BIKE: Re: Monorail, Transit, rails-with-trails

[Nawdry]

At 2004-07-20 16:24, Patrick Goetz wrote:
>PRT suffers from the same problem that the use of private automobiles 
>does:  Use of a (relatively) large container to transport a small number 
>of people.  Consider what would happen at rush hour - would hundreds of 
>people just line up waiting for their turn to get into a PRT vehicle? In 
>some ways this is actually even worse than the use of automobiles, as 
>there is only "1 lane" and everyone is trying to get in their "cars" at 
>roughly the same location.  Also, consider the logistics of stopping at 
>stations.  Does every PRT vehicle stop at every station?  If not, how 
>would the switching be handled, especially with hundreds of such vehicles 
>on the track all at the same time?  The switching thing seems like it 
>would be a big problem for this technology, at least give the state of the 
>art.  And if every vehicle stops at every station, why not just clump 
>groups of users together into larger vehicles for the sake of 
>efficiency?  Oops, we're back to monorail.


Well, for the most part, I agree with Patrick on this.

There's a fairly good article on Light Rail Now! which analyzes most of the 
salient issues associated with PRT:

Personal Rapid Transit - Cyberspace Dream Keeps Colliding With Reality
http://www.lightrailnow.org/facts/fa_prt001.htm

However, it's important to realize that PRT advocates do have responses to 
many of the criticisms which Patrick raises.  For example:

> >>PRT suffers from the same problem that the use of private automobiles 
> does:  Use of a (relatively) large container to transport a small number 
> of people.  Consider what would happen at rush hour - would hundreds of 
> people just line up waiting for their turn to get into a PRT vehicle? In 
> some ways this is actually even worse than the use of automobiles, as 
> there is only "1 lane" and everyone is trying to get in their "cars" at 
> roughly the same location.<<

PRT proponents envision lines of small PRT cars or "pods" waiting at each 
station as passengers arrive.  According to their computer simulations, 
passengers rapidly and efficiently board these little pods, the doors seal, 
and off they go.  Obviously, the first pod in line needs to be filled, then 
the next, etc.  This can happen simultaneously, per simulations, enabling 
an entire "conga line" of cars, head to tail, to leave the station.  Then 
the next "conga line" can enter the station.

Since pure (tiny-vehicle) PRT has actually failed in every real-world 
attempt at implementation, the closest approximation to this kind of 
operation would be something like a "Tunnel of Love" (or "Cave of Horrors") 
ride at an amusement park, where little cars come in a row for boarding by 
a line of riders, then proceed, one after another, into the tunnel.

PRT proponents then claim that their little cars can run up to high speed, 
virtually bumper-to-bumper, in complete safety, because the calculated 
chance of a failure, or accident, in the simulations is measured in tens of 
millions of years - i.e., the sun will probably supernova before there will 
ever be an accident.  Likewise, you don't need evacuation facilities (like 
a catwalk) since passengers will never face an emergency and the need to be 
evacuated.  Buy PRT, and you're clearly buying a dimension of perfection 
that has hitherto been unknown on our planet.

>Also, consider the logistics of stopping at stations.  Does every PRT 
>vehicle stop at every station?

Definitely not, in the PRT concept.  All stations are "offline", off the 
main guideway.  They are accessed by station sidings, onto which the PRT 
pods must divert or switch.

>If not, how would the switching be handled, especially with hundreds of 
>such vehicles on the track all at the same time?  The switching thing 
>seems like it would be a big problem for this technology, at least give 
>the state of the art.  And if every vehicle stops at every station, why 
>not just clump groups of users together into larger vehicles for the sake 
>of efficiency?

Most PRT concepts (and there are at least dozens) envision onboard, 
vehicle-mounted devices which I have dubbed "branch-grabbers" (rather than 
switches) to yank the little vehicles over onto the divergent branch - 
e.g., into a station siding.

Patrick is right to suspect a problem - PRT systems seem to require an 
enormous number of switches.  None of these has ever been tested for 
durability and reliability in repetitive operation over time at high 
speeds, where rather significant stresses would surely be encountered.  But 
of course, no problem - the computer simulations assure us that there would 
never be a failure, and that perfect service performance can be attained 
(onboard automatic glitch-detection would catch weaknesses in the 
"branch-grabbers" before they would emerge as a perceptible problem).

Incidentally, one of the more interesting features of state-of-the-art PRT 
concepts is how the station-capacity issue is handled.  What happens when 
an entire track in a station is filled with a queue of little pods, while 
others are waiting out on the mainline to enter the station?  Ah, no 
problem.  The "waiting" pods are simply sent looping around the PRT 
network, over and over, until they can join another "conga line" entering 
the station.  All controlled with smooth, flawless precision by 
coordination between the central and onboard computers.

And this raises the other prevalent concept - all PRT lines are 
single-tracked (one-way).  So, to go back the same way you came, you must 
first travel from your station in the same direction you originally arrived 
in, and wait for the first branch over to a line headed in the opposite 
direction.

By dint of these features, one PRT vendor (Taxi 2000) proclaims its little 
3-person pods, at headways of 0.5 second or less, can handle peak passenger 
flows of up to 21,600 in each direction.  Lower the headway to 0.1 second, 
and you can handle 108,000 passengers per hour per direction.  All our 
capacity problems are solved.  So, Patrick, we can toss both monorail and 
LRT in the land fill.

LH