Testing an Integrated Land Use and Transportation Modeling Framework for a Small Metropolitan Area

This paper describes the implementation of a land use and transportation modeling framework developed for Chittenden County, Vermont, to test for differences in modeled output when employing a dynamically linked travel demand model (TDM) versus an assumption of static regional accessibilities over time. With the use of the land use model UrbanSim, two versions of a 40-year simulation for the county, one with a TDM and one without, were compared. In the first version, UrbanSim was integrated with the TransCAD four-step TDM; this allowed regional accessibilities to be recalculated at regularly scheduled intervals. In the second version, TransCAD was used to compute year 2000 accessibilities; these values were held constant for the duration of the model run. The results indicated some significant differences in the modeled outputs. In particular, although centrally located traffic analysis zones (TAZs) reveal relatively little difference between the two models, the differential within peripheral TAZs is both more pronounced and more heterogeneous. The pattern displayed suggests that some peripheral TAZs have higher modeled development with a TDM because the TDM accounts for the increased proximity of destinations, thereby making them amenable to development. Meanwhile, some peripheral TAZs have lower modeled development with a TDM because they already have good accessibility (e.g., access via Interstate), but the model without the TDM does not account for increased congestion.