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Validation and Assessment of Activity-Based Travel Demand Modeling Systems Ram M. Pendyala, University of South Florida Chandra R. Bhat, University of Texas at Austin T he past decade has seen the rapid development of across the study or model area and for specific planning activity-and tour-based travel demand modeling districts or market areas. In addition, model-predicted systems. Several metropolitan planning organiza- volumes are often compared with observed ground tions (MPOs) in the United States and metro areas in counts for major corridors and across screenlines and Europe have implemented such systems to take advantage cutlines. Thus, the traditional notion of model validation of the derived nature of travel demand and interdependen- has centered on replication of observed base-year travel cies among trips. Despite the appeal of these models, their conditions within a margin of acceptable error. Existing widespread implementation appears to be hindered by the four-step models that are in use to develop long-range absence of a detailed validation and assessment of this new transportation plans and undertake major investment wave of model systems. Many MPOs will not adopt such studies have been subjected to such validation proce- models until they are tested. These sentiments were dures to replicate base-year travel conditions. expressed 10 years ago in New Orleans at the Travel Model Activity-based travel demand models, like trip-based Improvement Program (TMIP) Conference on Activity- models, could (and may have to) be adjusted so that they Based Travel Modeling and more recently in e-mail forums replicate base-year travel conditions. Otherwise, it is such as the TMIP Listserv. The conference in Austin will unlikely that MPOs will be motivated to make the tran- bring model developers and MPO staff together to discuss sition to innovative model systems. Areas that have tran- validating and assessing activity-based models. sitioned to tour-based or similar model systems have subjected their models to validation procedures to ensure that the model predictions replicate a host of base-year VALIDATION OF ACTIVITY-BASED TRAVEL travel conditions. DEMAND MODELS If activity-based travel demand models are validated to base-year travel conditions (similar to existing four- Validation of travel demand models involves the refine- step models), two questions arise: ment and adjustment of model components to ensure that predictions replicate base-year travel conditions and 1. Should activity-based travel demand models be statistics within an acceptable margin of error. There are held to a higher standard of validation? numerous measures against which model predictions are 2. Should activity-based travel demand models be often compared; these include vehicle miles of travel, able to replicate base-year travel conditions with fewer vehicle hours of travel, mode split (by purpose), trip adjustments and refinements (or none at all) when com- length distributions (by purpose), and total trips and trip pared with existing four-step models? rates (by purpose). These measures may be compared 157