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differences can be occupancy or, conversely, vacancy, the aggregation of distinct land use types into more general categories, and local variations. Retail rates are based upon a medium size shopping center. Typically a smaller retail establishment will have a higher trip generation to employee ratio than a large shopping mall. In addition, the ITE national average, or NCHRP 187 and 365 rates, assumes the same trip generation rates at each development. During the actual system peak hour, this is not necessarily the case. For example, one industrial development or office may dismiss their employees during the peak hour, while another, located elsewhere in the model area, will have a slightly earlier (or later) discharge time. Adjustments were made to the 2008 model to reduce trip generation in the residential areas that may have seasonally vacant vacation homes, homes away from the urban areas resulting in lower peak hour trip generation rates, and to increase the generation for retail, office, and service uses. Rates were adjusted slightly for balance between origins and destinations and to account for locations in the model where overall trip generation appeared too high or too low. The factors were applied to the collected land use information and stored in the origin- destination files in VISUM. These files contain the origins and destination values for all trips generated by all land uses and external zones. External Zones Origin and destination totals for external zones were set at the base-year peak-hour traffic volumes. These were based upon data from the previous models and WSDOT data. As with internal zones, traffic generated externally is also apportioned among different trip purposes as show in Table 6. Trips generated by external zones fall into two categories. Traffic that travels from external zone to external zone, or through the network, is called a through trip. These movements are designated as X-X trips in VISUM, which stands for external to eXternal travel. The primary characteristic of these trips is that they travel through the network but do not stop or start within an internal or perimeter zone. In the Kittitas County model the best illustration for this movement is the trip that starts in Seattle and ends in Moses Lake or Yakima without making a stop in the Kittitas County model area. The second trip type generated by an external zone is the one that begins at an internal zone and ends in an external zone, or vice versa. These trips, often designated as I-X Kittitas County, Washington Transportation Model-May 2009 Page 33