Systems, methods, and programs create a road map data structure, the data structure including road network data in which a road is represented by a plurality of links connected in a link string. The systems, methods, and programs assign a link ID to each of the links of the road network data. For each link, the link ID has a numerical range set in accordance with both a length of the link and a road attribute, and the link ID is set sequentially in a connection order within the link string.