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VA L I D AT I O N 51 idation process will include model estimation tests and VALIDATION AND ASSESSMENT OF model application tests. The anticipated validation tests for ACTIVITY-BASED TRAVEL DEMAND the model components include (a) checks to ensure that the MODELING SYSTEMS model component is producing the correct results, (b) com- parisons of model parameters to comparable parameters in Ram Pendyala and Chandra Bhat similar models in other areas, (c) disaggregate validation of all model components, (d) testing of each model's sensitiv- Ram Pendyala discussed validation and assessment of ity to variables through controlled modification of those activity-based travel demand models. He described vari- input variables, (e) comparisons of the model component ous approaches to validating and assessing activity-based output to the results from the survey data set, and (f) com- models, potential issues, and topics for further consider- parisons of base year outputs from model components to ation. Volume 2 contains a paper on the topic.2 The fol- available independent observed data. lowing points were covered in his presentation. · The second set of tests will focus on model appli- cations. The model application tests will address aggre- · While there continues to be growing interest in the gate comparisons to the Travel Behavior Inventory (TBI) use of activity-based travel models, actual application of survey results, comparisons to independent data, and these models has been limited. The lack of detailed vali- assessments of the individual model components. dation and assessment of these new models may be a · A number of issues may need to be addressed in contributing factor to their slow introduction. Given the developing and conducting validation tests of new activity- costs associated with the new models, information on based models. Possible issues include the lack of experience their benefits is needed for widespread use. with activity-based models and available comparisons and · Validation of travel demand models typically the lack of established standards, acceptable error ranges, involves the refinement and adjustment of model com- and elasticity standards. Other possible issues may emerge ponents and parameters to ensure that the forecasts repli- during the aggregate comparison to the TBI survey results cate base-year travel conditions and statistics within an and the aggregate comparison to independent data. acceptable margin of error. Examples of measures fre- · The overall model validation will be performed for quently used in the validation process include aggregate the model estimation year 1997 and for 2005 against measures of travel such as vehicle miles traveled (VMT), independent observed data. Examples of elements to be vehicle hours traveled, mode split, trip length distribu- included in these traditional checks are the root-mean- tion, and total trips and trip rates. The traditional square error of modeled to observed traffic volumes, approach has focused on replicating observed base-year matching observed vehicle miles of travel with approxi- conditions within a certain margin of acceptable error. mately 1% error, matching highway and transit screen- · At a basic level, validation of activity-based travel line volumes, and matching total transit boardings. models will focus on replicating base-year travel condi- Other traditional checks include park-and-ride lot usage, tions comparable to those achieved with existing four- matching of peak and off-peak roadway speeds, toll road step models. A number of issues may need to be usage, and highway volumes on individual freeways. addressed in validating new models. There may be an · The validation process includes validating the new expectation that a higher standard of validation should model against observed travel data for 1997 and 2005. be used with activity-based models and that fewer While these tests are important for model validation, adjustments and refinements will be needed. Currently, they do not address the potential true value of activity- there is an absence of performance assessment standards based models, which is the ability to provide better for validating activity-based models. There is a need to assessments and travel forecasts based on a more appro- develop techniques and approaches for comparing the priate representation of the actual decision-making results from traditional four-step models with the results process. Two approaches will be used to test the sensitiv- from activity-based models. ity of the activity-based model. · While it is important to consider the accuracy of · The first approach for testing the sensitivity of the replicating base-year conditions, one of the benefits of new activity-based model will be to compare the forecast activity-based models is their use in analyzing a wide year results to those results obtained by the calibrated trip- range of policies and scenarios. To address these elements, based model. The second approach will focus on develop- an assessment of an activity-based model might focus on ing a model that is more sensitive to policy variables. a variety of policies and scenarios. Examples might These policy-oriented tests, which will be subjective, will include examining changes in land use, socioeconomic include evaluating transit-oriented development areas, dif- ferent regional development densities, development in 2 See Pendyala, R. M., and C. R. Bhat. Validation and Assessment of known industrial areas, development of specific greenfield Activity-Based Travel Demand Modeling Systems. Volume 2, pp. areas, and redevelopment efforts in targeted areas. 157­160.