Wednesday, October 7

Not really cars, not really transit, what is it?

I generally do not indulge in high-tech fantasizing, mostly because I believe that an expectation of technological salvation too often substitutes for doing the best with what we have right now. Traditional ideas and machines (bicycles, passive solar design, organic growing) have evolved specifically to be energy-efficient and they're cheap solutions. But I'm going to now break that rule.

The Boston Globe published a nice overview of the the resurgence of Personal Rapid Transit (PRT), a concept that has floated in and out of vogue for the last fifty years. For those unaware, the idea is to construct a system of individual pods, like cars, that will drive themselves between various nodes on the network. The occupant need only submit a destination and let the computer sort out the route. The pods would be powered electrically, and they would be equipped with sensors to coordinate movements with other pods. Because a new system is set to open in Heathrow Airport and a few others a being proposed, PRT is starting to raise eyebrows.

I'm not exactly sure why the tracks have to be elevated, other than adding some futuristic cache and avoiding current roadways, but that seems to add some unnecessary expenses and aesthetic problems. Otherwise, leaving aside the issue of feasible implementation, this does seem to match many of the positive attributes of transit with benefits of cars. Since the stations would be fixed, you'd think the land use effects would be more like transit than automobiles, promoting walkable density around the cores. Furthermore, eliminating the need for parking is an incredible benefit to facilitating transit-oriented development around stations.

The energy consumption would probably be similar to light rail. On the one hand, you have lower capacity cars. On the other hand, you only have to run them in proportion to fluctuating levels of demand. Apparently, someone secured a grant to study the potential of building solar panels into roadways, but it's difficult to provide protection from heavy trucks. Perhaps the idea of a solar-powered roadway would be more suitable for PRT. The power would go directly into the pods themselves.

Safety seems like a potential concern, but I'd have to defer to the engineers on that one. It couldn't be any more deadly than highways currently are.

And, of course, the convenience of not having to wait for departures and the ability to travel directly to any destination station cannot be underestimated. Unlike owning an automobile, PRT users would not be able to display their social status through the type of vehicle they drive, although I'm sure that human nature will be able to adapt to that hurdle with, say, very expensive clothing.

I'm not an convert yet by any means, just batting around the idea. Do you think PRT would work in certain metropolitan areas?


PRT Strategies said...

For more on Personal Rapid Transit (videos, links, studies):

PRT is recommended elevated to grade-separate it from streets and rail systems, allowing it to operate in its own space, free of surface congestion and traffic signaling. This allows non-stop, direct transit between origin and destination with no delay, and no need to stop at other stations along the way.

By elevating guideway at approximately 2nd-story levels, station portals can also be built INTO structures to reduce costs and increase convenience for a variety of venues -- apartment and condo buildings, offices and big-box retailers. Hospitals are excellent candidates for PRT portals.

PRT can be built "at-grade" though, as ULTra operates at Heathrow Airport. It simply must be protected (e.g. fenced) to isolate the vehicles for safety.

Elevated PRT guideway can also be mounted with solar panels to offset electrical consumption during daytime operations. Guideway can present approximately 12-15k square feet of photovoltaics per mile.

Stuart said...

West Virginia University operates a PRT system in Morgantown, there is a lot of information online about their experience with it.

Seth said...

There is much assumed about how PRT would operate... some quick questions:

1. How many cars need to be stocked to accommodate fluctuating demand? Public Transport systems currently park a good portion of their fleet during the day waiting for the commute hours. We're given the idea that PRT would be accessible on-demand as you arrive at a station, to accomplish this systems need extra vehicles parked there.

2. Cost: guideways look sleek, but it's more expensive than you think to build. Each pod would likely cost as much as a car. There's nothing more cost effective than a BRT that takes advantage of infrastructure that's largely built. And we can easily program lights to minimize delays for buses.

3. Street life: people accessing sleek office buildings at the second floor mean that people don't have to cross paths with others ever again. Doesn't sound like a city I want to live in, even if it's futuristic. Mobility and convenience aren't the ultimate ends of man.

Mike C said...

Energy consumption for the Heathrow ULTra system is less than 1000 BTU per passenger mile, based on reasonable assumptions of 1.4 passengers per trip and 40% empty pod movement.

By comparison, the most energy efficient US light rail system is San Diego, which consumes just over 2000 BTU/passenger-mi, and the average US light rail usage is more than 7500 BTU/passenger-mi. So PRT would be much more energy efficient than US light rail.

(see this page, figure 2.2).

Elevation is not strictly required, but exclusive right-of-way IS required, and elevation is just the most common way of creating an exclusive ROW. For example, tunneling, cut-and-cover, and highway medians would all be viable options for PRT.

I've been a PRT proponent for many years now. The upsides are very significant (availability, convenience, energy-efficiency, financial sustainability, accessibility, convenience, urban-friendliness), and these positive factors far outweigh the few negatives like up-front capital costs and aesthetics. The aesthetic argument is certainly valid, but with good design I'm confident that a PRT system can integrate well into most city environments without destroying the aesthetic.

Daniel said...

At this point, I'm hesitant to compare projections PRT with actual LRT numbers. Seems better to wait until the Heathrow service is up and running.

And Seth makes a good point about street life. second-floor service within buildings sounds like 60's "skywalk" planning. Just last week I walked around the corridor system in Crystal City which was supposed to be an ultra-convenient connection between office buildings and the shops. Now Arlington is trying phase it out and bring the energy back out to the street level.

Stuart said...

Here is a 2008 report to the governor and G.A. called "The Viability of Personal Rapid Transport in Virginia"

Their finding? "PRT is not yet ready for commercial application. Any decision to implement PRT at a
location in Virginia should recognize the significant time and funding that would be
required to fully develop and test a pilot system."

Mike C said...

Daniel: Certainly projections are to be looked at with a skeptical eye, but the simplicity of the PRT operating model makes it much easier to project energy usage.

The absolute worst case passenger load factor for a PRT system is 0.5 passengers per vehicle. This corresponds to the simplest routing case where every vehicle carries a single passenger and makes a full round trip from source to destination, then back to source.

ULTra's assumptions are not far removed from this theoretical worst case. They assume that 40% empty vehicles and 1.4 passengers per vehicles. In other words, at any given time, 40% of moving vehicles are empty, and the other 60% of moving vehicles average 1.4 passengers per vehicle.

This corresponds to an overall (0 * 0.4 + 1.4 * 0.6) = 0.84 average load factor, not significantly more than the absolute worst case of 0.5.

The raw vehicle energy usage calculation is similarly uncomplicated. Speeds and accelerations are very precisely controlled, and there are no stops during the journey. There are very few unknowns.

These factors make PRT energy efficiency calculations much more straightforward than those for scheduled transit systems, in which energy efficiency is very much dependent on load factors which can vary over orders of magnitude, depending on how well schedules match demand. This uncertainty is reflected in the US DOE data I linked to earlier, where the worst light rail system (Galveston) consumes 15 TIMES the energy per passenger-mi than the best (San Diego).

Having said all that, even if we double ULTra's estimate, it still fares better than the best US light rail system, and it does so even in light demand. This is not a knock on rail, just a demonstration that PRT can do better than good (rail) even in low-to-moderate density settings.

Alberto Zayas said...

The future of automated freight delivery and personal transportation is neither a delivery truck nor a car, the Intelligent Transportation Systems (ITS) initiative main flaw is assuming it will!

Why is anyone trying to figure out how to make a truck and a car drive themselves after they are designed and built to be driven? I suppose the reason is that we already have the vehicles and the roads; but, has anyone considered developing and implementing a fully automated freight delivery and personal transportation system using vehicles no one has to drive from the start? I did and submitted a U.S. Patent Application for it. No other transportation system in use today can claim driverless door-to-door freight delivery but my proposal does. Same thing for personal transportation. No need to go to a station then wait in line to get a PRT (Personal Rapid Transit) pod car that only takes you to another station. You can go to my blog to read more about it:


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