This isn't just to notify oncoming drivers of a cop, either. It's used to warn of any hazard .....in my neck of the woods it's used more to warn people of deer or loose livestock in the road.
I was on my way to work yesterday on a long two lane stretch. Three cars flashed their lights at me and I though it was odd because they literally NEVER run radar here. But I slowed down. Good thing. Over a hill the entire road was blocked by fire trucks handling a hay fire on a trailer.