. >> reporter: catie and matt stegmoyer heading for ohio with their children. drive from sunshine into a wall of fog. >> i mean, fog like i had never seen it. it was so thick that it was actually, i had to use the wiper blades, because it was condensing on the windshield. >> reporter: police dashcam video of i-10 that very morning. in an instant, daylight to danger. through the fog, catie stegmoyer spots a truck dead ahead. she stops just in time, but all around, other drivers fly blissfully right on by. >> i kept telling matt, they don't know, they don't know, they don't know to stop and then they started piling up on each other. >> reporter: ahead, a mazda has rear-ended a chevy tahoe. it's a simple fender bender. but it sets off one of the biggest chain reaction crashes in history. >> a massive crash. >> mangled metal as far as the eye can see. >> bam, you could hear it for miles. >> reporter: somebody described it as a demolition derby. >> i think that would probably be a fair assessment at that point in time, yes. >> reporter: a sense of the bedlam in texa