Positive feedback : exploring current approaches in iterative travel demand model implementation.
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2012-01-01
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Alternative Title:Project title: Positive feedback : an examination of current approaches in iterative travel demand model implementation
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Edition:Technical report; Sept. 2010-Aug. 2011.
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Abstract:Currently, the models that TxDOT’s Transportation Planning and Programming Division (TPP) developed 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 data used in the different
stages of the process that may result in: (1) TDMs, which do not accurately reflect system-wide or corridor-level travel
patterns, (2) travel times in alternative analyses that may not reflect accurate results, and (3) inaccurate results being
used for the air quality determination process.
To resolve these differences in the model sequence, researchers proposed an iterative feedback mechanism as
an approach. This project researched current trends, practices, and tools in implementing a feedback approach for
potential implementation in the TxDOT TDMs. In general, past research and practice underscores the importance of
incorporating feedback loops in the travel demand process, especially in regions with moderate to high levels of traffic
congestion during certain times of the day. Since the current TxDOT TDM model does not include any impedance or
accessibility measures in the trip generation step, the best current approach is to feed back the output from the traffic
assignment step to the trip distribution step. Overall, critical consideration should be given to the relative impact that
implementing feedback might have on existing TxDOT-TPP practices. A feedback mechanism necessarily includes
additional training and increases the calibration time associated with delivering base year travel models; and possibly
could include additional data processing (and collection) related to time-of-day implementation.
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