Don't know what happened in ATL but over here in Birmingham here's what happened.
About a half hour before the snow started to fall, ambient air temp was hovering around 45 degrees. When the snow started to fall the temp dropped to 18 degrees in about 15~20 minutes. The roads already warm from both air and use, instantly melted any snow that landed on them, then the super frigid air instantly turned it into ice. What we wound up with was a 2~3 inch sheet of very solid ice on the roads. I drove in it from around noon until almost 9PM before I gave up getting constantly turned around by blocked roads and wound up at a wally world less than 5 miles from my destination.
The forecast was for a light dusting for about a half hour before the (fast moving) front passed so there was no warning when everyone at work, school, etc., suddenly found they had about 10 minutes to get home before every road in the area was covered in a nice thick sheet of ice. That the temp never rose above 20 until around noon today just made it worse as ice melting from being run on just kept on refreezing.
I've driven all over America and as far north as Victoria BC and it was a serious challenge to keep my pickup truck on the road yesterday. Typical of this situation, in a 2wd with street tires, coming from a dead stop is the most difficult part and if you are on an upgrade it just gets harder, until you reach a point where you just can't overcome the combination of grade and wheelspin to get moving. You can move all day long going slow and steady but once you stop it could be your last until the ice goes away.
To be perfectly honest, considering we get this kind of weather once every 30 years or so, I was surprised it didn't go bad a lot sooner and didn't get any worse than it did. When you factor in the fact that everything in this town is on the side of a hill or smallish mountain, it just made a bad situation worse, but again, I was really surprised at how well most folks handled it, but it only takes one panicky driver to block a lane, then before you know it the road is blocked by cars that can't get moving on the ice. I was lucky in being able to keep my speed adjusted constantly so I never had to stop my forward motion, even if it was just above a crawl. I got caught a couple times but managed to work my offside tires on to the dirt or grass to get enough traction to get moving again. Had it not been for stalled vehicles I believe I'd have got home fine, but it just was not in the cards.