In every case that the symbol image 37, 37′ of the travel course includes any of images in FIGS. 7, 10A, and 10B, it is preferable that the color thereof is distinctly different from colors that are normally used for traffic signs and the like on roads. In consideration of that the symbol images 37, 37′ indicate normally scheduled travel course, it is preferable to adopt blue or green color series that may provide a relative sense of safety to an occupant.