Where do traffic jams come from?

Traffic jams caused by tollbooths, accidents and rubbernecking rank high on the list of ways to ruin a summer vacation, but for sheer exasperation, it's hard to beat congestion that has no visible cause. You're cruising at 55 and suddenly you're bumper to bumper. You inch along for a mile. Then, daylight: just as suddenly you're back up to 55 (or more), never having seen even so much as a salacious billboard to explain the slowdown. Once such delays were merely annoying. Now their cost is mounting. Hours lost to traffic jams exceed 2.7 billion a year; idling cars and stop-and-go driving add to air pollution and guzzle gas. Before highway designers can prevent traffic jams, though, scientists have to understand where they come from. "We have equations to explain traffic," says Michael Cassidy of the University of California, Berkeley, "but many are wrong. Only a new approach -- looking at traffic as wave motions over time and space -- might reveal the cause of bottlenecks