i don't know if it is the same in every state but in Pennsylvania the fog lights must have a relay that shuts then off when high beam headlights are on. I have always used headlights w/ fog lights during daytime with the logic that maybe someone would see me that might not have. My previous car and this one with auto headlights i just trust that the daytime running lights will be enough.There really no point in having fog lights on during the day. The purpose is to provide more low level light in fog at night to prevent reflected light from limiting your view at night as it would with your bright lights. My bmws would switch them off whenever I used brights and vice versa. During the day the only advantage might be for someone to see you, but that's what the daytime running lights are for and those are always on.