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functions. Roadway or link data includes traffic count data for links and turning <br />movements at key intersections. Travel characteristic data includes the land use <br />inventory, trip generation rates, external volume data (volumes entering, exiting, and <br />traveling through the model area), and trip length frequency distributions. PTV <br />America conducted a field review of major facilities within the county including some <br />locations with Kittitas County staff. <br />Upon completion of data collection, development of the model's mathematical "rules" <br />began by coding the information and readying it for entry into VISUM. Essentially all <br />entered data is numeric. Each entry, such as speed limits for links, capacities for nodes, <br />and collected land use data, is used by the model to estimate network or street volumes. <br />The VISUM software contains many equations and algorithms that help the traffic <br />volume computation process. Therefore, given the amount of data in the transportation <br />planning model, it is advisable to group like data together and assign uniform values. <br />For example, link and node capacities are assumed to be uniform for links and nodes of <br />similar classifications and types throughout the model area. In actual practice, these <br />capacities are unique to the location and road conditions. The method for developing <br />these "blanket" values is considered part of the model's rule -building process. <br />Calibration <br />After all data has been collected, coded, and entered in the VISUM program, the <br />calibration process begins. In this task, the data and the model rules are refined so that <br />the model closely simulates existing travel patterns and volumes on the roadway <br />network. Calibration is performed by conducting a series of simulation runs and <br />evaluating the results. The calibration is considered complete when the results of the <br />simulation runs are statistically similar to the traffic count volumes and other measures <br />of travel behavior. <br />Distribution and assignment are the two steps undertaken during a typical model <br />simulation run. Distribution is the process of allocating trips between various zones <br />within the network. The product of the distribution is a trip table that lists the number <br />of trips between the model's zones. The distribution for this model was computed with <br />the VISUM/TMODEL form of the gravity model. <br />Application of the gravity model in transportation modeling is derived from earlier <br />work with economic interaction through a study of social physics. The idea, simply put, <br />Kittitas County, Washington Transportation Model - May 2009 Page 6 <br />