Wednesday, March 23, 2016

A Simple Question

I have a simple question for Google, Tesla, Apple, GM, BMW or any of the other makers (or potential makers) or self-driving cars. It goes something like this:

Let’s suppose you put 4 robocars in a parking lot, and they all want to leave at once. Who goes first, and why? Taken a bit farther, if a couple of your competitor’s self-driving cars enter the mix, then what? Can I bully my way out first if I am the only human driver? What if I yell, “My wife’s water just broke!”  What if one is a bus? Do large or full vehicles get priority over a vehicle with a single passenger? Or here’s one for Amazon. What if a bunch of delivery drones arrive back at the warehouse at the same time? What about when multiple robo-forklifts rush pallets to a waiting truck? How is all of this coordinated?

What we are talking about here is something very fundamental, even in nature… When herds navigate a river crossing or when bees enter a hive, there must be a pecking order. Extended to movements en masse, there are behaviors such as flocking, schooling and swarming. This is an essential building block that must be integrated into the programing of any autonomous robot that moves around amongst other mobile devices. It is more complicated than simply allowing the first in line to take the lead. As noted earlier, there are mitigating circumstances than can change the priorities. “First come first serve” won’t work, for example, if the first in line is a nimble little vehicle but the second is very much larger and/or moving faster. These parameters must be mathematically quantified in computer code.

Whenever autonomous vehicles (of any type) approach each other there must be some kind of mutual greeting, and exchange of self-descriptions, and declaration of current and intended movement. In the simplest example, two equal vehicles are traveling in straight lines and those lines cross up ahead. A check of velocities determines that both will arrive at this crossing simultaneously, so one or both must adjust speed accordingly to prevent a collision. This greeting need not be last-minute in either time or space, but rather should occur as far ahead of time as is practical.  After all, if an automated train pulling thousands of tons is one minute away, the other vehicles that will be crossing that track would be well advised to anticipate this arrival! In the automotive realm, convergence of large numbers of cars at a single choke point will form gridlock, so advance warning of this would obviously be advantageous. In other words, what is collision prevention close-up is traffic prevention if done ahead of time. A cloud-based, queryable master timetable can be generated by monitoring such intervehicular communications, so all upcoming interactions on any particular vehicle’s itinerary can be foretold by simple sorting. In other words, every vehicle can know who it will encounter on a journey and when.

Because of the many manufacturers and types of robotic vehicles that might be involved, “interspecies” traffic management will eventually require a holistic approach, and so a number of standards will need to be established. Otherwise muscular Teslas will be bullying Google cars and neither will understand why a bus keeps stopping. Aspects of such protocols will be widely applicable to various forms of human controlled transportation as well, providing an additional layer of safety for road, air, harbor or rail traffic. (Existing standards and practices will have to be folded in.) Drones could simply be “repelled” by air traffic corridors when they are active, road traffic would automatically start pulling over with the approach of an ambulance and so forth. Even inanimate objects could participate.  Perhaps someday stop signs may literally stop you!  

And what, pray tell, does any of this have to do with Personal Rapid Transit? First of all, collision avoidance and traffic control are problems that equally apply to PRT, and no such software for autonomous PRT vehicles currently exists, even though it would have huge advantages, such as eliminating any chance of a systemic failure. The fear of such failure has been one argument against compact track designs that are otherwise cheaper and less visually intrusive, since they cannot double as an emergency walkway.  Autonomously controllable vehicles could reroute, backup, or even push a disabled vehicle, making any such problem much more manageable. In short, a system based on autonomous vehicles has the robustness demanded in today’s world.

Secondly, such software will have to be written anyway, as it is the core of any system that can coordinate self-driving cars or other mobile robotic systems. It is that last assertion that is key: The software will have to be written anyway, because will likely be done before any comparable code is written specifically for PRT. The problem for PRT, besides the wait, is that there is no particular reason to believe that the developers of self-driving cars will tackle this problem as a holistic platform until they are forced to do so. It is much easier to stick with “first come, first served” until forced to do otherwise. Until you ask the parking lot question, the matter appears to be a simple extension of obstacle avoidance – a mere footnote in the code. Someone needs talk to these folks and point out that there probably is a sizable DARPA grant in there for whoever takes this on as an independent field of R&D. It has incredibly important military, as well as civilian, applications. 

The good news for PRT is that it just happens that this platform can, in most cases, be researched more easily with PRT than with cars or other steerable robots. To illustrate this, let’s start with connectivity.

The same year as I started this blog, Kevin Ashton coined the term “internet of thingsreferring to a world of pervasive internet where everyday devices would communicate with each other, the cloud, the owner, and/or the manufacturer. Such a world is slowly coming to be, from doorbells that ring your phone to, of course, GPS navigation in your car. Yet fast and completely reliable real-time connectivity is still a problem, something that the upcoming 5G cellular standards will try to address. In the meantime, for automakers, this limitation adds to the constraints in developing the kind of vehicular coordination discussed earlier. Between the unpredictable street environment, the lack of connectivity, and the lack of other cars with which to communicate, it is likely that traffic coordination directly between vehicles will become mired in, conflated with, and given a back seat to these other issues. PRT, in contrast, is a perfect testing ground for such coordinated control because it lacks these complications. “Leaky cable” (as proposed by Skyweb Express, Flyways, and others) is a method of extending the range of wireless communications that is basically protocol agnostic; Perforated coaxial cable acts as a track-wide linear antenna for both broadcasting and receiving wireless signals, and inside of a shielded track at a range of mere inches it can provide super-clean transmission of wireless Ethernet, Bluetooth, or experimental protocols designed to exploit or supplement next generation cellular technologies. In other words, Google’s cars might not be able to reliably chat with each other from long range now, but with leaky cable equipped PRT they could simulate that future connectivity in a way that they can’t out on the streets.

To prevent collisions (or to merge PRT vehicles) we need more than a “handshake”. We need to act on real-time velocity and trajectory information. The parking lot example illustrates just how complicated this is. The cars can be initially positioned and aiming anywhere, and their preferred paths can be curved by any amount or into an “S,” for that matter. Meanwhile they can speed up or slow down by any amount, all while trying to maintain a comfortable ride and watch for obstacles. In short, the possible combinations are nearly infinite. Track-based PRT reduces this to absolute bare-bones. Trajectory information is nil (or zero, for straight ahead) because the vehicle is captive on a track, and changeable only to 1. (to switch tracks) This means that while still needing to be written into the program, all of that complex turning radii and angle information is reduced to one single, binary “bit.” - Little more than a place marker -  I told you it was easy!

Velocity information for PRT is also highly reliable in comparison to self-driving cars since, out on the streets, moments of uncertainty rightly cause moments of deceleration which results in unpredictable travel times. In the controllable environment of a track the interactions between vehicles can be programmed to milliseconds even when it will be many minutes before the encounter occurs. This simplifies experimentation with algorithms for shaping traffic of any type including air, harbor, warehouse, rail, etc. One immediate and valuable use of such software is to more accurately simulate proposed highways and modifications to them.

PRT is also the ideal way to experiment with adding “the cloud” into the mix. If two vehicles will cross the same merge point in 5 seconds, that interaction should be transacted locally. If the encounter is 3 minutes away, however, that is something that should probably be cloud-based. The interaction of cloud and local communications and processing resources is hard to study out on public streets, but is essential for making robots behave as a team, not just for transportation but for fields such as space or undersea construction, search and rescue, or for military uses - anywhere where swarms of small robots acting in concert could gainfully replace teams of humans.

Moving from simulations to making occupied vehicles actually cooperate takes testing in the physical world, and this is expensive. For such testing, it is very hard to imagine a mechanical system that is simpler than a PRT bogie which, since it doesn’t have to steer or handle bumps, can be reduced to the absolute minimum of moving parts possible for a self-mobile device. This simplicity and affordability enables experiments that require large numbers of vehicles. For scaled-down, indoor model testing, track could be something as simple as a one piece extrusion. Such experimentation is not frivolous. Consider the problem of sequential merges, for example, such as would happen with multiple on-ramps feeding a single lane. How many vehicles can you add and how fast, when they are all “thinking” for themselves? Can you throw in a simulated emergency vehicle? Mastering this ultra-simplified, track-based environment would be a valuable precursor to doing the same on the open road.

The bottom line is that to get those cars out of the parking lot, you need to write a program, and that program needs structure and a foundation. To go about their business efficiently, self-driving cars, (or other forms of mobile robotics) need to coordinate their movements by communicating and planning ahead. Doing this effectively across a broad range of devices constitutes a huge but necessary challenge, and a chance for interested companies to create standards and otherwise influence a whole new and emerging, yet fundamental platform. To get this right, they will need to develop and test a system whereby autonomous vehicles will resolve conflicting routing intentions in real time and circumvent future conflicts by some method of fair prioritization. Doing this for drones, in full 3D, will be an astronomical challenge, and even coordinating vehicles in open parking lot or warehouse space involves a nearly infinite number of possible paths, not to mention timing. Doing this for rail-based PRT would, perhaps, be the simplest testable starting point, because the vehicles are as elemental is possible, the steering is binary, the rolling surface is smooth and controllable, and the wireless communications are free from interference and distance limitations. A near-perfect laboratory.

For PRT, this presents an interesting business model. What if, for example, a city were to agree to partner with a Google or Apple or Uber (or a consortium) to create such a laboratory in the form of an actual PRT system. In this case the city and its local partners would be primarily responsible for track and stations, which I have demonstrated can be quite minimal. The majority of the system could be developed with funds already earmarked for similar experimentation. This combination also falls neatly into federal funding programs already underway, and we should not underestimate the influence these tech giants have in setting the agenda for such funding. The US Department of Transportation may not be interested in PRT per se, but they are clearly interested in everything that makes PRT tick!   Also, the (PR) optics of the whole program would be great for everyone involved. To these companies (unlike the government) bold is beautiful! Such a scheme squarely addresses PRT’s three greatest problems, those being [1] the “PRT-needs-a-large-network-to-be-profitable” problem, [2] the “government-bias-toward-pavement-based-solutions” funding problem, and [3] the “reluctance-of-cities-to-try-anything-new, especially-with-an untried-startup” problem.

I have one more, simple question for the tech giants, not to mention our respective governments. “Do you want to shape this coming paradigm or do you want to cede this field to someone else?” 

Sunday, December 20, 2015

Modularizing PRT

Let’s Hope, for Santa’s sake, that the nations of the world make 2016 a banner year by actually acting on the agreements reached at the recent international climate summit! PRT is too good of a solution to languish for long in a world that is finally getting serious about carbon reduction!

One problem that is holding up the implementation of PRT, ironically, is the speed of technological progress. In eras past, such big projects could be tackled with the assurance that they would remain cutting-edge for years to come, so any government contributions would remain relevant and financial stakeholders would enjoy a fairly long-term leg up on the competition as well. These days, better techniques, materials and methods come along almost faster than whiteboard ink can dry. In particular, the progress toward electric and self-driving cars, better batteries, robotics, communications and so forth creates the impression that any major innovations in transportation field may well be obsolete or unneeded soon anyway. After all, many of us still have our share of VCR players, film cameras, and cassette tapes somewhere in the basement, and what mayor or transit chief wants to create a highly visible, far-flung legacy that ends up looking as modern as an eight foot home satellite dish? Like it or not, cutting-edge means little-explored, unvetted, vulnerable to being leapfrogged. With the speed of technological change being as fast as it is, is it any wonder that neither government (local, state or federal) nor private investors want to stick their necks out on tech ventures that involve so much untested ground?

Making sure that the PRT infrastructure remains useful for decades to come requires a careful analysis of the problems that PRT is supposed to solve and designing the machinery, software and architecture with an eye on that distant future. To do this, we must bet on various assumptions, many of which are, luckily, quite likely. For example, it can be wagered that land for stations in the inner city will not get cheaper or more plentiful. It can be assumed that the expectations for comfort, speed, quietness, will become more demanding over time. It is not good enough to come up with the best option currently available. What is needed is a solution that will evolve to be better and better, and will never be eclipsed by some new technology that unexpectedly becomes polished and cost effective.

Too bad the challenge isn’t confined to the just the vehicles. Daily use always reveals design shortcomings, and new additions to a fleet will surely sport many improvements. Evolution happens more quickly for parts of a system that are frequently replaced. Conversely, (and this is the problem) since the track should last for many decades, we are seemingly stuck with whatever we start with.
The obvious downside of making such wrong choices was a motivation for establishing this blog and a call for consensus and standards. It soon became clear, however, that various PRT developers would always differ on what design approaches would win the day, and that it hardly mattered anyway. Financial backers have little interest in anything beyond getting an initial contract and ownership of proprietary technologies. Yet perhaps there is a way to address this whole problem of obsolescence that makes sense for all stakeholders in both the long and short term; Modularization!

Modularizing PRT (or any other system) ensures that only a barest minimum of components cannot be easily swapped with updated versions. As readers of this blog know all too well, a tremendous amount of work has been done herein on bogie designs that were later scrapped. The bogie, of course, fits within a mating track, so if the bogie is not perfect all of the track for years to come will have the same shortcoming. It is like introducing a bad gene into an organism that will be carried down through generations of offspring. By modularizing the mating components, this risk can be minimized. In particular, PRT track, of the sort we are dealing with here, can be divided into two functions. The first is to provide rigidity across a span, like a bridge, and the second is to provide the interior with rolling surfaces, electrical and data carrying structures, and so forth. This second aspect can, in whole or in part, be replaceable, adjustable, or otherwise be modifiable. This is not an intuitive way to treat track design, the first instinct being to make the track as simple, strong and affordable as possible through tight integration. There are, however, natural boundaries within the track that can be exploited. Issues of thermal expansion, sound and vibration isolation, third rail and cable maintenance, etc. all tend to favor fastened inner structural elements over direct, permanent (welded) attachment to the structure. Note also that, within the SMART framework, there are already specialized track “innards” for climbing or making tight turns. Even the structural shell has some modularization, first, because lengths of track would most likely be manufactured off-site in prescribed lengths and turning radii, but second, because the left and right halves from a length of track would optimally be detachable from each other so as to facilitate the insertion of switch points or just for ease of access.

Modularizing elements of the track also makes the initial design of the bogey much easier because it can be designed for what is most practical and cost-effective today without as much fear of introducing some demon-seed design detail that will be impossible to exorcise at a future date.

Modularizing key track elements also facilitates adjustability. Rather than setting internal dimensions in stone, why not build-in a range instead? Especially if it only involves punching a few alternative bolt holes – sort of like adjustable shelves? 
This illustration shows just how simple modular PRT running surfaces can be. Two pieces of angle steel, (red) and two inner guide rails are the only continuous components in this (evolving) “Mama Bear” configuration, other than the “third rail” electrical contacts and “leaky cable” Ethernet communications. (Not shown.) In other words, if a city were to, for some reason, change to some other system, there need not be a whole lot to swap out.

Today, PRT’s advantage shouldn’t be seen so much in terms of robotic vehicles, but rather as the most practical way to enter, exit or bypass the stop-and-go ground-level congestion that is an unavoidable part of urbanized life. It is the exploitation of unused “air-corridors,” and it can only be accomplished in such an affordable and compact fashion (and with such high vehicle density) using automation, or it would have been done long ago. We may well even be entering an era of “If you build it, they will come,” when it comes to PRT track, since exploiting these corridors represents a quantum efficiency advance in the same vein as the ongoing efforts of companies like Amazon, Google, Apple, Tesla, etc.       

In a future where ground-based automated vehicles are commonplace, PRT will still add an important, complimentary contribution, but besides being affordable and long lasting, it must be versatile. Making track components modular and adjustable seems like a vital first step in making sure that first iteration systems do not become obsolete in the face subsequent designs and this, of course, is prerequisite to being seen as a good long term investment for a city. In other words, any PRT system isn’t seen as future-proof is toast because nobody will take a chance on it in the first place! Well, back to the drawing board...

Happy Holidays from Dan the Blogger!

Wednesday, October 28, 2015

There Will Be a Quizz.

In the last post I referred to PRT as a kind of “Urban Wormhole” and spoke of how self-driving taxis could never replace PRT. I would like, in, this post, to follow up on that a bit. I think it is important to make the relevant arguments clear and on the table for all who will listen. An entirely new and untried transportation infrastructure is a tall order, yet I believe there are unassailable arguments why such an augmentation to our current systems is necessary and inevitable. I have included a short quiz in this post, designed to (hopefully) win someone over. Now, if I could just come up with a really catchy slogan….
The robotic car push reminds me of that stage of the internet revolution that centered on perfecting the dial-up modem, which of course, can only be as good as the phone wires it is connected to. In the case of mobility, at least in urbanized areas, the main problem is that there is an inherent cap on our surface transportation’s efficiency. It is, at best, 50%. The reason is simple. Simple physics dictates that if 50% of traffic wants to go east/west, and the other half wants to go north/south, each will have to yield right-of-way 50% of the time, once a line for each direction has developed. Nothing but passing over or under that crossing traffic can improve this dismal number. And that is the best case for the intersection itself. Were we to measure efficiency based on the time difference between “making the light” in no traffic and what can typically happen in rush hour for individual cars, that efficiency number would be much, much worse… 15 – 20% perhaps? And that is only one, and they add up! In what other endeavor would we put up with such inefficiency?

This is all unfortunate because, with the possible exception of bike trails or sidewalks, there is no current surface transportation infrastructure that can be elevated or buried economically enough to generally allow multi-level, non-stop traffic flow. If it WERE economical, we would not have such gridlock today!

Vehicles, and the surfaces constructed for them to roll upon, (this includes rail) have evolved to be too big, cumbersome, and expensive to be the only alternatives in space constrained, urban areas.  How ironic that, in a quest for the efficiency of carrying more load per vehicle trip, we have accomplished exactly the opposite! In all fairness, bigger really IS better, once you get out of town a bit – yet gridlock is the elephant in the room when it comes to urban mobility and robotic cars can only be of peripheral help in the current context. Any real solution demands an infrastructure specifically designed for non-stop travel in all directions at once, and logic dictates that this architecture be smaller, not bigger. The fiber optic cable of transportation! Actually, it is within this framework that automatic driving technology can really shine. With a faster, yet smaller “pipeline” the driving decisions would come at a much faster rate as well. No place to be texting!

So here is the quiz. It is more for making people think and to stimulate conversation than anything else. Maybe it will help win a few converts!

1] Non-stop ground transportation can only be achieved by incorporating overpasses/underpasses but this is impractical for widespread use because of the heavy loads that our current roads and rail systems are designed to carry.  True/False
2] The majority of vehicles on the road are carrying payloads, including people, that weigh just a tiny fraction of what roads are built to carry, and this represents a waste of resources if more appropriately scaled infrastructure is possible. True/False
3] The ONLY way for most travel to be non-stop within an urban/suburban environment is to create a new, more affordable infrastructure which is necessarily aerial and sized appropriately for lighter payloads, such as people.  True/False  
4] The technologies for automatic vehicular and traffic control to utilize such an infrastructure have now come of age.   True/False 

There. Saying “False” to any one of these will hopefully start a thoughtful debate at least. “Grass roots” movements have to start with consensus, and consensus must start somewhere!   

When I started this blog, part of the mission was to create some standardization, particularly with regard to the track. Seen in this light, it can be better understood why so many PRT bogie/track designs have been explored on this site. The world needs an infrastructure for urban/suburban mobility that allows non-stop travel. Dual-mode? Fare based? Privately owned? In a way it really doesn’t matter. To me the question is, “What are the mechanical/architectural underpinnings that will best encourage the development and proliferation of such a system?”  We need the combination of present practicality and boundless future possibilities if we want to propose it as a solution worthy being added to skylines across the globe. I believe a PRT track can be designed that includes very little beyond the architectural structure that is required for spanning between support posts. Sort of a standard PRT building block, which (as most of you know) I have dubbed the SMART (Suspended Multi-Axis Automated Rail Transport) platform.

A brief progress report is, perhaps, in order. In the design pictured up top, I have continued with the theme of using off-the-shelf-parts, and experimented with putting the four motors between the drive wheels instead of off of the ends, and this shot highlights a new cam-driven lever-type switching-guide-wheel mechanism.

Right this minute, the holdup is this – I love the idea of being able to remove the side of the track that normally switches off (right if you are American, left if you are British) without diverting PRT traffic. This would allow switches to be added or removed with minimal disruption in instances where no alternative routing exists, such as is likely to happen again and again on the outer edges of any growing network. Unfortunately this entails running on the left wheels alone and doubles the load on them. If the wheels are to be soft enough to cushion vibration and smoothly handle expansion joints, and especially if they use ordinary, safety-rated pneumatic tires, they will compress under the load and maybe even deform side to side, adding substantial complication to what could otherwise be a super simple switching scheme.

In these last two pics first note that the steering wheel guides have been turned upside down (compared to the one on top) and there are no longer two sets per side, like in previous posts. It looks like this would be sufficient, even in this "half-track" mode, if we wanted to use custom solid tires designed not to rock or compress too much. Unfortunately I would like to make this cheap and straight forward enough for an individual, company or university to build for experimental purposes…, hence the “off-the-shelf” trailer tires. (Smaller, higher pressures and stiffer sidewalls than automotive)

Finally note how the track’s spine is missing the opposing C channel, since that side has been removed. That otherwise sandwiched plate is a splicing means, a potential hanger for cable stays and placing an upside down “U” channel over it can create a waterproof seal, like in a standing-seam metal roof, when used with the sheet metal sheathing, which is curved to stiffen it between structural ribs.  Work continues! 

Monday, August 10, 2015

168> Self-Driving Cars and the Urban Wormhole

In our last exciting episode I promised to explore cooperative robotics, but there is an unavoidable side issue that is worth at least one post in itself, one that high-jacks the whole subject. What I am referring to is self-driving cars. It is not just that PRT and automobile automation have such obvious similarities that makes the subject a prerequisite to discussing PRT control. It is also the fact that self-driving cars may actually change the definition and purpose of PRT.

Self-driving cars are not just about following directions and staying on the road. Spinoff features such as automatic braking for collision avoidance are already widely available in everyday production models. Since self-driving features often involve technologies that clearly can enhance safety, there is an “arms race” in this regard.  With communication between vehicles (“I’m slamming on the brakes, so you better, too!”) and awareness of real-time traffic, (such as could be monitored, compiled and reported by the vehicles themselves) it is not hard to see how cars could be made to operate within a sort of “hive mind,” to the benefit of the group. This is basically a PRT operating system and is a wakeup call to self-steering PRT systems like ULTra or ToGethere, whose technologies are getting leapfrogged by this trend. Notably, the self-driving paradigm is proving that a high degree of autonomous control is doable, challenging the centralized control schemes of only a few years back.
Does self-driving auto technology render PRT irrelevant? Do self-driving cars eliminate the same problems that PRT was to solve?

The short answer is no. The question does, however, illustrate how various flavors of PRT have very different challenges in this new technological landscape. What, for example, should the business model be for ULTra in a world transitioning to self-driving taxis that may not need guideways or stations? Of course that may be quite a ways off, and maybe it is Google who should be approaching ULTra. After all, every cent ULTra has made is a cent more than Google has made on its robocars, with Google putting all of their eggs in a business-model-basket that is still a bit of a head-scratcher and is contingent on many  unanswered questions when it comes to safety.

So far Google and the others have seemingly only taken their vehicles out in decent conditions, weather-wise, content to garner impressive numbers of miles without incident. But how do they handle being behind a vehicle that is dropping debris? Or on patchy “black ice?” Fresh snow or street flooding can completely obscure where the road is. Timid response, in these instances, that would prevent lawsuits is also the kind of driving that would snarl traffic. Imagine a car that is afraid to go through ankle-deep water and so just stops! Can a robocar understand when weather conditions are deteriorating too much to enter the freeway or understand the significance of a funnel cloud? Humans usually know when they should stay put or go back. Will Google be able to give that much common sense to their cars? What about morality? Will they know to hit the truck to avoid the woman with the baby carriage?

It has been reported that, at least some states, they cannot dispense with manual controls (such as a steering wheel) nor the human seated so as to operate them. This leads one to wonder under what conditions the authorities would permit such use. Beyond that, what is the profit model that trumps the obvious, added liability issues? Do drivers really want to relinquish all control and do manufacturers really want to sell mechanically stripped down cars with no sex appeal?
A safe starting point would be a special lane or only going at fairly slow speeds. (i.e. a mode similar to current pavement-running PRT systems) After all, many localities allow golf cart type vehicles on public roads, and these need no horns, airbags, seat belts, etc., as they don’t go that fast. While this would probably be allowable, it does little to solve urban congestion and so, by itself, risks irrelevancy.

Interestingly, there are flavors of PRT that have just the opposite problem. SkyTran, for example, is really built more for speed than for serving little stations in every nook and cranny of a typical city. Suspended systems, in general, have the potential to move people from one side of town to the other quite quickly using an inexpensive, minimalistic track, but like all PRT, suffer from the potential problem of not having a sufficient number stations and walk-up customers to create the cash flow to pay-down the system components and still provide a return on investment. It’s the old first and last mile problem. Could self-driving automobiles be the answer to aggregating more PRT passengers at fewer stations? Quite possibly.

Uber has expressed interest in self-driving cars, and car sharing schemes like Zip Car raise interesting questions about ownership. Why garage a vehicle that could be gainfully employed elsewhere while you are not using it? Why own a vehicle at all – especially if there is one parked close by that will come to you when summoned?
Once upon a time, PRT offered a unique combination of benefits that could not be had otherwise. It was automated, elevated, personal, fast, but it was only practical within an interdependent framework of technologies. Over time, developments in sensor, communications and computing gave advantage to a variety of methodologies enabling new designs that emphasized varying missions and business models. The emergence of autonomous cars, it seems, puts a focus on one of the original themes, which is a network of fast, elevated, non-stop corridors that are unencumbered by the gridlock below. This mission is unthreatened by the self-driving car revolution and, indeed, may well benefit from it. PRT, at this point, needs to be considered not as a fleet of automated taxis, but as a network of personally navigable urban wormholes. The automated taxi aspect is now only a means to an end, not the end itself.

Originally the idea was to space PRT stations so that each was reasonably within reach by walking. Now perhaps each station could garage a half-dozen automated taxis, giving such stations much broader reach. This associated service would not have to extend very far to cut the number of stations to a fraction of what would otherwise be required. This scheme works perfectly with car sharing, carpooling, or even private ownership. Even if each fully self-driving “car” can only go a few blocks, with similar vehicles and capabilities at each end of the PRT “wormhole,” the combination could be very synergistic. The timing and payments between the two systems could be integrated, even if under different ownership. On such limited routes where the speed limits are low there is much less reason to require a driver, meaning self-driving cars are clearly ready for this NOW. Where special lanes could be created, ULTra is also ready to go as well.  
Side note: The above combination offers less advantage for the SMART PRT designs shown within this blog because SMART is specifically designed to address the “last mile” problem with vertical capabilities and stationless pickup and drop-off capabilities. Of course any infrastructure, surface or elevated, will undoubtedly face obstacles on certain routes, and so alternatives are always welcome. That being said…

It is important to note that slow, short-haul, self-driving “taxis” also offer synergy with other forms of mass transit, such as light rail, subways, scheduled shuttles and so forth. Even with limited routes, (There might be too much traffic on public roads and no room for a special lane) phone apps could be used to locate the nearest suitable pickup point. Such pickup points could enhance the value of properties that are otherwise inconveniently remote from mass transit. Short hauls to such existing transit hubs seems like a good business model for such a product/service right now and deployment of such FULLY robotic vehicles, even at slow speeds, might serve similar R&D aims as the current, unpaid efforts and would offer a potential opportunity to cement a leadership position in the field while generating revenue.

Self-driving cars, capable of full city and highway use without special lanes or a standby driver are still a long ways off for general use, and the eventual payoff for the producers of such vehicles is questionable. Short range, slower vehicles face no particular obstacle to immediate adoption, however, and everybody from ULTra to Google to Uber to Zip and the various auto makers should jump on this opportunity to become leaders in this transitional space. Meanwhile PRT wannabes need to take note that the current technological backdrop no longer supports slower, station-intensive, short range systems. Instead, PRT companies should concentrate on designs that foster the cheapest, fastest network of elevated track possible, filling that one niche that automation itself cannot address. 
Elevation has always been the answer to surface traffic congestion, but has always been prohibitively expensive (not to mention ugly and in-the-way) when scaled for heavy vehicles, and so gridlock continues. Like fiber-optics compared to copper, some form of light, affordable, high-speed “pipes” for moving people are inevitable, and the first barrier, autonomous yet cooperative automation, is falling away fast. Like that fiber-optic cable, the second barrier is what happens at each end – how data is efficiently transmitted and received. Hopefully self-driving vehicles will help provide comparable, easy-to-implement solutions for either end of the mobility solution that cities need so badly - that Urban Wormhole technology called PRT.

Wednesday, June 17, 2015

167> Easier Every Day

Creating a PRT system is getting easier every day. Increasingly, it is a matter of employing off-the- shelf components in ways that have already been established for other purposes, using software that basically provides a “fill in the blank” framework, and communications that work reliably at blazing speeds using ordinary protocols. These days each individual PRT vehicle will have far more computing power than yesterday’s entire system, and what used to be worrisome safety issues can now be addressed by a redundancy of cheap processors and sensors that can check and recheck everything under the sun hundreds to times per second, using self-diagnostic AI software that is ever more capable.

When I started this blog one of my primary worries was that cities would never accept a whole new infrastructure that would be dependent on all kinds of exotic, proprietary technologies and specifications, understood only by the PRT system developer. At the time, only a precious few corporate giants had the technological credibility and depth of resources to give much political cover and comfort to local transit officials. Those days are fading fast. While any project can turn south, particularly when attempted by an untested company, at least the next generation of PRT systems will be understandable and serviceable by ordinary professionals, no doubt with considerable free help from component vendors. Indeed, the idea of an automated taxi service, what with self-driving cars already roaming our streets, only seems truly futuristic in regards to the uncertainties of the street – the darting dog, the patch of ice and so forth. Such a service within the controlled confines of a track hardly seems like a technological challenge anymore, just a monetary, logistical and political one.

With that in mind, there is a case to be made for setting aside any remaining system components that are overly exotic. Although self-turning wheel motors are absolutely perfect for robotic transportation, at present the selection of such motor/controller packages in the appropriate power range is still quite limited.  Therefore, it is not a great idea for me to tightly couple my open-source designs with these motors, as though the designs are dependent upon them. Mechanically, a self-switching bogie for carrying a suspended load (such as PRT passengers) is almost ridiculously simple, even with off-the-shelf motors. I have therefore used four ordinary BLDC motors in the above design. SMART (Suspended Multi-axis Automated Rail Transport) designs require a motor for each wheel because switching from cruising to climbing (and vice versa) requires the front and back wheels to momentarily rotate at different speeds, and turning in very tight quarters calls for the left and right wheels to rotate differently as well. The redundancy of four motors also dovetails well with reliability, continuing a philosophy of completely eliminating any possibility of a single vehicular breakdown, let alone anything systemic.

Shown are Golden Motor’s 5000w offerings, for a combined total of about 26 hp. It is reasonable to assume that these motors would be powerful enough for full “SMART” functionality, such as even climbing straight up if necessary, while still being “geared” to reach highway speeds, or close to it. (Contingent on final aerodynamics/permitted load.) Continuing with the “You can build it in your garage” theme, I decided to use trailer tires, which, despite their small size, are designed with tougher sidewalls and higher inflation pressures than automotive tires while still being load and safety tested. In production solid tires would be preferable, although there are various strategies that could allow this vehicle to limp along, even with multiple flat tires. Note – although I picture a 16”OD tire, as is clear from the end view, the raised steering guide wheel assembly ended up considerably higher, defeating the reason for that size wheel. With a few modifications, a 20.5 OD drive wheel could be used that will have a greater (1050 lb. each) load range.)

From my 3D simulations, it appears that this bogie can achieve turning radii (horizontal and vertical) of under 60 inches, keeping with a design philosophy of enabling PRT to go essentially anywhere with minimal conflicts and expenses regarding right-of-way or station placement. A whole PRT vehicle could easily pull a U-turn within the floor space of a small two car garage!
Briefly, a description of the drawing - Wheels A only come under load when stabilizing a turn, unless encountering gale-force side winds, or helping to support vehicle in the event of a deflated or over-weighted tire. (running slowly on only the two left wheels enables the switching side of the track to be modified or even removed, such as to add a junction, without completely detouring or halting all traffic, but this would tend to compress pneumatic tires until wheels A are sharing the load.)  Wheels B have opposite rotation because they hold the bogie down, (engaging the bottom side of the channel that A uses) especially at high speeds into gusty head winds, where the aerodynamic “pushback” would otherwise lift the back wheels, or when climbing “cog” style, using the pinion gear (or sprocket) C. Note that the pinion gear C and the drive wheels F are driven in unison while wheels B are free-turning. Normally engaged, oppositely rotating wheel pairs D keep the bogie centered. Raised steering guide wheel E does not normally contact the track because the guide rail is discontinuous and tapers into contact at junctions only. Note that steering guide wheel E gets raised and lowered with the upper wheels A of the OPPOSITE side, not the same side, as in previous designs, since the steering guide mechanisms (shown in violet) now crisscross each other. This more securely holds the bogie to one side or the other by basically scissor-clamping that side of the track, while hooking the top guide rail, so as to completely support the vehicle, if need be. The channels that hold these upper guide wheels (A) would, by the way, have a liner (shown in blue) that would only be thick enough to make forceful engagement at sharp turns and junctions. Although this bogie might, at first glance, seem to be bristling with various wheels, most only see use for a tiny fraction of the time. Their various sizes are proportional to the durations and load of such engagement. Not shown are the electrical and communications interfaces between bogie and track. Contacts for third rail” electrical feeds would be side-mounted. “Leaky cable” Ethernet could go almost anywhere. I have not yet worked on emergency brakes for this design. Also, the track is shown as a simplified representation of the various contact surfaces: It is not some miracle extrusion!

Finally, I would like to reiterate the rationale for the “Mama Bear” design. I can sympathize with readers who have, for years now, seen design after design come forward from this site, only to be replaced by yet another, presumably better one. Hey! The Model T was a great design as well, but that does not mean it was the “last word” in automotive functionality! In the case of prior PRT designs, there was always an inherent conflict between the holy grail of minimal track size and the obvious merits larger, automotive-sized wheels for heavy loads, higher speeds, and less maintenance. While simply splitting the difference is one solution, this essentially means that future designers must “round up” both the track and the bogie size to accommodate the largest and fastest anticipated vehicle and impose that same design on neighborhoods where such a system could prove unpopular or too costly. But the only other alternative is to limit the potential speed or size, which does not seem like a good starting point either. A high speed, 6-10 person, scheduled shuttle between an airport or a park&ride and downtown seems like a practical arrangement, and would be better yet if the track is PRT compatible. But let’s keep such “muscular” track out of peoples’ front yards! Simply eliminating mechanical reliance on a track’s “ceiling” height allows this flexibility, so that the smallest vehicles, designed for using equally small track, can still larger track wherever needed. Smaller track, we should remember, is much cheaper and so has inherent advantages in addressing the “last mile” problem, (or simply providing more coverage for a given cost) and we should not dismiss places like campuses, that don’t need a family-sized vehicles in the first place.

In conclusion, I would just like to point out how mechanically simple this whole thing is. There are two moving assemblies that go up and down on a cam, mounted on a frame (yellow) that holds four motor powered wheels. Other than that, what is left is basically an exercise in cooperative robotics, a technological environment that is becoming increasingly commonplace. I will be addressing aspects of that next time. 

Tuesday, March 10, 2015

166> Cruisin' for Burgers and Batteries

As I wrote in my last post, to pay its own way and generate additional profits to reward stakeholders, a system must achieve a certain threshold of passengers, and these are, by definition, pedestrians – a relative rarity in many (especially younger) US cities. I illustrated the problems of gaining passengers with many low volume stations with a heat map. What I left out, however, were the special cases where there are natural congregations of people for some reason. There is an obvious disadvantage in only catering to such hot spots, even if almost all cities have them. To do so is like planting daffodils too far apart. They may fill in someday, but it could take decades. Still, it is worth listing some of these “special cases” as many are actually quite common.

>A bridge – The cost of building a PRT bridge or putting track on an existing bridge is almost trivial compared to adding a lane for ordinary traffic and getting through such a bottleneck quickly can potentially create high volumes of passengers.

>A large topographical obstruction – a related issue is when a city is located on a lake, ocean or river or even against a mountain, where straight-line travel is obstructed. A good example would be a city on a bay, where the only routes are C-shaped and through highly developed areas.

>An existing, pedestrian-based transportation infrastructure. There is obvious advantage to sharing stations with other networks where possible. This includes bus stations, airports, subway stations, etc.
>Park & Ride locations - Get passengers where they are leaving their cars anyway!

>Tourist destinations and parks – It should be noted that many times parks front rivers and streams, which may even provide a ready-made route for PRT. Parks may be advantageous from the city’s point of view as well, involving less red tape and/or alternative funding possibilities.  

>Privately owned commercial real estate – this could include office buildings, strip centers, big box stores and so forth.
If I have left out any categories, post ‘em in the comments section! For the moment, though, I would like to concentrate on this last one, because it is both universal and well-distributed, and so should influence a PRT system’s design. I will skip going through multistory buildings for the moment because there is already a lot to cover.

In the US, it is common to have “big box” stores and strip centers running along freeways, accessible by the freeway’s one-way feeder roads. This tends to create elongated commercial zones … little “Main Streets.” What used to occur in the town center now often happens here. Moreover, highways reserve overpasses for only the major cross streets. This makes these overpasses the only portals between adjacent neighborhoods and stimulates commercial activity on these crossing routes as well. The corners between highways and these cross streets tend to have a lot of traffic and are eventually prime sights for multi-story buildings.

I regard these sites as extremely important to PRT design since they often have terrible traffic, both on and off of the highway, and it is a particularly difficult situation to remedy by conventional means such as road expansion or widening the roads a block over. These sites have added significance to the commuter, as they tend to occur “on the way,” and so would seem to be one way to extend PRT out from downtown areas to allow suburbanites a way to access a downtown PRT system without driving all of the way there first. They are naturally distributed in key areas. If PRT is to have a real impact it must be able to spread, to join the various hubs and trafficked areas, and so such areas are essential footholds.

One requirement that PRT should have, to service such areas, is the ability to cross the highway. Here suspended designs have a huge advantage over supported ones as they can, in many cases, simply run under the overpasses, using their ability to quickly change elevations and tilt into turns to comfortably navigate what is likely a very serpentine route. These advantages come into play even if the only route is over the highway, as this would be extremely high (supported designs would need very long ramps) and would ideally use some sort of cable-stayed or suspension design, which is also very well suited for suspended systems.

The next requirement PRT must have to service such areas takes some consideration regarding ownership of these properties. (I am not including using public easements in this discussion, for brevity.) First of all, the retail establishments probably do not own the land. Such land parcels are often owned by developers and whoever has their store there at the time is simply babysitting it… paying the bills and generating some return while the land appreciates. Therefore putting PRT stations on such land is contingent on what serves those developers’ needs. For this reason, it is worth exploring what strategies could be employed to entice these land developers to allow stations on their property and under what terms. 
One thing that needs mentioning is that these “developers” are probably not individuals but rather partnerships or corporations, often set up expressly for a particular property. As such, they may have certain bylaws, have silent and managing partners, and so forth, so any kind of agreement regarding adding a PRT “station” should be as simple as possible, as they have their own internal politics. This would involve some kind of monetary benefit for them and provisions for eliminating risk and complications. On the cash side, the solution seems easy enough. One way that a city can incentivize corporations is with a tax break, something that is increasingly done to attract new businesses. This way cities can knock down the cost of the project, at least on paper. Never underestimate a city’s willingness to “rob Peter to pay Paul!” For the landowners, they see their tax bill reduced, their tenants see higher foot traffic, so leasing values go up. Win win.

The next criteria is to make it risk and hassle free. This is where the physical PRT design comes in, particularly in regards to the matter of permanence. No landowner wants to permanently relinquish sovernty over any section of his property, nor does he want to be tasked its upkeep, utility costs, security and so forth. He also won’t want to be responsible for its removal and repair of the site if he decides to repurpose or sell the property. This means that ideally the “station” would be powered from the track and require minimal anchoring or other foundation work. For example, holes could be augered into the parking lot for subterranean concrete anchors, where removing bolt-on components leaves only small holes to fill and patch. The idea is to make the entire spur and station easy to install or remove.

Also note that many of these properties have super-sized parking lots, the owners being understandably reluctant to subdivide the land. This raises the possibility of creating a mini “park and ride” lot. It also raises the matter of ensuring convenient parking for store patrons is not sacrificed for commuter parking. This will take some creative problem solving! Luckily the system will probably know who the passengers are.

Another related instance of private ownership is the strip center. These tend to be smaller lots, and usually have somewhat less parking. They may also have vacancies, raising the possibility of using indoor space for a station. Again, this would take a PRT design capable of tight, serpentine track configurations.

Then there is great American mall…Many of these are dying and have parking galore, indoor space and so forth. These folks should jump at the chance to have PRT, and have “Park & Ride” written all over them.

The original creators of PRT envisioned a citywide transit solution, not a system designed for just a few niche markets. They also lived in a time when people believed in big government funded projects, and cities where not so spread out. These days, if PRT is to demonstrate its worth, we will have to find a formula where it can spread, station by station, by the benefits each new station brings to everyone involved, from cities to investors to passengers, to merchants along the way. One overlooked source of revenue is the immersive experience of the trip itself. After all, the pod knows who you are and where you are going, and potentially much more… It can talk to you, play games with you, even tell you about the attractions along your route.

Imagine, for instance, the pod itself offering special sales, issuing coupons, etc. for upcoming stops. “Battery sale? Next stop?” “Half-priced fries for the next 30 minutes at McDonalds? – Honey, better hit that button for station 22!” For the merchants, it’s like home delivery in reverse. It is a big machine to automatically attract and deliver customers to the front door, and at an exact time of their choosing! That item you looked up on Amazon last night might just “happen” to be on sale just a few stations down. A personalized, temporally-tuned billboard… You could hunt for (or potentially even negotiate) custom tailored deals along your way!

One thing that has, so far, characterized SMART PRT designs is that they are not cheap. The track, yes. The stations, yes – but the vehicles have capabilities that far exceed other designs. I have always considered that the vehicles themselves are best left to a deeper pocketed partner – that they should be, in the words of Steve Jobs, “Insanely great.” This business model – the intelligent delivery of customers directly to merchants – is not well supported by slower systems that need big loops, large stations, long ramps, etc. If you are going to stop at the store on the way home, it needs to be a fast, red carpet experience…which, by the way, is the same experience we need for PRT to become the long-lasting paradigm that it deserves to be.  To do this right, we really need an Apple, a Google, an Amazon -  and the right business model to get their attention. Maybe this is it. You listening, guys?.. Tim? Elon? Sir Richard?...  Call me!! 

Wednesday, February 4, 2015

165> Tips for Designing PRT

Recently I have been corresponding with a new PRT developer, and I was struck by how similar his logic is to the many who have come before.  He initiated his communications by saying that the first step to a successful solution should be a fresh look at the problem.  He then stated his list of design criteria, tying back each decision to the problems that it would solve.  Before very long, he had narrowed his design to something very similar to the SMART platform, yet quite different in many important ways, and I found myself defending the specifics of my approach.  I thought I would share.

Design differences don’t just come from differing views on the practicality of one mechanical solution over another, but also from the original assessment of the problem itself.  PRT is the would-be solution to a host of different grievances.  To some, it is primarily an energy/climate saver.  To others the main emphasis is downtown mobility, while some would try to solve the problem of gridlock on our highways.  The various PRT solutions that are out there, it seems to me, are all pretty well crafted for their intended purposes.  What worries me is they stick to their respective missions a little too tightly. 

Current PRT offerings are not versatile enough to harvest sufficient passengers from the varied, real-world cityscapes to be economically viable until they have reached a critical mass of coverage.  The cost to achieve this break-even coverage is simply larger than what cities can afford or, at least, are willing to risk their careers over.  Here is a brief synopsis of the problem:

For PRT (or any other public transportation system) to be a serious contender, it must pass two tests. First, it must not lose money.  Second, it must perform its function better than alternative choices. When PRT can clearly meet these two criteria, it will be adopted, pure and simple.  Starting with the first test, let’s imagine the simplest, cheapest system, a simple loop between two stations.  PRT costs money, and that only gets returned through fares.  The separate costs of track, stations and vehicles must each be amortized though their respective lives and these numbers (along with operating costs) tell us how many passengers need to use those assets daily to make the system break even.  Here’s a hint.  It takes lots and lots of passengers to make the payments!  With two stations, the only people who will use the system are those who need to go to one of those two destinations. While you can buy fewer vehicles, and you can make smaller stations, you can’t reduce the track costs to where only the occasional pod will pay the bills, so that component (the track) bleeds money from the stations and vehicles, leading to its logical elimination.  Now we have another shuttle bus on the streets.

As for the second criteria, such a simple route lends itself better to a larger, scheduled vehicle anyway.  Since everyone is going to the same place, why not ride together?  Even a few stations between would not represent much added inconvenience.
Above is a “heat map” of 5 layouts, representing increasing numbers of loops and stations.  The warmer colors represent greater traffic.  These are not representations of city blocks nor would it ever be possible to have such evenly spaced stations.  Passengers are only where you find them, and then only some of the time.  What is shown, though, is how stations are interdependent for passenger traffic.  Each station is both source of new passengers and a destination for others who would not otherwise ride. In a full-sized system, (not shown) the vast majority would be hot yellows and oranges, and only the outer perimeter would be the unprofitable purple or blue.  PRT clearly can’t start too small.

Needing more closely associated network nodes to be effective is not unique to PRT.  The same heat map and lack of profitability at the outer edges can be seen with a bus system or parcel delivery, for example.  Yet networks are not necessarily all equal when it comes to the ability to grow “from seed.” The key is making each part of the network viable in its own right.  No doubt the builders of the very first stretch of railroad already had a pretty good idea that it would be profitable on its own, and each subsequent branch surely met this test as well.
When it comes to priorities for designing the perfect PRT system, this business of needing a whole network from the start is a whopper.  It is the elephant in the room.  Yet with proper design I believe a formula for growth can be found.  There’s more.  Next time you are somewhere that you think would benefit from a PRT system, try identifying the next closest station sites, and estimate how many passengers each such station would attract.  If your experience is anything like mine, you will soon see that good sites are not at all easy to come by, and those with lots of foot traffic are usually on private land or are squeezed by roads that have been widened as far as limited easements will allow. At best they are in patches, separated by areas that don’t need PRT at all.  I pity the poor city planner tasked with finding station sites every half-mile that have enough pedestrian traffic to warrant an elevator equipped station.

The mobility problems faced by the urban and suburban populations differ greatly, and often PRT designers are biased by their own experience.  There are differences due to geography (such as climate) and even city-to-city topographical differences, such as water frontage or hills.  Political differences are important because they effect funding and attitudes toward public works in general. There are even historical differences.  One that I am often reminded of is (then) President Eisenhower’s “cold war” strategy of initiating of the US interstate highway system, resulting in urban neighborhood segmentation and the suburbanization of an entire nation.  Now, 1 out of 4 miles driven in the US is on an interstate, often at maddeningly slow speeds! The point is that these are among the factors that individualize a city’s needs, even as the economics of starting a PRT company push toward a more “cookie cutter” approach, and therein lies the rub.

This is where “SMART” comes in.  “Suspended Multi-axis Automated Rail Transport” is not a PRT system per se, but a development platform.  Any system that can move vehicles in 3D can also work in 2D.  Any system that can travel at 90 mph can also travel at 30.  If it can turn in a 6’ radius, it can certainly manage any street corner.  The idea to avoid “baking in” needless limitations.  The SMART vehicles are very capable and full featured because someday they might be expected to be that way. After all, people will eventually want, from PRT, the same qualities they would expect from a manually driven taxi service, and investors, equally, will prefer a fast, efficient and comfortable fleet to maximize returns.  In the meantime, there are tough choices in store.  SMART, with all of its “bells and whistles” is, admittedly, an extremely (if not prohibitively) expensive way to start.  Yet its basic architecture, especially the track, addresses the above problems squarely by being uniquely versatile.   
With PRT requiring a whole network to operate profitably, and with the vast differences between different cityscapes, the problems that predicate a PRT designer’s job are immense and varied.  Some examples include how to put dirt-cheap stations everywhere and anywhere; how to take the shortest route between lucrative sites instead of being detoured by worried landowners; how to piggy-back other services, like carrying utilities or street lighting; How to give local planners more and better options; How to get property owners to allow stations on their parking lots and in their lease spaces; How to attract the most riders without cutting fares, how to encourage night traffic and freight, how to minimize operating costs, and so forth.  

Why PRT has not entered the mainstream is not simply a matter of suspended vs. supported design, linear vs. rotary motors, or third rail vs. battery power.  It is the big structural issues regarding network size, versatility, and the business model, and translating these overarching issues that into physical form should be “job one” for the PRT designer. 

Saturday, January 17, 2015

164> Papa Bear, Mama Bear, Baby Bear

As promised in my last post, I have modified the SMART PRT (Suspended Multi-axis Automated Rail Transport – Personal Rapid Transit) design yet again, this time to accommodate featherweight “pods” and enclose the bottom of the track, as well as fine-tuning certain proportions.  
There is also a secondary rationale for considering ultra-light PRT systems, which is the matter of parcel and light freight delivery. Companies such as Amazon are clearly putting heavy emphasis on speedy delivery these days, and the ability to integrate a steady stream of varying vehicles is, unlike days past, essentially a given. Couple this will the fact that such track would be many times cheaper, and a very good case can be made for exploring the matter further. 
In response to this challenge I submit the pictured multi-sized approach. In the top picture, the comparatively tiny profile of the “Baby Bear” track can be seen, which does not even reach the man’s knee. 
Before going any further, though – a disclaimer: The pictures are to illustrate component geometry only. The track is shown as if it can be simply extruded, which it can’t, and I just thickened the top of the larger sizes to show where ribs would most probably go.
Also, I’m sure that there are some newer readers who may have never considered the matter of switching suspended PRT, and who may be confused by the various end-views. To them, I offer this more complete, yet still simplified, end view and explanation.

To switch suspended PRT, the part of the vehicle that rides inside of the track (the bogie) must grip one side of the track or the other, so that when the track routing diverges, the vehicle’s drive unit (bogie) does not fall out of the track’s ever widening slot. In this end-view, the pair of steering-guide wheels on the right “clamp” into rolling engagement with the track, while the left ones spread apart to release that side. In other words the bogie, shown in blue, can be fully supported by either side of the track and so can follow either as they split apart into different routes. The weight forces of the hanging passenger cabin are shown by the three red arrows. 

It is also noteworthy that certain parts of the track are eliminated where there is no switching. In this picture, the steering guide wheels are not quite contacting anything. Side-to-side guidance is maintained by the large, counter-rotating horizontal wheels; to do otherwise would simply add wear to the smaller wheels for no reason. The actual contact rails are only at switch points and taper into contact gradually for a smooth transition. Also, particularly observant, long-time readers will note that some track parts will need to be modified or removed for tight radius turns, both side-to-side and up-and-down.
Now, back to the top pictures. In these two illustrations, BTW, the steering guide wheels are engaged as if they were approaching a “Y” in the track.

The “baby” track is sized for speeds below 30 mph, (48 km/h) with weights below 500 lbs. including vehicle and passenger/cargo. It has 8” (215 mm) solid drive wheels. (It could handle more with steel wheels and very frequent supports but this would be inconsistent with most applications) The track width is 13.5”. (343 mm) 
To the person’s top-right are the “baby” wheels within a larger (mama-bear) track. Directly below are the wheels for the “mama-bear” vehicle within the same, matching track. This size (30” or 762mm wide) is consistent with the 3.5 person vehicles that have been shown in earlier posts, weighing in at about 1300 lbs., fully loaded. The performance would be generally auto-like, with quick enough acceleration to allow fairly short on-ramps, (an often overlooked design consideration!) and speeds approaching 50 mph. (80 km/h) It is not impossible that faster speeds would be practical…Only testing will tell. 

The second picture above  illustrates (left to right, clockwise) the “mama bear” bogie in the “mama bear” track,  the “mama-bear” bogie in the “Papa bear” track, and the “Papa bear” bogie in the “Papa bear” track. This larger wheel and track would be for still higher speeds and/or loads. Not much bigger than the “Mama Bear” combo, (35” or 890mm wide) the steering guide wheels get an extra two inches of diameter while drive wheels become auto-sized. The guide wheels of heavy vehicles can also be made of longer-lasting, harder polyurethane formulation or even steel, as sound is less of an issue because such routing would typically be along highways, more than fronting homes of businesses. Being designed for long distances and for larger (GRT) shuttle vehicles, this larger sized track would not universally assume full 3D routing and would be encased in a heavier structural elements. High speed sections would require some additional interior space to allow air to slip around the speeding bogie. Some very large cities might opt for the “Papa Bear” track throughout, to accommodate faster personal vehicles designed for typically long commutes. It is, after all, only 5 inches wider and height is more a function of span/vs load than wheel size.  I am somewhat dubious about the utility of making this largest track universally “baby bear” compatible, and one approach would be to design such track to be easily convertible for this purpose, rather than to include this feature initially, like the pictures shown above. This is primarily because of the speed differences, since any quiet and smooth running 8” wheel couldn’t tolerate continuous, heavily loaded, high speed use for very long, and so must be confined areas that are appropriate for low speeds anyway. For lighter parcel delivery, however, in, say, the 25 kg load range or less, it is a different story. Here small vehicles could almost fill the track spaces between large GRT vehicles, not slow them down at all, and still be nearly silent when they get into neighborhoods… without needing new wheels every thousand miles. 

What is regrettable is that the Mama Bear track isn’t smaller and cheaper – more of a mid-point between the larger and smaller sizes. Unfortunately once you can physically over-pack a vehicle with weight, (as is unpreventable with the interior cabin space required by disability laws) you can theoretically load each vehicle just as much, and as long as there is the remote possibility of a sudden stopping of traffic, where suddenly such loaded vehicles are bumper to bumper, applying all of that weight to a span, or even all applying emergency brakes at once on a curve, (thus pulling the track forward and sideways and down) … there is the need for some pretty beefy track, at least that is how it will be regulated. 

Ironically, the baby bear track can be small because it is small… If track and vehicles are cheap enough, it becomes easy to pay for, even without heavy ridership, which permits longer headways and slower speeds, so everything can be lighter-duty. Of course the “baby bear” can’t exist as a transit system alone, since it does not meet the criteria mentioned above. The Mama and Papa Bear systems need to really pack the tracks and run as fast as possible to pay their way, especially until the business model proves itself and costs begin to drop. This kind of traffic is best promoted by providing a certain degree of comfort…And comfort adds…you guessed it… weight. In PRT, there is no such thing as a free bowl of porridge!

Next step - Adapt this wheel geometry to actual stock steel, practical fabrication methods and truss design, as well as to finally address the long-neglected issue of “third rail” placement.