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 The following excerpt was taken from the Transportation Planning Handbook published in 1992 by the Institute of Transportation Engineers (pp. 114-115). Mode choice models are usually the most complex of the sequential model structure. Typically these models estimate how many persons will ride public transit and how many will use private vehicles. Further sophistication of these models may include identifying submode choice among different transit services and estimating the number of car pools or van pools of various sizes for high occupancy vehicle facilities. . . . Logit Model Mode choice models are found in numerous formulations, but the most common are based on the probabilities estimated by some variation or sophistication of the logit function. . . . The common logit mode choice relationship states that the probability of choosing a particular mode for a given trip is based on the relative values of the costs and levels of service on the competing modes for the trip interchange being considered. The level of service provided by a particular mode for a specific trip interchange is usually represented in part by the travel time for that interchange as computed from the transit and roadway networks. The travel time components used to represent level of service include the in-vehicle travel time for each mode and the out-of-vehicle time required to use that mode, such as walking to a transit stop or from a parking lot. The level of service also includes the waiting time likely to be experienced, either to board transit or to transfer. The delay due to roadway traffic congestion is included inherently by using attenuated speeds for congested roadway network links. . . . The travel time and cost of a trip are usually combined using an estimate of the cost of time to convert either cost or time to the terms of the other. The cost of time is usually a variable, based on the economic level of the traveler. Although the mode choice model may be developed using the economic level of individual travelers, forecasts of mode choice are prepared for different economic groups, such as high, medium, and low income travelers. The resulting combination of time and cost is commonly referred to as the "utility" or "generalized cost.". . . . The logit formulation is not a complex mathematical function nor is the utility function it employs. The difficulty in developing a logit model is encountered in estimating the considerable number of parameters for variables in the utility function. The estimation is accomplished using one or another multivariate statistical analysis program to optimize the accuracy of estimates of coefficients of several independent variables.
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