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and X-1 trips (for Internal to eXternal, eXternal to Internal) can be illustrated by the movement from Ellensburg to Yakima. Trip distribution is typically only performed for I-I (Internal-Internal), 1-X, and X-1 trips. The remaining X-X trips are placed in a trip table. This trip table, listing the number of direct movements between zones, is a manual distribution of the X-X traffic based upon some known parameters. External-external traffic is difficult to simulate (or in this case, distribute) with the gravity model. Therefore, the modeling process with VISUM includes a step for "manually" distributing X-X traffic to the external stations. For this model, the External-External traffic was derived from the previous modeling efforts and then tested in the model. These were revised in this process because several ramps with count data were too high in the model. It was determined that more through trips would reduce these to more reasonable values. The X-X trips were placed in two through trip table matrices, one for Cars and one for Trucks. The Truck matrix was derived using WSDOT ADC counter locations and an average of 25% trucks was used at all external locations except for SR 821 which assumed 10% trucks. The remaining trips associated with the external zone's 1-X and X-I movements were added to the trip generation portion of the modeling stream and then combined with the model's origin-destination file for the model runs. The model stream run module used in VISUM automatically adds the manually distributed X-X trips to the trip table created from the origin-destination file during the gravity model distribution process. Combine Productions and Attractions and Balance Data from the external traffic zones were combined with the internal zone trips to form a complete origin-destination file for the Kittitas County model. After the I-X and X-I trips were added, origin and destination sums by trip purpose were automatically balanced to the average of the number of productions and attractions to be equal. The trip generation rates were previously checked with land use totals to insure that trips would be balanced. The primary purpose for checking equivalencies was to ensure that for each production or origin generated by the model there was an attraction or destination. (Transportation planning models are closed systems, meaning that every trip on the network must have an origin and a destination.) Care was taken to closely correlate to target percentages by trip purpose from NCHRP 365. A trip generation rates strategy such as the one used to develop the Kittitas County Kittitas County, Washington Transportation Model -May 2009 Page34