It's both, Jed and Stewie. People panicked but they were driviing slow when the density was low as is evidenced in my vid. I should put the whole drive up. I made the trip quickly because I drove 60 mph while traffic was travelling at 40 mph. I wove in and out in the gaps. I'm sure that I looked crazy. People thought the roads were bad but they weren't. The density gradient was compressed as a result. The roads weren't emptying as fast as normal; so, like a tsunami, the gradient increased rapidly. And, as the roads actually became slick, the motion stopped.
At one point midway through it all, newsmen measured the flow rate. It was 100 ft in 40 minutes. Hence, a disaster resulted.
