96> Control Issues, part II

Let’s demystify control with an overview. Background on this subject can be found in posts 75-77. I’ll start with navigation. Obviously the first choice in any navigation system is to simply go in a straight line. The mitigating factors are availability of track and traffic on it. If there is a possibility of some traffic being slower than other traffic, then that adds a third factor. Let’s start with the straight line. Imagine the network as a chessboard, with you at one corner and your destination at the other. It should be a simple matter to identify all squares directly between the two points and rate them highly. Squares less direct, but still nearly in the line would score a bit lower, and so forth. Now within those squares are actual tracks. Some go East/West, some North/South, or somewhere in between. Here again there can be a rating system based on how close the direction of a route segment comes to the direction that would be the optimum straight line. A final factor would be expected traffic and speed. Traffic can be derived from ticket data or real time reporting from the vehicles themselves. This data then is crunched, preferably by the vehicle, and it can set out. I say “preferably by the vehicle” because if the vehicle makes these decisions, the route can be refigured along the way. Every route is only a series of left or right turns separated by a straightaway. At each diverge point, the whole fastest route strategy can be recalculated. This information would be available wirelessly, possibly even via standard internet means.

Let me back up a bit and clarify the concepts of speed and traffic. I do not like the concept of line speed, except perhaps at merges. While line speed will probably be the inevitable result of having vehicles not holding each other up or rear-ending each other, there are arguments for allowing vehicles to decide their own speed where possible. If we are designing a control architecture from scratch, let’s start with a wish list and see if those wishes can be fulfilled. Personally, I like fast. I drive fast, and if I haven’t arrived yet, then I’m probably in a hurry. Hazard of years of self-employment, I guess. I want all timid, motion-sick wimps to get out of the way! Hey, I’m on the clock here! PRT can, and should be, fun, sexy and exiting. Why not? Also, one way to keep a track segment free of traffic congestion is for the people in front to speed up and make room for those bringing up the rear.

The stations, it seems to me, should talk to each other as the first level of traffic management. This would be where the vehicles would get information about expected traffic and speeds, and where they would report their prospective routes. Getting the OK, (or rather a confirmation that the trip is doable in a reasonable timeframe and the destination station is able to receive the vehicle) they would leave the station and drive themselves autonomously. If conditions change, they would be capable of changing routes, but since this would be a rare occurrence, the central system would generally be pretty accurate. This is after all, essentially a traffic reporting function, not so different than the morning “Jam Cam” on local TV.

A second system could be vehicle-to-vehicle communications, sort of like a trucker’s CB radio. (“Breaker Breaker, good buddy!”) Such a system would keep track of the positions and speeds of all vehicles within a relevant range. The most important aspect of this function would be at merges. Vehicles would negotiate for precise time-based “reservations” for crossing the merge point. Especially important would be communications between the vehicles equidistant from the merge that must adjust themselves to each other to avoid conflict. Early upstream communications between many vehicles would enable the traffic to be “shaped” so as to maximize throughput. For example, if track A has twice the vehicles as Track B and they are to merge, the best way to “shape” the traffic would be to get Track A’s vehicles in evenly dispersed groups of 2.

There must be a backup system for this. One idea is to enable a vehicle on track A to disable the power to the corresponding section of track B. The frontrunners would always get preference. This could be built into the track with a few Reed switches and relays, and would not tend to wear out because they would be switching off sections of track that are not supposed to have any power draw anyway. (When vehicles lose track power they slow to walking speed.) A final, emergency backup solution would be to slow everybody down and just take turns, the PRT equivalent of a malfunctioning traffic light. “Treat it as a four way stop.”

The vehicles’ headway control would work by each car reading its position on the track (optical bar codes, magnetic markers or RFID tags) and reporting its velocity by broadcasting it wirelessly through ordinary wireless LAN protocols. The position sensing would be enhanced and checked by counting wheel rotations. A back-up system would be to use a pair of overlapping wave guides, perhaps a hundred meters in length. (I am counting Leaky coaxial cable as one type of wave guide.) The limited length would limit communications to relevant traffic only, and would eliminate demultiplexing of many extraneous communications streams. In closer ranges optical or ultrasonic methods would be added. (The chip they use in cameras is under $50.)



So that’s pretty much it. It’s not a particularly difficult to avoid driving into a vehicle in front of you, nor taking the correct turn, nor maintaining a speed to cross a merge at an exact (to the second) time. So I say let the vehicles control themselves. That way control will automatically get updated as vehicles wear out and are replaced, except for station controls, which can be monitored and serviced very easily and need WiFi and line power for the kiosks anyway. Almost any task that can be done by a central wayside computer can be done by combining the computing resources of vehicles connected by a wireless LAN. In this architecture all control decisions come from the vehicles, although zone and station traffic is monitored and shared. The exception is the power scheme for merge points that I mentioned.

This architecture is designed for an open source system. It presumes that vehicles are designed and continually improved by a nonprofit organization (NPO) but built by independent contractors. The track and station construction would be contracted to locals, and the station communications and kiosks would be Open Source (NPO developed) but installed and maintained by local contractors. The role for a PRT company is minimized. I think this is a good thing, because no PRT companies have a long-term track record anyway. This is fundamentally different than having a PRT company design, build and operate an integrated system, which can blur control responsibilities between vehicles, stations, zone controllers, and even control room operators. No customers for anything want to be locked into a single vendor.

The other emphasis is on extensibility. Whereas the ordinary PRT model would have the vendor needing to quickly add staff and production capabilities to service a growing demand, this model minimizes these problems, which would only, in the end, make city officials look bad when they are forced to explain delays and problems to the public. In this model skill sets are organized in a way that allows much more rapid growth with minimal growing pains. The brains are in the cars, more than the track and stations. This is also a more familiar model just on its face, because it is more like traditional roads and automobiles. Hopefully this will make PRT somewhat easier to embrace. The last aspect to mention is that the model is nearly unbreakable. There are no parts that can malfunction that can affect whole groups of vehicles, except the merging track deactivation that I mentioned, and hopefully a better (in-vehicle) fail-safe can be devised.

That’s it. Oh! My laptop has recovered famously from yesterday’s surgery. Thanks for asking…

sorry folks,

My laptop has just been through a very long day of surgery. I just had to solder in a new power jack, and its after 11 pm. I'll be posting soon, but this computer problem has set me back a few days. I still have a blown diode somewhere in the battery charging system, so I'm writing this with a small short circuit which may melt my handywork or kill the power brick at any time. Too bad. This has been a good computer...Time to start shopping...

95> Call the Cable Guy!

Whenever I see an artist’s conception of something futuristic I always have to muse at the attributes they give the materials of the future. While up at the cabin, I stumbled on this old magazine. Note the total lack of support for the track of the main vehicle. (I won’t even get into the giant ULTra vehicles)

While this is common practice, it’s pretty tough to compete with designs made out of super alloys straight out of science fiction. In the case of PRT advocacy, the main job is to sell the concept of a whole new infrastructure, overlaid on the existing one. This is obviously much easier to do if it is minimalized. On the other hand, there is also a credibility issue here. If the system you are sold on bears no resemblance to what you are really going to get, how can any further claims by a PRT advocate (or vendor) be trusted?

I have given a lot of thought to PRT trusses, and I just want to make an observation. I have never seen any system meant to carry people that is more than about 35 times as long as it is high. Box beams, I beams, trusses, whatever. They rarely approach this mark and are usually much less. There is some pretty good reading on the subject in the Wikipedia entries on beams, bridges and trusses.

The bottom line is that PRT needs to have as skinny a support structure as possible, much more so than any other application I can think of. In other situations, generally, something needs to be built and the buyer has little choice but to take the recommendations of the architect or engineer. Here they can just decide not to involve themselves with PRT in the first place. 

Next time you pull down your wooden attic stairs or have a wooden stepladder handy, note how they reinforce the steps. You will see that they have a thin metal rod beneath each step that can be tightened, squeezing the step end-to-end. The step then becomes a compression member resting on a tension member, resulting in a very strong “beam”. Tension members (cable) are also employed to great effect on pre and post-tensioned concrete in a very similar way. Below are some drawings of cables that are integrated into a traditional truss. I am quite confident that this type of addition would increase span substantially, beating that 35 to 1 ratio.  

 

Fig. 1 shows geometry similar to a suspension bridge. Support points can be had by periodic attachment along the length of a stretched cable. In a Suspension bridge secondary cables can hang straight down to support the bridge decking. Suspension bridges must have the ends of the cables be anchored to the ground, however, although multiple spans can attach together instead. Pulling the cable tighter makes the decking arch upward. Figs. 2 and 3 show how cable could be stretched within a truss and terminated in a spool, which could be tightened. Tightening both spools equally (with a BIG torque wrench) would be essential, or the support would be pulled over. Straight runs would need to terminate by ground anchoring, just like a suspension bridge. The cable could also be continuous. (Between ground anchors) Fig. 4 shows how cable-stayed bridges differ from suspension bridges. Instead stretching a cable from two ground anchors and hanging a bridge off of the cable, the cable-stayed design balances cantilevered loads on a support column. At these slight angles there would obviously be a lot of compression put on the truss itself, pushing it toward the support posts. What is interesting to me though, can be seen in FIG. 5. Note that the truss also becomes a tension member, because the cables work to pull the structure in opposite directions in the middle of the span. This gives it characteristics similar to the continuous cable in Fig 3.

Finally, to really confuse the reader, figure 6 is a full crossbreed. Depending on how the cables are tensioned, this can be either a suspension or a cable-stayed design. Confused? I know I am. But I have used similar techniques to make impossibly long and thin unsupported shelves with great success, and even took the sag out of a roofline once. I know it would work to some degree. Also, never underestimate the wisdom and practicality of the farmer. They use cable to keep stuff from sagging all the time, like this irrigation system. 

 One final photo. It is nearly impossible to down a single telephone pole, since they are all cabled together at the top. I have seen them broken in half by trucks, and the snapped pole just hangs there. Such a quality would seem to be ideal for PRT from a safety point of view. By the way, cable is, relatively speaking, dirt-cheap. Each half-inch steel cable has a tensile strength of over 20,000 lbs. It would sure make ME feel better in an earthquake!

94> Say Cheese!

Sometimes consumer technologies march forward and open possibilities in completely unrelated fields. Such a situation seems to exist with the problem of position and distance sensing as it might apply to PRT. Traditional technologies include SONAR, LADAR, (similar to sonar but with lasers instead of sound) as well as magnetic position sensors and RFID tags. Meanwhile, digital photography has taken off, and computing speed has made interpreting camera information essentially instantaneous. The following is a brief exploration of the possibility of using ordinary “webcams” for this purpose. This is made possible by the relatively controlled lighting conditions in an enclosed track. The following assumes a light on the front and a hollow square reflector on the back of each bogie. My sample webcam has been simplified to the point of having almost no resolution, with only 100 pixels. In real practice the light sources and cameras would be in pairs, offering redundancy.     

 

Here you can see the reflected square as captured by the camera. I have made it off-center to simulate an approaching curve in the track downward and to the left. Because of curves in the track the camera will not always be aimed directly at the leading vehicle. There can even be blind turns, an issue I’ll avoid for now.

In the example above, the pixels 52-55, 62, 65, 72, 75, and 82-85 are activated. It would be a simple matter for the computer to recognize the patterns, since the horizontal lines are characterized by consecutive numbers and the vertical lines are characterized by incrementing by tens. In either case, (up & down or across) the count is four. 

 

In the second picture the longest string of numbers (consecutive or by tens) is three digits, not four. The box is smaller, as it would appear if the lead vehicle were further away. 

Here is a nearly blind turn. In this case the only information that the computer can use is that the pixels 30, 40, 50, 60, and 70 are red. The computer would rightly interpret this as a five. Now it is apparent why a chose a hollow square reflector. As long as there is at least one straight line with a beginning and an end, the distance to the lead vehicle can be determined.

The example above is a very primitive, I know, but it would work about the same with a higher resolution system. For example, the lowest resolution “webcam” that I found online was 640 by 480 pixels, about a third of a megapixel. I found a “two-pack” of 1-megapixel cameras for $33. (U.S.)

Consider how such resolution would apply to distance determination. Assuming a bit of edge-blur from the optics of, say, 3 pixels, that would still differentiate over two hundred different sizes/distances. Again, this is with the very least available resolution.

Video is commonly captured at 30 frames per second, so this gives you an idea of the sampling speed. Were a vehicle to be stalled, for example, such sampling would establish a possible problem by the second frame, confirm it by the third, and double check the results by the fourth, activating a braking routine. I would assume that such cameras would also be able to receive track location information as well, perhaps by simple shape or pattern recognition, like a bar code. 

But these handy little cameras do even more. They can also serve as a WDM style demultiplexer. WDM stands for “Wavelength Division Multiplexing” and is a method of cramming multiple simultaneous data streams into a single fiber optic cable. As long as each stream has it’s own frequency, (color) the streams can be unscrambled at their destination. In our case different colors could simply mean different things, just like the way traffic lights communicate with red, yellow and green. Since the computer is already equipped to recognize color information from the camera, there can be many input “channels” that can be utilized, all with their own color. Of course this is all still very primitive, especially because 30 frames per second is too slow for meaningful serial communication. Yet between simple shape recognition, color differentiation, and the (painfully slow) serial transmission, this gives an autonomous redundancy/backup capability to vehicles otherwise directed by more complex and capable wireless communication methods. And it’s hard to beat the price!

93> In Search of PRT’s “Killer App”

I want to dust-off a topic I have posted on before, and add some new thoughts I have had on the matter. The subject, or perhaps the question, regards what kind of routing layout PRT is best suited for, or should start with. Unfortunately I need to get down to some greasy details to make my point.

First I want to point out something about PRT control. In the very early days of PRT, back in the days of the Aerospace Corporation’s involvement, sensor, computer and communication technologies were in their infancy. The logical approach to vehicle control was to have a big computer manage all of the cars like one big machine. That way not much was required in the way of sensors or computing power on board. (Now, of course, we have more computing power in our cell phones than their whole system had.) Merges were conceived in terms of vacant or occupied spaces that were all moving at the same speed, something that can clearly be seen in the video. This required a uniform “line speed.” I am not sure about Vectus, (seems like someone told me it has dynamic speed control) but I believe everyone else pretty much assumes this sort of set speed. (If my information is dated, please correct me!) The going wisdom seems to be that to be financially viable, the track needs to be packed at all times, and so the system must be confined specific, highly urbanized areas. Therefore the routing involves close distances and so the system doesn’t need to go fast.

Then there is the matter of headway limitations. There is a notion out there that vehicles can only go so fast, because the headway requirements increase as the speed increases. Therefore, more cars can pass a point traveling slowly and close together than by going fast and being more spread out. There is a formula that “proves” this. Unfortunately this has led some to conclude that PRT, as a rule, can only go so fast. I have heard specific speeds mentioned.

I believe the arguments listed above are wrong-headed. First, about the packed track/financial viability thing…Perhaps the reason the track needs to be packed is because it is downtown, moving slow, has many stations, only short trips, etc. This all jacks up cost or constrains revenue. Amortizing this cost requires either high fares or a packed track. The per mile/kilometer cost estimates for PRT generally include several elevated stations, assume many long spans across streets, high construction costs because of traffic, buried utilities, etc. Has anyone, ever, given a quote to go across an empty field? Of course not. PRT is too slow to be useful for long haul, and it is too expensive to go anywhere where the track won’t be filled. See where I’m going with this? It’s a circular argument. Maybe it’s expensive because it is downtown and is downtown because it is expensive.

Let’s go back to, before moving on, to that argument about top speed and headway, since nobody is going to trade a 45-minute car ride for a 65 minute PRT ride. It, too, is a false choice. This is because the numbers going into the formula can be changed by simply designing the vehicle differently, and then the answer changes as well. I looked at the formula and found that the variables that must be entered refer to common sense considerations like braking efficiency, response time, and crashworthiness. Therefore any suggested optimal speed or headway distance is the result of plugging in numbers for a particular system’s capabilities. Nobody, (including me, so far) has plugged in the numbers for a system like I have proposed, but I guarantee that the safe headway would be way, way less than for a system where the first part of the vehicle to make contact in a collision is the passenger compartment itself, which is the case all of the systems currently on the market, yet need not be. They don’t need crashworthiness because they don’t go fast, because of, well… more circular arguments.

As far as the controls go, the technology for dynamic speed control is really not an issue anymore. Thousands of calculations can be done in thousandths of a second and transmitted and received with similar speed, although some intrepid programmers need to step forward and write an open-source version of the control software. An example of a demonstrated system (That even works with truly antique computers and sensors) is the PATH program. Insofar as at least some of it was funded by taxpayer money, I sort of hoped that they would at least respond to my requests for the code they used, but, alas, I’m just a lowly blogger…Anyway, with variable speed, another of the factors holding back PRT from being a commuting tool will have fallen.

One note, however… There is the matter of controlling vehicles that run on simple pavement, like ULTra and 2getthere. There is definitely a slippery pavement issue when it comes to going fast for these guys. None of my arguments apply to them. They really do need to keep to routes appropriate to more limited speeds, at least for the time being.

There is the matter of motion sickness, but again, with dynamic speed control, cornering speed would be based on factors including what is comfortable. The individual could (theoretically) even specify the kind of ride they prefer. So the last of the arguments against commuter PRT has been answered. Well, sort of… There is the matter of where to go on the suburban end.

Another thing that has changed since PRT’s inception is the proliferation of “Park & Ride” systems. These are essentially parking lot/bus stops in the suburbs, which are sometimes used in combination with HOV (High Occupancy Vehicle) lanes. The commuter can take the bus or carpool to bypass the clogged freeways. These lots are ready-made outlying destinations for PRT. But why not just take the bus? Because if there is no HOV lane, the bus gets just as stuck in traffic as the rest of the commuters. If there IS an HOV lane, it too, will become clogged over time, when (in some cases) it will magically morph into a toll lane. (Funny how that happens!) Also, upon exiting the HOV lane, the bus cannot take passengers to all of their respective destinations efficiently. And HOV lanes are usually one-way. (reversible) The buses face ordinary traffic on the return trip. Anyway, these Park and Ride lots are generally on inexpensive land that is very close to the freeway and would be cheap to connect to. True, the passengers would have had to drive to these lots, but don’t forget, the downtown traffic comes from somewhere. Typically, the first part of a morning commute goes pretty fast. It is the last five miles or so where the traffic gets really bad. This is nipping it at the bud. Otherwise the ironic alternative might be that the traffic downtown is from people looking for a place to park so they could use the great PRT system! That is one aspect of “short-haul” PRT that has always puzzled me – What does it really save if the passengers have to commute in to use it? (But then again I live in a city where almost nobody lives downtown)

It was recently remarked that PRT needed a network to be effective, that a simple loop or straight line would be a waste. Whereas this is largely true, especially compared to a large network, it is should be pointed out that even on a two-way straight-line configuration travel time can be improved by PRT’s off-line stations, meaning you can go non-stop to your destination, and the fact that PRT is available on demand. This cuts travel time to a fraction of light rail and a fraction of a fraction of bus travel time. In the case limited routing outlined above, though, many passengers would undoubtedly still need further transportation. The Achilles heel of buses is the many stops they must make, both for passengers and for stoplights. But even a very simple PRT loop could eliminate a huge portion of this wasted time. Personally, I wouldn’t like taking a five-minute bus ride to finish my commute, but I would do it if I had already saved enough time getting to the downtown area in the first place. If the last part of the journey were to be taken from a downtown terminal, however, (where an express bus would drop you off) that last leg might be and agonizing 20 minutes instead. I guess I am suggesting a possible symbiotic relationship between the downtown and commuter legs of system.

Finally I want to point out that the cheapest configuration for routing on freeway medians would be actually be a bottom supported design like Skyweb Express, because it would be so easy to construct low-rise track. This is not all that different from the cost/structural dynamics that enabled the spread of traditional railroads. For example, such track could be supported with gravel instead of deeply anchored supports. It is hard to imagine such a configuration costing very much more than a million dollars per mile. In most cases the Park and Rides have structures in place that could be converted into the required elevated boarding areas. In defense of hanging systems, I think they would be much preferred for these very large parking lots since they could pretty much come to your car.

Amortizing a million dollar a mile track is much, much easier than the inner city routes on which it would depend. (OK, it would probably be more, but I like round numbers) Consider amortizing the track over 10 years, with 33 cents per mile going to this purpose. Payoff is 3 million trips, 300,000 per year, or 822 trips per day. That is 34 trips per hour (averaged over 24 hrs) or about one every two minutes. Obviously they are mostly during rush hour, but equally obvious is that Holy-Grail benchmarks like two-second headways probably need not be reached here. More to the point is that a comparable HOV lane costs 5 times as much, and also the size of the parking lot needs to be considered. This may not be PRT’s “killer app” but keeping that many cars out of city center in the first place certainly seems like a worthwhile goal.

Finally, to my friends in the U.S… Happy Independence Day! This is a time when we can all come together and enjoy the sights and sounds that result from the enormous, ultimate, instantaneous release of CO2! :o) Oh well, be Happy. It’s the 4th of July!

92> Crack in the Track, Jack...

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

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

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

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

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

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

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

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

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

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

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

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

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

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

91> Dan the Blogger was Writing ...

“Dan The Blogger” was writing a comment (to alert reader Andrew F) and decided to just go ahead and make a post out of it. (I refer to myself in the third person as sort of a joke, as well as a way to be found slightly more easily on the Internet) The comment was about putting utilities in the track, and here are some observations:

Electric companies keep themselves pretty busy repairing electric lines that are damaged by ice, wind, auto accidents, etc. Recently, in Louisville KY, the tail end of a Gulf hurricane darkened much of the city for weeks, and then, a couple of months later, a severe ice storm did the same thing. There was a public outcry. The electric company reported that the only real solution would be to bury the wires, something that they estimated would cost about one million per mile. That was that. The poles remain.

Actually, in spite of the cost, buried lines carry a fairly large percentage of the electricity in the U.S. But the wire is not totally safe there either, and one city reported a digging accident, on average, every single day. There are also problems with water intrusion and corrosion. So what I take away from this is that even with the cost and problems of buried lines, it is still widely considered worth it. They have never dreamed of the kind of sheltered accessibility that the PRT track would provide.

Meanwhile, in San Francisco, a lawsuit was recently filed against the electric company for polluting the ground water. Wooden telephone poles have been treated to prevent rot with a host of nasty chemicals, mostly illegal these days, and are known to leach these poisons into the soil. Millions need replacing annually because of rot, which, if you think about it, is actually the process of the wood itself turning into soil, preservative and all. At some point this practice will stop. Any electric company that would proactively take out the poles and the dirt around them before they were forced to do the job under regulatory scrutiny would be doing their shareholders a big favor. And they would look good doing it.

I wonder then, about the prospect of offering to house a mile of those electric lines safely for a mere, say, $900,000? That’s about $170.00 per ft. enough to pay for all of the steel in the trusses I showed last post. Those poles need to come out eventually. It is just a matter of time, and the contaminated area is expanding as we speak.

Then there is the matter of street lighting. Most streetlights are outdated, by and large, both in electrical efficiency and design. As a person who flies a fair amount, I often stare down in wonderment at the sheer volume of energy used to light all of those city streets below. Although the typical sodium vapor is quite efficient in many respects, LED lighting has the potential to vastly reduce energy costs over all. Apparently, as sodium vapor lamps age, they require high and higher voltage to stay lit, eventually exceeding the capacity of the ballast, and the light will begin to flicker, then flash intermittently, then go out altogether. But another aspect is the whole notion of always flooding a large area with bright light even when it is not needed. You can’t put dimmers on sodium vapor streetlights.

Way back in post 14  I illustrated PRT track based street lighting. It uses many little directed spotlights to illuminate objects on the ground without blinding drivers. When drivers are not forced to stare into the light they need less of it to see, hence less wattage is required. Furthermore, much less light is needed at, say, 3:00 AM than at 10:00 PM, because there is almost no traffic. Having many smaller lights enables energy saving schemes where, for instance, half of the lights are turned off in those wee hours or are on motion detectors. More savings. And of course there is the matter of not needing a ladder truck to change the LEDs, which last longer in the first place.

The point is that the city, too, has a potential financial benefit from PRT track, aside from the transportation angle. By the way, this money is your tax dollars at work. In Los Angeles, for instance, each 50’ property is assessed $70.00 per year to keep those streetlights on. Not only will PRT make a street quieter, (over adding a traffic lane) it could directly save the effected property owners money.

Another aspect is the possibility of using solar cells on top of the track to eliminate the electrical costs altogether. Even amorphous (cheap) solar cells produce about 5 watts per square foot. This means the track could produce, say, 10 watts per linear foot. I would guess that this is pretty close to sufficient for the kind of system I have in mind.

I must say that, as an era, this is not one where the “winds of change” are blowing very hard. Perhaps people are a bit overwhelmed by the pace of advancement in communications and computer related fields. It’s certainly not like the sixties, when every car needed fins and taillights that looked like booster rockets to celebrate our entrée into the “Space Age”. At any rate, it seems people are in no mood for their government to try anything new with their tax money. Can you imagine mega-projects like the interstate highway system or rural electrification getting passed today? PRT is going to need to be seen as having multiple, tangible, short-term benefits as well as those harder-to-explain ones that most of us already understand. It will need to be structured in a way gauged to gain friends in high places and to have multiple levels of public appeal.

No black outs and tax savings – That’s a start.

90> Let's Take It From the Top

I want to go back to the beginning in my discussion of PRT track, specifically track for hanging style PRT, for the benefit of the majority of readers who have not followed the ongoing development of the corresponding SMART (Standardized Modular Automatic Rail Transport) specification.

The most fundamental concept that needs to be understood is that of switchless steering. Unlike a railroad, which requires a track segment to be mechanically moved to provide switching, PRT designers have generally agreed that this can be more easily done by the cars themselves by means of engagable/disengagable guide wheels. These require a vertical surface to engage upon. Since the main wheels (or even a Maglev system) require a horizontal running surface, the two, in combination, look something like this.

 It should be noted that only one half of this book-matched arrangement is theoretically needed, except for places where tracks are merging and diverging, though leaving the redundant half in has advantages (load distribution, simplicity of symmetry) which I believe out-weigh the disadvantages. This picture,
from post 54, illustrates the concept.

The second concept is the combining the functions of multiple wheels into one. A look at a roller coaster wheel setup  shows how multiple wheels can hold a vehicle tightly to a track. A simpler approach, however, can be achieved by the use of flanges, as on a train wheel, or dished in or out wheel or track profiles. The angle of contact, though, in such systems, should be as close to a right angle as possible or the flange will tend to wear out, since it introduces a point of friction to the system. A quick look at my older posts will show various examples of radii on both wheels and track.

A third concept is that plate that keeps appearing in my track designs, pictured below.

Whenever a tube-like structure is bent, it tends to flatten out in the process. If a section of track is spanning a great distance and is bearing a great deal of weight, the downward forces will tend to want to flatten, then fold the track under that weight. Stopping this flattening arrests this tendency and strengthens the track. In the case of an encased track with a slot in the bottom, the initial flattening would cause the sides to fold inward or outward, thus narrowing or widening the slot. These plates, placed periodically, prevent this. In the second picture you can see how such bracing can be connected by long plate steel to make a complete box beam. Personally I find this design ugly and worry about single wall designs for PRT because of noise, condensation, and uneven expansion.

A fourth concept is modularity. Almost all discussions on PRT touch on the possibility of movement of goods as well as people, particularly at night. Additionally, many envision a variety of vehicles, and the current diversity of motor vehicles that populate our roadways would tend to bear out the desirability of this approach. Such ideas are not put forth by actual PRT vendors, however, since it currently would be counterproductive for them to develop and offer a confusing array of alternatives. Then there is the matter of routing. Long commuter type routes would call for faster vehicles or possibly vehicles for groups, if there were sufficient numbers of people with common origins/destinations to support this scenario. Therefore there would seem to be a very strong case for creating a track that is compatible for all or any of these vehicle types or speed ranges. This logic leads to a track profile where smaller vehicles are compatible with the track of the largest anticipated vehicle type. I submit that this “largest vehicle” would be GRT. (Group Rapid Transit) In the U.S. many cities have “Park and Ride” systems already in place, which aggregate passengers in outlying suburban locations. I would not want to rule out shuttling in these passengers in groups of, say, 10 or 12. Although GRT is a contentious issue in PRT circles, it should be noted that from a track design point of view, there is very little difference dimensionally between the two. Heavier gauge steel, more frequent supports, or simply spacing heavier vehicles more sparsely are ways to accommodate such a possibility. Whereas PRT track could be made smaller (in profile) than track for GRT, the size of the truss that is required to span reasonable distances can fit either. I would suggest that any such heavier track be used for arterial routes only, because cost is everything when it comes to expanding limited routes into meaningful networks, which, after all, is what PRT is all about. This modular approach recognizes that no one set of engineers is liable have all of the answers as to the very best vehicle design.(s) any time soon. Therefore I suggest removable running surfaces. This has the added advantage of reduced noise, a smoother ride, and essentially no chance of condensation. It also looks to a future with possibilities like Maglev. In order to accommodate the largest number of vehicle types I suggest something like this general shape.

The design allows for several centering means through the use of either convex or concave surfaces. My only concern is the fabrication of this profile, especially radiused sections for turns, because the required roll-forming equipment for that is not universally available on a local basis. Although I have tried to keep my designs easy to produce anywhere, this may be something that would need to be shipped in. I suspect, however, that many local job shops would be happy to fabricate the shape by splitting stock pipe and making curves by a tack-and-bend-as-you-go method.

Although I promised some further thoughts on use of tension cables to extend the span of track sections, I would like to put that off for the moment and return to standard truss design. As far as box trusses go, the round tubular type seems to be acknowledged as the strongest, and was the choice for the Skyweb Express demo track. I believe 90 ft. was the maximum span.

It has the disadvantage, however, of being relatively complicated to fabricate, because the pipe ends must be cut to match a curved surface. (I have seen the tube ends are flattened in some applications) I have wondered about this for some time, as it seems that square tubing would be so much easier, since it just needs to be mitered. Nonetheless, in most demanding situations, such as cranes, these complex joints are almost universally used. Here is a picture showing he complex cut. These have just been tack-welded.

In the illustration below I have taken a similar box truss but substituted those plates for the verticals. Because the plates are only rigid in one direction, I have stiffened them by joining the upper and lower gussets into a single piece. I have also used simple miter cuts on the diagonal pipes, again, using gussets to reinforce the joint. This is not exactly to scale and the diagonal and horizontal pipes don’t even touch in this picture, (much weaker) but this is just a conceptual drawing. The general design should, however, be very strong, cheap and easy to build anywhere where CNC flame-cut plate and pipe is available.

In the next picture the red surfaces indicate the running surfaces for a PRT bogey, taken from Post 83.

If it looks like I haven’t decided exactly what becomes part of the truss and what becomes removable, you’re right! I have not. There’s also a radius missing. It’s a work in progress. Also not shown is the skin, and provisions for electrification, communications, carrying utility wires. There’s plenty of work to do, and now you all know were I stand at the moment. For shorter spans or hanging sections, BTW, the design would be completely different, but, as they say, “One step at a time.”

89> On the Road Again...

I am in the civilized world once again. Well almost. The bus I took from Dartmouth to Boston had wi-fi – just no AC receptacles. My poor old laptop only holds a charge for about half an hour these days, so the planned two-hour bloggathon had to be cancelled. That made me wonder what will happen when those upcoming electric cars start stopping on the road for lack of juice. Yeah, I know, they will be have low battery alarms, quick swap battery packs, and there will be charging stations… But that won’t do it.

With gasoline, when your tank is almost empty, you have a certain amount of time to get to a gas station, and it is a fixed amount. A limited amount gasoline will always get you roughly the same distance, even if your car is old. Not so, batteries. When they get old their performance falls off of a cliff. That means that just because a 60% charge got you home with juice to spare 4 months ago, there is no guarantee that a 75% charge will get you home today. Couple this with the high costs of batteries, people struggling to make ends meet, and human nature, and it will be roulette by the masses, with many making shorter and shorter trips, until one day it happens to them, just like it happened to my laptop on the bus. I was almost finished with my research when BOOM! Windooze lived up to its name. Involuntary hibernation. I predict that the only meaningful way to solve this dilemma will be to automatically slow the car down to walking speed before the battery is finally exhausted. That will force people to deal with the problem or at least get them on to the shoulders. Someone should patent that. Oops.

Another thought occurred to me, sitting on that bus. I was considering the effort that was taken to blast away the granite hills to make way for the road. And such a wide swath of land.  I guess the wide median is to make oncoming traffic lights less blinding. And now it will need mowing forever more. It made me consider the hypocrisy of “saving the rainforest” without starting first a little closer to home. When my computer died I was trying to learn a little something about “carbon credits”. The whole concept of paying someone not to burn to offset the burning that you must do would seem to be a concept with some relevance to systems like PRT. I wonder, for instance, about the carbon emissions of a highway, about viewing a given stretch of highway as a CO2 producing system, including factoring in the loss of CO2 sequestering forest that such a system requires. (I wish they could also factor in habitat loss and fragmentation)  Isn’t a highway a bit like a coal burning power plant? What if highway land had to be offset with carbon credits? Thought of in that way, elevated PRT starts looking pretty good.   

As I approached Boston, computer tucked away, I couldn’t help but notice how expressive cars are. These days there doesn’t seem to be much taste for expensive, formal clothes, so that leaves the car as one of the few mobile ways to resister our social status to the world. That’s a very basic primate behavior and not easy to break. And it runs so much deeper than just status. Carmakers can make us feel masculine or feminine, outdoorsy, socially conscious, sophisticated, elegant, fun loving, daring, young, reasonable, powerful. Take your pick. You’ll feel that way and project that self-image to the world. That role is pretty hard to replace with a public transit system. Even one with wi-fi and AC.

88> Gone Fishin'... sort of...

Sorry folks, I’m without internet and with very limited power, at least for a while. As many of you know, I normally flee the hot climate of Texas in the warmer seasons, when possible. I am in New Hampshire working on ongoing projects that are related to my cabin here. Since my power here is limited to a few solar cells, and I have no internet access, I usually rely on a particular comfy chair at the trusty town library for any computer related activities, including this blog. The problem is that the library is closed for remodeling. If it were a rainy day, I might be inclined to travel a few miles further and make an afternoon or an evening of it at some other library, but work calls me back to Texas soon, and I need to make the most of the good weather and my limited time here.

Sorry about the broken link to the video in the last post. I don’t know why it doesn’t work. If you search YouTube for it, you get the same video at the same address. Go figure. I also apologize for all of the unanswered emails. The library is supposed to be finished in a week, and rain is forecast, so stay tuned!

87> PRT Track and Bridge Design

I have been trying to devise the absolute best PRT track design I can come up with, with the help of you, the readers. My goal is to offer an alternative transportation infrastructure that beats the pants off of roads by every metric, save for (initially) the network effect. In my opinion, only then will politicians and transit decision-makers have the political cover to move forward with a PRT program.

The search for the best track design brings up some inevitable questions, such as “How far apart can the supports be before the stability or economy of the system is compromised?” and “How high should the track be?”

The first question leads to an examination of bridge design. Trusses with minimal girth, such as Pratt, Howe or Warren trusses) seem to top out at about 100 feet. The most economical way to span long distances, however, lies in the introduction of tension elements, such as is seen in a suspension or cable-stayed bridge. The most extreme case of a tension maximizing (compression minimizing) design would be the cable/tower arrangements that support ski-lift gondolas. Some have proposed such means as an urban transit system, and they certainly would have something there were it not for the need for multiple origins and destinations.

 It would be an easy matter to design a flexible track and simply pull it tight, though sagging in the middle would be both unavoidable and unacceptable. Pulling this sag up with cables would seem to be the cheapest way to span long distances with a minimum profile track. 

 

Suspension Bridges can traverse the longest distances, but cable-stayed bridges (above) offer other advantages. A good explanation of how the two compare is found about halfway down this Wikipedia page. ttp://en.wikipedia.org/wiki/Cable-stayed_bridge.  The gist of  it is that suspension bridges have vertical cables from which the load bearing structure hangs, while cable-stayed bridges’ hanging cables are angled from a support column so that the load bearing structure is compressed by its weight.

Many of you may have seen the late Hans Kylberg’s dramatic use of cable-stayed PRT track in the Bubbles and Beams video or in this illustration. Below is a picture of a curved bridge section supported by cables.

The use of cables does present a question of aesthetics. Whereas I think most people would find both types of cable supported bridges generally attractive, there is a question of too much of a good thing. Instead of a minimalist design, there would be support columns and cables everywhere. What looks good crossing a river might not look so good close-up on your street. I have given some thought to reducing the tower height and I will post my ideas for that in the near future.  

Then there is the question of track height. One obvious way to mollify the NIMFYs (Not I My Front Yard) is to have the system so elevated that it is unobtrusive. An advantage to the system I have been advocating is that it can easily and steeply climb or descend to any desired level. There is the matter of emergency evacuation, but I think that is manageable. So how high is too high? Should this be a system that can whisk you along above the trees? That would be my preference, but everyone has his own sensibilities. What are yours?

86> Can’t We All Just Get Along?

I recently read an article called “Robocars vs. PRT” posted on ITT’s PRT Debate page, which directly relates to my last (and many other) previous posts. Whereas I found more than a few dubious assertions in there, I’ll confine my response to just one. Don’t get me wrong. There is a whole lot of good stuff in the article, and I would recommend reading it. I mostly disagree with the premise. That is that there can only be one urban transportation platform that “wins,” and that will be the robocar. Is it any wonder that, in some imaginary bipolar battle for acceptance, the winner is cars/roads? They are already here. The “robo” part is just evolution.

Virtually nothing is said as to why such a competition exists. Are there also “Robocars vs. Light rail” or “Robocars vs. Buses” articles in the pipeline? This is a false choice. One of the first clues to this fact can be heard in the author’s statement, “New dedicated right-of-way is, of course, wonderful for any transportation system…” Exactly. That hits the nail on the head, because it leads to the obvious question, “What is the cheapest, most efficient, most compact, least intrusive, most flexible way to produce this “dedicated right-of-way?” Score one for PRT. His argument, however, is that roads are actually more doable for political reasons. This is certainly true, but it doesn’t make it right. This is the kind of logic that perpetuates obsolete designs and practices of all kinds. The obstacles he sites that are faced by PRT are, however, spot on. His arguments against PRT should sound a cautionary alarm to all would-be system vendors. Nobody ever said it was going to be easy. 

 

PRT can be seen as the most efficient means of alleviating traffic congestion by bypassing it. Robocars and “smart” traffic management techniques are means to decrease that traffic in the first place. Both are sorely needed.

The PRT community needs to hone its message and its product offerings. The virtuous combination of being electric, automated, and point-to-point is not exclusive to PRT anymore. PRT must be a reflection what it does best – It must be (and bill itself as) the most efficient way to move stuff (like people) using the least expensive, most efficient and versatile infrastructure.

Steerable free-roaming robocars can never be as efficient as rail-based PRT. They need MUCH bigger (and heavier) batteries, time to recharge, and those batteries eventually need replacement. They need softer tires. They cannot go as fast because they have nothing to grab on to in an emergency stop situation, and could skid or roll over for the same reason. They are more at the mercy of adverse weather, again effecting speed. The road or guideway cannot be as compact or as lightweight. Moreover, privately owned robocars will evolve into as big and energy hogging a form as the law allows. If they can drive themselves, they’ll come with wide screen TVs in no time. Energy saving electric cars will become complete offices-on-the-go. Sometimes a little “top-down” infrastructure control is a good thing.  

Rail based PRT need not go everywhere. Every mile of it that exists is saving the commuter and the taxpayer time, money and resources with every trip. It effectively uses an underutilized resource - the space over medians and sidewalks. It is ideally suited for being routed in 3D thus avoiding land-hogging ramps, as I mentioned in my last post. It is a minimalist solution that recognizes scarcity. Tell me THAT has no future.

Cities, states and countries are up to their eyeballs red ink and we live in a very competitive, increasingly used-up world. Doing more with less faster is the ONLY answer, so we as citizens, as communities, as countries, and even as a civilization, must get down to the business of replacing our various inefficiencies, or face a pretty dire future. We need both PRT AND Robocars. Hopefully there is somewhere in the world with the political will.

85> The Enemy of the Good

I just wanted to expound a bit on the subject matter that was touched on in the comments section of Post 82. The conversation centered on the pros and cons of the ULTra system. (Actually mostly cons, in regard to weather, speed and aesthetics.) One statement by alert reader Bruce, in particular, got me thinking -  “I think it would be rather more regrettable if the perfect were allowed to become the enemy of the good. The ULTra design is quite good enough for a wide range of transit applications.”

This has always been a worry of mine. As one who is probing the possibility of a standards-based design architecture, I am particularly averse to imposing arbitrary limits on those standards. The fact is, though, that if I had real-world budgets, deadlines and targeted customers, I, too, would have to dial back the system capabilities to get the job done. I do not want to create unrealistic expectations in regards to what I am doing or cast doubt on present systems. In other words, I do not want to be the “enemy of the good.”  On the other hand, there is also the possibility that the “good” could become the enemy of any and all PRT, if it doesn’t measure up to expectations.

PRT used to have the advantage of being the only practical way to move individuals and small groups electrically. Battery technologies have changed that. Now we can expect the door-to-door convenience of a private car with the energy usage formerly attributable to PRT alone. Suddenly PRT has something else to compare itself to beside gas-guzzlers.

I believe a very strong case can still be made for PRT, but some embodiments make the case better than others. Recently this NY Times article was posted on the Transport-Innovators site. It illustrates how damaging it can be to choose the wrong PRT system for a given implementation. This has given a black eye to all PRT. The layman will read this article and assume that the PRT concept was proven unworkable. At the very least, they will take away that it is “buyer beware” when it comes to PRT. In actuality the problem wasn’t PRT per se but more with what this design was to ride on… pavement. If we lived in a paved labyrinth of levels and ramps, relatively slow robocars would be an excellent choice. But in a 2D world of limited surface area, pavement riding PRT designs must compete with electric Scooters, Segways, bicycles, pedestrians, regular electric cars, not to mention gasoline powered vehicles. Is PRT really the best use of pavement? If so, by what measure? Energy usage? Passenger throughput? Time to destination?  Will it remain that way into the future?

There was a time when one main object, it seems to me, was to free up the pavement to reduce traffic and get a bit more green space. True, pavement roving PRT vehicles are smaller than the average car, so the track for such vehicles is more economically elevated. But such track could also be used productively by opening it up to ALL small, motorized vehicles. This would encourage downsizing.

At the risk of getting sidetracked, I wish to reiterate the point about being smaller and therefore more economically elevated. This is no small deal. All ground -based travel, from pedestrians to supertankers, is subject to interference based on differing directions of travel. That is a fundamental fact of 2D travel. The fact that ordinary roads must sometimes support very heavy trucks makes overpasses, (the non-stop solution to 2D interference) much more expensive. Nevertheless, making them anyway has revolutionized our way of life and greatly increased our prosperity. Imagine, for a moment, turning back the clock, and replacing all of the freeway overpasses in your town with stoplights. This would effectively draw many cities to a halt. This is a revolution that has not come down to the neighborhood level, however. We all still pay homage to the good old red light.

PRT carries the promise of cutting through the busy urban landscape like nothing ground-based ever could. With a system like Ultra or 2getthere, there is the flexibility to have

the system either ground-based or elevated. Ground based is cheaper, and so has that as an inherent attraction. Both companies point this out. But when once you consider that the track must be fenced, and that it will block any cross traffic from pedestrians or other vehicles, this becomes a false choice. It seems painfully obvious that this is partly what the designers at Masdar are now discovering.

 Another promise of PRT is (like most automated systems) to achieve speed by eliminating human error. But we have become accustomed to dangerously small headways between very fast moving vehicles when it comes to cars, yet are extremely unlikely to ever allow such headways on automated systems that rely on simple tire traction to steer and stop. This is especially true considering the possibility of wet or icy pavement. So this promise, too, of PRT is unlikely to ever be realized in such systems. The system’s users will have to be content to go at school-zone speeds for the entire trip.

True, these problems are of little consequence for applications like airports or campuses. Creating a profitable business model around these platforms would seem to be a positive first step for PRT.  But many people are holding these systems up as the urban/suburban transportation of the future, using arguments borrowed from faster, all-elevated (and sometimes purely theoretical) systems. Somewhere in the definition of PRT is the implicit supposition that the system is a viable means of urban transportation. With top speeds that are 10mph less than the current speed limit for un-posted city streets, I really have to question that, at least for the sprawling cities I know. I very much worry that such a system will be tried and then fail to live up to expectations. Imagine what the folks from light rail would say then.