Engines of Our Ingenuity

No. 433:
TRAFFIC

by John H. Lienhard

Click here for audio of Episode 433.

Today, a note of hope in a traffic jam. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

It's a crowded morning. You're driving along when a stop light turns red, far down the road. The first cars halt and -- one by one -- the cars behind it close up. You're 25th in a row of unmoving, bumper-to-bumper cars.

The light turns green and the front cars start moving. By the time you can move, the light is red again. The cars stack up a second time. The process repeats. You've just been part of something called a kinematic wave.

It's a peculiar kind of wave. It only moves upstream. Waves in water and waves on violin strings travel both forward and backward. Try it. Pluck a clothesline in the middle. The waves move outward in both directions. Drop a rubber duck in the center of the bathtub, and waves go both ways. Once they're started, the mass of water, or of the string, moves them.

But the waves in traffic aren't caused by the mass of your car. They're caused by your mind. You know you're unsafe if your car's too close to the one in front of you. So you create a spacing that depends on your speed. You and the other drivers on the road agree to stay one car length back for every ten miles an hour you move.

That has an interesting side effect. Cars on crowded highways always settle down around 35 miles an hour. That's where the combination of speed and spacing lets the most cars through.

These traffic waves are long. And the smallest things can set them off. Drivers slow down to look at something along the road -- an accident, a bear, a man with a funny hat. A wave of cars that've slowed down and crowded together stretches back for a mile into the oncoming traffic. A stop light turns green, and a thinning-out wave reaches as far as cars are backed up. But these waves can only move back. We always drive away from them -- we leave them behind. They can never catch up with us.

Kinematic waves in traffic are a strange delight. They look like pure physics. But they're something else entirely. They're the fruit of a social contract. They represent an agreement among drivers. We all hate the tailgater. He violates that contract and threatens our lives.

But he's a rarity. Traffic waves are as reproducible as sound waves or ocean waves. They're formed by our instinct for living harmoniously together. The next time you're angry and impatient in gluey traffic, relax and watch those wonderful waves. They remind us that we're a saner and more civilized people than you might have first thought.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)





The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.

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