As the federally-designated Metropolitan Planning Organization (MPO) and the state-designated Regional Transportation Planning Organization (RTPO) TRPC is responsible for developing, maintaining, and applying the regional travel demand model. The county-wide regional model includes the Lacey-Olympia-Tumwater metropolitan area as well as the more suburban and rural portions of Thurston County. The regional model has a high level of detail to support all local planning and concurrency analysis efforts.
The current TRPC model was developed to replace a T-Model2 implementation that estimated only vehicle trips based on national average travel data rather than a region-specific survey. The region needed to plan for transit, demand management strategies, the state-mandated Commute Trip Reduction (CTR) program, and other issues, and found the T-Model2 tool inadequate. The agency made a conscious commitment to both develop the model and manage its necessary data acquisition in-house, and hired qualified staff to do so.
The resulting model development effort took place during 1997-2000, beginning with the 1997 I-5/US 101 external origin-destination survey (supported with WSDOT funding) followed closely by the 1998/1999 TRPC Household Travel Survey (supported with Intercity Transit funding). Model development proceeded during the entire time, resulting in an operational model by early year 2000.
The current implementation is a trip-based, 4-step, typical-weekday model built using INRO's EMME software platform and based upon the data described in section 3.2 above. With three peak-hour assignment time periods per day, six trip purposes, six passenger modes, and a truck mode it is well-stratified compared to models in similar regions elsewhere. It uses multinomial logit models for destination choice and multinomial logit models for mode choice. It performs a multi-class vehicle assignment and a multi-path transit assignment in each of the three modeled peak hours (AM, midday, and PM) and includes LOS skim feedback mechanisms to trip distribution and mode choice. Appendix D and the TRPC documents cited in the footnotes provide more detail on the current model.
The Thurston region has experienced significant growth and demographic change since the current model was first developed. A new household travel survey will be conducted in the fall of 2012 to collect data on the current demographic characteristics and travel behavior in the region. TRPC will be updating its base year and forecast year travel demand models using, among other resources, the new household survey.
As already mentioned in Section 1.3 TRPC's overall goal for seeking a TMIP peer review was to receive a transparent, un-biased, external expert review to inform its model update work. TRPC's application to TMIP expressed the belief that a peer review would help ensure that the agency's model improvements will be representative of the state-of-the-practice in travel demand modeling. Having a state-of-the-practice model is a TRPC agency goal and in the interest of its regional partners.
To the knowledge of this panel TRPC has not previously held a formal TMIP model peer review.
 Thurston Regional Planning Council. Thurston Region Multimodal Travel Demand Forecasting Model Development. 2000. p. 2.