  Travel demand forecasting is an essential part of transportation planning and
management. The four-step travel model is the traditional and most-common
procedure utilized for travel demand forecasting, and many models have been
proposed in the literature to describe each step separately. However, there is
still a lack of a unified modeling framework that can successfully describe the
collective choice behavior of travelers interacting with each other at
different steps. This study uses the free utility model, whose objective
function is mathematically consistent with the Helmholtz free energy in
physics, to separately and simultaneously describe travelers' mode,
destination, and route choice behaviors. The free utility model's basic
assumption is that the travelers will trade off the expected utility and
information-processing cost to maximize their own utility. This model provides
not only a unified modeling framework for traveler choice behavior, but also
provides a new perspective for understanding the user equilibrium model in
transportation science and the potential game model in game theory.
