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