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)
