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Innovations in Travel Demand Modeling, Volume 2: Papers (2008)

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Suggested Citation:"T57054 txt_154.pdf." National Academies of Sciences, Engineering, and Medicine. 2008. Innovations in Travel Demand Modeling, Volume 2: Papers. Washington, DC: The National Academies Press. doi: 10.17226/13678.
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focus group discussion best summarizes DRCOG’s philosophy: Researchers and practitioners have not thought carefully enough about the criteria for validation of models. Researchers have the habit of asking practitioners to believe that activity- based methods will produce better impact assessment and fore- casts because such models more appropriately rep- resent the actual decision process (we plead guilty to this charge). There is a good basis for this line of thought, but researchers need to go beyond this argument. They need to develop clear validation criteria and demonstrate the value of activity- based methods in ways that are easily understood. (2) Because the ABM development process for the Den- ver region has just begun, this paper focuses on the ini- tial plans for the validation and sensitivity testing of the models. DRCOG ACTIVITY- BASED MODEL APPROACH The ABM approaches for the Denver region will be based on those used in other parts of the country, partic- ularly the San Francisco Transportation Authority. Most of the components will be nested or multinomial logit (MNL) models, sensitive to person and household demo- graphic variables and transportation level-of-service variables. Anticipated model components include: • Synthetic population generator; • Regular workplace location choice model for each worker; • Regular school location choice model for each student; • Household auto ownership choice model; • Daily activity pattern choice model for each person- day; • Number of tours choice model for each person day; • Work- based sub- tour generation; • Tour- level destination choice; • Tour- level mode choice; • Tour- level time- of- day choice; • Trip- level destination choice; • Trip- level mode choice (conditional on tour mode choice); and • Trip- level time- of- day choice (conditional on time windows remaining after all previous choices). Several components will be transferred or adapted from the existing four- step model for the region. Exam- ples include the area- type and parking cost models, and traffic and transit assignment procedures. DRCOG VALIDATION AND SENSITIVITY TESTING APPROACH A plan describing the validation tests to be conducted for the ABM components and the overall model system has been developed with the specification of the ABM for the region. It includes the standards by which the tests will be evaluated, such as: • Checks to ensure that the model component is pro- ducing the correct results (i.e., verification of computa- tions); • Comparisons of model parameters to comparable parameters in similar models in other areas; • Disaggregate validation of all model components estimated using disaggregate methods and comparing the model outputs to the estimation data; • Testing of each model’s sensitivity to variables through controlled modification of those input variables; • Comparisons of the model component outputs to the results from the survey data set; and • Comparisons, where data are available, of the base year outputs from each model component to indepen- dent observed data (e.g., comparisons of mode choice model outputs to linked trips estimated from transit boarding counts). The above tests are typical of model validation tests that have been recommended in documents such as the Model Validation and Reasonableness Checking Manual (1) and should be performed for all model development efforts. Because there are more components in the proposed ABM than in a conventional trip- based model, there will be significantly more component testing. It will be important to design validation tests that are appropriate for each component. While some tests will be analogous to those performed for components of trip- based mod- els, others will be different. Examples of similar tests include comparisons of modeled trip length frequencies with those from the household survey (tour lengths must also be compared) and comparisons of modeled and observed mode shares. Examples of tests to be performed without comparable trip- based tests include the number of trips per tour by purpose, amount of time spent in activities versus traveling on tours by purpose, and the number of activities performed by each person. The DRCOG model validation plan (3) provides a list of all tests to be performed. One difficulty in performing the tests is the lack of experience to determine standards. For example, how close should the modeled number of activities per person be to the observed number? In some cases, established standards for trip- based models may be used to inform the choice of standards for the ABM. In other cases, the 154 INNOVATIONS IN TRAVEL DEMAND MODELING, VOLUME 2

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TRB Conference Proceedings 42, Innovations in Travel Demand Modeling, Volume 2: Papers includes the papers that were presented at a May 21-23, 2006, conference that examined advances in travel demand modeling, explored the opportunities and the challenges associated with the implementation of advanced travel models, and reviewed the skills and training necessary to apply new modeling techniques. TRB Conference Proceedings 42, Innovations in Travel Demand Modeling, Volume 1: Session Summaries is available online.

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