An aggregate approach (traffic analysis zones, TAZs) has been used to conduct conventional transportation-planning analysis. The impact of a TAZ (sizes and boundaries) on traffic demand estimates and evaluation of transportation systems, however, has not been addressed adequately in the literature. In this paper I will attempt to examine the impact of spatial data aggregation on transportation by generating TAZ alternatives and building linkages among land use, transportation, and GIS. This paper consists of two major components. In the first, attention is focused on the discussion of the GIS-based interface system which links land use, transportation, and GIS. The GIS-based interface system also includes a GIS-based human-interactive TAZ design algorithm that generates TAZ alternatives. In the second, I concentrate on the examination of the impact of TAZs on transportation. This is conducted by simulations, which create TAZ alternatives and report final estimates of traffic demand and evaluation of transportation systems. It is concluded that spatial data aggregation affects the outcomes of transportation-planning models significantly, particularly when the number of TAZs is small.