The (Il)logic Behind PRT

Post by Steven Dale

The Morgantown PRT. Those vehicles sure do look personal don't they? Image by Brian M. Powell.

Personal Rapid Transit (PRT) is a technology that’s been dreamed about in transit circles for roughly the last 60 years with little to no progress. For those unfamiliar, the concept of PRT is based around the following 4 principals:

  • Small, automated vehicles with seating for 2 – 8 people.
  • Vehicles available on demand at stations throughout the system.
  • Direct from origin to destination. Vehicles do not have to stop at intermediary stations.
  • Non-linear networks of stops, thereby eliminating the need for transfers.

The concept is that for public transit to compete with the private automobile, it needs to replicate the comfort of the car. Fair enough.

The transit-geek-gadgetbahn-aficionado in me would of course love to see PRT sometime in my lifetime. But that’s based upon blind hope and little else. The fundamental logic behind PRT just doesn’t work. Here’s five reasons why:


ONE. Vehicle Capacity. The appeal of Personal Rapid Transit is that it’s personal.

And yet if every vehicle were loaded with only one single rider, there would be plenty of wasted capacity and seats. As a PRT system typically has only one single guideway, the system would basically just be replicating a single lane of under-capacity cars. There is, however, a solution to this problem. Which leads me to my second problem:


TWO. Solving the vehicle capacity problem negates the whole concept behind PRT.

The only way to solve the problem outlined in the previous point is to enforce a ‘carpool’ mentality. How popular is carpooling? Station attendants would necessarily have to force riders to ‘buddy-up’. 8 person vehicles would be filled by 8 people whether they were traveling together or not. Suddenly it’s not personal. Suddenly you have 8 different people traveling to 8 different locations. Which leads me to the third problem:


THREE. Station Attendants will cost money.

Any cost savings that PRT imagines would be erased by the need for station attendents to enforce carpooling during peak hours. Despite having these station attendants, it’s unlikely that the attendant will be able to group passengers according to their destinations. As such, we have a fourth problem:


FOUR. People going in 8 different directions must travel in 8 different directions.

So now it’s rush hour and we’ve got 8 different people traveling to 8 different destinations. Now the algorithm used to control the vehicles must calculate a linear route that stops at each destination sequentially. And that would be utter insanity. Imagine if you and your fellow rider were traveling to destinations at the exact opposite ends of your respective city!

You could solve this problem by giving everyone their own vehicle, but to do so leads us straight back to point ONE. The only real way to deal with this issue is to institute fixed routes, which leads me to the fifth problem:


FIVE. The Appeal of PRT is the Elimination of Fixed Routes.

If suddenly every PRT system is a linear fixed route, then what we have is nothing more than an Automated People Mover that has the ability to skip stations. Note, however, that as 8 different people with (presumably) 8 different destinations are using this souped-up APM, riders will still be faced with the situation of stopping at stations different from their destination.

It shouldn’t surprise anyone that the most famous “PRT” system in the world is the Morgantown PRT in Virginia -which shares a surprising resemblance to the situation I’ve just described.


SIX (BONUS!). Google’s already invented PRT.

It works and is a driverless car.

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  1. Steven, PRT can still work for line haul applications. For instance, there are many LRT proposals where the passenger volume is quite low. Even current PRT technology, such as Ultra, could handle these applications. Worst case, you could supplement with buses for the 2% busiest times to reduce delays/number of vehicles required. More than that, PRT is nearly an order of magnitude cheaper than LRT, so in these marginal LRT applications PRT could provide equivalent or better linehaul service at lower cost. As far as real-world development, Ultra is expanding at Heathrow, and actively developing large urban systems in India. The critics won't be convinced until it is applied there, and even then they will say "It would only work in India", etc. Thankfully, your skepticism does not stand in the way of these developments.
  2. Marsden Burger
    I think these have been some of the best PRT discussions I have seen on the internet. Some thoughts on PRT vs Robocar and Robocar's future role: IMHO, Andrew F has the best of this "debate" without even getting to the issue the snow and ice issue. I think the work on driverless vehicles will lead to many positive things - just not automated vehicle networks in northern climates. One of the major problems of safe automated vehicle operation is control over the vehicle to "guideway" interface. There is no vehicle interface control over ice covered roads with normal rubber tired vehicles, hence the possibility of there ever being automated automobile operations on normal streets in areas where there is significant cold weather is a very low. Could an automated car ever be affordable if it is going to be able to dig itself out of being snowed in on the side of a street, which is typical for urban area automobiles across the entire northeastern part of the United States? Why buy an automated car if you need to dig it out and drive it under adverse weather conditions - will they be cheaper than a normal car that you need to dig out and drive? If you expect to go away from the streets and use special grade separated structures, then obviously, Andrew F's logic wins easily again. I think Google deserves a great deal of credit for forcing new technological advances on a transportation industry that has not seriously been interested in major change.
  3. Andrew, I think the question is, if PRT is simply operating as a "straight line" replacement of an LRT system, then what exactly is the advantage of the technology? As the Morgantown PRT system demonstrates, the technology cannot allow on-demand service during peak times as the system would be overwhelmed. As for London, it's really not much more than an APM system. India . . . let's see what they build. I'm not actively rooting against PRT, but I have yet to see a really compelling argument to suggest how it stacks up.
  4. Hi Marsden, I think the real question then is this: Hasn't Google already accomplished more with a single driverless car than the entire PRT industry has in 50 years of existence? Doesn't Google stand a better chance of creating a "true" PRT technology than others?
  5. Morgantown does not prove anything about PRT. It is an ancient GRT system. It's like saying Ford Pintos prove that modern cars are liable to explode. Marsden, I don't think snow and ice is a show-stopper for robocar. Robotaxis will be maintained by the operator, who would be responsible for digging them out from under snow (or they might just park them indoors when not in use). They may get stuck on snowy streets, which is admittedly a problem. I'm not sure it's that different than what happens today. Ice/slipping is probably no more an issue than it is for human drivers. There will of course still be accidents, but I expect robocars to be able to recover better than human operated vehicles in slippery conditions. I saw a very impressive demonstration with a Google Prius taking people on white knuckle test drives on a rooftop parking garage.
  6. True, Andrew, true. But then what do we have as a strong current example? London's system? It's basically two stations (yes, I know, it's three, but let's be realistic here) with a line capacity in the low hundreds. It took years longer than expected to build and does little more than what one of those rail-based old-timer car rides at a fairground can do. Again, my skepticism in the technology stems from a lack of credible and useful examples.
  7. Marsden Burger
    Hasn't Google already accomplished more with a single driverless car than the entire PRT industry has in 50 years of existence? I do not know if it was Mr. Page's intent to improve mass transit or to improve the ability of his company to photograph all of the streets in the world that led to the interest in the driverless car. If we are to accept his commencement speech at the University of Michigan a while ago, one sees that he had at one point an interest in PRT. Is that what is "driving" the interest in driverless cars? I would be skeptical that it is, and lean more toward the desire of reducing the cost of the manually driving needed to photograph streets, that initiated the effort - but who knows. I think Mr. Page has "Accomplished more" by being at the right place at the right time with the right knowledge, and most importantly, while others may not agree, "the courage" to step into the unknown and create the search engine and the company to make the "internet search" what it is today. That is a major accomplishment - a major creation of value! We as a society need all of this style of accomplishment we can get. Will the spin-offs from the dabbling in driverless cars be anywhere near the value of internet search... Has the driveless car had a positive impact on mass transit.... Has PRT had a positive impact on mass transit... What is the role that is envisioned for the driverless car as it relates to mass transit....?
  8. Marsden Burger
    Andrew, I believe that there will be many positives that will come from driverless car technology, and I could see that robotaxi fleets could be part of those. I am not sure of the value in mass transit, but if they could replace low density bus routs with robocar fleets, that would be a major advantage. Effectively applying PRT principles to the many low density bus routes - but at what cost in vehicles and maintenance? The problems with snow and ice seems always to be underestimated - not sure why. There are many issues here, but I have yet to see anything that deals with the related problem other than to indicate that solving these issues is farther down the development path. When a two way street turns to one lane ruts, how does a robocar know when to turn out in a game of chicken - and if it could, how does it get back into the ruts from the piled up snow...??
  9. PRT of the Ultra variety is really no more than simple robocars on a lightweight dedicated guideway and a dispatching system, so should be a lot easier than equivalent robotaxis on public roads. I can see these technologies converging, the differences evolving more in the service characteristics rather than in the underlying engineering. PRT has always suffered from the proprietary system problem when applied to public funded transit. With trains and buses you can replace and expand easily, purchasing from a wide range of suppliers, with PRT authorities have understandably avoided ending up locked into a single supplier for what has until now been a very complex concept. That is why there has been no PRT industry as such thus far. This is changing with the technology itself no longer being the major barrier.
  10. I wonder why is possible to think that PRT or Googlecar could be answer to the public transport demand. They could be alternative to the TAXI , no more.
  11. Marsden Burger
    GiorgioXT "the public transport demand" is a very blanket statement within which there many areas of significance where small automated vehicles can play a important role. When one looks at taxi usage in our major cities like New York, Chicago or Paris, taxis play a major role in the un-defined "transport demand". Below is a link to a study done as part of the efforts to construct the Cabintaxi small vehicle system in Hamburg in the late 1970's. If you have an interest in how small automated vehicle systems can play a significant role in real world situations, take the time to look at this in detail. The Hamburg effort was undertaken by a very broad high level team, which included all elements of transit professionals throughout the highly respected German transit world. Beyond question, still the most significant small vehicle systems effort ever undertaken. No major city transit property has ever come as close to the significant urban small vehicle systems installation which was under consideration by the Hamburg Hochbahn. While I concur with Mark that the "proprietary system" issue is a significant factor in the confusion that helps to block the introduction of these systems. In Hamburg, it was simply the fickle nature of government funding in general. http://faculty.washington.edu/jbs/itrans/big/Cabintaxi%20PRT-GRT%20Study.pdf [img]http://faculty.washington.edu/jbs/itrans/cabintaxi%20illustrations.htm[/img]