WEBVTT

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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.

NOTE Paragraph

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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 producing 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 ahead

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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.

NOTE Paragraph

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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.