47> Serial vs. Parallel

In my last post I brought up the idea of GRT as a way to get the first PRT track built because one of the problems with PRT is that it becomes less compelling with fewer stations. I think it is legitimate to ask, “At what point (in terms of scale) is PRT cost effective and profitable?” Is it worth building a mile of track with just a couple of stations?

This type of question got me to thinking about station design. Below (left) is a drawing showing the typical station usually associated with PRT. The vehicles wait in line for passengers, and then leave. In my example, they get off at one area and enter further down the line, cutting “turnaround” time. The problem with this design is that the whole station throughput is dependent on how fast the passengers enter the pods and seat themselves. In a very minimal system, this could limit the number of paying customers on the track.

On the right is a station with parallel berths for boarding, allowing the throughput of the station to be much higher. My original question was how minimal a system could be, and I will demonstrate my points with the following formula, where H is the headway between vehicles, T is the turnaround time, or the time I takes to enter a vehicle, get seated and get up to speed, and N is the number of stations.

Let’s consider the case, for example, of a system that could financially “break even” at 6 cars a minute, on (24 hr) average (10 second headway), but that means, for example, that the headways must be far closer during peak times, say, 2 seconds. Now let’s assume that the city wanted to start with only small triangle of track between 3 stations. If the stations were configured as in the left hand illustration, and the turnaround time (T) was, say, a minute, then it can be seen that H = 60secs divided by three, which is 20 seconds. Therefore the minimum headway is 20 seconds, or ten times as much as is required to break even. It can be shown that with the taxi-line style stations, it would take 30 stations to achieve the desired traffic throughput.

The second (right hand) illustration shows how parallel docking areas can be configured to allow multiple boardings at a time. In such a configuration the berths may be considered as separate stations, as far as the formula is concerned, but even with the additional berths the number of stations needed to break even is over 7. In a situation where the customer really only wants a 3 station loop, the stations had better be able to accommodate 10 berths each and the foot traffic had better be there to require them, or the project will lose money.

I think that there are important design implications to consider here, first and foremost being the importance of rapid boarding.

I, off-hand, do not recall seeing parallel boarding schemes in any illustrations other than the Mister System. Because of the slots that would be in the floor on most bottom track designs, I wonder how parallel boarding would even work, although two berths would be easy, because the elevator could be positioned between the tracks. In any case, designers should consider ways of speeding the boarding process as much as possible for limited system designs.