Posts Tagged: Personal Rapid Transit



The (Il)logic Behind PRT

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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