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Kittitas County Transportation Element 2018 Travel Forecasting Forecasting future roadway volumes requires knowledge of the current state of the transportation system and best estimates of future population and employment. Modeling software is then used to understand where new vehicle trips will be distributed on the road network. For a more detailed explanation of the travel forecasting used in this plan see the 'Kittitas County Travel Demand Forecasting' box below. Kittitas County Travel Demand Forecasting The Growth Management Act (GMA) requires that the Transportation Element support the land uses envisioned in the Comprehensive Plan. Thus, an important component of this plan was forecasting how the future land uses envisioned in the County, as well as regional growth, would influence demand on Kittitas County's transportation network. A description of the travel demand modeling process is provided below with more detail about land use assumptions in Appendix C. The Tool. As a part of previous planning efforts, Kittitas County created a travel model with the Visum software package. This model forecasts traffic volumes during the evening commute hour (4-6pm) along key streets and intersections. This tool provides a reasonable foundation for developing year 2038 forecasts, as the underlying land use assumptions have been updated to match the land use forecasts for the current Comprehensive Plan. • Estimate Land Use Growth. As a part of the Comprehensive Plan update, the County is planning for expected growth in population and employment over the next 20 years through 2038. Based on growth estimates from Kittitas County Conference of Governments, the County is preparing for 23,297 new residents and 11,155 new workers by 2038 . The County then allocates the growth throughout the region based on adopted zoning, observed development patterns, and other county policies. • Capture Regional Growth Patterns. Other communities throughout the region are going through this very same process . Since travel does not stop at a jurisdiction's borders, it is important to capture how regional growth could influence travel patterns throughout the County . • Translating Land Uses into Trips. The next step is evaluating how the regional growth assumptions described above translate into walking, biking, transit, and auto trips. The travel model represents the number of housing units and employees in spatial units called traffic analysis zones (TAZs). TAZs can be as small as a few street blocks to as large as an entire neighborhood. They provide a simplified means to represent trip making rather than modeling individual parcels. The travel model estimates trips generated from each TAZ (both inside and outside of the Cities) using established relationships between different land use types with trip making. These trips are then assigned onto the roadway network to estimate how much traffic would be on each street during the evening commute hour. 34 IP age