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18 Table 14. Logit model coefficient estimates. Coefficient Variable Estimate t-statistic Constant - Automobile 7.4401 5.798 Constant - Commercial Air -5.3354 -3.895 Constant - Air Taxi Piston 3.4218 2.588 Constant - Air Taxi Prop -0.6141 -0.278 Cost - Automobile -0.5573 -21.424 Cost - Commercial Air -0.0127 -11.361 Cost - Air Taxi Piston -0.0137 -15.137 Cost - Air Taxi Prop -0.0078 -7.422 Cost - Air Taxi Light Jet -0.0071 -11.970 Total Travel Time -0.0093 -6.062 Avg Time Between Departures - Commercial Air -0.0049 -10.923 Income - Automobile -0.0058 -0.340 Income - Commercial Air 0.0944 5.322 Income - Air Taxi Piston -0.0405 -2.201 Income - Air Taxi Prop -0.0354 -1.196 Based GA Piston + Prop Aircraft 0.9307 3.664 Based GA Jet Aircraft 0.8537 5.092 the overall sample is representative of the total population necessarily indicates the direction of effects. A more meaning- shares of each mode derived from the datasets. These existing ful interpretation can be gained by computing so-called "direct mode shares are as follows: mode elasticities," which reflect how a 1% change in the value of a particular attribute for a particular mode will affect the Automobile: 44.9%; likelihood of selecting that mode. For example, a price elastic- Commercial Air: 52.1%; ity of -2 for the commercial air mode means that a 1% increase Air Taxi, Piston: 2.0%; in the price of commercial air would lead to a 2% decline in the Air Taxi, Turboprop: 0.2%; and probability of selecting that mode. Direct elasticities for all of Air Taxi, Light Jet: 0.8%. the explanatory variables in the model are shown in Table 15. For the most part, the elasticity estimates have the expected sign. Increases in price, travel time, and average time between Statistical Results flights all lead to decreases in the probability of selecting the The coefficient estimates and statistical significance indica- associated mode. Increases in the number of based aircraft tors are shown in Table 14. T-statistics greater than about 2.0 (piston/props or jets) at GA airports are associated with in- in absolute value indicate statistical significance at the 95% creases in the probability of selecting one of the air taxi modes. confidence level. As can be seen, most coefficients are statisti- If in fact these based aircraft counts are reasonable proxies for cally significant, with the exception of two alternative-specific airport amenities such as hangar facilities, ground transporta- income variables and the constant for the air taxi piston mode. tion services, precision approaches, and so forth, the relatively All variables listed in Table 14 (even those with insignificant large values of the elasticities suggest that the ability of airports coefficient estimates) were included when making projec- to provide such amenities may in fact lead to significant new tions for future years. air taxi traffic. There is no easy straightforward interpretation for the co- Somewhat surprisingly, the elasticity estimates also indi- efficients themselves; not even the sign of the coefficients cate that a given price change for the air taxi modes will elicit Table 15. Direct elasticity estimates. Commercial Air Taxi Air Taxi Air Taxi Light Automobile Variable Air Piston Prop Jet Cost (Price) -0.70 -0.18 -2.52 -5.51 -3.17 Travel Time -0.32 -0.17 -0.48 -1.10 -0.50 Income (relative to AT light jet) -0.03 0.35 -0.90 -1.94 NA Avg Time Between Departures -0.09 Based Aircraft - GA piston/prop 1.54 3.89 Based Aircraft - GA jet 1.17