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160 I N N O VAT I O N S I N T R AV E L D E M A N D M O D E L I N G , V O L U M E 2 tory dependency in time allocation and activity partici- information with a view to stimulate discussion among pation, in-home versus out-of-home activity substitution conference participants on how best to validate and and generation, induced demand, and travel efficiency assess activity-based model systems vis--vis existing (say, through chaining of trips) are but a few of the con- four-step models: cepts that merit recognition. Behavioral processes and decision rules: Recent 1. Model validation guidelines for activity-based work in activitytravel modeling has focused on the travel demand models including information on what behavioral processes and decision-making rules that peo- base-year traffic conditions activity-based travel demand ple employ when scheduling and executing activities and models should replicate, the margins of error that are trips. Rule-based heuristics, in addition to random utility acceptable, and the extent to which adjustments of theory-based models, are being incorporated into mod- model components and parameters are acceptable; els to reflect these behavioral processes in microsimula- 2. Model assessment guidelines for activity-based tion frameworks. Understanding behavioral processes is travel demand models including the range-of-sensitivity key to developing robust model structures, specifica- tests, policy measures, land use scenarios, and technolo- tions, and forms. gies to which the activity-based models should be sub- jected, the model outputs that should be examined, and the acceptable ranges of responses in model outputs; Comparisons Between Model Systems 3. Model comparison guidelines for comparing activity-based travel demand models with existing four- The discussion so far has dwelled on how one might step travel demand models including the development assess the performance of an activity-based travel and presentation of a comprehensive matrix that clearly demand model system. However, the following questions shows how and where four-step models, tour-based remain: models, and activity-based microsimulation models are applicable to addressing a range of issues; and 1. How does one know or determine whether the 4. The design of comprehensive experiment(s) for activity-based travel demand model is giving the right performing controlled comparisons of activity-based answer or level of sensitivity for a particular scenario? travel model outputs and existing four-step travel model 2. In comparing the outputs (in response to a scenario outputs. A variety of scenarios have played out in the analysis) between an existing four-step travel demand real world, thus providing real-world data against which model and an activity-based travel demand model, how model predictions can be assessed. Both activity-based does one know or determine which one is right or more and existing four-step travel models can be applied to accurate (in cases in which it is not obvious)? these situations and the outputs can be assessed against real-world observational data. CONCLUSIONS AND CONTENT The presentation will also include results of model OF PRESENTATION validation and assessment exercises that have been undertaken using the Florida Activity Mobility Simula- This white paper raises important questions regarding tor and Comprehensive Econometric Microsimulator of the potential adoption of new and innovative activity- Daily Activity Patterns to illustrate how the guidelines based travel demand modeling systems in practice. The presented can be used to assess and validate activity- proposed presentation serves to deliver the following based travel demand models.