Cecil Adams, whose "The Straight Dope" column has appeared in alternative newspapers since 1973, answered the following question.
"Why does traffic come to a stop on a freeway that presumably offers nothing to stop it?" "In theory," Adams replied, "given the old rule about maintaining one car length ahead of you for each 10 m.p.h. of driving speed, the capacity of a single lane of freeway is 40 cars per minute (2,400 per hour) at 60 m.p.h. 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 m.p.h.
"At this critical juncture, drivers are extremely jumpy, and they will slam on the brakes at the slightest provocation--anything from an accident or a stall to a couple of 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 guy may stop altogether, bringing traffic to a halt. That is why you almost never find smoothly flowing freeway traffic at speeds below 35 m.p.h.--it is usually stop-and-go, or, at best, speed-up-and-then-slow-down-real-quick."