Some people don't seem to think about their turn signals very much, not good when you're doing 70 in four lanes of traffic.
Today, two cars almost hit each other in front of me on just such highway. The guy in my lane went left one lane while a car was passing. The guy in my lane did not use his signals so the car passing had no idea he was changing lanes.
The car passing swerved, slammed on his brakes and just barely missed the wall trying to avoid the guy leaving my lane. Meanwhile the guy that was in my lane also slammed on his brakes and swerved over into the next lane to the right. If there was a car there he would have hit it.
I had already dropped back when I saw the guy drifting across in the other guys path, I have seen a few multi car (chain reaction accidents) and don't want to be in one.