Explain to me the traffic jams

Absent any obvious cause (accident, stalled vehicle, anti-war protest, etc.) how do traffic jams happen?

Oh, I should clarify I’m more interested in highway situations than surface streets.

I heard this on the news once, so there is no cite.

At freeway speeds the normal driver will try to keep a reasonable distance between his/her car and the one in front. As more cars enter the flow of traffic, to keep a reasonable distance, the normal driver must slow down. So as the amount of drivers increases, the flow of traffic reduces, thereby causing traffic jams.

Lots of reasons. Basically, once the number of cars gets above a certain “density” people instinctively start slowing down. Then, if something unexpected happens (like a car suddenly cutting across lanes to get to an exit), they will come to a near or complete stop. This makes the cars behind them stop, and so on.

Cecil covered this in detail in one of his columns.

Animation of “traffic waves” in action.

There is some discussion of that in this thread. The OP specifically asks about long-lasting jams after accidents have cleared, but the discussion is more general. The gist is that even minor disturbances can escalate into a mass slowdown.

On Scientific American Frontiers (that Alan Alda PBS show), they had a computer model that simulated traffic jams.

Many times when you don’t see the cause, there was a cause, about a mile ahead of where the jam now breaks up.

The reason is that jams break up from the front. As it slowly dissipates, the point at which the traffic eases moves further and further back. Meanwhile, new cars are adding to the jam in the back. Often you’ll be entering a traffic jam where the cause of it was a couple miles away from where any part of the tie-up is now.

Here’s an excellent article from Science News:

This topic is cutting-edge physics, and a bunch of nonlinear dynamics projects sprang up in Europe in the 1990s. At the same time, traffic engineers ridicule the physics (possibly because it threatens their older style of thinking.) The above article mentions this, and I’ve encountered it first hand. “Planck’s Law” says that most experts can never accept revolutinary new concepts, instead they have to die and be replaced by younger experts who were familiar with the new ideas during their schooling.

Fear of flaming death. One of Cecil’s best. lines. ever.