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38 BREAKOUT SESSION Assignment Advances James Hicks, PB Consult, Inc. Richard Dowling, Dowling Associates Alexander Skabardonis, University of California, Berkeley Stephen Boyles, University of Texas at Austin Satish Ukkusuri, Rensselaer Polytechnic Institute S. Travis Waller, University of Texas at Austin Kara Kockelman, University of Texas at Austin A DYNAMIC TRAFFIC ASSIGNMENT MODEL BREAKDOWN James Hicks James Hicks described the use of a dynamic traffic assignment (DTA) model in Atlanta. He discussed the application of the VISTA DTA software package, the data requirements and specifications, and the analysis process and preliminary results. Volume 2 contains a paper on the topic.1 The following points were covered in his presentation. ⢠The Georgia Department of Transportation is con- ducting operational planning studies of different freeway sections in the Atlanta area. Microsimulation models of freeway sections are being used to evaluate operational alternatives. A DTA model provides a method to calcu- late realistic time- dependent flows through the areas. The DTA model uses input data from the regional travel demand modeling process and produces data required by the microscopic simulation method. ⢠DTA model inputs include the regional highway network, regional trip matrices, and traffic control infor- mation. The regional highway network includes posted speed and capacities by facility type. The regional trip matrices convert to discrete vehicle trips and assign departure times for trips. The traffic control input includes phasing and timing plans. DTA specifications include the demand period, the simulation period, and the assignment interval. The link- time aggregation inter- val, the results interval, the warm- up interval, and the cool- down interval are other specifications. ⢠The input data for VISTA DTA includes the Atlanta regional highway network described as a link table and a node table. Input tables also define the loca- tion and operational characteristics of signalized inter- sections in the network, as well as an input table for the demand to be simulated for the network. ⢠The vehicle simulation is based on the propagation of vehicles according to the cell transmission model net- work links, which are divided into cells. Vehicles are moved from cell to cell along links and between links. The propagation of vehicles depends on the posted speed for the links, saturation flow rates, and jam sensitivities for the links. ⢠A 1-hour warm- up period was used in this analy- sis. Three 1-hour analysis periods were used. These analysis periods were 6:00 a.m. to 7:00 a.m., 7:00 a.m. to 8:00 a.m., and 8:00 a.m. to 9:00 a.m. Flows were tab- ulated for these time periods and compared with observed 1-hour counts. A cool- down period sufficient to allow all vehicles to be simulated entirely from their origins to their destinations was also used. ⢠DTA models typically determine the equilibrium solution by first identifying a feasible or reasonable path set and allocating flow between those paths in a manner 1 See Hicks, J. E. A Dynamic Traffic Assignment Model Breakdown. In Conference Proceedings 42: Innovations in Travel Demand Modeling, Volume 2: Papers, Transportation Research Board of the National Academies, Washington, D.C., 2008, pp. 101â108.
