©2003
By Dr. J. Edward Anderson, Taxi 2000 Corp.

"Known as PRT... Its disadvantages are that it's costly and untried."

-- Laurie Blake Minneapolis Star-Tribune Aug. 17, 2003

Aug. 17, 2003-- THERE HAVE BEEN MANY SYSTEMS CALLED PRT. They vary from the 100-passenger Westinghouse SkyBus, to the hugely expensive Morgantown system, to the slightly less expensive Japanese CVS system, to the too expensive Raytheon system, etc. Saying that PRT cost is thus-and-so would be similar to saying that all automobiles cost the same, whether a Geo or a Rolls Royce -- an evident absurdity. The only reason a number of us have continued to pursue PRT is that we know from our own analysis and experience that PROPERLY DESIGNED PRT will be the most cost-effective transit system ever devised. Consider the paper "Optimization of Transit System Characteristics", which can be downloaded from www.skywebexpress.com. In that paper I show that, PROPERLY DESIGNED, the PRT concept -- minimum sized, minimum weight vehicles leading to minimum sized guideways, nonstop trips in private with one's own traveling companions or alone in an network of guideways -- results in the most cost-effective transit system that can be devised.

The reasons are basically two: minimum cost and maximum ridership. A major problem with the PRT systems I just mentioned is that the designers did not appreciate how to minimize the cost of the guideway, which is the major expense item in the system. Mainly, guideway weight is directly proportional to vehicle weight. This is why it is necessary to design a minimum-weight vehicle. Our target vehicle weight is 1000 lb. Our prototype vehicle is somewhat heavier, but we understand why and know from analogy and experience that we can meet our target weight in production units by using construction methods well known today. Secondarily, guideway weight depends on the height/width ratio of the cross section, on construction methods, and on other factors. It is well known that a truss is the lightest weight way to make a structure that spans supports by a factor of at least three, so we use a truss. We reduce weight further by using clamped supports (first suggested by The Aerospace Corporation) rather than simple supports. The result is that we have a guideway that weighs about 1/15th the weight per unit length of a guideway that would support a so-called "light-rail" vehicle, which in the case of the Minneapolis system weighs 105,000 lb. An elevated "light-rail" system costs roughly six times a surface-level "light-rail" system, which is why promoters recommend surface operation, notwithstanding that such systems create barriers to cross traffic and occasionally kill people. For example, I have an article from the Denver Post that says that they have had a "wreck a week" as a result of collisions between "light-rail" cars and other road vehicles.

The vehicle-fleet cost is the next factor. Data reported in the above-mentioned paper shows that the cost of transit vehicles divided by the people-carrying capacity is independent of the capacity of the vehicles. Notwithstanding more motors, wheels, etc. the small vehicle weighs no more per unit of capacity than the large vehicle. One very important factor is production quantity. Thus, the cost of a fleet of transit vehicles needed to move a given number of people is proportional to the total people-carrying capacity needed, which is proportional to the average trip time. The shortest trip time is obtained if the trip is nonstop, which is practical if one has, which we do, a switch system with no moving parts in the guideway. Then all of the stations can be placed on by-pass guideways, called "off-line stations," which permits the trip to be nonstop, provided the occupants of each vehicle are all going to the same destination. Thus private riding, i.e., with one's own traveling companions, minimizes the required trip time and hence the cost of the fleet of vehicles.

The operating cost is the next factor. Data from the same paper shows that the operating and maintenance cost per vehicle-mile of well-designed conventional transit system for any vehicle size divided by the vehicle capacity is not dependent on the vehicle size. This tells you that the operating and maintenance cost per passenger-mile is inversely proportional to the daily average load factor, which is the ratio of daily average occupancy to people-carrying capacity. In conventional transit systems the daily average load factor is inherently only about 10% to 15%. I say "inherently" because vehicles must continue to run at all reasonable times of day in all parts of the system in order to provide service whether or not there are any riders. In PRT, because the stations are off line, vehicles need move only when there are demands for service. This increases the daily average load factor counting empty vehicles to about one third in vehicles the size of those in SkyWeb Express. But SkyWeb Express does better than that because the small, light-weight vehicle is easier and cheaper to maintain. Facilities are much smaller and maintenance time is much shorter.

The bottom line is that the total cost for capital amortization and operation is substantially less in a properly designed PRT system than in a conventional system. What about ridership? The cost per passenger-mile is the total cost for capital and operation per year divided by the passengers per year. Passengers per day depends mainly on total trip time including walk time, wait time and ride time. Walk time is decreased by having a more wide-spread network with close station spacing, both of which are much improved with PRT as compared to conventional rail systems. We have shown that in a properly designed PRT system we can reduce average wait time to less than one minute, whereas wait time in conventional system typically varies from about five minutes to much longer in off-peak periods. Ride time in conventional transit is much longer than in an automobile because of all of the intermediate stops which are not of use to the rider. In PRT the trip is nonstop, which is the best the can be done. PRT has the additional advantages that the ride is private in seated comfort alone or with one's own traveling companions any time of day or night. Studies show that ridership on PRT may exceed ridership on conventional transit by factors of more than ten.

So, as you see, the argument is somewhat complex -- undoubtedly too complex for the average person -- but shows that an OPTIMALLY DESIGNED PRT SYSTEM, of which SkyWeb Express is the best example, will be far from "costly" in terms of cost per passenger-mile. Indeed our many studies show that our total cost per passenger-mile will be well below what can be charged as a fare per passenger-mile, thus permitting the system to be deployed as a private concession. This means that a private finance group can go to a city and say: "If you will cooperate with us we will install a SkyWeb Express system at our cost in return for the revenue, part of which can be taxed." Indeed, for downtown Minneapolis our studies show that the profit from operation of a SkyWeb Express system as a downtown circulator is likely to be sufficient to pay the expected operating deficit on the Hiawatha Light Rail line.

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