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Innovations in Travel Demand Modeling, Volume 2: Papers (2008)

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Suggested Citation:"T57054 txt_155.pdf." National Academies of Sciences, Engineering, and Medicine. 2008. Innovations in Travel Demand Modeling, Volume 2: Papers. Washington, DC: The National Academies Press. doi: 10.17226/13678.
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acceptable error ranges will be determined by estimating the variation expected in aggregate model statistics (such as vehicle miles traveled [VMT]) resulting from the devi- ation on the particular model component, in effect tying new validation tests that as yet have no standards to existing tests that do have such standards. Sensitivity testing will also play an important role in the validation of the ABM components. There are no established standards for reasonable elasticities for some of the newer model components, so this will be more of a reasonableness test than anything else. In such cases, it may not be possible to tie these tests to existing tests with standards already developed. In such cases, it may be possible to compare Denver model test outcomes with observed outcomes in other cities with conditions similar to those being evaluated in the sensitivity tests. Significant effort will be placed on the validation of the overall model system. Again, this will be comparable to previously recommended validation procedures and will include the following: • Reasonableness and logic checks of demographic and network data or skim data input to the models; • Traditional validations for the model estimation year (1997) and for 2005 against independent observed data. Depending on data availability, these traditional checks will include: – Root- mean- square error of modeled to observed traffic volumes by appropriate segmentation vari- ables (such as facility type, traffic volume level, and so forth) – Matching regional observed VMT within approximately 1% error – Matching observed VMT by facility type – Matching VMT by area type – Matching total transit boardings – Matching transit boardings by sub- mode – Rapid transit boardings by corridor, sub- mode, and station – Park and ride lot usage – Matching a series of at least 10 highway and transit screenline volumes – Highway volumes on individual freeways – Toll road usage – Acceptable matching of peak and off- peak speeds – Roadway speeds by several time- of- day periods (a.m. versus p.m. versus off-peak, and so forth); and • Tests of the sensitivity of the overall model system to input variables (similar to the procedures used for the model component sensitivity testing). The ABM will be subjected to the same validation stan- dards that were used for the recently refreshed four- step model. Results are expected to be as good as or better than those produced using the four- step model. While this may seem to be a rather lenient standard, it must be remem- bered that the four- step model was, in fact, calibrated to produce reasonable validation results for 1997 and 2001. One would expect that, because the ABM can consider more aspects of personal activity performance and travel behavior, the amount of “adjustment factoring” not tied to specific measurable behavior should be less in the ABM. Another validation activity under consideration is “back- casting” to a prior year (besides the model esti- mation year of 1997). This will be done if the necessary data are available and the resources to perform the back- cast are available. In addition to specifying traditional model validation standards, input and coordination with federal agencies will be sought in the validation and sensitivity testing of the ABM for the Denver region. This will be particularly important in the development of the ABM because oper- ational experience with them is limited; federal agencies may be expected to evaluate them closely for validity and for consistency with the calibration outcomes of the numerous trip- based models in existence. DRCOG intends to involve federal agencies early in the process, through oversight panels or other means, and will include its requirements in the calibration and validation plan at the earliest point. TEMPORAL AND POLICY SENSITIVITY TESTING The normal validation testing outlined above includes some temporal validation in that the model will be vali- dated against observed travel data for 1997 and 2005. Such testing is crucial for model validation but does not address the hypothesized true value of ABMs— the pro- duction of better impact assessment information and travel forecasts that will result from the more appropri- ate representation of the actual decision process. Two approaches will be used to test the sensitivity of the ABM. The first will be the application of the ABM for an existing forecast year and scenario and compari- son of those results to those produced by the calibrated four- step model. While the “true” results for such a fore- cast year cannot be known, the results from the four- step model provide one outcome that has been deemed rea- sonable. Several questions will be asked: • How similar are the results? The traditional valida- tion measures outlined in the previous section regarding traffic volumes and transit boardings can be used to mea- sure the similarity. • Which model produces more believable results? Two outcomes are possible: the forecasts from the two models are not substantially different or the forecasts are substantially different. In either case, an assessment will 155PROPOSED VALIDATION AND SENSITIVITY TESTING OF DENVER REGION

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TRB Conference Proceedings 42, Innovations in Travel Demand Modeling, Volume 2: Papers includes the papers that were presented at a May 21-23, 2006, conference that examined advances in travel demand modeling, explored the opportunities and the challenges associated with the implementation of advanced travel models, and reviewed the skills and training necessary to apply new modeling techniques. TRB Conference Proceedings 42, Innovations in Travel Demand Modeling, Volume 1: Session Summaries is available online.

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