I stumbled across this video that very clearly shows why snowchains and studded tires have their uses. Some common sense and restraint on the lead (and/or brake) foot is also useful in this situation.
http://www.kgw.com/sharedcontent/VideoPlayer/videoPlayer.php?vidId=114046&catId=131]15 cars colliding and caught on tape by man on rooftop - Footage from KGW
I've never seen such sliding - even on the slick driving course track I was on when I took my license, where they use chemicals to make it slippery enough, we could stop relatively controlled, compared to this. Looks close to zero traction... What tires are they using - Soap tires? Cars are sliding sideways for long distances.
Probably this is the result of some freaky, very local temperature conditions, as cars making it out of the road onto an intersection seems to be able to stop readily enough.
Salting the roads, combined with temperatures around -7C to -8C and some snow will create an unholy mess. The snow and salt mix will not melt, but create a very fine ice powder with ice on the asphalt beneath (especially when the ground is colder than the air), giving a situation with fine ice powder on top of thin, hard ice. That might be what has happened here.
The road goes downhill and looks to be pretty steep.