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16 Table 11. Relevant universe of automobile trips for 1995. Region of Departure Count Percent of Total Central 801,109 5 Eastern 2,961,779 19 Great Lakes 3,394,996 21 New England 684,720 4 Northwestern Mountain 764,044 5 Southern 2,543,010 16 Southwestern 2,138,512 13 Western Pacific 2,680,552 17 Total 15,968,722 100 Only domestic trips within the lower 48 states and between ton or turboprop aircraft types, so the same costs were used 150 and 400 miles (using the great-circle distance of the for both automobile users and air taxi/commercial air users. corresponding departure and arrival VLJ airports) were consid- Cost and travel time attributes were developed for each air- ered; these trips were then further filtered by keeping only those craft type based on data obtained from the Conklin & de trips taken by "high-income" business travelers, defined as Decker Aircraft Cost Evaluator database. individuals with annual incomes greater than $75,000 (in 1995 Party size is an important factor in the statistical model be- dollars). The logic behind imposing the 400-mile upper limit is cause the air taxi and automobile modes have capacity con- that any automobile trips longer than that probably indicate straints that could affect total costs depending on the party that such travelers have particular reasons for selecting pri- size (e.g., if more than one unit of the mode is required to vate surface travel (e.g., making multiple stops as a traveling accommodate the entire party). In addition, when the VLJ salesman, etc.) and would not be good candidates for VLJ mode is added, it is important to consider the availability of services. This resulted in estimates of relevant automobile per-seat service offers as this will also affect total costs. trips as shown in Table 11. For existing air taxi users identified in the ETMS data, it was It is important to note that the mode choice decisions assumed that all passengers on board (estimated by applying made by ATS automobile users reflect choices that were a 70% load factor to the aircraft passenger seat size) were part available to them in 1995. Consequently, the attributes of of the same travelling party and that the mode choice decision each mode alternative should reflect values relevant for that was made for the group as a whole and not at the individual time period. level. Current air taxi services are offered almost exclusively on As with the other datasets, the raw ATS data provide only a per-aircraft basis, so a reasonable assumption is that such limited information on the geographic distribution of pas- flights mainly reflect demand by groups of travelers where it senger trips. Again, Census population estimates were used to makes sense to rent the services of an entire aircraft for the generate synthetic observations of automobile travel at the group. Census tract level. For current (2007) commercial air users, the DB1B dataset does not include any information on party size. However, the ATS survey indicates that almost 75% of commercial air pas- Model Assumptions sengers on business trips between 150 and 800 miles fly alone, The data from the three sources described above--ETMS so a party size of one was assumed for all current commercial 2007 air taxi flights, DB1B 2007 commercial air trips, and air passengers. This is a somewhat optimistic assumption in ATS 1995 automobile trips--were combined into a single favor of VLJ per-seat services because it does not consider large dataset to reflect the potential universe of trips from that some commercial air trips are in fact taken by larger par- which a new VLJ mode would attract customers. To convert ties who might not realize the cost savings of per-seat services, flights into passenger trips for current air taxi users, an aver- but instead could realize savings as a group purchasing the age load factor of 70% was applied to the seat size of each rep- services of more traditional air taxi services that are sold on a resentative aircraft selected. per-aircraft basis. As noted earlier, the primary attributes used to distinguish Forcurrent (1995) automobile users, certain high-passenger- one mode offering from another are cost and travel time. A count records in the ATS dataset have large party sizes that representative aircraft was selected for each air taxi mode-- are probably not typical; consequently, a maximum party size piston, turboprop, and light jet. A review of aircraft-specific of two was used for automobile users in the ATS. This was cost data indicated that real (inflation-adjusted) costs had not also used as the capacity limit for the automobile alternative for changed much between 1995 and 2007 for representative pis- current users of other modes.