post-Raytheon? He didn't change his track design; Raytheon's engineers came up with the bigger track.

Andrew F said...

I was reading an anti-PRT site complain about the drip pans and pedestrian walkways that Raytheon's design required for safety. Do you think that these things would be required (even) for a suspended PRT? One of the challenges is definitely regulatory, where overly onerous and relatively useless safety requirements are applied to PRT, perhaps with an eye to blocking the technology.

And for something completely different, there is a high density new urbanist district in the proposal stage in a suburb of Toronto near where I live. It is on a 116 acre site that is at the confluence of a regional rail line, a planned subway, and a major highway. At any rate, the planning documents refer to the possibility of using PRT as a circulator as the site can only be developed to its proposed density if cars have a significantly lower mode share than usual. It's a pretty interesting proposal, and a good example of what can be done with a starter PRT system in retrofitting suburban areas around subway stations.

Project plan:

Dan said...

Dan the Blogger Responds -

Thanks, Cmfseattle, for setting the record straight. I should be more careful about historical accuracy. I’m going to rewrite the paragraph, when I get a few minutes.

Andrew F, I have one question. How, in a free-market economy, can a city get such a massive re-do? Aren’t there dozens and dozens of existing property owners with their own ideas?

Andrew F said...

Dan: it's 116 acres. A few developers have assembled the properties for redevelopment. The city is involved with respect to planning and approval of the zoning change. This will be a very dense development, at around 65,000 people per sq km. Interestingly, cities have more power in the US, with imminent domain laws. There's no real equivalent in Canada.