and no city in the world has ever adopted it as the basis of its transit system. Sure, various cities have individual, limited lines, but no city has a multi-line system that covers even a small area.

Do you know why?

I once talked with a local PRT advocate, and I was able to leave him speechless. Here's why:

Do a thought experiment.

Think of a city with a fully functioning PRT system, let's say trunk lines in all four directions radiating from the downtown area and branch lines into neighborhoods. Now think of rush hour. Thousands of people leaving downtown and getting off at various stations. How do you avoid backups with only one track in each direction? If you have "off-ramps" for the stations, how do you prevent backups at the off ramps for popular stations? What about backups at the on ramps to enter the main flow from popular stations? Once the people get home, you have a surplus of pods at their destinations that may not be needed till the next morning. What do you do with them?

Suppose inbound a bunch of inbound pods are making their way downtown from Mile 10. There are a bunch of people at Mile 5 Station who need to get to work, but all the pods are taken. What do they do as e pods pass them by? Do they flag the first empty one down electronically? If there's no off ramp, just one track, that means holding up the full pods from Mile 10. If there is an off ramp, how can the people in the station see that there's an empty one?

Note that a train (like the exemplary commuter systems in Japan) runs back and forth on its schedule, whether there are passengers or not. It meets a schedule and stops at each platform for a predetermined amount of time. No one gets left out.

Then there are the stations themselves. The sketch shows them with room for one person, with the station accessible only by stairs, which would make it awfully hard for the guy with the stroller to access it. Building stations with elevators and room for a lot of people to wait raises the price tremendously.

PRT works only when there's a single line with few stops. Adding extra lines increases complexity and raises issues that I've never seen a PRT advocate answer.

The biggest advocates of PRT are Republicans who want to waste transit money on systems that don't work but have the superficial appeal of letting white flght suburbanites imagine that they can enjoy convenience without encountering the people whom they irrationally fear.

That's actually a great thought experiment. We see it all the time with cars and how it fails.

Right now we don't have pods, we have cars. Cars run everywhere. So in your rush hour example, we gridlock with cars getting to work. Now they're all needing to be placed somewhere for the workday so we need all this parking. Then they have to leave to go home. Again, massive parking. And because all the vehicles are run by humans, there's accidents and slowdowns and death.

Here's how the skytran system works.

They determine peak traffic flow and the number of pods needed to support it. Those are added to the network as necessary. Offline cars are kept back at the depot.

Every stop has sidings for spare pods. You take a pod, the next spare slides up.

So, how do you handle a sports venue? The venue lets the system know the time of the event for scheduling purposes. As fans arrive their vacated pods go back into circulation. No parking lot is needed. What you do have is a heavy number of stops ringing the venue for hundreds of simultaneous offloads. When the event is over, pods have been prepositioning along those sidings. You don't need every pod for every fan then and there, more will be coming as pods depart. But in short order you are emptying the venue and everyone goes home.

Republicans tend to be the biggest detractors of PRT because it cannibalizes the existing customer base for cars with are a) 20th century state-of-the-art PRT and b) tremendously flawed. But it appeals to the American ownership model, I own my car, fuck you I got mine, jack.

The advantage with something like Skytran is it works with a lower passenger density than required for traditioanl mass transit like subways. You could lay a Skytran grid down on top of our low-density sprawlscape.

of PRT.

I hate sprawl as much as you do, but it's the individual nature of PRT that makes it just like an automated highway.

By the way, crossing lines at different levels LOOKS great until you think about the question of transferring from one line to another, because as one who has experienced rail transit systems in many cities, I know that's what people do in real life. Tokyo's subway lines run at different levels underground, but there are pedestrian walkways, escalators, and elevators between them. (The escalators are equipped with motion detectors so that they run only when someone is actually on them.)

You also have more faith in computer systems than I do, and even the best models can't predict every surge in demand.

I'd suggest a serious model of multiple lines overlaid on a real city, handling both rush hour and off-peak traffic and any special events, ensuring the proper supply of pods at each stop whenever needed, accommodating disabled people, and allowing transfers among lines. I've challenged many PRT advocates over the past ten years or so, and no one has come up with a working model yet.

All I've ever seen working models of is single lines or oversimplified animations with little dots moving around but no dealing with the issues.

The entire thing is packet-switched, like the internet.

What you're saying about transfering lines is necessary because the vehicle itself only goes on one loop. You transfer green line to red, you need a different train.

The Skytran system handles all that.

As for the PRT and Republican thing, any good idea can become the basis for a con. The Simpsons and monorail is the classic cartoon example but we can look at the speculative railroad bubbles of the 19th century. Was rail the future? Yeah. Were the early investors going to lose their shirts to scumbag promoters? Yeah.

So, which solutions do you favor?

with simulations of every situation that current rail systems handle routinely.

Because our new stuff was built without considering any other form of transport, end of story.

What would you prefer?

In the U.S. where it is underfunded and slowly being rebuilt after being destroyed in the 1950s? Or in Tokyo where millions take it without trouble every day?

I'll be bold enough to say that PRT could not physically accommodate Tokyo's or London's or New York's population of commuters (or even those in Los Angeles).

And really? That "we're so special, so there's no comparable model" is an evasion. If you can't build a model of how PRT would work in Los Angeles or Denver or Minneapolis or Kansas City, how can you be trusted with a real city?

Transit needs critical densities with which to operate. The car system fails under heavy load but works in sprawl because it's individual vehicles going places. But the sprawl is economically unsustainable.

Current transit rail models fail because they need higher densities than we've built at. It works great in dense areas like the northeast. We don't have the densities for it in places like South Florida. My suspicon is we'd need a massively-revamped bus service to really provide mass transit here. Subways can't work, we're not dense enough.

Do they work in Tokyo? Yeah. They're built to that density.

I'm in South Florida. We have Tri-Rail. It can't do the job. Our only other option is private vehilces. It fucking sucks. But we're so heavily committed to low-density sprawl, we're painted into a corner. How do we fix this?

Oh wait, that's right, white suburbanites won't take the bus because they're afraid they'll have to sit next to a black or Latino person. If you don't believe me, just read the online opinions about transit on any newspaper website in the U.S. "The buses are full of gangbangers and illegal immigrants." "We can't have a rail line here because it will bring crime and drugs into the neighborhood."

So you tell them that you're all for them riding around in their own little private pod, never mind that if you wanted to make that kind of transit work in a non-dense situation, you'd have to have even more little branch lines than any rail line.

OK, create a model for Oklahoma City or Albuquerque or some other sprawly mid-sized city, one that functions both for rush hours and for getting people without cars (elderly, disabled, children) around town during non-peak hours and for special events.

It's literally as slow as walking. I had to get my car in for servicing and decided to walk back home that day, walk back to the dealer the following day. 7 miles as the crow flies, I think I did it in about three hours given the lay of the roads. Bus would have cut it down to 2.5 hours. For serious? To get to work is a straight shot down a main drag, 11 miles. Takes 20 minutes, plus minus for traffic. Bus transit would take me two hours. Walking the distance is about four hours. I haven't biked it but I could probably just about half the bus time.

As I believe I said above, a better bus transit system would probably be the quickest fix for my area. I guess you're seeing a lot of white bias in your area. I haven't really heard much from people I talk to but maybe they'd shoot down mass transit for seeming too poor or having to hang with too many ethnics.

I don't think you've read anything about the skytran model so you don't understand the difference in scale between their lines and traditional rail. Rail is like a highway and you only run highways where it makes sense. The smaller skytran lines could penetrate into less dense areas, a residential road compared to a highway.

From what I can see, we're still doing our level best to fuck over mass transit in this country. the decision-making process has nothing to do with the rational but is more about status symbols and the percieved good life. People like cars, people like instant transportation, and they've been inculcated into this culture. The younger generations aren't quite so enamored. We may see some real demand shifts in time.

What I don't think you've paid any attention to is that I'm not a PRT-or-bust guy. I like the promise of PRT but am more than happy to entertain any other idea. The only point I won't budge on is that cars fucking suck and I want alternatives.

Show me a simulation for a typical American sprawly city, one that actually WORKS on multiple lines, and I'll be on your side. Till then, it's all just recreational arguing.

Meanwhile, let's stick to ideas that have worked for others throughout the world. If you think that overhead pylons would be less "fixed in place" than a rail line or would take less room or wouldn't be subject to vandalism (I can easily see yahoos in trucks thinking it great fun to knock down one of those pillars), then you haven't thought this through.

Your bus experience is a matter of bad planning, most likely by people like the crew that runs Twin Cities Metro Transit, who one bus-dependent acquaintance characterized as "having contempt for the riders."

A good bus system has frequent service and express runs on all arterials, for one thing. I lived comfortably in Portland without a car for ten years, depending on a combination of light rail and buses.

If you aren't even bothering to read what I'm writing. I think you just skim the first sentence and then go with a canned response.

Can't find a good example of the layout that isn't super-huge. But you can see how they can nest loops together.

I like the Skytran concept but am not wedded to it.

My position is this: cars are a terrible fucking idea. Sprawl is obscenity. Highway interchanges and stripmalls and suburbia are hateful to quality of life. We can surely do better. So what's better?

The problem with PRT is simple, how do you a LARGE number of people on one better then a Light Rail Vehicle (or even a bus) OR provide more service then simply walking?

Transit throughout the world fall into several overlapping categories:

1. Heavy Rail, multi car movement permitted the movement of hundreds of people at once.

2. Light Rail (the name Light Rail was invented in the 1960s due to American/Canadian English using the term Streetcar for what the rest of the English Speaking world calls a "Tram&quot is designed for areas where 50-100 people at a time constantly. Many LRVs run empty as they return to pick up more passengers but the purpose is to pick people up and move them down the track. Please note some cities have Bus rapid transit, bus lanes instead of rails, but used the same as an LRV, to move large number of people up or down the bus lane. Such Bus Rapid Transit are more like LRVs then the buses mentioned below, but by NOT being tied in with a rigid rail system able to exit the bus lane, but at the cost of moving less passengers slower (and it is due to the fact LRVs can move more people faster then a bus lane is why LRVs are making a comeback over the last 40 years).

3. Buses. Ideally where you have 25-50 passengers and thus can NOT justify a separate transportation line. Today, often used in areas where LRVs should be operating (and then the LRVs should be on its own line) but in areas where an LRV can NOT be justified in many ways ideal, more flexibility but at slower speeds (and the slower speeds is not only tied in with being mixed with automotive traffic, but that many LRVs have they own right of way and thus do NOT have to pick up passengers and then wait for a light to change, a problem many buses suffer from. Please note, many of the old Streetcars systems in the US were more like modern bus lines then LRVs in that they did NOT have their own right of way.

4. Automobiles. Yes, Automobiles MUST be considered when discussing mass transit, for it is the first options for many when they is no other option. The problem is parking the car, both at home, when shopping or working. On the other hand, able to go anywhere where a paved road exists. Automobiles are the first choice in Rural Area (and was the prime area of Automobile growth in the 1920s).

5. Walking. Slow, but sure, can go anywhere any other means of transportation can go (except maybe planes and boats). People can walk three miles an hour, thus one mile in 20 minutes. Yes, a lot of people today, take their car instead of walking a mile, but walking is NOT that much out of the running for such short trips (and if you tie in bicycles you are into a 3-10 mile maximum range).

Where does a PRT fit into the above? Technically it should replace the automobile, saving the needs for parking spaces, but then you lose the ability to drive it right to your home, even if your home is in the middle of no where. In heavier populated areas, buses, then LRVs, then Trains (And finally just walking) became better options. Thus park and rides are very popular for people living in areas that can NOT support buses, LRVs or Trains just by the people around the stops for buses, LRVs or trains.

In a PRT you are stuck with what ever track your PRT is on, like a Bus, LRV or train, but unlike an Automobile. PRT also can not handle the volume of people a bus can, let alone an LRV or a Train. Everything a PRT can do, something else can do better, and that is the main problem with the whole concept, it is an attempt to provide an automobile on a track system like a LRV and you end up with something that is NOT as flexible as an Automobile, nor can handle the mass of people a bus or LRV can handle.

PRT is a solution looking for a problem, the problem of people living in low density communities wanting a high tech transit system, rather then a bus or an automobile, when the later two are just a better fit for such low density areas.

PRT can NOT solve the problem of people wanting to get away from crowded urban centers for PRTs can NOT handle that level of population (again buses can do it better, through LRVs and Trains do it even better). PRT can not solve the problem of access to mass transit for low population areas, better the a bus or the Automobile. The PRT can not even address the problem of people moving from low population areas to high density population areas, better then driving your car (or walking or riding your bicycle) to a LRV, bus or Train stop and taking such Bus, LRV or Train to and from the high population density area (i.e. where people work and shop). PRT is a solution looking for a problem to solve.

South Florida is a real sprawlville. If we were to try an off-the-shelf solution tomorrow, I think it would have to involve a wide mix of buses, mini-buses and golf cart trains. Ever see them at a theme park? A drive cart in front, four or five tow cars connected in back.

Trains simply can't work because of the low density. Well, the might work for long-haul trips. West Palm to Miami? Bus to station, train between station, bus to destination. I don't think we even have rail service to Orlando. It does run to Tallahassee, though. But as it stands there's no good way to get to and from the station.

Regarding the shortcomings of past PRT designs, they're not really designed the same way as Skytran. Whether Skytran can deliver on promises made is an open question.

Stepping away from the question of which technology to use, I think the clear answer is that any mass transit solution needs to be:
1. Close to the time convenience of a car
2. Lower cost than the car
3. Not a bigger hassle than everything associated with the car.

In practical terms I think this means setting a maximum walk time from door to ride. 15 minutes seems to be the standard the cities shoot for.

The driverless car technology could be the breakthrough we need to make this practical, especially if we can use computers for selecting destinations and queuing vehicles. Need a ride? Hit the app. The bus will tell you when it gets to you. It could be as small as a minivan, it could be a glorified golf cart depending on where you're at. It'll pick up as many people as need rides along the way. Don't have a phone? Solar-powered bus stop signs will be everywhere. Swipe your transit card, tell it where you want to go, a vehicle will be summoned. If you can make this half as expensive as actually owning a car and have a ZIP car service for the few times a year you need a dedicated vehicle, people will do it.

THe problem is simple, the entire system is the problem. PRT can NOT fix that inherent problem. Pedicars would be a better retrofit. I always joke about my old home town of Pittsburgh PA, it was designed for people to tie you canoe up on the river bank and then walk into town. Streetcars were a good retrofit, for they could move a lot of people five to ten miles so they could disembark and walk to another Streetcar or walk to where you are going. Automobiles were a bad retrofit, for that require wider roads, faster roads that had the secondary feature of reducing population density, making Streetcars less competitive.

In Suburbia you do NOT have any large group of people going in any one direction. Given that situation for a PRT to work, you will have to duplicate the entire road system AND maintain the present road system. That double cost is what is the real killer behind any PRT. Which system gets priority? Auto drivers including truck drivers will demand that it be the highway.

Now, the same complaint can be made about LRVs, but LRVs are intended to move a large number of people from one area of high population density to other areas of equal or less population density. if fed by people who walk can easily move thousands. If the system add park and rides, can move even more people. PRTs, if design as a true PRT (instead of a small LEV) can do the same, but you need a decent population density to justify it to people whose first choice is the public highway system.

For example, if I was designing a transit system for a Modern City. I would look at the present highway system to see where are the bottlenecks and then try to figure out why they occur. More often then not you have people wanting to go through these bottle necks to go to or from a common area, more often then not a "Downtown" or similar business/Commercial/Government Center. I would then put in an LEV system from such center along (or under) such a highway to the other side of the bottleneck and install a Park and Ride at that location. To purpose is to get people to drive to the Park and Ride not to the business/Commercial/Government Center. I would have buses also running to such Park and Rides.

Inside the Bottleneck I would also run a series of elevated what in Pittsburgh was called "Skybus", an automated system, with LARGE cars that picked people up every couple of blocks. Unlike the LRVs that would have few stops, these "Skybuses" would stop frequently and then always at an LEV stop. Some of the "Skybuses" I would run par rel to the LRVs, but most would run between LRVs lines to connect them up. People would have to move from an LRV to a "Skybuses" or even to walk to get to they final destination. I would also look into automating the LRV, all to reduce operating costs.

Here is a Picture of the Skybus automated system, proposed in the 1960s to replace the last Streetcar Line in Pittsburgh. When a Study was actually done, it was found that replacing the old PCC Streetcar system with a rebuilt LRV was cost effective, thus Skybus was never built but an LRV system was built instead:

http://www.brooklineconnection.com/history/Facts/Skybus.html

One of the problem with Skybus was it was intended to replace a Streetcar system that even today beings in 10% of all workers into Downtown Pittsburgh AND traveled on its own right of way, i.e. independent of the road network. Because it existed, peopled settled near it and a good Park and Ride system existed even in the 1960s along with extensive sidewalks to get people to the streetcar stops. If it had been used to replace the normal streetcar that traveled on public highways, it would have been a success, but it was NEVER intended for that purpose, the local transit service preferred regular buses on those routes. On the last Streetcar line, if it was abandoned and replaced by buses, any trips would have doubled or triple in time (even today, if you get on the LRV at its last stop, and someone else drives his car from that stop to Downtown Pittsburgh, the Car will beat the LRV if you exclude the time needed to park the car and walk from the parking lot to actual downtown).

Now, I would also build Skybuses from various areas outside the bottlenecks to the LRVs stop in the other-side of the Bottlenecks. Within the Bottlenecks, all service from the LRV stops would be by foot, bike or Skybus. Outside the Bottlenecks access would be expanded to buses and automobiles.

Such an integrated system would be roughly the equivalent of the present highway system, i.e. Interstates and other limited Access highways are the equivalent of LRVs, local streets are the equivalent of Skybus within the Bottlenecks, and Buses, Skybus, Automobiles, bicycles and walking.

PRT is an attempt to replace buses, trains, "Skybuses" LRVs with one system, it is like replacing Limited Access Highways, Four Lane Highways and local roads with two lane highways (To fast for local housing, but to small to move what an limited access highway can do). Thus the problem of PRT, to small to move a large number of people, and to large to replace walking. Thus a solution looking for a problem to solve.

I'd be curious to see what a fully built-out system like yours would look like.

I agree that we built stupid the first time around but I don't know if we will ever have the capital to fix it. I'm still shocked at how willing we were to destroy functional neighborhoods by running beltways through them.

I look at our built environment and see this tremendous misallocation of resources. It's frustrating and sad.