Sunday, April 3, 2011

121> Solving Traffic with 3D PRT

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?  

9 comments:

Juho said...

The second weakness identified in the article said that single more vehicles are captive to the guideway. You used word "urban" in the definition of the PRT system. I tend to think that regular (single mode) cars are captive to the current road system. The captivity problem of the PRT systems is related to the expected limited area covered by the first PRT guideway systems. They will probably be limited to one urban environment only, and to a limited number of guideways within that urban environment. But in the long run PRT networks need not be limited to urban environments only. Urban environments are just places where PRT systems are most likely to appear first (and start their growth).

One should count the cost of building light weight PRT guideways and compare that to the cost of building new roads. In the future it might be that it is cheaper to extend the PRT guideway network than to extend the road system. Current road systems already cover almost all places where people live, but still PRT guideways might turn out to be the most practical way of connecting new places to the traffic network at some places. And eventually, maybe some day, regular (single mode) cars would lose some of their attractiveness because they are captive to the old and not so well maintained and non-automated ground level road system.

The first weakness identified in the article, the need to construct new infrastructure, is maybe the key reason why PRT is still as weak as it is. This is linked also to the second weakness. And this is strongly linked to having standard guideways. One reason why nobody wants to start the business is that there is a high risk that the system that one builds would not be compatible with other PRT systems, and as a result there would be no proper market of the PRT systems, and no connectivity to neighbouring systems either.

PRT has still many standards. Are there any open or multi-provider standards available yet? The beam guideway approach of this blog is anyway one serious candidate with many benefits. I hope this initiative ("Open PRT Specification Project") will grow to something practical and generally accepted. I'd say the lack of standards is one additional key weakness that hinders the growth and emergence of the PRT systems.

Andrew F said...

I'm starting to think that the only way this initiative can see the light of day is through an act of philanthropy. Either a wealthy investor who wants to help solve the world's problems or more of a grass-roots effort (like Bill Gates or Sergei Brin). You probably also need to patent significant parts of the system and put it into the public domain.

Maybe a really progressive government could take it on? Sweden seems inclined.

You're right to emphasize that robocars aren't a paradigm shift in transportation technology. They can provide some improvement, but in they won't be a game changer in addressing urban congestion.

akauppi said...

If Sweden doesn't do it, we'll do it in Finland. :)

Dan said...

Dan the Blogger is in Louisville, headed for Seattle by car -

Hi guys, my son got a job with Microsoft, and I'm helping him move. Redmond here we come! Hopefully I'll be in touch along the way, and knowing my kid, he won't go long without internet once we find a place...Thanks for the comments!

John Hutt said...

The other benefit of the captive, public system over a dual-mode system with private vehicles is that the vehicles will be used by multiple travelers on many more trips; hence, fewer will be required and you won't have to find parking for the multitude of private vehicles that mostly just sit around. Parking is a huge waste of real estate, is very expensive, and makes cities much less walkable and PRT friendly.

qt said...

Nice post, Dan. This is the kind of public-relations thinking I've been trying to wrap my head around, while trying to get others to do the same.

Now to come up with the sound bites.

Hm...

Imagine a world without stop signs

or...

Freeways for the rest of us

or...

No more stop-and-go

Or something.

Those are the first ideas that popped into my head. I'm sure somebody could do better. And needs to.

Dan said...

Juho, thanks for putting my post more in the perspective that long-term followers of this blog know to be my idea of the “big picture.” It is quite true that much of the thinking behind the designs we have been talking about is motivated by a realization that the problem extends well outside of the “urban” areas alone. As far as comparing costs with roads, that is a VERY important topic, one that will certainly be the subject of future posts. Clear, unambiguous quantification of the advantages of such a system is obviously something that requires some work. Otherwise, our “common sense” is someone else’s idea of crazy. As for standards, this has been identified as a weakness in almost all of papers I have run across where the author was actually being funded, (usually via research grants) by government. (As opposed to pundits writing for the media)
Andrew, you are quite right, I think, and this goes to the heart of standards thing. It leads logically to the need for a non-profit organization to accept such donations and channel the money productively.
Well, I have to get on the road… There’s a lot more to say and even more to figure out…
Lastly, Good point, John. That should have been part of my original post. Qt, Akauppi, I gotta go… Next stop Redmond!

Juho said...

One more comment on the cost of roads vs guideways. I guess also the maintenance costs of guideways are competitive. Especially in the north one could save some since the winter maintenance costs are lower. No snow plowing and no damages caused by the freezing and melting roads. And no slippery roads either.

Mike-in-Hull said...

Here's a proposal that was a winner in MIT's 2011 Climate CoLab contest...

http://climatecolab.org/web/guest/plans/-/plans/contestId/5/planId/15207

Install connected Personal Rapid Transit grids over the urban and suburban areas that house the densest 50% of the US population.

A large set of national problems are caused by our car-based transportation system. It is directly expensive for individuals, second only to housing. It is time-consuming especially due to the slow speeds in congestion. It causes air pollution which causes respiratory disease and global warming. Car accidents cause 40K deaths and hundreds of thousands of injuries per year. The demand for foreign oil causes wars. (As I write this, the US is fighting 3 hot oil wars with, I suspect, more on the way.) We can eliminate cars, buses, subways and short-haul airplanes from our urban areas with a grid of 1-mile square cells of magnetic-levitation guideways, 20 feet above a city containing 2 person 'pods' that are waiting for customers at an average distance of just over 1/4 mile from every point in the grid. These pods use less energy and money per passenger mile than any practical electric car, bus or rail (light or not) and are safer and faster. At the top speed of 150MPH, they are faster city-center to city-center than current air travel for cities several hundred miles apart. They beat high speed rail not because their top speed is faster, but because you don't have to wait for them or travel so much to stations. At a fare of 10 cents per passenger mile, (compared to more than 50 cents for cars and even more for existing mass transit) the system can pay for its capital and operating expenses off of rider fares with NO government subsidy, unlike all mass transit in the US today.