What is phantom traffic and why is it ruining your life?

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You're cruising down the highway when all of a sudden endless rows of brake lights appear ahead. There's no accident, no stoplight, no change in speed limit, or narrowing of the road. So why is there so much traffic? It's due to a phenomenon called a phantom traffic jam. Benjamin Seibold explains why it happens and how we might prevent it in the future.

Lesson by Benjamin Seibold, directed by Cris Wiegandt.

You’re cruising down the highway
when all of a sudden

endless rows of brake lights appear ahead.

There’s no accident, no stoplight,

no change in speed limit
or narrowing of the road.

So why the @#$%! is there
so much traffic?

When traffic comes to a near standstill
for no apparent reason,

it’s called a phantom traffic jam.

A phantom traffic jam
is an emergent phenomenon

whose behavior takes on a life of its own,
greater than the sum of its parts.

But in spite of this,
we can actually model these jams,

even understand the principles
that shape them—

and we’re closer than you might think

to preventing this kind of traffic
in the future.

For a phantom traffic jam to form,
there must be a lot of cars on the road.

That doesn’t mean
there are necessarily too many cars

to pass through a stretch
of roadway smoothly,

at least not if every driver maintains
the same consistent speed and spacing

from other drivers.

In this dense, but flowing, traffic,

it only takes a minor disturbance
to set off the chain of events

that causes a traffic jam.

Say one driver brakes slightly.

Each successive driver then brakes
a little more strongly,

creating a wave of brake lights
that propagates backward

through the cars on the road.

These stop-and-go waves
can travel along a highway for miles.

With a low density of cars on the road,

traffic flows smoothly
because small disturbances,

like individual cars changing lanes
or slowing down at a curve,

are absorbed by other
drivers’ adjustments.

But once the number of cars
on the road exceeds a critical density,

generally when cars are spaced
less than 35 meters apart,

the system’s behavior
changes dramatically.

It begins to display dynamic instability,
meaning small disturbances are amplified.

Dynamic instability isn’t unique
to phantom traffic jams—

it’s also responsible for raindrops,
sand dunes, cloud patterns, and more.

The instability is
a positive feedback loop.

Above the critical density,

any additional vehicle reduces
the number of cars per second

passing through a given point on the road.

This in turn means it takes longer
for a local pileup

to move out of a section of the road,
increasing vehicle density even more,

which eventually adds up
to stop-and-go traffic.

Drivers tend not to realize they need
to break far in advance of a traffic jam,

which means they end up having
to brake harder to avoid a collision.

This strengthens the wave of braking
from vehicle to vehicle.

What’s more, drivers tend to accelerate
too rapidly out of a slowdown,

meaning they try to drive faster

than the average flow of traffic
downstream of them.

Then, they have to brake again,
eventually producing another feedback loop

that causes more stop-and-go traffic.

In both cases, drivers make traffic worse

simply because they don’t have a good
sense of the conditions ahead of them.

Self driving cars equipped with data
on traffic conditions ahead

from connected vehicles
or roadway sensors

might be able to counteract
phantom traffic in real-time.

These vehicles would maintain
a uniform speed, safety permitting,

that matches the average speed
of the overall flow,

preventing traffic waves from forming.

In situations where there’s
already a traffic wave,

the automated vehicle
would be able to anticipate it,

braking sooner and more gradually
than a human driver

and reducing the strength of the wave.

And it wouldn’t take that many
self-driving cars—

In a recent experiment, one autonomous
vehicle for every 20 human drivers

was enough to dampen
and prevent traffic waves.

Traffic jams are not only
a daily annoyance–

they’re a major cause of fatalities,

wasted resources,
and planet-threatening pollution.

But new technology may help reduce
these patterns,

rendering our roads safer,

our daily commutes more efficient,
and our air cleaner.

And the next time you’re stuck in traffic,

it may help to remember that other drivers
aren’t necessarily driving spitefully,

but are simply unaware of road
conditions ahead— and drive accordingly.