Saturday, April 20, 2013
One of the problems with PRT is that it requires a network to function at its full potential, and cities and investors alike are reluctant to fund such a complex, untried undertaking. Unfortunately a small, trial loop reflects nothing about how a bigger system would work. It’s like giving some pickup trucks and a circular track to Amazonian tribesmen. Vehicles are only as enabling as their routes will allow.
I have, for this reason, given particular focus on the problems of minimal networks, since this will always be the starting point, like it or not. Configurations such as a single loop, unfortunately, do not make a very compelling case for PRT when compared to more traditional alternatives. Yet this is a battle that should not be ceded, because businessmen need the prospect of near-term profits as well as hopes of a long term bonanza. Therefore a design that is superior in this respect is the more lucrative offering.
To illuminate this point, let’s consider a starter loop with, say, four stations. Our imaginary system goes 35 mph, and has one second headways between vehicles. Let’s further stipulate that the stations are in pairs, say two stations downtown and a pair two miles away, say, in a hospital/hotel district. This is so we can better count the passengers, who will be assumed to be traveling between the two main areas, making a two mile trip.
That means that at maximum capacity, a vehicle will come into one of the two stations at the end of the loop every second, so if there is full, equal demand, there will be an arriving vehicle at each station every two seconds. That means, to keep the whole thing going, there must also be a vehicle leaving each station every two seconds as well.
That is challenging logistically. In order to have a vehicle leaving every two seconds, there will need to be many people boarding a large number of vehicles at once. For example, let’s suppose it takes 30 seconds to get access and get seated. There will need to be 15 parties boarding simultaneously, just to keep up. There will also be 15 parties leaving the vehicles – hopefully a bit faster, let’s say 20 seconds. Let’s say there is a 10 second delay between people leaving and people boarding. (After all, at least sometimes the vehicles will have to move up.) That means that if each vehicle is in the station for only 60 seconds, the station will still need 30 births. And that is with everything working like a clock.
Unlike these illustrations, obviously such a station would be a beehive of activity, and everything would have to be working perfectly to get those vehicles in and out in sixty seconds! I imagine that there would be green and red lights and self-locking gates to regulate access to the vehicles.
For the curious, here are some additional, extrapolated numbers. 35 mph = 184,800 feet per hour = 3080 ft. feet per minute = 51.3 feet per second, which gives us our headway distance. At this full capacity there are 102 vehicles on each mile of track, or 204 each way on our two mile example. If there are always 30 in each station, that equals 120, so 204 x 2 = 408 +120 = 528 separate vehicles.
It is easy to see how people would regard PRT as hopelessly impractical after reflecting on these considerations. After all, a single light rail train can carry 400 people at once. Let’s do the math. If each pair of stations does, indeed, deliver a vehicle each second between them, that is sixty per minute, or 3600 per hour. Assuming something like the 1.2 passengers per vehicle average that is common for auto traffic, that is 4320 passengers per hour, each way. That is just over what a four-car light rail train can carry with just ten trips per hour, or one train every six minutes. Buses, on a designated “busway”, running with headways of about a minute, can also boast similar capacities. Either seems much easier than PRT.
But let’s look at the other side of the coin. First of all, where can you find enough pedestrians to even walk through the doors of such a station at that rate for more than a couple of hours a day? Disney World? Niagara Falls? It will be far more typical to spread the load out with more, smaller stations. So this example greatly favors large capacity vehicles. Furthermore, in such a “back and forth” layout, it would be highly unlikely that people would want to pay a premium for a private vehicle for such a short trip. It would be a simple matter to use at least 75% of them as shuttles, at a fraction of the fare, taking four passengers at a time. That boosts capacity to almost 12000 passengers per hour. Use of GRT (group rapid transit) vehicles would probably result in still higher capacities, although headways would have to be greater, and track would have to be beefier as well.
The cost for light rail is phenomenally high, like 60 million USD per mile just for the track. Compare that with the often quoted estimate of 15 million per mile for PRT, and it’s pretty easy to see where the money for all of those vehicles could come from!
Adding only a few additional stations starts to really break down the advantages that buses and trains enjoy. These large vehicles are slow to start and stop, dropping their round-trip speeds down with each new station. (On the other hand, 35 mph is conservative for PRT, especially for the SMART system depicted, which can easily achieve highway speeds and is designed to mitigate G forces) Large transit vehicles block traffic, and are very expensive to elevate to avoid this. Every stop inconveniences the vast majority of passengers who not boarding or getting off. Once there are a dozen or so stops, it’s really a mess. From a simple perspective of real estate usage, large vehicles MUST accommodate large crowds to justify their presence. The advantage of scale (of LRT and BRT) becomes a disadvantage when it comes to offline stations or other means of passing. In fact, it could be argued that even simple bus stops, in congested areas, often do more harm than good. While the bus is stopped, or is nearly so, it is using valuable real estate that could better be used for facilitating the flow of ordinary traffic. This is especially true if there are only one or two passengers using that stop. With light rail a similar condition exists. Every stop must justify the wait for all through-passengers, as well as pedestrians and vehicles that would otherwise be able to cross the tracks. Therefore light rail should have as few and as large stations as conditions will permit. How few and how large? That is debatable, but it seems to me that holding up 400 passengers to let a dozen or so off and on is real waste. This certainly gives a clue as to how to make these heavier forms of transit more effective. It seems that there should be a limit on how small of a station should be permitted, as well as a limit on the number of stations, period.
I think this raises an interesting question, particularly in regard to light rail. Could PRT actually be good for this form of transit instead of competing with it? Let’s start with some parameters. Say we want to limit the number of stops and increase the passenger usage at those stops so that there is no less than 10% turnover at each stop, with 10 stops total on a loop, so most passengers would only have to wait through 5. If the train has a capacity of 400, that is an exchange of 40 at each stop. Let’s say that we want to provide the convenience of a train every 5 minutes. That’s 480 pedestrians boarding at each stop per hour, or 8 “walk-ups” per minute. Even this, except for rush hour and perhaps lunch time, seems like a lot for most areas. Could PRT be used to deliver those passengers?
It’s a provocative notion. After all, light rail is often run at less than capacity, and is generally constrained by the lack of routes with sufficient riders to justify its enormous cost. It would seem that they might have much to gain by offering a more global transportation solution. Do light rail venders even suggest multi-modal approaches? For example it would seem like a no-brainer to link “park&ride” buses with certain light rail stops. After all, in order to have good geographical coverage along a route, there are liable to be some stops that bring in fewer passengers. Why not make such non-performers into interfaces with other systems, like buses or PRT? That way every station is worth the time and trouble, every time.