Sunday, April 3, 2011
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?