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You’re cruising down the highway when
all of a sudden
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endless rows of brake lights appear ahead.
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There’s no accident, no stoplight,
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no change in speed limit or
narrowing of the road.
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So why the @#$%! is there
so much traffic?
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When traffic comes to a near standstill
for no apparent reason,
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it’s called a phantom traffic jam.
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A phantom traffic jam is an emergent
phenomenon
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whose behavior takes on a life of its own,
greater than the sum of its parts.
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But in spite of this, we can actually
model these jams,
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even understand the principles
that shape them—
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and we’re closer than you might think
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to preventing this kind of traffic
in the future.
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For a phantom traffic jam to form, there
must be a lot of cars on the road.
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That doesn’t mean there are necessarily
too many cars
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to pass through a stretch of
roadway smoothly,
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at least not if every driver maintains
the same consistent speed and spacing
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from other drivers.
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In this dense, but flowing, traffic,
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it only takes a minor disturbance to set
off the chain of events
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that causes a traffic jam.
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Say one driver brakes slightly.
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Each successive driver then brakes
a little more strongly,
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creating a wave of brake lights that
propagates backward
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through the cars on the road.
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These stop-and-go waves can travel
along a highway for miles.
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With a low density of cars on the road,
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traffic flows smoothly because
small disturbances,
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like individual cars changing lanes or
slowing down at a curve,
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are absorbed by other
drivers’ adjustments.
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But once the number of cars on the
road exceeds a critical density,
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generally when cars are spaced less
than 35 meters apart,
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the system’s behavior
changes dramatically.
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It begins to display dynamic instability,
meaning small disturbances are amplified.
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Dynamic instability isn’t unique to
phantom traffic jams—
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it’s also responsible for raindrops, sand
dunes, cloud patterns, and more.
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The instability is a positive feedback loop.
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Above the critical density,
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any additional vehicle reduces the number
of cars per second
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passing through a given point on the road.
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This in turn means it takes longer for a
local pileup
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to move out of a section of the road,
increasing vehicle density even more,
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which eventually adds up to
stop-and-go traffic.
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Drivers tend not to realize they need to
break far in advance of a traffic jam,
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which means they end up having to
brake harder to avoid a collision.
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This strengthens the wave of braking
from vehicle to vehicle.
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What’s more, drivers tend to accelerate
too rapidly out of a slowdown,
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meaning they try to drive faster
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than the average flow of traffic
downstream of them.
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Then, they have to brake again, eventually
produce another feedback loop
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that causes more stop-and-go traffic.
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In both cases, drivers make traffic worse
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simply because they don’t have a good
sense of the conditions ahead of them.
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Self driving cars equipped with data on
traffic conditions
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ahead from connected vehicles or roadway
sensors
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might be able to counteract phantom
traffic in real-time.
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These vehicles would maintain a uniform
speed, safety permitting,
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that matches the average speed of the
overall flow,
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preventing traffic waves from forming.
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In situations where there’s
already a traffic wave,
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the automated vehicle would be able
to anticipate it,
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braking sooner and more gradually
than a human driver
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and reducing the strength of the wave.
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And it wouldn’t take that many
self-driving cars—
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In a recent experiment, one autonomous
vehicle for every 20 human drivers
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was enough to dampen and
prevent traffic waves.
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Traffic jams are not only a
daily annoyance–
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they’re a major cause of fatalities,
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wasted resources, and planet-threatening
pollution.
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But new technology may help reduce
these patterns,
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rendering our roads safer,
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our daily commutes more efficient,
and our air cleaner.
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And the next time you’re stuck in traffic,
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it may help to remember that other drivers
aren’t necessarily driving spitefully,
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but are simply unaware of road
conditions ahead—and drive accordingly.