Sunday, September 21, 2008

Personal rapid transit fixes the wrong problem

There is a conference at Cornell about the viability of Personal Rapid Transit as a future transportation technology. The proponents of PRT argue that their preferred mode is environmentally friendly and generally a forward looking progressive approach to personal transportation. While I admire their goals, there is no reason that their main concerns can't be addressed on the existing transport system.

Mostly we shouldn't be looking at a system that requires brand new infrastructure. We can better manage what we have and simultaneously encourage cleaner and more efficient vehicles. Overhead lines will harm the sidewalk and road environment by reducing the views and sunshine while increasing noise. In addition, the energy expended to build a brand new transportation infrastructure will greatly diminish any potential environmental gains, even if we (wrongly) assume that the only way to have a system of electric vehicles is through PRT.

Perhaps PRT has a role for niche markets such as airports and maybe colleges. As a large scale transit system, however, it will likely fail to achieve the stated goals, and it will divert precious resources in the process leaving us with a transport system that is worse off than we have now.


A Transportation Enthusiast said...

A few points in rebuttal:

(1) What "existing transportation system" can eliminate all local emissions while achieving Kyoto targets today in consuming one half to one quarter the energy of trains, buses, or a Prius - even while providing continuous on demand service 24 hours a day, 7 days a week? PRT can do all that. If an existing system can do the same, or even come close, I haven't seen it.

(2) "Brand new infrastructure" - we are building new infrastructure every day: new roads, wider roads, rail and BRT lines, etc., and our existing infrastructure is decaying under the weight of ever increasing traffic. PRT can alleviate that with a 3ft diameter support post every 50 feet.

(3) "Energy expended" - there is no transit system that requires less infrastructure and energy to build per unit capacity than PRT. Even bicyclists and pedestrians require roads and sidewalks. The PRT guideway is significantly smaller in size than even a pedestrian walkway, yet carries the capacity of a freeway lane.

(4) "Reduced views and sunshine" - Look at this rendering - is that tiny amount of obstruction so intrusive that we will continue to endure endless gridlock, more freeways and neighborhood-destroying road-widening projects?

(5) "likely fail to achieve stated goals" - how so? Do you have any basis for this prediction?

Anonymous said...

No one said the only way to have electric vehicles is with PRT. Obviously you can have electric automobiles, and companies are indeed beginning to make them. The questions is what kind of electric vehicle is most efficient, convenient and cost effective.

The problem is that, since automobiles are privately owned, you need to make a lot more of them to transport the same amount of people. Only a fraction of all automobiles are being driven at any given moment--the rest are just sitting parked somewhere, a wasted resource. And not only are you wasting an expensive piece of equipment that's just sitting idle, you're also wasting the land it's parked on, which is often quite expensive and could be used more fruitfully in other ways. A PRT system only needs capacity as great as the maximum demand. That means that a lot fewer vehicles need to be manufactured to provide the same number of people with all-electric transportation. Since there are fewer vehicles, the vehicles are smaller, and a parked vehicle doesn't need extra space around it (you can essentially double-park every single vehicle, in all three dimensions, since all you need access to is the first one in line), total parking required is much less than for automobiles, meaning more land or space is available for other purposes, such as commercial, residential or public space.

Another issue is that automobiles are (currently) driven by humans. Not all humans can drive; of those who can, not all of them want to. Many would much rather be relaxing in a vehicle that someone else (or a computer) is driving. And of course humans also often make mistakes while driving, leading to expensive accidents, injuries and deaths. Not to mention that humans drive inefficiently, which wastes energy resources. All of this adds to the cost of driving.


Anonymous said...


Electric automobiles, generally, need to carry their energy source on board. Thus current electric or hybrid electric vehicles carry heavy and expensive batteries on board. This increases cost and reduces efficiency. And since current battery technology doesn't allow for rapid recharging or the amount of distance between refill that we are used to, it also makes electric vehicles less convenient and desirable. Perhaps this can be remedied with a few more decades of research. But PRT already has a solution for this now--it uses line electricity, like other electrified forms of transit. Since the energy doesn't have to be stored in the vehicle, vehicles can be much lighter and cheaper. There is also no need to worry about recharging the vehicle.

Finally, at-grade surface transportation is inherently inefficient because vehicles have to wait for other vehicles to pass at intersections. In fact, the most efficient parts of our road system are the freeways--roadways that do not intersect with cross traffic, thanks to overpasses and underpasses. This is the same principle that PRT uses; it just takes it to an extreme (complete elevation) that is necessary for uninterrupted urban or suburban travel. Of course since PRT vehicles are smaller and lighter than automobiles, they are elevated at much less expense.

So those are the upsides of PRT relative to cars. What are the downsides? Well, the closest vehicle will be a block or two away instead of in your driveway (urban drivers without driveways will already be used to walking a block or two to their car). Perhaps a little inconvenient, but most people can use the exercise. If anything it will be more convenient for the disabled because they don't have to get someone else to drive them. If they need help getting to the PRT station, they likely need help getting in a car too, even if they are able to drive once they're in it. Mature PRT designs have plenty of room for wheelchair users. And there will be some elevated guideways on a few streets. But they will take up much less space than the roadway underneath them, which, if enough people switch to PRT from cars, can once again be used for cyclists and kids playing stickball or basketball (and will also have more room for emergency vehicles that need to get through). And in places that already have two-story construction, the guideways aren't really going to be blocking any views.

Anonymous said...

As for existing public transit modes, people are not going to willingly switch from driving to public transit unless it gives them better service--that is, faster and more convenient service. But very few places have transit systems that provide faster and more convenient service than cars. And where they do, it's usually because the streets are full to capacity or parking is expensive or scarce (nobody drives in New York, there's too much traffic!). PRT could potentially get people where they want to go faster and cheaper than cars. That means many people will actually prefer to take transit, rather than be forced to because of traffic or lack of access to a private automobile.

Yes, PRT results in elevated guideways that some might object to. But electric PRT vehicles are much quieter than buses or trains, not to mention cars. If you live on a street that gets a lot of car traffic you're already used to the noise; if that traffic switches to quieter PRT vehicles I'm sure you'll be grateful. As for the guideways, well, they shouldn't be any more of a visual obstruction than any of the two-story buildings around them. And of course every type of transit has trade-offs. So do you want some annoying elevated guideways with your fast, safe, cheap and efficient PRT or do you want traffic jams, noise, slow speeds and accidents with your other ground-based transit options?

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