You come screaming over a hill at 70 mph and discover a parking lot where the highway used to be. You lock 'em up, join the creeping pack of cars and crane your neck, wondering when you'll see a conflagration of Camaros. You keep craning. You keep creeping. Then, as suddenly as it stopped, traffic rolls again. You're back to 70 mph.

When a highway is packed to its max, it doesn't take much to start a chain reaction: a bend in the road, a truck changing lanes or a driver slowing to root around for a hankie. There may, indeed, have been a burning Camaro here, too. Or a policeman writing a ticket, a chicken crossing the road or a misguided Martian spacecraft. (All of the above, by the way, are known in the traffic industry as "incidents.") Any incident that can inspire a single driver to hit the brakes will do.

The sorry state of human reaction times will take care of the rest.

When, for example, a guy in a Camaro bolted into the left lane two hours ago to get around a slow truck, he almost clipped the Buick behind him.

The guy behind the Buick, see, had less time to react to the incident, so he hit the brakes a bit harder. And the gal behind him really had to stomp on her brakes, and the guy behind her had to mash them. Eventually someone had to come to a full stop to avoid biting the bumper in front of her.

Back down the road ("upstream," engineers say) the incident travels. Car after car after car stacks up in a traffic jam that can stretch for miles.

And you are stuck in what is essentially the ghost of the incident. That ghost can last as long as traffic is heavy enough to keep piling on cars at the back end of the chain, even though nothing prevents those at the front ("downstream") from zooming up to speed again. Had traffic been lighter when the woman in the Buick touched her brakes, the man behind her might not have been following so closely and might not have needed to slow down at all. None of this would have happened.

But now this incident has taken on a life of its own. And the longer it persists, the greater is the chance that a 16-year-old kid in a 17th-century Gremlin is going to come popping over the hill and peg an 18th-century Pinto. And that would definitely be an incident.