From what I've read so far, there is no national rule/law governing fog lights and their usage. A couple of states have laws regulating them, but most do not. The states with these laws ,for the most part, are generally poorly written and difficult to interpret. For example, in one paragraph of the Oklahoma law it states that fog lights can only be used when visibility is down to half-a-mile or less, but then goes on to say that they can be used whenever the headlights are on if they are equipped with a switch that allows this type of operation.