An integrated framework is proposed that models the regional transportation system including three principal modes of transport – road transport, commercial air transport and a hypothetical on-demand air service mode. The main objective of the work is to explore the demand for such an on-demand, point-to-point air service. Demand forecast modeling is based on the traditional four-step process. A multinomial logit model is used to model the mode choice behavior of the travelers. Network assignment is carried out on a ‘composite network’, consisting of all the modal networks. Different scenarios involving varying on-demand air service network topologies and price structures are presented, as case studies. These case studies illuminate the most appropriate service networks for ODAS in case of different aircraft performance and price structure. In case of a typical Very Light Jet aircraft parameters, the study shows that most of the demand for ODAS lies in the short-range trip (~200 miles) domain. The framework also demonstrates a viable analytical tool for studying transportation systems with multi-modal interactions.