Traffic is created by idiots
In theory, given the old rule about maintaining one car length ahead of you for each ten miles per hour driving speed, the capacity of a single lane of expressway is 40 cars per minute (2,400 per hour) at 60 MPH. In practice, however, drivers instinctively begin to slow down at loads higher than 25 cars per minute (1,500 per hour). At 33 cars per minute (2,000 per hour), average speed drops to 35 MPH.
At this critical juncture, drivers are jumpy, and they'll slam on the brakes at the slightest provocation--anything from an accident or a stall to a couple extra cars trying to merge into traffic at an on-ramp. The first guy slows down a little, the second guy slows down a lot, and the third, fourth, or fifth guys may stop altogether, bringing traffic to a halt. That's why you almost never find smoothly flowing expressway traffic at speeds below 35 MPH--it's usually stop-and-go, or, at best, speed-up-and-slow-down-quick.
It also explains why relatively minor increases in traffic volume, such as those caused by mass transit strikes or fare increases, can cause chaos on the highways.
The Straight Dope
I've observed the same thing in the past.