Land-use inventories and household travel surveys, which are often used to collect socio-economic and traffic data for urban transportation planning models, are costly and time-consuming. The required time and cost can be reduced by using an aggregated Traffic Analysis Zone (TAZ) structure and network model since they are closely related to detail of the TAZ structure and network model. However, it is important to determine the aggregation levels, depending on the purpose and subject of the research, because the use of the aggregated data could lead to aggregation bias. The main of this paper is to analyze the spatial aggregation level effects of TAZ structures and network models on traffic assignment results using an urban transportation planning model based on data from Seoul City. The second objective is to propose two new aggregation levels for the planning assessments of higher-class roads. Several evaluation indices are available for analyzing the aggregation effects of TAZ structures and network models and for investigating the proposed aggregation levels. Results have shown that the proposed TAZ structures and network models can produce traffic assignment results within a reasonable range of error and also reduce the time and cost required for surveys and analyses. The results of this study can provide transportation planners with a quantitative guideline and objective, rational data for use in developing proposals, and preliminary feasibility studies of transportation facilities.