"The road is frozen solid", means it has already happened: the road is icy, and very dangerous to drive on it. The second one, "The road is freezing over," means this is currently in progress but maybe hasn't finished happening yet. "The road is frozen so hard." means the same thing as saying it has already frozen solid---the first one. It's just a different way of saying the first one.

If you are talking about a water on the surface of a road, you can say things like The road is icy. The road is covered with a sheet of ice. There are patches of ice. The road is freezing over. (The water on the surface is starting to freeze.) Be careful. The bridge can freeze over even though the rest of the road is wet.