FIGS. 10A and 10B illustrate modified examples of the symbol marks 37, 37′. A symbol image 42 in FIG. 10A is formed by continuously arranging a plurality of similar triangles with dimensions thereof gradually decreasing toward the travel direction. Similarly, a symbol image 43 in FIG. 10B is formed by continuously arranging a plurality of similar arrow-head portions with dimensions thereof gradually decreasing toward the travel direction. Naturally, not limited to the images in FIGS. 10A and 10B, the symbol image 37 may be modified variously.