Sunday, May 9, 2010
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.