Multi-layer network approach for modeling general combined-mode trips

This paper describes a multi-layer approach for modeling general combined-mode trips. The model formulation is based on an extended network framework, termed a multi-layer network. Two major problems arising from the specification of a multi-modal network equilibrium model are addressed, i.e. route (or path) enumeration and travel choice in a network with multiple transport modes. It is shown that the multi-layer network approach can facilitate the generation of feasible routes in a multi-modal setting. A feasible route in the multi-layer network is characterized by a combined mode, which indicates the feature of the used modes or mode combinations along the route. All feasible routes between each origin-destination (OD) pair are grouped, according to the combined modes of routes, into four subsets: auto, transit, auto-transit, and pure walking. The travel choice model is given by a nested logit model where the upper level is focused on combined-mode choice while the lower level on route choice. The multi-modal network equilibrium problem is mathematically formulated as a variational inequality (VI) problem. In addition, interactions among various motorized modes sharing the same road space, as well as nonlinear transit fare structures are taken into account in the multi-modal network equilibrium problem.