Travel Demand Model - OKI

OKI/MVRPC Travel Demand Model – Version 6.04. Highway Assignment ValidationThe validation year for the consolidated model is 1995. Model results for the base year werecompared against ADT traffic counts circa 1995. For validation purposes, an ADT traffic estimateis constructed by adding up the estimated volumes of all four time period assignments. All linkbasedvalidation statistics are calculated for directional ADT counts and estimated volumes. Thissection discusses the final model validation. For comparison purposes, validation results for theOKI Model v54 and MVRPC 1994 Base Model are included in Appendix A.Three criteria are used to assess the adequacy of the model validation: percent VMT error,percent VMT root mean square error and percent Volume root mean square error. Thesemeasures of performance are calculated separately for the OKI and the MVRPC regions, andacross two link classifications: facility type and volume group.As part of the model validation process, a number of adjustments were made to the model andhighway networks:• Because model convergence now requires more feedback loops than initially assumed, it wasnecessary to recalibrate the trip distribution and mode choice models with the highway andtransit skims estimated after the implementation of the model convergence application.• Trip generation estimates were scaled to correct for a global VMT overestimation, as well asfor OKI-to-MVRPC (and vice versa) trip overestimation. The scale factors vary by trippurpose, production/attraction end, and by region. Please refer to the Part III (TripGeneration) of the Model Development Report. The need for trip generation scale factorsstemmed from the differences in model validation statistics resulting from the updated skims.• Bridge travel time penalties were added to a number of bridges in the OKI Region, to correctan overestimation of trips between Northern Kentucky and Ohio. Please refer to Part IV (TripDistribution) of the Model Development Report.• Transit Agency –specific constants were added to the mode choice models, to improve thetransit validation for TANK and the City of Hamilton transit agency.• Speeds, capacities and/or speed/capacity classification codes were revised for selectedroadways in the MVRPC network. This was necessary because this network showed severalinconsistencies between free-flow speeds and functional classification. The MVRPC free-flowspeeds were taken from their posted speed limits. While in general there is generalagreement between these limits and reasonable free-flow speeds, in many instances localand minor collector roads showed speeds of 40 mph and above. Not surprisingly, the initialvalidation overestimated flows on these roads by more than 50%. There were also a fewinstances where the functional classification did not agree with observed volumes, forexample a collector that had volumes more typical of an arterial. An accurate functionalclassification is important because the OKI model uses facility-specific volume-delayfunctions; the wrong classification can severely over or under estimate a particular facility.Please refer to Appendix B for a list of the changes made to the MVRPC network.The results reported in the following sections already incorporate all the adjustments listedabove, and hence represent the final model validation statistics.Assignment and Validation - Highway Assignment Validation 9