Modal Choice Analysis
After completing the Trip Distribution Analysis, we need to determine what
transportation system each of those travelers will use. Mode choice models estimate how
many people will use public transit and how many will use private automobiles. The most
common form of the mode choice model is the logit model. The logit mode choice
relationship states that the probability of choosing a particular mode for a given trip is
based on the relative values of a number of factors such as cost, level of service, and
travel time. The most difficult part of employing the logit mode choice model is
estimating the parameters for the variables in the utility function. The estimation is
often accomplished using one or more multivariate statistical analysis programs to
optimize the accuracy of estimates of the coefficients of several independent variables.
In regions where there are several alternative modes available, the mode choice model
may require a special form called the "nested" logit. This form attempts to
represent the choices presented to the traveler in a more structured manner. Nesting is
necessary when there are major competing alternatives within, as well as between,
principal modes.
Logit Model
Where:
Pit = probability of individual t choosing mode i
Uit = utility of mode i to individual t
Ujt = utility of mode j to individual t
For example:
Uauto = 1.0 -0.1 (TTauto) - 0.05(TCauto)
Ubus= -0.1 (TTbus) - 0.05(TCbus)
Uwalk=-0.5 - 0.1(TTwalk)
TT = travel time by mode in minutes
TC = travel cost by mode in dollars
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