My commute is mercifully low-density. I used to have to "run the Park" (i.e. drive through the 3-mile section of Research Triangle Park, wherein 50,000+ researchers and corporate drones ingress at 8am and exit at 5pm, creating a hellacious traffic jam) and it was not fun.
Although the absolute worst traffic snarl I've ever seen was in Raleigh several years back, when we had a light snowfall about noon. The weathermen had called for flurries, but it came down looking like a blizzard. Didn't last all that long, but the sight of giant, fluffy flakes drifting down from the heavens triggered something buried deep in the Southern psyche, and we all done lost our collective shit. Schools announced they were closing, people called into work from their lunch break and said they were taking the rest of the day off, etc. Bottom line, a couple hundred thousand people all got into their cars and hit those snow-dusted roads, which had not yet been salted. The continual compression and melt/refreeze quickly turned much of that inch or so of snow into a glass-like coating of ice. Which then promptly caused accidents at several key locations in the traffic grid.
Raleigh, like many large cities, has a radial hub-and-spoke traffic system. We have a Beltline that goes in a circle through the city, and major roads which intersect it from various directions. Rather quickly, the Beltline filled up. The entry/exit ramps were so ice-coated and treacherous that the police closed most of them. Which meant the Beltline became a giant vortex: once you got on, you were trapped. Like the great worm Ouroboros, you wound up stuck in a queue 50 miles long which was, in turn, stuck waiting behind you.
This cause traffic on the major roads to back up, as many of them were waiting to get on the now-inaccessible Beltline. Once all the major roads backed up, it began to clog the side roads and feeders. By 5pm, when I tried to leave work, I too was trapped. In one direction, the side road fed onto a major road which was now a wall of angry, honking metal. In the other direction lay a steep downslope, followed by a steep upslope on the other side. This little U-shaped valley became a flytrap of its own, as cars slid down the hill but lacked the traction to climb up the other side. Thank God I decided to reconnoiter the situation on foot first. There was a pile of at least half a dozen cars at the bottom, some feebly trying to move and looking for all the world like hapless insects struggling to escape a spider's web.
I just laughed at the insanity of the situation, walked back to work, walked across the street to a restaurant, walked up to the bar, and began racking up a tab. The front window of the place looked out on a major road, so I figured I'd know when the traffic had begun to break loose. Wound up leaving about 10pm. Probably could have been busted for DWI that night, if not for the fact that nobody was going over 5mph, and I decided to take one of the side streets that had been abandoned because it was hilly. In my little lightweight Toyota Echo, I just got a good running start and then coasted for miles. Essentially, I ice skated home in my car.
I hear they use our situation as a worst-case scenario in traffic engineering courses now.