You're probably thinking "Way too many cars on the road, duh!" But the real answer is a bit more complicated, and counterintuitive. Tom Vanderbilt explains in a 20-minute presentation.

"Tom Vanderbilt, author of Traffic, gave a great 20-minute overview on the counterintuitive science of congestion at the Boing Boing: Ingenuity conference in San Francisco last month," writes Eric Jaffe. "Turns out a lot of the problems we ascribe to poor roads or other drivers are really our own fault. '[T]he individual driver cannot often understand the larger traffic system,' says Vanderbilt."
Among the nuggets: "A big reason for traffic is that too many cars are trying to occupy too little space on the road," notes Jaffe. Aha, see I told you you were right!
"But that's not the only problem. A human inability to maintain a steady speed and following distance on the highway makes traffic a lot less smooth than it could be."
FULL STORY: A Fantastically Clear, Concise Explanation of Why Traffic Happens

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