Sunday, February 14, 2010
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)