It is increasingly apparent that combining the travel demand modeling and micro simulation modeling techniques can improve model quality and promote modeling productivity. The project is intended to provide Wenatchee Valley Transportation Council (WVTC), the metropolitan planning organization (MPO) in central Washington State, with a transportation improvement plan for the State Route 285 North Wenatchee Avenue Corridor, a fast-growing area along the Columbia River in Wenatchee, Washington. The project team proposed a tri-level modeling approach to incorporate the strengths of all three types of prevalent traffic analysis tools, namely macroscopic regional travel demand forecast modeling, mesoscopic dynamic traffic management modeling and microscopic traffic simulation modeling. WVTC has a long used and well tested a four-step travel demand forecast model in VISUM, which was further updated and calibrated based on the 2008 land use and traffic operations and bus ridership data. The static 4-step model provided the basis to analyze and project the urban growth to the transportation systems. However, as generally recognized, the static four-step modeling approach does not work well to trace the temporal evolution of the transportation network flow patterns and their changes under various traffic management and control measures such as intelligent transportation system (ITS) options. In this project, this problem is overcome by integrating a dynamic traffic assignment (DTA) model with the traditional travel demand forecast modeling. DTA is capable of computing the network flow patterns not only based on typical V/C ratios and resultant delays but also queues, dynamic intermittent delays, and shockwave propagation within the network. It also offers multi-modal operations within a general transportation network including bus operations, and signal optimization modules that can be used to investigate the effect of various control schemes as well as an interaction with the dynamic traffic assignment if so desired. Due to its capability of dynamic modeling of the traffic flows and simplicity over detailed vehicle trajectory modeling as microscopic simulations, DTA has served as the screening tool for multiple improvement projects that may potentially eliminate the choke points identified from the travel demand forecast modeling. Once the most promising projects (two had been scheduled) were chosen out of the above dynamic modeling process, they were be refined in microscopic simulation models for final technical assessments. Important in this process is the seamless transfer of the dynamic traffic assignment results in the form of time-dependent path flows from DTA into micro-simulation. At the final micro simulation level, the detailed infrastructure improvement and ITS measures can be assessed, and the final transportation plans will be recommended to the WVTC board. The authors conclude from our coordinated 4-step modeling, dynamic traffic assignment modeling and microscopic traffic simulation modeling that combining macro and micro models with an intermediate dynamic assignment layer yields great benefits in dealing with both engineering and planning needs.
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