121> Solving Traffic with 3D PRT

The other day I read a paper promoting “robocars” and related technologies and I ran into the following quote:

“There are two weaknesses in the PRT idea.

1.       1. The need to construct new infrastructure. PRT proponents argue that the guideways would be light, requiring little space.

2.      2. There is no good reason why the vehicles must be held captive to the guideway.  Vehicles captive to the guideway are called Single Mode (SM) and those capable of operating off the guideway are called Dual Mode. (DM)  There has been considerable discussion of the merits of each approach.”

Let me quickly address the author’s second assertion.  I would point out that the obvious reason for holding a vehicle captive on a guideway is speed, which clearly relates to issues like safety and weather.  The author indicates, through this assertion, that he envisions improvements that are, at best, incremental.  OK; on to the main topic.

The article got me thinking about something that many PRT advocates seem to get, but many otherwise rational and educated people completely miss.  I will state it as bluntly as possible, because it occurs to me that only very distilled concepts seem to get traction (and funding) in our society.  You know, “war on terror,” “no child left behind,” etc.  Effective leaders understand the power of a sound bite.  I know that I am “singing to the choir,” and I know my readers are much more astute than most, but I have heard too many discussions  where even the most well-read PRT people stumbled around on this issue.  So here it is, boiled down to two pithy sentences, ready to pull out at the next opportunity.

Efficient urban transportation at ground level is a physical impossibility.  Therefore the best transit solution will necessarily require a whole new infrastructure.       Period.   End of story.  

It is simple physics.  Objects moving in different directions on a single plane will either bump into each other or have to wait for each other.  This is the universal truth behind traffic.  By moving in groups this effect can be minimized somewhat but never eliminated.  The best solution to urban congestion, by far, is to move in three-dimensions.  This, and only this, gives many-fold, rather than fractional improvement. 

Consider that once-modern, transformative invention, the superhighway.  Can you imagine eliminating all of the overpasses and putting stop lights in their place?  Each and every overpass can, in a sense, be thought of as a wormhole, a portal, that effectively eliminates a standstill condition in two directions.  A highway can even be thought of as a string of traffic solving overpasses, with the higher speeds just being a byproduct of this linear arrangement.  

It is the ability to leave ground level – to go from 2D to 3D - that makes efficient high-speed urban transportation possible.  You can have a 200 mph bullet train, but without getting off of the ground, there will be people waiting for it to pass all along the way.  And all of that wasted time adds up, even if it is distributed.  By the way, this example illustrates two important points, which I will call “Herd behavior” and “Saturation”. 

In herd behavior, which is the 2D version of flocking or schooling, many move as one.  This is the animal kingdom’s mimicry of fluid dynamics.  In the case of the bullet train many passengers are moving as one, like a herd, and very fast.  This is a great dynamic but it becomes decreasingly effective as cross traffic becomes denser.  At a certain point the traffic slowdown created by cars that must wait for the train to arrive and then pass creates compounding gridlock that would otherwise not exist.  A related problem, found along freeways, is the fact that underpasses are often few and far between.  There is just no free lunch with fast ground-level travel in the city. The slowdown is just distributed in a way that obscures the cause and effect.

   

Or take the example of a four-way crossing.  If there are only a few cars in town, obviously there is little chance of delay.  An intersection might only need a yield sign.  With moderate traffic, timed lights and other means can help greatly by moving groups in unison. (Like a herd).  To illustrate, consider how fowled-up things get when a traffic light is broken, so it is treated as a four-way stop.  Yes, moving groups in unison really works, and all kinds of clever routing and timing schemes are in common use.   With high-density traffic, however, a point is reached, which I will call saturation, when nobody, in either direction, “makes” the light.  In this case each stoplight necessarily cuts traffic flow by over 50%.  The avoidance of this degree of saturation should be the first object of any remedy.  Indeed, all of the potential benefits of robocars, intelligent lights, contraflow lanes, etc. fall into this category.  They don’t even attempt bidirectional non-stop movement.   

It is the difficulty and cost of building a 3D road infrastructure that is responsible for our traffic.  Roads and the vehicles that travel on them can only go up and down gradually, high-speed or sharp turns lead to skidding, and roads must carry extremely high weight loads.  It is impossible to use overpasses and cloverleaf interchanges on every block.  So nothing that runs on roads, as we know them, will ever be more than a “Band-Aid” solution...  The cost and space constraints of roadwork will keep cars largely earthbound and in each other’s way, and this will limit speed and efficiency.   

So the logic is very simple.  We need to move under and over each other to get around a city efficiently.  It may be cost prohibitive to do this with cars and trucks, but people are light and easy to lift, and account for almost all traffic.  Therefore it stands to reason that a very good solution to our problem is to start with the creation a 3D infrastructure for moving people. (and not, say, cement trucks)  If that is the starting point, logic ends up dictating the rest of PRT design.  I would even go so far as to say that a measure of the effectiveness of an urban transit infrastructure is the ease with which it can utilize multi-level routing. 

The PRT message has become muddled.  Now, when people think of PRT, they increasingly envision publicly rentable robocars.  Having a designated guideway that is really a roadway in disguise misses the point.  Any efficiency gains that such a system produces will almost certainly be at the expense of other forms of transportation.  I do not believe a network of golf cart lanes is the answer for in most cities, and having the self-driving vehicles for them doesn’t change that.  In any case, if it is non-stop, it is either elevated or it makes someone wait.  PRT, in my opinion, should no longer be all about automation or being electric.  The real key is that PRT is the only model which can cost-effectively initiate the transition toward a fully 3D urban transportation network.  Personally, I think the question of private vs. public vehicles, PRT vs. dual mode, is secondary as well. 

No, constructing a new infrastructure is not a liability.  It is an opportunity to take a quantum leap in efficiency by specifically designing for 3D space.  And making the vehicle captive would mean much higher speeds in any weather, and allow electricity to be fed to vehicles directly, eliminating the efficiency losses associated with batteries.  This infrastructure would, at last, be appropriately sized for the job, and so would move many more people per dollar spent, and do so with almost no physical  footprint.  It would be blazing fast to construct and could also provide invisible housing for unsightly and weather-vulnerable utility wires, as well as house next generation street lighting.  It could be engineered to allow silent vehicle movement, even with highly efficient hard wheels.

But this new infrastructure, unlike improvements such as fiber optics or gas pipelines, will not be decided in boardrooms, and so requires a degree of generalized public understanding to get traction.  We need sound-bites so simple even that the “experts” will get it.  We need to win the war of the pundits.  It is they who echo ideas until they become commonly accepted by the masses.   

A NON-STOP URBAN TRANSPORTATION NETWORK… Not a too bad a phrase.  If only we could get industry and academia busily pursuing this as a goal, they would return again and again to what we already know to be the answer.  Then maybe some of the funding that is going to improving legacy technologies worldwide could be shifted to where it would do more good.  We already can’t afford to maintain the pavement we have. How long are we going to continue to throw good money after bad?  

120> Back to the Drawing Board

Well, it is “back to the drawing board.” In the course of the continuing debate about dual mode I have started to lean more and more toward the idea that the shape and functionality of the vehicle is secondary to bogie function and design. If the control system is shifted to the bogies, then the vehicles can almost be viewed as simple containers. As such, the main concern is how much they weigh and little else.  

Of course there must be some degree of control from the passenger compartment. An emergency “abort trip” command comes to mind. It wasn’t previously so clear to me, though, where the computers and communications equipment would primarily reside and why. In post 56 I raised the possibility of autonomous “engines” that could live within the track and be called upon to boost the speeds of otherwise slower PRT vehicles. Clearly this would require command and control that is sometimes free from the vehicle below. Now I am contemplating taking this idea to its logical conclusion, which is to have a mobile, standardized “skyhook” that can latch onto a passenger compartment. Primary communications and driving functions would be from the bogie, which is only networked to the cab. (Please bear with the simplistic nature of the “hook,” as shown in the illustration. There is a lot to consider design-wise, and I haven’t gotten very far.)  

  

The approach enables multiple, concurrent business models. For example, privately owned vehicles could “hitch a ride” right along with public PRT vehicles. Freight vehicles could be little more than containers with an RFID tag. It also enables some promising schemes that can only be accomplished via privately owned, dual mode vehicles. Since one or two seat vehicles could be robust enough for some general road use without being overly heavy, they could play an important role in a transportation mix. Such small vehicles would not pass ADA compliance rules for public transportation, yet clearly should be encouraged for environmental and energy efficiency reasons. With this scheme they could be developed and sold by vehicle makers directly to individuals.

In another model there could be multiple taxi or limousine fleets. I particularly like the idea of separate business entities competing for the most comfortable fleet of vehicles. “Fit and finish” issues have always been a weakness in public transportation, since there is little competition in the field. By putting the brains into the bogies we simplify the challenge of creating a great, aesthetically pleasing and ergonomic passenger vehicle. This is no simple matter. Modern cars employ assembly lines many miles long to assemble tens of thousands of parts. Since there is so much to it, why not ensure that this part of the project is completely within the core competency of a wide and competitive field of companies?

I do not mean to totally confuse the PRT world with endless choices. But the “last mile” problem is real and not going away, and I doubt the notions of dual mode or private ownership will either. From a design point of view it is a question of “Why not?” 

Ultimately, the obvious business model is one of collecting a fee for using the track and the auto-navigating bogies within it. The “chicken and egg” problem would seem to mean that the service would start out as purely public transit, meaning the cabs are “rented” as well.  The company responsible for this service would have to keep vehicles clean and in good working order. I would think the cab interior, save the seat cushions, would be bare-bones, of hard, scrubbable materials. A taxi or limousine company, on the other hand, would pay for bogies only, at a discount, and then charge passengers a premium price for riding in cabs fitted for more comfort. Rigorous passenger screening or even memberships would minimize vandalism of the amenities required for a truly luxurious ride. 

Privately owned vehicles can be introduced even if they are not dual mode. “Pods” could be centrally garaged, for a fee, and made to arrive at any station upon request. (Failure of the owner himself to arrive on time would have to result in a penalty charge.) Eventually privately owned and garaged dual mode vehicles might appear, but I question how they would compare with regular cars, which themselves might be automated at that point. After all, if that is the case they could simply drop you at a station and go back home. An automated taxi could be waiting for you at your destination. (No, robocars alone CANNOT replace PRT, which is specifically designed for 3D, non-stop travel. Robocars will never be able to get across a city as fast unless they sprout wings!)

But back to the sky hook. A quick look at the illustration above makes another point about the design we have been developing. That is that the swing-arm itself is a fairly complex gizmo, which is a bit troublesome. Seeing it without the vehicle, though, gives a clue about who might want to make it. Below is a gantry robot. I am very glad these things are getting much cheaper, although they are not exactly following Moore’s Law.  

In fact, here is a clip of the machine in action. Doesn’t it seem like this system would be a lot more useful if it were untethered? Such an arm connected to a bogie would clearly need to clamp the track for precision positioning, but other than that..

Finally, when I say back to the drawing board, I mean it. Below is an example of how the three-wheel design from the last post might play out. Such a design can “land” on a flat surface like an airplane. The back wheel can be jacked up to pivot the front down creating a front-loading boarding ramp. The wheel size would be dependent on the anticipated use. Were it to remain permanently attached to the bogey, they would be very small. For dual mode they would be replaced with larger, motorized ones, and the maximum passenger weight would have to be restricted accordingly. I figure I might as well share it, because I plan to shift my focus back to the bogey for a while. Chances are it will end up buried deep in the dustbin that is my hard drive, at least in its current form.

119> Further Thoughts on Dual Mode

I want to express a few more thoughts on the subject that I raised in the last post, that being using something along the lines of the EN-V as a dual mode PRT vehicle. The first conclusion that I have come to is that the ability to balance on two wheels is not really that advantageous for PRT. This is especially true in countries that have the equivalent of the US’s “Americans with Disabilities Act.” ADA requirements for wheelchair accessibility mean vehicles need to be longer than the very short configuration that the self-balancing hardware was meant to enable.

I realize now that what enthused me most about the EN-V was not the self-balancing capability but rather the maneuverability afforded by the side mounted, independently engageable drive-wheels. Actually, steering in this way is not at all new. Inspired by aviation design, geodesic dome pioneer R Buckminster Fuller developed and prototyped what he called the “Dymaxion Car” back in the early 1930’s to address some of the same weight and efficiency issues that we are concerned with today. This mammoth eleven passenger vehicle got 22 mpg!

 

I remember reading anecdotes about amazed onlookers staring wide-eyed as the vehicle made a U-turn and parallel parked into an impossibly small spot in a single motion by using the full 90 degree pivoting ability of its single rear wheel. The same geometry is widely used today in the form of “piggyback” forklifts, primarily for the maneuverability it affords. This rear-wheel steering concept should not be confused with the many other “reverse trike” designs out there that have front wheel steering. Front wheel steering is undoubtedly better for roadworthiness at higher speeds but the space and position requirements tend to highjack a vehicle’s design more than Bucky’s layout. 

Those following the comments section of the last post have already heard my opinions on the dangers of adding anything more than the most modest weight gains to a vehicle. I still believe that the concept of cheap light track should be the primary design consideration because the main advantages of PRT only really manifest themselves within a network. Configurations of simple loops or figure eights would be better served by GRT, shuttles, etc. If dual mode capabilities compromise this priority… Well, in Texas we call that “Lettin’ the tail wag the dog.” There needs to be some limit to how much weight that ground travel capability imposes on the system design, and that limit is a painfully small amount. 

I think it is time to consider the matter though, because certain aspects of the whole system design may be contingent on the results. For example, having wheels on the sides strongly suggests having a front-loading door, and that influences station architecture. Any ground clearance creates an elevation that must be navigable by wheelchair. Seating changes could influence weight limits and distribution, possibly changing bogey design. So here are a few thoughts.   

The case for 3 or 4 wheels is not completely clear. With a four legged table, if you remove a leg it may or may not balance on the remaining three. In any case it won’t immediately crash over. A four wheeled vehicle behaves similarly when going over a pothole. The momentary removal of support has little effect. This is not so with fewer wheels. At least with the two in-line wheels of a motorcycle you can steer around bumps. Not so true with trikes or Segways. This leads to the design choice (for 2-3 wheeled vehicles) of larger diameter wheels that can better span dips. This can also be accomplished by wider or double wheels but this adds weight quickly. For four wheelers, the wheels can be smaller, but only with all four wheels being highly steerable can you match the maneuverability of those two side mounted wheels. 

None of these limitations bode well for the goal of speedy, comfortable, or long range dual mode. The question becomes one of how much hardware one is willing to haul around everywhere. Way back in Post 50, I brought up the idea of a drive-by-wire skateboard approach to address the problem of carrying this deadweight. Obviously this is a complicated solution, but one that completely addresses issues like larger batteries, robust suspension, etc. There is a tipping point where carrying around integrated dual-mode hardware becomes impractical weight-wise and the skateboard becomes the better choice. In my opinion this threshold is reached well before the vehicle is roadworthy. This is not to say that seldom-trafficked residential streets wouldn’t be drivable, just that busy streets are not safe for such vehicles just as they aren’t safe for golf carts. 

If you preclude busy streets, with their crazy drivers and potholes, and assume that trips will be fairly short, then the PRT vehicle could carry around the needed hardware without too much extra weight. The emergency battery could be split between the bogie and the cab, and that cab portion could be sufficient for short trips. If travel is mostly on paths specially paved for the purpose, suspension requirements are minimal. If the trips are short, minimal tires will last an adequately long time. If trips are short, speed is not a concern, so motors can be small and light. If speed is not a concern, wheel and load balance geometries can be used that would be less than satisfactory for ordinary driving, such as the Dymaxion car design.   

In conclusion, I think the best balance may be in the old Dymaxian car design, with a clamshell front door. Two large diameter (but thin) wheels (think dirt bike) with “in wheel” motors would fit into a pair of skinny wheel wells. In the rear would be an external (but shrouded) steering wheel. Some means would be needed to rock the vehicle forward to facilitate entry for wheelchairs. The wheels could be designed as modular, removable components and vehicles without wheels might coexist within the system. I would shoot for under 50 kg of total added weight. I was hoping to include some preliminary illustrations, but these things take a lot of time.

118> GM and Segway’s Unintentional Dual Mode Platform

As many of you know, I have never been a big fan of dual mode.  The problem is simple.  If a vehicle is light enough to be part of an effective elevated PRT system, it is too light to be a robust road vehicle.  Conversely, if it is sturdy and comfortable enough to not seem ridiculous as a car, it would require a track that would be unacceptably massive and costly.  You tend to either have a bad car or bad PRT or both. Yet if you thread the needle just right, they are off by tantalizingly little . Lithium based batteries, ever-shrinking computing power, carbon fiber technologies and miracle plastics are nudging things forward, but what is needed is something really dramatic.  Something to knock off half of the weight form the start. 

The other day I was thinking about all of this, or at least how to transport people that last mile.  I was considering bicycles, scooters, and Segways, and wishing for something that would have a roof to keep the rain out.   I was even considering what technology would be involved in matching PRT to a Segway, instead of the other way around.  I even have a picture to prove it.  

It was around then that I decided to do an image search for a rain-proof Segway, and I stumbled onto something that had passed beneath my radar when I first heard about it.  It is the EN-V concept car, which, as it turns out, which may well be the “best-yet” dual mode platform.  It is a joint venture between GM and Segway, and it does two things that really reduce the weight problem.  First of all, it runs on two wheels instead of four.  Roadworthy tires and wheels are heavy, after all, especially if you include durable shocks and springs.  This vehicle also lacks a mechanical steering wheel and all associated linkages.  Like the ULTra, it is self-navigating, or at least, “drive by wire.”  That brings up the intriguing notion of having the vehicle drive itself back to the station after dropping off a passenger.  The side-by-side, two-wheel arrangement enables steering without any pivot assembly, and also allows 360 degree rotation in place, something that might add considerable flexibility in station design.  

The relationship between the EN-V and PRT seems symbiotic: PRT can’t go the “last mile”, and the EN-V can’t go all that far.  The EN-V’s shortcomings in speed and battery life could be rectified by an electrified track.  Equally promising is that the EN-V weighs in at under 500 kg, and that is for a version with a much bigger battery and motor than would be required for dual mode use.  Also, as long-time readers well know, I have my doubts about how good of a PRT vehicle can be designed and constructed by any fledgling company without seriously deep pockets for R&D.  GM and Segway have dumped a lot of money and knowhow into this project.  They have based the vehicle on what they call the “Puma” platform, which is literally just that… a versatile, self-balancing platform slung between two wheels.   

I really think this combination deserves some serious consideration, more than I have time for within the context of a single post.  Look forward, therefore, to more on this subject in days to come.

Here are some related videos: This short clip shows the “Puma” platform without the passenger compartment. 

This second video shows a bare-bones version in action as well as a simulation of a city street designed for using the vehicle as an ULTra-like PRT.

The EN-V comes in three flavors, as shown in action in these vids.

http://www.youtube.com/watch?v=-fyIGp2qrmw

http://www.youtube.com/watch?v=hCGUmsdMifo&NR=1

117> Snow Day Musings

Well, we’re having a snow day in Houston…Well not really a snow day, actually it is an ice day. The buses are not running. Everyone is being advised to stay home. At the moment, all of the freeways are closed. It seems like a good moment to curl up with a warm laptop, and tap out some thoughts about this epic winter.

Being a mere six hours from Mexico, we are not well prepared for these kinds of weather events. We have no salt trucks or snowplows, but yesterday a truck preemptively applying a deicing solution caused a great traffic jamb, of which I was a part. It did no good.

It boggles the mind to think of the calamity that these weather systems are causing across the US (and Europe?) this year. In the last one, there were even fatalities in New York because ambulances couldn’t make it through. Enough, already! Is this really the best we can do?

The ongoing recession should serve as a “teachable moment” that illustrates the effects of a few percentage points of reduced economic activity. Clearly, these weather events must work against our collective well-being, event though we may not make the association.  Such shutdowns further compound the wasted productivity caused by simple traffic, illustrated in the chart below.

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Being paralyzed like this should point out the consequences of having all of our transportation “eggs in one basket.” For example, the rise of radical Islam makes me wonder if anyone has really considered what would be the effect of a sustained campaign of sabotage against road-based travel in a modern society. After all, a single disabled vehicle can nearly freeze a whole highway. Imagine the effect of terrorists simply targeting the tires of moving vehicles on a continuing basis… or even traffic lights for that matter. (I’m glad my readership is a very small and constructive group, or I would not share such notions)

There is also the warm weather counterpart to the “snow day,” which is street flooding. PRT systems can be specifically designed with this in mind. As ridiculous as it sounds, many urban areas around the world are built on floodplains. What would have been, for example, the result of a PRT system in New Orleans? If predictions of climate scientists are correct, we are in for lots of major weather events of all types in years ahead.

Anyway, my point is that society has reached a point of unprecedented interdependency, and is very vulnerable to any disruption in travel. These are not the days when everyone had canned leftovers from their large gardens and a cord of firewood on hand. If our transportation stops, our means of survival (and escape) does too.

Astute reader Lars Endre recently referred to raised PRT as exploiting “the virgin third dimension.” I love the phrase. It occurs to me, however, that it is not really virgin at all, at least around here. Here we have lots of spaghetti-like highway interchanges that are many stories tall. The interesting thing is that these forays into that third dimension are the very reason this city has drawn to a halt. Raised roadways and overpasses freeze first. We have many miles of raised HOV (High Occupancy Vehicle) lanes, connected to “Park and Ride” parking lots. Our embryonic light rail system continues to work, but nobody can get to the stations.

Too bad we can’t exploit the nice dry undersides that such elevated structures enjoy. Oh wait… We can! Yes, it’s the underside of that virgin third dimension that offers the possibility of completely weatherproof transportation. (You knew I would turn this into a shameless plug for suspended PRT sooner or later, right?) 

While we're talking about snow, although this is a bit off-topic, I would like to take this opportunity to mention the troublesome act of repeatedly salting roads. It reminds me of the age-old notion of dumping waste into the ocean or atmosphere because “It’s just so darn big that it can’t be hurt.” We know better now, but we continue to salt the earth on a mega-industrial scale. (10 million tons per year in the US, which works out to 66 lbs. annually per person) It works its way downward, eventually, to the water table, where it migrates “away.” Some readers may have seen these structures along roadways and not known what they are. They are structures for storing all of that road salt. I only wish PRT could solve this dilemma too.  

Finally I just thought I would share these pics that I happened upon. As you can see, the need to remove snow from our 2D transportation systems is not confined to just roadways.

Well it’s a day later and the sun is shining and the roads are clear once again. Time for me to wrap this up…

PRT cannot mean an end to roads or the costs of maintaining them. There still will be the need to move heavy loads, that last mile problem, and the whole countryside beyond. But an extensive, all-weather PRT network could, in times of crisis, be a very important backup system to have. We sure could have used it yesterday.   

 

116> Parkin' On the Hill...

This post is in response to comments made regarding the last one. In order to illustrate my points, I have used a modified version of the following picture, which I had originally intended to use in a different manner, so even though it is a bit off-topic, let me start with an explanation this illustration first.

This picture is the result of a design exercise, the object of which was to create the highest capacity station possible with the smallest footprint. In order to do this, I used elevators with curved doors, so that they can retract without needing much space in the walls. There are two of them, one for entering and one for exiting the station. This station is not “off-line,” but would rather be bypassed by a track that is not shown. Four cars can be loading while four are unloading, and (guessing a time of thirty seconds to get seated and on your way) the capacity of the station would be one car per 7.5 seconds, which works out to 480 vehicles per hour. It was designed to be ADA compliant, yet has a footprint of only about 50 square ft. One thing to think about is that if 8 of these stations where operating at capacity, the track they would be feeding would need sub-second headways to handle the passenger load. A station like this would be factory-built and delivered to the site in several pieces. Obviously the design is a bit misplaced in this setting, which isn’t exactly downtown, (so a footprint small enough for a crowded sidewalk isn’t really needed) but I had no other jpeg to “shop” the model into.

This leads me to the next picture, which shows a two-way variation of the same station. In this one, each elevator handles both arriving and departing passengers, with one elevator being for each direction. I drew this in response to alert reader Lars Endre, who suggested the possibility of using sloping track to capture the energy lost in deceleration. While this would be impractical for most PRT designs, it’s a concept that is well suited to self-leveling, hanging systems. The idea rests on the recognition that it takes a great deal of energy to get a vehicle up to speed and that it wastes a lot of kinetic energy to get it to stop. Parking atop a hill, so to speak, addresses both issues. This picture shows such an arrangement.

An alternative (frequently mentioned) approach to the problem is regenerative braking. As the vehicle slows, the momentum of the vehicle turns the wheels, which rotates the motor faster than it wants to go. This turns the motor into a generator and a brake at the same time, and the power is fed back into the track to be reused elsewhere. Sounds good when you say fast. I am not, personally, completely sure that this is an efficient process that is practical to exploit, what with electrical transmission losses, etc., especially with minor voltage supplementation in a DC system. 

Regenerative braking raises another fundamental question. How much braking do you want to do? After all, if the system is smart enough, it ought to have vehicles coasting more and braking less, right? The problem boils down to the need for speed. To some, it is assumed that assumed that PRT has a natural speed limit. Studies have shown that as speed increases, the safe spacing between vehicles must increase as well. Thus a system with a densely populated track going slower could move more people than one with faster, more widely spaced vehicles. The problem with those studies is a glaring fault in logic. It assumes that nothing can improve braking ability or crashworthiness of the vehicles. Fix that and you can both pack them tighter and go faster. But then we need brakes, and must deal with those mechanical inefficiencies. Consider the off ramp leading to a station. Making the split-off ultra gradual and giving a very long lead-up track is not very practical. 

 

Here is a different angle to show more track. While my first instinct was to think that raising the boarding area was a waste of materials, I soon realized that this cost could be offset by allowing shorter on/off ramps. Obviously this is more of an attractive option for fast, densely populated systems than for slower ones with few vehicles.

Another consideration is the “Umbrella Effect,” where overhead structures block the sky, a concern for landowners along the route. While this is less of a concern for minimalist track systems like I advocate, in a bi-directional station like the one shown there is still a lot of track up there, as can be seen. (Imagine the ULTra track four lanes wide!)  Long acceleration lanes represent additional visual obstruction. If raising the station can shorten these ramps, that would seem to be a plus for public acceptance as well as cost.  Even the station itself would appear somewhat less imposing by being higher, as more light would get in beneath it, and individual areas would remain shaded for less time. 

Astute reader Andrew F further pointed out that in tight turns, a sloping track could also be used to “bleed off” speed. (and give it back again after the turn) There are plenty of tight turns in a city environment, so this is something to consider. The negatives here are about ride quality, the way the system looks, the extra engineering, etc. Clearly, going very fast downtown would require a system that would be designed like a roller coaster, and I doubt we really want to go that far. On the other hand, in a system fast enough for commuting from the suburbs, there will always be the interface into the slower urban environment, just like freeway exits feeding downtown streets. Such an approach should certainly be in the toolbox. The case for using slopes to slow or speed a vehicle naturally arises, I believe, from the fact that it is so easy to do, considering that the track is raised anyway and the vehicles are designed to handle slopes and turns with minimal discomfort to the rider.

115> Swing Low, Sweet PRT...

Sometimes a technology pops up out of the blue that unexpectedly solves stumbling blocks that have been around for years. In this case I will refer the reader to the 3-axis accelerometer, a nifty little device that you probably own a few of. Own a car? There’s probably one in the airbag controller. Own a digital camera? Probably one there too, to help you take a clear shot with shaky hands. Got a smart phone? That’s how that nifty feature that keeps the screen upright works. Game controllers. The Segway. The list goes on.

What I am exited about is how the device can operate as a level, a feature that was exploited to create the iPhone app above. You see, because gravity and acceleration are essentially interchangeable, an accelerometer senses gravity as constant acceleration. Zero this force out with software, and your accelerometer senses “acceleration” every time you tip it in any direction… a thousand times a second.  Voila! A level! Or actually, to be more precise, an inclinometer!

Meanwhile, in the world of motion control, engineers are redesigning the electric motor. In the old days, it was realized that rather than putting bunches of electromagnets in a large circular array to make a motor, just a few would do, if the rotation were faster. That way, the same magnets could come around and around again, faster. More power, less materials. Magic! Only problem was that many people didn’t want fast rotation, and a century’s worth of bulky and inefficient reduction gearboxes ensued. Recently, a revolution has been taking place in the world of motors, the conversion from mechanically switching the electromagnets off and on (brushed motor) to using an external controller. Now, rather than simply rotating at a given speed, motors can be made to stop, change speed, reverse, hold a position, etc. The modern servomotor has been born.

Now a new generation of brushless, direct-drive motors is emerging which return to large diameter magnet arrays for torque, rather than gearboxes. These offer powerful and accurate rotational control without requiring that a machine be designed around standard gearboxes and motor mounts. I am referring, specifically, to frameless torque motors, which have reduced this architecture down to a simple pair of concentric rings. They are simply inserted between the machine and the shaft to be turned, like a bushing or ball bearing unit.

So here’s how it all comes together. Pictured above is the swing-arm for the PRT vehicle that I have detailed in previous posts. Two pair of frameless torque motors (shown in red) are controlled by an accelerometer. These keep the vehicle in line with the normal gravitational forces. The accelerometer-torque motor combination can, in theory, eliminate any errant, sideways G forces. The idea is to emulate a free hanging system, without really being one. Why not just let it hang? Unbalanced or shifting loads, sudden gusts of side winds, or continual rocking back and fourth are all effects that need to be canceled. Other than that, a free hanging design has the wonderful effect of self-canceling motion-related forces from acceleration, deceleration, or turning. It’s like a bucket on a rope. No matter how you swing it around, water in the bucket won’t spill, because all gravity gets shifted toward the bottom of the bucket. With a vehicle-mounted accelerometer, any forces that it senses other than “downward” (in a relative sense) would cause the motors to lock up to arrest that movement, with the exception of a slight dampening, to control of the tendency to swing repeatedly like a pendulum.

I have mentioned in previous posts how the swing-arm design is extraordinarily safe, because rather than throwing occupants through the windshield in a head-on collision, the cab would swing forward, absorbing shock and transferring the direction of momentum so that it would essentially push the occupant into the seat instead of out of it. In this system, extreme forces will initially simply break the magnetic bond, allowing this forward swing. As the swing continues toward its apogee, however, the relative strength of the torque motors increases geometrically, applying ever greater braking force. Meanwhile the cab has gone from traveling forward to traveling upward, so it is additionally fighting gravity. All of this absorbs the force of impact without any mechanical damage to the vehicle. Combine this with bogey-to-bogey bumpers, and you have an extremely effective crash protection system. There are theoretical and mathematical ways demonstrate that split-second headways are not dangerous for PRT vehicles, but it’s pretty hard to beat coming out of a crash test damage-free to drive the point home.

Bottom line:

 You could set down a full cup of coffee and be whisked away at high speeds without spilling a drop. There is no reason why PRT can’t put any luxury car to shame in ride quality. An added bonus is unprecedented safety.  

114> In Search of a Cheap Lunch

One of the dirty little secrets of “green” electric cars is that the batteries have consumed a lot of energy and created considerable greenhouse gases before they are even installed in the vehicle. The real environmental cost of batteries goes all of the way back to the mines, where diesel fuel is used in large quantities to extract ore. Fossil fuel is an ingredient in the plastic battery cases. Refining the ore into metals and useful compounds often is extremely energy intensive. It takes fossil fuel to ship the materials to the battery maker and still more energy to assemble them. Of course then they need to be shipped to distributors or to the vehicle manufacturers.  More energy lost. The real energy costs should probably even include the energy budgets of all of the employees of all the companies involved insofar as those expenses are directly tied to the manufacturing process. (A miner’s gasoline costs getting to the mine, for example) Then there is the energy to move the electric vehicle’s extra battery weight, and eventually everything involved in the steps of removal and recycling. Then there is the fact that fossil fuel is burned by utilities to generate electricity to recharge the batteries, but let’s leave that one aside for the moment.

It is a reasonable to ask, “How much energy is actually saved over simply fueling vehicles with gasoline directly?” After all, gasoline has one thing going for it. The pipeline between the well and your car is very efficient. This is something to consider with other supposedly “green” products as well. Solar cells, for example, are notoriously energy intensive to make and, likewise, do not last forever. It’s like the oil used to make the fertilizer for the corn to make cleaner burning ethanol fuel. There is no such a thing as a “free lunch…”

I am certainly not saying that this stuff is a waste of time to pursue, but that it should be considered in the design considerations of nascent technologies like PRT. This applies to all design choices, not just whether to use batteries. In particular, I would point out that my call for a minimalist track profile is not purely for aesthetic reasons. We ought to be asking ourselves, “What is the greenest possible medium for moving from point A to point B within the urban/suburban environment?” This, as luck would have it, will also probably be the cheapest, and least objectionable to look at.

I submit that a power-carrying micro-monorail system is the greenest alternative, all things considered, unless we can invent a way to make ski lifts have branching routes and off-line stations. It should be as close to invisible as possible and use minimal materials. It should allow very flexible routing options including tight turns, steep slopes, etc. If it can’t be run somewhere, then people can’t use it. I further submit that it should be thick enough to be a “workhorse” that can take fast vehicles and span wide streets without shaking or sagging. Being too thin mandates closely spaced supports, which can also be a disadvantage. On balance, this trade-off puts me squarely in the Ed Anderson camp, size wise, of about a meter high and about two thirds of that in width. Long-time readers of this blog know how much I have agonized over these dimensions. One advantage to a self-leveling suspended vehicle, I would note, is that it can transition in elevation easily, so that the main routing need not be on the same level as the stations, enabling track that can be higher and more out-of-the-way, if that is what the community demands. We don’t want to cut trees to put PRT in.

PRT has been caught up in kind of a “Gee-wiz, I’m so futuristic!” mindset, even though there is nothing, in this age, futuristic about it. But it is still about being green. My last post was about how free-roaming robocars had co-opted the PRT moniker, and we’ve been having a lively debate on better names. I would just like to add this thought to that debate. If PRT is the physical equivalent of the internet, then the track is the equivalent of telephone wires or fiber optic lines. I say, “Let’s go broadband from the start!” Furthermore, let’s make that infrastructure as green as it can be. That means not being designed to be scrapped, but rather being modular, so it can be moved and reused rather than melted down; It should contain zero fodder for the landfill.  PRT, of the powered rail variety, isn’t just another green transportation alternative. It is the ultimate green alternative, bar none. (I’m not counting open-air or human powered vehicles) So maybe it should be presented that way, by the infrastructure, and not the vehicles or the difficult-to-explain operational characteristics.

After all, if you are promoting “elevated microrail transit,” then the whole rest of the PRT paradigm becomes implied. 

“Automated or involving lots and lots of drivers?”  - Automated.

“Make everyone wait behind a stopped vehicle or have off-line stations?” - Off-line. You get the idea.

In the end, being green, being efficient, and being prosperous are all one-in-the-same. Battery powered electric vehicles, though not a complete red herring, do start with substantial energy deficits that should not be ignored, so environmentalists should be made aware of the fact that powered rails are a much more efficient option.

Elevated, line-powered, mini-monorail transit: To me, it’s a no-brainer. There should be non-profits promoting it, universities developing vehicles for it, the works. It’s where we need to go. Delay in doing so is simply squandering resources, including our land, our raw materials, our fuel, our time, (spent in traffic) our time (spent building and unbuilding stuff) our (still not totally carbonated) atmosphere, and of course, our money. 

PS - If there’s anyone who can find a link to actual studies on the energy used in the life cycle of batteries I would be grateful if you would share… I have only found this paper, which is so outdated that it doesn’t even have figures for Lithium-based types. Finally, I would like to share this video, listed as “300 years of fossil fuels in 300 seconds”.

113> 2010 – Historic!

It has been said, “History is written by the victors.” Well it appears that the victor, in the war of competing PRT standards, is the ULTra/2Getthere steerable design. It is reasonable to assume that the systems will work as advertised… well enough to get some new contracts while continually improving. Therefore:

“PRT” now means automatic, driverless cars that travel on pavement. It doesn’t really matter anymore that PRT started out as something different. PRT is, and will be, a short-range automatic shuttle service for airports, campuses and the like. PRT will require snow plowing, battery charging and replacement, and will compete for space with cars, bicycles, golf carts, or pedestrians. Get used to it.

The fact that some of us have tried (or are still trying) to craft PRT as a next century transit solution is beside the point. That is not what PRT will be in the minds of most people a decade from now. As the folks at ULTra proudly point out, they have more people working on “PRT” in their company than all other PRT companies combined. To those of us who see PRT’s potential as a means to a more environmentally sustainable and efficient future, I can only say we had better retool our message, and do it fast.

Of course Masdar and Heathrow will have one beneficial effect. They will demonstrate the viability of computer-directed traffic management for small, automated vehicles. At least that is one hurdle out of the way…but honestly…was that outcome ever in doubt?

Our goal will be, then, to widen the discussion to include less pavement, not more, higher speeds, longer distances, more efficient ways to deliver power to the motors, much larger scale networks, etc. It will be hard to talk about it, though, because the term “PRT” has switched from being overly inclusive to being downright misleading. Now discussions about PRT deployment for a city will logically begin with providing ground-level right-of-way, similar to mapping out potential bicycle lanes. Discussions about elevating significant portions of this “track,” I predict, will end pretty quickly, as the logistics become apparent.

I think I like the term “microrail PRT.” (Not to be confused with “MicroRail,” a small gauge train from MegaRail Transportation Systems Inc.) It is reminiscent of the word “monorail” but obviously refers to something smaller. Remember, most people have no idea what PRT is, and it takes a while to explain. Say “microrail PRT” and they might get a picture of a tiny monorail in their heads. (or maybe something on a roller coaster track) Either way, it’s minimal and elevated. PERFECT!

I think PRT, as it was originally envisioned, was really a multipart invention. It synergistically combined the concept of many small computer-controlled vehicles with the concept of an electrically powered light rail system that could be economically run above street traffic. 

 

The cost of free-spanning, beam-like support structures, you see, is reduced exponentially (I’m using the term informally) as their weight bearing requirements are reduced. It’s like fleas jumping 200 times their body length. Some things are possible only at smaller scales. The lightest human-carrying vehicles are in a weight range where a single-beam track can be almost ridiculously cheap, especially when compared with the other options in densely populated areas.  It is true that access-for-the-disabled laws, or any scheme that enables capacity much beyond the average (110kg for autos) occupancy greatly increases track costs. That was a clear lesson of Raytheon’s PRT debacle. Nonetheless, carefully designed vehicles can still allow track costs that would enable a true transportation revolution… of that I am convinced. This is both the challenge and promise of PRT… er… microrail PRT…hmmm… microrail podcars?  Automated Microtrack Transit? Autonomous Minirail Transit?   HELP! We need a new name!                 

                                                   Happy New Year!

112> Interview with Santa

 

This just in!… After persistent rumors of a historic PRT deployment above the Artic circle, intrepid reporter Dan the Blogger goes to the North Pole to interview the big guy himself. That’s right, folks. Santa Claus himself has made a “ringing” endorsement of PRT technology at his northernmost campus…Here are excepts from the interview…

Dan the Blogger: “Santa, I must say I’m a bit surprised to see you adopt such cutting-edge technology. What made you turn to PRT?”

Santa: “Simple logistics Dan, I run a tight ship here. It’s a pretty big enterprise. We’ve got daily shipments coming in…one hell of a payroll…I can’t be wasting time stuck up to my keester in snow, trying to get the reindeer hitched-up. What’ya think? I should snowshoe all the way from the house to the workshop? And it’s worse for the elves, ya know…I used to have to issue periscopes and shovels or they’d get lost entirely!

Dan the Blogger: “It looks like you went with Vectus. Any particular reason why?”

Santa: “Have you ever tried to fit a reindeer in one of them “Skyweb” things? Damn uncomfortable for the reindeer, I’ll tell ya.”

Dan the Blogger: “Wow, it seems like you really put PRT to good use.”

Santa: “That ain’t the half of it, Dan. I even got track runnin’ between the warehouses and the docks.”

Dan the Blogger: “the docks? I thought you…”

Santa: What? Fly everything from here in a single night? What are you in? The 1860’s? The stuffs gotta be staged, ya know…I got warehouses all over. Plus I got ships comin’ in from China almost daily.”

Dan the Blogger: “I….I thought the toys were made by elves…”

Santa: “Chinese elves, Dan. “…do a wonderful job.”

Dan the Blogger: “My readers are probably wondering what you think of having the linear motors in the track instead of on board… I guess that probably saves weight and space as well, doesn’t it?

Santa: Don’t need motors, Dan. I just tie some cars together and put a reindeer in the front one. Works like a charm…. Hey, I gotta get back to it, ‘less you wanna see 5000 little girls get headless dollies…

Dan the Blogger: Thank you Santa, for this interv….

Santa: “yeah, yeah,.. Keep your stick on the ice, fella… oh, and Dan…”

Dan the Blogger:  “Yes, Santa…”

Santa: “You know damn well I can’t give you THAT kinda thing for Christmas…”

Dan the Blogger: “Yeah, I know…”

Dan the Blogger: MERRY CHRISTMAS EVERYONE!

111> Capacity

For some time now I have advocated designing PRT for more than strictly CBD (Central Business District) use. Even though the “flight to the suburbs” in the U.S. has led to the need for urban renewal, and PRT would be a terrific tool for just that, the suburbs and highway fed satellite communities already exist, and need service too.

Companies wishing to get a PRT product to market do not have infinite time and development dollars, and their efforts necessarily must start from the basic building blocks of stations and loops. There is little point in promoting a vision that they are not yet prepared to follow though on. Unfortunately, a scaled-back, slow version of PRT is a lot like a scaled-back, slow version of the Internet. Eighty people being able to dial up a dozen sites won’t exactly make you thunderstruck by the possibilities. Yet that is a stage that was, at one point, a future vision. 

So here’s a little peak at a PRT “trunk” line, which, of course, can only happen when there are massive networks on either end to feed it. In this picture there are four tracks going in the same direction. Reversible lanes would offer up to five possible configurations, direction-wise.   

Here are some sample numbers. With a one second headway, and vehicles traveling at 60 mph, (88 ft. spacing) and the U.S. average of 1.2 passengers per vehicle, the capacity would be 17, 280 passengers per hour. Not bad for fitting over a strip of grass no wider than a residential driveway! With vehicles that can swing forward from sudden deceleration and bumpered bogies that clamp the track for extra stopping power in an emergency, still shorter headways and higher speeds can be expected without much technological challenge, especially if platoon strategies are employed. I think what is informative here is the capacity vs. the minor amount of steel and concrete needed to achieve it. By the way, with rubber wheels inside of a sound-insulated track casing, the system would be nearly silent.
                                    ______________________

It occurs to me that when I invent, I frequently start at an endpoint or finished product and work backwards toward the present. In a world of cause and effect, one can start at the desired effect and try to envision the causes that could create it.  This is why I have been advocating a standards-based open-source PRT solution. Can you imagine a single company ever being able to successfully scale up to this kind of volume? I can’t. The only way I can see it is if the track is a simple and standardized design, buildable by local firms, the control system is infinitely extensible, and multiple manufacturers can compete for business with certified, standards compliant vehicles. The funding needs to come out of the highway budget, and there needs to be a non-profit organization specifically tasked with organizing and fostering the public/private partnerships to make the whole thing happen, as well as developing and maintaining the standards. 

I do not rule out one company building a starter network and growing substantially from there. I just do not think that the design choices that are best for the company’s shareholders in the short or medium term are the best choices for PRT as a long-term technology. PRT can have extremely positive effects on society and the planet and still be the basis for very profitable businesses, but there is no reason to believe that a system designed to be profitable and saleable today will resemble the design that has the most transformative potential. In particular, I think current designs have traded away flexibility in a rush to have a market-ready product. These designs have limitations in terms of speed, station layout, turning radius, scalability, safety, track pitch, and a host of other issues. Once deployed, these limitations become set. If they later become burdensome, there is not necessarily any way to remedy the situation while maintaining compatibility with the legacy system and its installed infrastructure. 

P.S.   I have added a Table of Contents. Now you can enjoy one-click access to all 110 posts in the archive! Also, there have been no malware alerts connected with this site per se; The one reader who has reported a problem apparently only gets a warning from the use of his “Google Alerts” redirect function to this site and not the site itself, so I’m not going to worry about it.

110> Google's Robocars

First this note; I have recently been informed that this blog has twice triggered a virus alert for one reader. Has anyone other than this one reader had any malware alerts when visiting this site? I take this matter very seriously and have not been able to identify any malicious code. I have removed the “recent comments” feature from the sidebar, (again) since, in the Blogger forums, it has been suggested that such third-party “widgets” could be to blame, although no mention has been made of this (most popular) one specifically. Anyway, if you have received any alerts, please tell me about it, via email or the comments section. I am hoping it is a false alarm.

While I was out of touch last month, up at the cabin, an extraordinary thing happened that I just found out about. Apparently Google unveiled small fleet of driverless cars that had secretly logged 140 thousand miles of varied California driving with essentially no human assistance. It seems that the age of the Robocar is really here – at least as far as the science goes.

I am an unsuccessful contestant in a recent Google contest, which was about world-changing ideas. I can’t believe they picked the “Shweeb” concept over open-source, standards-based PRT. Now they have invested in Robocars. These people are no strangers to PRT, I just wish they would jump in with both feet. 

It seems to me that Google has “out-ULTra-ed” ULTra. After all, couldn’t Google’s modified Toyotas make the runs around Heathrow with ease? For that matter, it seems like they could just take you the rest of the way to your door. Back a few posts, I speculated that the real value proposition for a four-wheeled, pavement-driving designs like ULTra is really dualmode. Now it seems pretty clear that Google could clean their clock in that, and their present business too, if they wanted to.

In a way, my worst fears have come to pass. I believe that much (if not most) of the promise of PRT lies in the establishment of an alternative to roads, rather than making vehicles automatic. Regular roads are designed to carry huge loads, making them very expensive to elevate. Yet it is wildly inefficient to remain on the ground, where there are constant conflicts between people and cargo going different directions. There is also limited space. In most cities, well over 30% of the land is devoted to vehicles, either for roads or parking. This huge, paved landmass is an environmental nightmare, atmospherically, climatically, and hydrologically, not to mention a giant waste of very valuable real estate.

Because of the public’s resistance to having substantial overhead structures in front of their houses or businesses, it is essential for PRT track to have a minimalistic profile. This suggests something in the shape of a simple beam, and certainly not a wide, flat running surface, which would form an umbrella over the real estate below. (particularly at junctions)  Robocar makers may pay lip service to raised track, but practical realities say otherwise. Additionally, the removal of any way to “hook” into the track automatically makes travel in icy conditions something that just can’t be done at reasonable speeds.

With PRT being broadly defined to include four wheeled, steerable vehicles, advocates, (such as myself) are faced with the assumption, by many, that PRT is simply robocars on a designated roadway. This is defining PRT away from its original premise. Under the broadest definition the vehicles could even be gasoline powered. This is very, very far from the promise of PRT as envisioned by the early developers of such systems. No wonder people consider driverless cars as being an equally viable alternative to PRT. The PRT that they are familiar with has been stripped of most of its advantages. 

I just hope the good people at Google understand the unintended consequences that are inherent in their technology. Private driverless cars will encourage greater fuel consumption, because being free from driving will encourage other ways to pass the time, such as eating and drinking, watching TV, doing office work, etc. The resultant mobile office/living room will mean bigger, not smaller, vehicles. The comfortable and productive ride will encourage longer, not shorter commutes. It will encourage more pavement, not less.

Having robotic cars, especially with special lanes, may seem tantamount to a true PRT system to many, and this fact endangers PRT adoption. You can’t morph private cars on roads into PRT. PRT is meant to replace the need for more cars and roads. It is, at its core, efficient public transportation, which, in turn, encourages the building of truly efficient communities, leading to reduced environmental impact and greater prosperity.

Robocars will only help alleviate traffic, without cutting down on driving. It will make private car ownership even more desirable in emerging countries like India and China. This will eventually encourage even MORE driving and consequently MORE sprawl, just as faster highways have ended up doing across the U.S.  And mark my words, if they are privately owned, they will be BIG, powerful and very comfortable. I’ll take the camper!

109. Harnessing "Anti-Gravity" for PRT Control

Recently astute reader Ken MacLeod suggested that we consider certain computer-generated steering behaviors for merging PRT vehicles, and included this link.When I looked at them, (He specifically referred to the queuing example) I was first reminded of a thought I expressed in post 41, where I touched on the possibility of flocking or schooling behavior to help alleviate traffic, although I really didn’t know what it was called at the time. And so I responded by saying that the behaviors seemed more suited to traffic management. Now I get it, however.

Flocking (and the other related behaviors) is actually extremely relevant to PRT because of how it is generated. It is the interaction of imaginary gravity and antigravity between objects in motion. Specifically, it happens when there is very strong close-range anti-gravity and weaker, longer-range gravity. The gravity creates a group or “flock” and the antigravity keeps the “birds” from hitting each other. A favorite demonstration of this is “Boids,” which, as it sounds, supposedly came from the word “birds” combined from someone’s strong (New York area?) accent. In this example, the programmer periodically changes the parameters, (number of fish) leading to stronger schooling or flocking (gravity) forces. Anti-gravity is localized to the area immediately around each fish.

When we drive our cars there is a similar effect going on, except that it is highly linear. The anti-gravity part is dominant and stems from driver reluctance to follow a vehicle too closely. The closer you get, the faster and harder you are likely to apply the brake. When you follow at a safe distance, you are liable to maintain an equilibrium between braking and accelerating. When you see the gap getting larger, you are free to apply the gas petal, discounting other factors like speed limit. In auto traffic, there is no appreciable gravity effect, however, as there is no benefit to clumping cars together, (with the possible exception of convoys of speeders.)

Note that the “anti-gravity” at extremely close range (like actually touching) can be set to infinity. Therefore brakes are applied to maximum extent possible to avoid touching bumpers. In PRT, it seems that something similar is almost unavoidable. The question is whether collision avoidance through some kind of proximity sensors should be an add-on for redundancy or emergencies only, or integrated directly into the control processes.

As I mentioned in the beginning of this post, it has been suggested that such algorithms could be used in PRT for merging as well as headway control. In such a scenario a computer could consider where, in a line of vehicles, the merging vehicle would be expected to eventually fit, and then gradually apply more and more anti-gravity to that spot. Thus it is like a ghost vehicle slowly materializing and repelling the vehicles around it. It increases this short-range anti-gravity toward mathematical infinity until it has forced a space for the merging vehicle. This is an interesting approach because all decision-making can be localized to the vehicles involved. It also doesn’t need too much or particularly fast communications. Each vehicle simply responds to the proximity-induced repelling forces while trying to maintain forward speed.

One problem with this approach, however, is that it lacks intelligence in the choosing of where, in a line of vehicles, a merging vehicle should be placed. For example, what if a single merging vehicle is near the front of a group of 15 vehicles with plenty of space in front of the group. It seems like it shouldn’t split them up, but rather speed up and merge in front them, if there is time. At least it should delay as few vehicles as possible. But then again perhaps the anti-gravity could be set so that the group collectively repels the single car into such a choice. Or perhaps the group is self-dispersing, because the anti-gravity’s range is sufficient to “de-clump” the 15 vehicles in the first place.

In such a system there is no problem controlling traffic in the event that a section of the system becomes “over-booked.” The traffic would simply slow down. In an earlier post I outlined a control method based on precisely synchronized timing, where vehicles could “book” a split-second “reservation” to go through a merge point without conflict. The system begins with fuzzy time estimates at the point of departure, and but reschedules with more and more precision as the vehicle nears a merge-point and utilizes track markers to time itself.  Such a system has automatic traffic control; it can be set to never allow overbooking of a route. It becomes more problematic, however, as fast and heavy traffic pushes the limits of communications and reduces the margin for error. Perhaps the two methods can be combined. Thanks again, Ken. You’ve reminded me again that many heads are better than one!

108> Putting a Foot Down for PRT

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.

107> An App for that.

In the world of TCP/IP, and even Ethernet, and especially any wireless version of either, there are transmission delays that limit what such technologies can do for synchronizing PRT vehicles. That is not to say that such delays cannot be overcome. The problem is that off-the-shelf solutions are only just emerging. Building an IP-based PRT network infrastructure that doesn’t rely on proprietary techniques is, for the time being, still fairly complicated.

Modern computer networking involves lots and lots of flexibility, security, compatibility and legacy issues. After all, computers are used for all sorts of things that were developed over time by many players. Anything resembling a standard communication procedure needs to address many unknowns. Remarkably, the present systems are designed to handle messages of unknown length, coming in at unknown speed, which will be forwarded along an unknown route. The original designers of these systems wisely prioritized accuracy and completeness of the data being sent over speed, and the protocols reflect this. Hence these systems (as they analyze and sort bits of information like a postal worker sorting letters) produce the delays we refer to as latency.

Luckily PRT vehicle control is not the only application that requires minimal network latency. Two others come to mind. The first is industrial processes. Machinery must often interact with split-second precision. The ubiquity of Ethernet as a way to network computers has created a market for methods to achieve “real-time” process control using the same equipment. One interesting open-source project worth exploring is RTnet, which has advanced to the point of having suites of development tools written for it.

A second area driving change is VOIP. (Voice Over Internet Protocol)  You see, VOIP cannot tolerate latency. Unlike the data packets that will re-assemble into an Email or even a streaming movie, the packets that comprise your telephone conversation must arrive in order and on time. (relatively speaking)  With new “smart phones” appearing daily, higher quality VOIP is becoming essential. Getting there involves a reversal of the previous priority of completeness and accuracy. For example, if a split-second part of a conversation is lost or scrambled, it should just be tossed out, leaving a click or silent spot in the audio. In ordinary TCP/IP, the system would detect the damaged data and ask that it be resent, even if it came in two seconds late. The bottom line is VOIP is forcing a rethink of this and all of the other latency producing steps used in communication and networking. Furthermore, telephones are inherently mobile. When a person travels down the highway talking on the phone he is forcing the network to find a seamless way to “hand-off” his call from one antenna (network access point) to another. This is similar to the problem PRT designers face if they want to minimize the number of vehicles in any subnet and want to minimize the number of hops any transmission must take.

Telephone standards have been referred to in terms of which “generation” they come from. We are coming to the end of the 3G era. Aspects of 4G are already appearing. Whereas some 3G systems had legacy analog capabilities for better phone reception, 4G is 100% packet based. Previously cellular carriers had little incentive to adhere tightly to standards such as the various flavors of IEEE 802, preferring to compete with proprietary protocols. The convergence of corporate data networks, internet, smart phones and the like will surely lead to more “Plug-and-Play” solutions for mobile users interacting with private networks. And that means more standardization between carriers. And, speaking of IEEE 802.xx, the next generation of standards (such as 802.20) looks pretty awesome. But what is “state of the art” now?

There is a lot of activity surrounding the 802.16e standard, which is being commercialized as “WiMAX.” (successor to WiFi) Supposedly latencies are in the in the 40 ms range, but I’m skeptical as to whether real world performance is anything like that. I assume, for example that that is a one-way number, which would double in a round-trip message that included a confirmation. But I am no expert. 

I also am not sure what, specifically, can be done with a guaranteed round-trip transmission in, say, 50 ms that can’t be done with a turnaround time of a second or more. It seems to me that for real split-second control issues, 50 ms is still an eternity. It is possible that real-time Ethernet (like RT net) could be adapted for “leaky cable” but that seems like a lot of effort for the hypothetical portion of the PRT control that needs 10ms to one second access times. After all, there is still direct sensing and switching, which is essentially instantaneous. I guess it would be worthwhile to take a good look at the specific maneuvers that a PRT vehicle must make in terms of actual time requirements, from control signal to electro-mechanical response, before getting too far in terms of favoring one technology over another. More specifically, I question whether sub-second communications are really necessary for pre-merge maneuvers. This question leads to track layout and capacity considerations as much as anything else. Should vehicles be packed so closely together and run so fast between closely spaced merge points as to necessitate split-second communications? It seems to me that a little traffic management and prudent track design could go a long way toward allowing a bit of latency. And there is also my often-stated preference for putting more control at the vehicular level, limiting the need for top-down control.  Meanwhile, technology marches on. Maybe PRT should run on the Android OS!