Positive Feedback: An Examination of Current Approaches in Iterative Travel Demand Model Implementation

(PI: Phillip Reeder (TTI), CoPIs: Chandra Bhat, Karen Lorenzini, and Kevin Hall)

Metropolitan area travel demand models (TDMs) are a critical quantitative analysis tool used to support the development of long-range transportation plans and air quality analysis. The Texas Department of Transportation Planning and Programming Division (TxDOT-TPP) provides TDM development support to 22 Metropolitan Planning Organizations (MPOs) in the state. Currently, the models developed by TxDOT-TPP are traditional three-step models (i.e. trip generation, trip distribution, and traffic assignment) that are sequentially applied. A limitation of this sequential approach is an inconsistency between the travel time date used in the different stages of the process, which may result in: TDMs which do no accurately reflect systemwide or corridor-level travel patterns, travel times in alternative analyses that mat not reflect accurate results, and inaccurate results being used for the air quality determination process. Proposed as an approach to resolve these differences in the model sequence, an iterative feedback mechanism "feeds back" output from traffic assignment for use in the trip distribution step of the next loop, or iteration, of the TDM. Model iterations are continued until specific criteria are met indicating TDM convergence. This project will research current trends, practices, and tools in implementing a feedback approach for potential implementation in the TxDOT TDMs.