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Dynamic Traffic Assignment Model Breakdown James E. Hicks, PB Consult D ynamic traffic assignment (DTA) models are provide further insight into how the DTA solution algo- being implemented more frequently in practice. rithm functions. Next, the knowledge gained from the An entire Sunday afternoon session at the 2005 analyses performed is described, and an approach to TRB Annual Meeting was devoted to presentations on overcome the problems highlighted and the impact of practical experience with DTA models. Furthermore, implementing this approach are given. Finally, the find- commercial software vendors are making DTA products ings with regard to implementing a large-scale DTA available in their traditional transportation planning model and the insights gained from breaking down the packages. Several new commercial software releases problems encountered and understanding those prob- occurred during the past year. Many presentations of lems in the context of the algorithmic steps involved in the successes achieved in DTA implementations and solving the DTA are summarized. demonstrations of the capabilities of DTA models will likely be forthcoming. This paper, however, examines a less successful DTA model. The emphasis will be on IMPLEMENTATION DETAILS describing the development of methods for analyzing DTA model results, understanding the detailed interac- The Georgia Department of Transportation is doing tions in the software, detecting relationships among operational planning studies on sections of its freeway data elements that produce various results, and synthe- system to guide decisions concerning the programming sizing an implementation approach that tries to over- of improvements. Its plan is to use focused microscopic come or avoid obstacles. The goal is not only a set of traffic simulation models of sections of its freeway sys- network simulation results that can be compared with tem to evaluate operational alternatives. A DTA model observational data but also evidence that the results are implementation has been identified as a means to calcu- logical and that the model has worked as intended. This late realistic time-dependent flows through areas where paper will discuss the use of one DTA software package, the DTA model uses input data from the regional travel the Vista package (see, and its demand modeling process and produces data required application in a project in Atlanta, Georgia. First, a brief by the microscopic simulation methods. DTA models description of the project identifies the scope for which represent individual vehicle movements, and aggregate the DTA model is intended. The data requirements and link performance characteristics gleaned to determine manipulations for the DTA model are then outlined, dynamic route choices and network equilibrium condi- and the model results are discussed. An example of the tions. A traffic flow solution determined at such a meso- buildup of congestion on a network and its effect on scopic scale is more appropriate for specifying traffic flow patterns is given. Potential underlying causes time-dependent traffic flows through a focused area than for the excessive congestion are addressed, which will a traffic flow solution defined at either a macroscopic 101