Airport Ground Access Mode Choice Models (2008)

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82 Air party—group of air passengers traveling together. Airport access trip—travel by air passengers, airport em- ployees, or others to an airport from their home or other local trip origin. Airport egress trip—travel by air passengers, airport em- ployees, or others from an airport to their home or other destination. Alternative-specific constant (ASC)—constant term in the utility function for a specific mode in a mode choice model that reflects the attributes of the mode that are not accounted for by the other variables in the utility function. Calibration—see model calibration. Causal variable—factor that is believed to influence the out- come of some process, such as a characteristic of a deci- sion maker or a measure of the situation faced by that de- cision maker that influences the decision that is made. Coefficient—terms in a model that assume fixed values and interact with the independent variables to predict the value of the dependent variable. Composite alternative—an alternative in a choice model (typically a nested choice model) that represents the com- bined effect of several discrete sub-choices. Continuous variable—variable in the utility function for a specific mode in a mode choice model (or indeed a vari- able in any model) that can assume any appropriate value. Dependent variable—term in a model the values for which are predicted by the model based on the values of the in- dependent variables. Deplanements—number of passengers getting off aircraft at an airport. Disaggregate choice model—model explaining the choice made by an individual (or group of individuals) in terms of the characteristics of that individual or group and the pre- vailing conditions under which the choice was made. Dummy variable—variable that can only take the value zero or one. Enplanements—number of passengers boarding aircraft at an airport. Estimation—see model estimation. Explanatory variable—independent variable describing some external factor or attribute of the process being mod- eled that is believed to influence or has been shown to in- fluence the outcome of the process being modeled. Independent variable—term in a model that can take values that can be varied independently of the values of other terms in the model. Logarithmic transformation—conversion of a variable to the logarithm (usually the natural logarithm) of its value. Logit model—particular form of mathematical model describ- ing the probability of choosing an alternative from among a set of possible choices, in which the natural logarithm of the ratio of the probability of choosing one alternative to that of choosing a second alternative is equal to the difference in the value of the utility functions of the two alternatives. Mode choice—process by which a traveler chooses the mode of transportation to use for a trip. Mode of transportation—specific form of transportation (e.g., automobile, taxi, and public transit). Model calibration—process of adjusting the formulation or parameters of a model so that the predictions of the model correspond to an observed outcome of the process being modeled. Model estimation—process of determining the values of the parameters of a model that result in the model giving the best fit to a given set of data describing the process being modeled. Monte Carlo simulation—an analysis technique in which a series of pseudo-random numbers are generated by a com- puter program (pseudo-random because numbers generated by a computer program cannot be truly random) and then used to select values from one or more defined statistical distributions to generate a new dataset that conforms to the statistical distribution(s). Multinomial logit (MNL) model—form of logit model with more than two choice alternatives in which all the alterna- tives enter into the choice process at the same level. Nested logit (NL) model—form of logit model in which some choice alternatives are considered to enter into the choice process as sub-choices of a higher-level choice alternative. Originations—number of passengers boarding aircraft at an airport who have traveled to the airport by ground and are starting their directional air trip. Pivot-point analysis—a technique in which the coefficients of a mode choice model are used to predict the proportional change in mode share from a change in some transportation service variable and then this proportional change is ap- plied to the existing mode share to predict the new mode share. Revealed preference model—model of behavioral choice process estimated from data on choices actually made by subjects together with data on factors believed to influence their choice process and descriptive data on alternatives available to subjects from which they made their choices. Stated preference model—model of behavioral choice process estimated from data in which subjects select options from among hypothetical sets of alternatives for which they have been provided with comparative descriptive data. Utility function—mathematical expression combining the effect of several causal variables into a single measure of the perceived attractiveness of a given alternative within a set of possible choices. GLOSSARY