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Proposed Validation and Sensitivity Testing of Denver Region Activity-Based Models David L. Kurth, Parsons Corporation Suzanne Childress, Parsons Corporation Erik E. Sabina, Denver Regional Council of Governments Thomas Rossi, Cambridge Systematics, Inc. T raditional four-step travel modeling procedures have major changes to the underlying transportation system, evolved over the last half of the 20th century. Many it is difficult to test the veracity of the underlying models improvements were made incrementally and sub- (unless the test proved that the models were, in fact, jected to either formal or informal validation and sensitiv- poorly calibrated). ity tests. Formal validation tests were normally applied at In 1997, the Denver Regional Council of Govern- the end of the model calibration process and, quite fre- ments (DRCOG) initiated the collection of travel survey quently, focused on the "super test"--the concept that data to update its traditional four-step travel model and reproduction of observed traffic volumes and transit board- for the longer-term development of state-of-the-art mod- ings at some reasonable level of aggregation somehow eling techniques. The "refresh" of the traditional model showed that the models were, in fact, valid. Informal vali- using these data took place from 2002 to 2004 and dation and sensitivity tests, unfortunately, too often con- included model component validation, validation to the sisted of discovering modeling problems after illogical 1997 base year, and validation to travel conditions in travel forecasts were produced. 2001. Between 1997 and 2001, Denver's light rail sys- In the late 1990s, FHWA commissioned the develop- tem more than doubled in length with the opening of the ment of a Model Validation and Reasonableness Check- 8-mi-long Southwest LRT line, enhancing the effective- ing Manual through the Travel Model Improvement ness of transit component calibration and validation. Program (1). This manual summarized validation stan- As with its current trip-based model, DRCOG is dards and recommended a process that focused on the committed to rigorous validation and sensitivity testing validation of the individual four-step model components of its activity-based modeling (ABM) system that will be as well as the traditional overall model system validation developed over the next 18 months. The system is focused on traffic volumes and transit boardings. designed to make use of the most recent developments Sensitivity testing has been somewhat less formal. It in ABM theory to better represent the travel decision- has frequently focused on the sensitivity of individual making process and provide reasonable sensitivity to a model components using measures such as elasticity. Sen- wider range of future travel options and constraints. sitivity testing of modeling systems by validating model The downfall is that there are a number of places where results over time has not been as common because it the models can fail. DRCOG has addressed this concern requires observed travel data from more than one point by committing approximately the same budget to the for the same region. Even if such data did exist, many validation and sensitivity testing of the ABM that it regions do not have measurable changes in their trans- committed to the entire refresh of the existing tradi- portation system such as the addition of new roadway tional four-step travel model. Perhaps a statement made capacity or the opening of a new transit line. Without by Chandra Bhat and Frank Koppelman in a recent 153