  This paper proposes a method of auto-generation of a centerline graph from a
geometrically complex roadmap of real-world traffic systems by using a
hierarchical quadtree for cellular automata simulations. Our method is
summarized as follows. First, we store the binary values of the monochrome
image of target roadmap (one and zero represent the road and the other areas,
respectively) in the two-dimensional square map. Second, we recursively divide
the square map into sub-leafs by a quadtree until the summed-up value of pixels
included inside the leaf becomes equal to or less than one. Third, we gradually
remove the distal leaves that are adjacent to the leaves whose depths are
shallower than the distal leaf. After that, we trace the remaining distal
leaves of the tree using Morton's space-filling curve, while selecting the
leaves that keep a certain distance among the previously selected leaves as the
nodes of the graph. Finally, each selected node searches the neighboring nodes
and stores them as the edges of the graph. We demonstrate our method by
generating a centerline graph from a complex roadmap of a real-world airport
and by carrying out a typical network analysis using Dijkstra's method.
