It looks like they had limited visibility, came over an overpass, downhill in the snow and piled up until traffic could see them on top of the overpass. What I don't understand is why someone in the first vehicles does not run back up the road to high ground to stop traffic. Or why someone in the opposite lane did not pullover and run across the median to flag down the traffic. Why is that bastard taking video instead of flagging down traffic?
I hate to say it, but it's a bad idea to try to wave to people from the edge of the road to get them to stop for one of the same problems that caused the accident: limited visibility. If they can't see the pileup forming, they probably aren't going to see a person until they're right up on you, and that's assuming they can figure out what you're waving your arms about. You could be putting yourself in harms way and not really be helping the situation.
Unless you have road flares that you can lay out (or a similar indicator that is easy to understand), your best place is to be well away from the road (in case a vehicle's gas tank explodes) and on the phone to report this to 911. Or if you're qualified in First Aid, go back once it's safe to do so and help the people who were hurt.