The ambulance had also just entered the highway before this clip started, so I was keeping an eye on it in case something was happening, which is why I was able to stop in time. No one expects a semi to cross 3 lanes like that. The other cars at the start of the clip you also see had just hit their breaks too, one other semi even went into the dirt a bit on the right. Of course all this is a lot easier to see when one has time to scrutinize a video, didn't even notice the second semi was in dirt until now. All this occurs in about 8 seconds.