73> Overwhelmed by an Underpass

I couple of weeks ago I was asked to quantify how tightly I thought a PRT vehicle would need to turn. In the case of PRT, the vertical turning radius must also be considered. (going into, cresting, or coming out of a slope) I have always considered that these radii must be fairly tight, but I had not really examined how and why I drew this conclusion.

I started my inquiry with a Google Maps starting with a place I have gotten stuck in traffic in the past. 

 

It is a place where an older road leading out of town intersected with a highway “loop” around it. Then time passed, and they built a mall and widened both roads a bunch of times. Now it’s a nightmare, but with good restaurants. Sound familiar? How would the various PRT systems compare as a solution?  The bottom picture shows the view looking east from “point B” and the smaller inset (point A) shows a small open-air bus stop, typical of the southern U.S.

The bus stop is included because it is part of the problem more than a solution.

On a typical afternoon, the traffic builds sufficiently so that it takes at least three full traffic light cycles for a bus to even reach this stop. There it blocks traffic further as passengers board - unless it just happens to reach the stop in synch with the rest of traffic stopping as well. Going straight, it blocks vehicles that might be able to turn right on the red light. If buses didn’t run through this intersection it would be better for everyone - especially the bus passengers, (who have to spend nearly ten minutes on this one intersection) but the alternative routes are nearly as bad. It seems obvious that the bus routes through here are very expensive to operate, and there is no place to fit light rail. The east/west road is as wide as will fit, and the loop was just widened - again. (Note the five-lane feeder road.)

PRT could cut through this mess like a hot knife through butter. I guess my main question is whether going over forty feet (12m) high to clear a raised highway is acceptable. As a devotee of all things futuristic, I personally have no problem with it, but will it sell at city hall? If the answer is no, it exemplifies a lot of what I have been saying about sharp curves, steep slopes, variable speeds and hanging vehicles. Hanging podcars, as I envision them, would have no problem going either over or under. The concept of a uniform line speed is challenged, however, because the PRT vehicles would need to bunch up and slow down to make the abrupt elevation drop and/or turns. I guess a two second headway would be about all we could get through per track, though.  If going over is the thing to do, the stations may need to be moved back from the intersection quite a ways, depending on how steep of a slope the system is designed for.  

I Googled around a bit, looking for other examples and found this. It is Main Street, Houston, TX as it passes under I-45. Pictured are the tracks of the new light rail.  I guess the car traffic has been largely diverted to other roads. On the face of it, it looks like they are squandering enough space to move a heck of a lot of people. Again note the need for PRT to go over or under. Actually there is virtually no way out of downtown Houston that avoids this overpass dilemma. Then there is there is loop 610 farther out, with the same thing…  And then there is the outer loop. Anyway, the situation is the same in lots of cities across the globe, so systems ought to be designed to handle the situation gracefully, whether it’s going over an underpass, or under an overpass! ;o)

58 > Defining The Problem

One of the hallmarks of PRT/PAT travel is that the vehicle takes you from your origin directly to your destination with minimal waiting time. There are no transfers, no stops for other passengers, etc. The track, being for small vehicles, can be so light and inexpensive that it can be built into very extensive networks very cheaply.

In practice, however, the initial track layout will be extremely limited and it will take many years to spread citywide. Therefore it probably won’t take you from origin to destination. Furthermore to really make an impact, PRT should address the commuter, not just travel around the central business district. What is the point of PRT if you need to drive 10 miles and pay for parking to get to it? Do the models that various PRT venders are promoting address the crowded freeways? Is any PRT model that addresses the freeways even close to the model that addresses the central business district? What about the suburbs?

If the need were only in the Central Business District (CBD), I think I would give the edge to the general design parameters envisioned by ITNS/Skyway Express models. This will surprise many, as I have a record of advocating a hanging system. The difference is, in the CBD, the main emphasis is footprint. Speed, and cost of track and stations are secondary. Having elevators in every station is actually a pretty good way to economize sidewalk space, and it takes less energy to lift a passenger than a whole vehicle. In an environment of all multi-story buildings, second floor stations can be accomplished with little more than a balcony.

The situation changes markedly as the system expands outward from downtown, however. Here, the expense of the track and stations becomes critical as individual station ridership begins to drop. In these areas, bus rides to town are short and convenient. Putting up elevator-equipped stations on every block is far less attractive than it was downtown. Now the advantage, in my opinion, shifts to hanging systems, because they are generally more versatile in terms of slopes and curves and multiple speeds, and can accommodate open-air stations that may double as bus stops. Because of population density, stations should still be within walking distance of each other, but many passengers will just be passing through.

Next comes the suburban sprawl. This mixed-use area goes on for miles and is punctuated by mini urban centers, residential neighborhoods and distribution parks. There is enough housing and employment that many of the residents never go downtown. Here distance, and therefore speed, starts to become a real issue. The potential riders currently utilize a variety of road types to get around. Often there is a freeway nearby. The challenge for the PRT planner is to provide a system that can match speed and convenience with the combinations of freeway and back street shortcuts that are utilized by the drivers living and working here.

Because of the sheer size of the suburban sprawl, it is unlikely that any form of PRT will blanket such an area for quite some time. This suggests a “low hanging fruit” strategy, where major “hub” areas are accessible but many low volume routes are not incorporated at first. City buses (horrendously inefficient for long trips because of the many stops) could, none-the-less, be a reasonable option for going a few blocks to get on the PRT grid. Here it would seem to make sense to have some kind of PRT express lanes that go quite fast to connect community hubs. This is an entirely different model than the CBD, both in terms of preferred track and stations and preferred vehicle. Speed, versatility and track cost would seem to be the main design factors.

Finally there are the outlying suburbs and satellite communities. Commuters typically travel at posted speeds until they get close to town and then traffic backs up. These people generally have large engine vehicles because aggressive driving, for them, is somewhat of a survival skill. Keeping these cars out of city limits would do a city a lot of good. Here 30 mph PRT would be useless. It must be much faster to compete with the freeway. People in outlying communities cannot expect to have PRT vehicles come by their homes. Dual mode vehicles would be a poor substitute for the pick-up trucks and SUVs that get them around now. To be perfectly honest, I have serious doubts about whether PRT is the right tool for this job. It certainly calls for fast vehicles on a fast track, but why individualized vehicles? A park-and-ride GRT (Group Rapid Transit) station on the out-of-town side and an ad hoc drop off scheme might be a more efficient. Here is some thinking on this.

The main drawbacks with group travel are waiting and having the station locations that are catered to the average passenger but are not anyone’s exact origin or destination. I believe, however, that with automation and an intelligent system these problems can be largely solved. For example, the problem of fixed scheduling and associated waiting is largely a communication problem, as is inconvenient transfer locations. (Passengers and transit have had to meet at a prearranged time and place, because it is presumed that they can’t talk to each other.) If the “system” knows the complete itinerary of every passenger, the right size vehicle can be sent at just the right time for the group. Is transferring a really the problem if the time waiting for the transfer vehicle is eliminated? After all, PRT vehicles can swarm incoming GRT vehicles moments before arrival, and the “system” can decide where this meeting would take place. GRT requires heavier track, but the other choice may be requiring all PRT vehicles to be more costly and robustly configured than would otherwise be the case. Building super fast PRTs to go slow is a waste. Building high-speed high-capacity track might be a better investment. After all, this might find dual use for freight. There is also the matter of spreading the weight. Perhaps a “fast lane” with greater headways between vehicles and greater spacing between bogies would not need to be that much more expensive.

The track profile I have been working on is specifically designed to be adaptable for multiple weights and speeds. This brings up the question of track “permissions.” Obviously a heavy vehicle must not use light track, but light vehicles could use heavy track. Clearly slow vehicles should not hold up fast ones, but fast ones might want to use a slow track, on occasion. These questions are for a different day, but, to rap it up, I suspect automated transit is not a “one-size-fits-all” technology, and different parts of a city have differing transit needs, and therefore different optimal designs. I also wonder… What starting configuration gives the most bang for the buck?

Next week: The long awaited grand opening of the design collaboration site.

13> A Mid-afternoon Dream

I was sitting in traffic the other day behind a Wal-Mart truck. We both were creeping along together, both going to the same place. Actually I was going to Northern Tool, but still in the same “mixed use” retail area, kind of a mall of sorts, but with no coordination and lots and lots of traffic. The fact is that 75% of that traffic was bound for one of the many sprawling parking lots in this area. It occurred to me that Wal-Mart had two problems. First it was burning Petro-dollars, engine life and driver salary trying to make a delivery, and second, the traffic Wal-Mart alone generates tends to keep potential customers away. If there was a place I could park my car, grab a PRT and be dropped at their front door, I might just stop in for a six-pack of socks. But what if delivery pod cars brought those socks right into the building, maybe right to the correct aisle from a warehouse that also was fitted with indoor PRT style track? Well for one thing, out of stock items could be replaced in a matter of minutes, even during rush hour. And Wal-Mart would be the king of “green.” (environmentally AND cash wise) Now the big question. What would Wal-Mart pay? Would they buy or share track? Contract for rights of usage? Subsidize a station? And what about McDonalds? (Hmmm… refrigerated pod cars. Now there’s a thought.) Can you imagine NOT having a station just down from Wal-Mart? And what about the other merchants? I could see an area I like this becoming a much more attractive destination because of PRT. And wouldn’t a track going to the nearest high density residential area be a logical next step?

This is one reason I believe that “pods just gotta hang.” They are much more parking lot and warehouse friendly. It is also why I now believe reverse is a must for any standardized drive unit. PRT is only as useful as the network of tracks is extensive. Finding uses other than public transit and adding alternative sources of funding can only be a good thing.