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The recent several decades have witnessed more and more three-dimensional (3D) intersections in urban traffic infrastructure. Therefore, modeling road network in 3D space becomes an essential step in traffic study, visualization, traffic micro-simulation, etc. Unlike modeling planar road network in which roads could be simply represented by their centerlines, a 3D line-based road network model tends to mislead readers, while representing roads with 3D surfaces might overcome this drawback to a certain degree. 3D road surface model could be built based on 3D road centerlines. However, existing commercial software is incapable of automatically generating surfaces from 3D lines. In view of this, we develop a new approach to the problem. A method including two key algorithms is developed to model 3D road surface. It contains three aspects. First, 2D lines representing road centerlines are digitized, and elevations are assigned to key points along these lines. Then, Lagrange interpolation algorithm is employed to obtain elevations of other points, and a series of 3D lines are generated. Based on the lane number of each road segment, a 2D buffer algorithm is applied to the 3D lines road and elevations are assigned to corresponding points composing the resulting buffer zones. The buffer zones represent road surfaces. The above procedure is implemented by a C# program with DirectX. The approach is employed in modeling Shanghai expressway system and the visualized result is pleasant.