By Michael D. Setty

 The website for the personal rapid transit (PRT) system claimed by proponents as closest to revenue operation, operations, the 2.5 mile "ULTra"  PRT parking lot shuttle currently undergoing testing at Heathrow International Airport outside London, recently released more information regarding construction costs. In most applications, ULTra estimates costs of $7 to $15 million per one-way guideway mile. After reviewing this cost discussion, it is highly probable that ULTra proponents have underestimated likely PRT capital costs per mile by at least a factor of two to three.

The ULTra website is located here: http://www.ultraprt.com/cms/index.php?page=cost-per-mile-7m---15m

If the vehicles as designed actually hold up under the relatively daily heavy usage anticipated by ULTra at Heathrow, their estimates of capital cost per mile appear to be reasonable, at $7 to $15 million per one-way guidway mile. Since the technology requires 100% grade separation, in most applications ULTra theoretically will require primarily elevated guideways; given this likelyhood, the estimate of $15 million per mile is a reasonable floor for realistic cost estimates.

However, the $15 million per mile minimum captial cost figure is contingent on vehicles actually remaining operable and available on a daily basis. The size and weight of fixed guideway vehicles--whether PRT or various garden-variety rail technologies--is a major determinant of guideway cost, particularly for elevated and subway sections. As currently designed, the 4-passenger, 25-mph ULTra vehicle is reported to weigh approximately 1,800 pounds (see http://www.ultraprt.com/cms/ULTraDescriptionMay09.pdf), about the mass of a mid-1970's Honda Civic (later Civic models have gained significant bulk during the past three decades). In order to save weight, the ULTra vehicle is built on an aluminum-framed undercarriage, includes major suspension components made of aluminum, and uses a wide variety of generally light duty automotive technology, including tires with 13" rims, steering mechanisms, and other components.

The initial 2.5 mile ULTra installation at Heathrow is projected to attract 250,000 annual passengers, riding from a long-term car park (in "Americanese:" a parking lot) to Heathrow Terminal 5. Assuming each passenger riders round trip, the ULTra line would serve 500,000 annual boardings. This is an average of 1,370 daily boardings, likely peaking at 2,000-2,500 boardings on the busiest days. ULTra will have a fleet of 18 vehicles at Heathrow; assuming normal transit spare ratios for backup and regular preventative and other ongoing maintenance, if the system proves as reliable as other forms of fixed guideway transit, it will be able to keep 14-15 vehicles in regular service during each day's peak periods, e.g., morning departures and evening arrivals.

Based on the above calculations, each peak PRT vehicle would be serving an average of 91-98 boardings per vehicle per day, and up to a range of 133-179 boardings on the busiest days. In contrast, typical New York City taxicabs--built with steel, not aluminum unibodies and newer SUV-based cabs on steel frames--served 241 million passengers in 1999 with 13,000 medallion (licensed) vehicles, according to http://en.wikipedia.org/wiki/New_York_City_Taxi_&_Limousine_Commission. That is, each NYC taxicab served about 62 riders on a typical weekday, and slighly more riders per vehicle, allowing for taxicabs used for spares and in maintenance. 

ULTra personnel have reported an expected life of 8 years for the ULTra vehicle design. Assuming an average of 1.2 passengers per one-way trip for each ULTra passenger at Heathrow plus 50% allowance for movement of empty vehicles to stations where needed, the 2.5 mile network would operate about 2,150 vehicle miles daily on an "average" day, and up to 3,100 miles on peak travel days. These estimates result in an estimated 785,000 annual vehicle miles, equivalent to 43,600 annual miles per vehicle, say 45,000 annual miles each--allowing for extra mileage on Heathrow's peak travel days.

Assuming an 8-year vehicle life, it appears that the lifetime mileage of the ULTra vehicle may be around 350,000-400,000 miles. This should be considered in comparison with typical European minibus designs such as the Mercedes-Benz "Sprinter." The Sprinter is a medium duty commercial design with a steel frame and all-steel body designed to carry 13-14 passengers with a lifetime of 300,000 miles+/-.

While the proponents would probably point out that the ULTra guideway is a relatively benign operating environment compared to Manhattan streets, the durability of a relatively light, aluminum framed, aluminum-suspension, automotive components-based, and automotive-based interior and exterior materials-based vehicle design remains to be seen. If the heavy usage per vehicle actually materializes, there is a strong possibility of metal fatigue and cracking developing early with these vehicles, starting with the suspensions and proceeding to the frames as the vehicles continue in daily use. If such vehicle durability problems materialize, very quickly British safety authorities are likely to "red tag" the vehicle fleet, forcing an early system shutdown.

This potentially fatal design flaw could be mitigated by retrofitting steel frames and heavier suspension to the vehicles, at the price of substantial weight gains. In turn, such vehicle weight gains would require substantial reinforcement of the guideways, in some case, probably outright replacement. Constructing heavier guideways would certainly drive up ULTra's capital costs from the stated range of $7-$15 million/mile, to the much more likely range of $20-$40 million per one-way guideway mile.

Should the above calculations prove correct, PRT is highly unlikely to ever meet the contradictory economics of very high cost fixed guideways that carry low travel volumes. PRT such as ULTra and the Swedish/Korean "Vectus" technologies may prove “feasible” with larger volumes of passengers, but so is BRT, LRT, and other forms of rail. At the higher volumes of riders, PRT offers little advantage over these other, proven technologies. At such prices as shown in my "worst case" scenario for PRT above, "rapid streetcars" (http://www.lightrailnow.org/features/f_lrt_2007-02a.htm) may be a much cheaper, more cost-effective option; certainly at the low volumes projected for the Heathrow PRT parking lot shuttle, sticking with shuttle buses and providing a few key bus traffic priorities--such as "queue jumpers"--would be the best solution. 

The Vectus PRT vehicle design (http://www.vectusprt.com/) appears to be potentially much more durable, generally due to an all-steel undercarriage design and steel running rails. However, the guidance and switching mechanisms appear to be much more complex compared to ULTra vehicles, possibly meaning its own unique major maintenance and operational difficulties. But my take on Vectus is another long post... A third competing design is the PRT proposed for the so-called ecocity" of Masdar in the United Arab Emirates, but little information is available compared to ULTra and Vectus.