Conventional thinking posits that Traffic Analysis Zones (TAZ) should be delineated by travel barriers such as topographic features, hydrological features, railroad tracks, and major roadways. This traditionally leads to a model region being “split” into component parts based on these geographical features. However, it is counter intuitive to the goals of transportation modeling to utilize roadways as travel barriers when they act to facilitate travel.

An alternative approach is to view roadway facilities as catchment areas akin to rivers in a hydrologic system. Low capacity roads funnel traffic to high capacity facilities in a similar fashion to streams joining to form rivers. Rivers define their catchment areas as water sheds and in the same light transportation facilities could view their catchment areas as travel sheds. Using this alternate TAZ definition paradigm RSG, Inc. leveraged ArcGIS and Python to develop automated workflows in order to create a Statewide TAZ set for Tennessee DOT. The process identified roads that reflected the center of travel sheds based on relatively high average annual daily traffic (AADT) and then used an iterative buffering technique around those roadways to aggregate more than 240,000 Census Blocks to form 3,293 TAZ.

This presentation will describe the process that was developed to automate the TAZ creation for 95 urban and rural counties for the state of Tennessee and discuss the resulting benefits of this TAZ development approach for other regions and states. Benefits include the following: by basing the process in a spatial modeling technique, the process provides transparency and parsimony to the TAZ building process rather than depending on a priori knowledge and judgment. Second, this process builds TAZ from facility AADT and thus bases the TAZ building process in a quantitative context. Third, the generation of centroid connectors is simplified because the TAZ is centered on the main facility to which traffic must and should ultimately load. Lastly, Tennessee is a large state with multiple municipal areas. Developing an automated system to develop TAZ promotes repeatability, organization, and continuity across the state for the DOT.