WEBVTT

99:59:59.999 --> 99:59:59.999
You’re cruising down the highway when 
all of a sudden

99:59:59.999 --> 99:59:59.999
endless rows of brake lights appear ahead.

99:59:59.999 --> 99:59:59.999
There’s no accident, no stoplight,

99:59:59.999 --> 99:59:59.999
no change in speed limit or 
narrowing of the road.

99:59:59.999 --> 99:59:59.999
So why the @#$%! is there 
so much traffic?

99:59:59.999 --> 99:59:59.999
When traffic comes to a near standstill 
for no apparent reason,

99:59:59.999 --> 99:59:59.999
it’s called a phantom traffic jam.

99:59:59.999 --> 99:59:59.999
A phantom traffic jam is an emergent 
phenomenon

99:59:59.999 --> 99:59:59.999
whose behavior takes on a life of its own,
greater than the sum of its parts.

99:59:59.999 --> 99:59:59.999
But in spite of this, we can actually 
model these jams,

99:59:59.999 --> 99:59:59.999
even understand the principles 
that shape them—

99:59:59.999 --> 99:59:59.999
and we’re closer than you might think

99:59:59.999 --> 99:59:59.999
to preventing this kind of traffic
in the future.

99:59:59.999 --> 99:59:59.999
For a phantom traffic jam to form, there 
must be a lot of cars on the road.

99:59:59.999 --> 99:59:59.999
That doesn’t mean there are necessarily 
too many cars

99:59:59.999 --> 99:59:59.999
to pass through a stretch of 
roadway smoothly,

99:59:59.999 --> 99:59:59.999
at least not if every driver maintains 
the same consistent speed and spacing

99:59:59.999 --> 99:59:59.999
from other drivers.

99:59:59.999 --> 99:59:59.999
In this dense, but flowing, traffic,

99:59:59.999 --> 99:59:59.999
it only takes a minor disturbance to set 
off the chain of events

99:59:59.999 --> 99:59:59.999
that causes a traffic jam.

99:59:59.999 --> 99:59:59.999
Say one driver brakes slightly.

99:59:59.999 --> 99:59:59.999
Each successive driver then brakes 
a little more strongly,

99:59:59.999 --> 99:59:59.999
creating a wave of brake lights that 
propagates backward

99:59:59.999 --> 99:59:59.999
through the cars on the road.

99:59:59.999 --> 99:59:59.999
These stop-and-go waves can travel
along a highway for miles.

99:59:59.999 --> 99:59:59.999
With a low density of cars on the road,

99:59:59.999 --> 99:59:59.999
traffic flows smoothly because 
small disturbances,

99:59:59.999 --> 99:59:59.999
like individual cars changing lanes or 
slowing down at a curve,

99:59:59.999 --> 99:59:59.999
are absorbed by other 
drivers’ adjustments.

99:59:59.999 --> 99:59:59.999
But once the number of cars on the
road exceeds a critical density,

99:59:59.999 --> 99:59:59.999
generally when cars are spaced less
than 35 meters apart,

99:59:59.999 --> 99:59:59.999
the system’s behavior 
changes dramatically.

99:59:59.999 --> 99:59:59.999
It begins to display dynamic instability, 
meaning small disturbances are amplified.

99:59:59.999 --> 99:59:59.999
Dynamic instability isn’t unique to 
phantom traffic jams—

99:59:59.999 --> 99:59:59.999
it’s also responsible for raindrops, sand 
dunes, cloud patterns, and more.

99:59:59.999 --> 99:59:59.999
The instability is a positive feedback loop.

99:59:59.999 --> 99:59:59.999
Above the critical density,

99:59:59.999 --> 99:59:59.999
any additional vehicle reduces the number
of cars per second

99:59:59.999 --> 99:59:59.999
passing through a given point on the road.

99:59:59.999 --> 99:59:59.999
This in turn means it takes longer for a
local pileup

99:59:59.999 --> 99:59:59.999
to move out of a section of the road, 
increasing vehicle density even more,

99:59:59.999 --> 99:59:59.999
which eventually adds up to 
stop-and-go traffic.

99:59:59.999 --> 99:59:59.999
Drivers tend not to realize they need to 
break far in advance of a traffic jam,

99:59:59.999 --> 99:59:59.999
which means they end up having to 
brake harder to avoid a collision.

99:59:59.999 --> 99:59:59.999
This strengthens the wave of braking 
from vehicle to vehicle.

99:59:59.999 --> 99:59:59.999
What’s more, drivers tend to accelerate
too rapidly out of a slowdown,

99:59:59.999 --> 99:59:59.999
meaning they try to drive faster

99:59:59.999 --> 99:59:59.999
than the average flow of traffic 
downstream of them.

99:59:59.999 --> 99:59:59.999
Then, they have to brake again, eventually
produce another feedback loop

99:59:59.999 --> 99:59:59.999
that causes more stop-and-go traffic.

99:59:59.999 --> 99:59:59.999
In both cases, drivers make traffic worse

99:59:59.999 --> 99:59:59.999
simply because they don’t have a good
sense of the conditions ahead of them.

99:59:59.999 --> 99:59:59.999
Self driving cars equipped with data on
traffic conditions

99:59:59.999 --> 99:59:59.999
ahead from connected vehicles or roadway
sensors

99:59:59.999 --> 99:59:59.999
might be able to counteract phantom
traffic in real-time.

99:59:59.999 --> 99:59:59.999
These vehicles would maintain a uniform 
speed, safety permitting,

99:59:59.999 --> 99:59:59.999
that matches the average speed of the 
overall flow,

99:59:59.999 --> 99:59:59.999
preventing traffic waves from forming.

99:59:59.999 --> 99:59:59.999
In situations where there’s 
already a traffic wave,

99:59:59.999 --> 99:59:59.999
the automated vehicle would be able 
to anticipate it,

99:59:59.999 --> 99:59:59.999
braking sooner and more gradually 
than a human driver

99:59:59.999 --> 99:59:59.999
and reducing the strength of the wave.

99:59:59.999 --> 99:59:59.999
And it wouldn’t take that many 
self-driving cars—

99:59:59.999 --> 99:59:59.999
In a recent experiment, one autonomous 
vehicle for every 20 human drivers

99:59:59.999 --> 99:59:59.999
was enough to dampen and 
prevent traffic waves.

99:59:59.999 --> 99:59:59.999
Traffic jams are not only a 
daily annoyance–

99:59:59.999 --> 99:59:59.999
they’re a major cause of fatalities,

99:59:59.999 --> 99:59:59.999
wasted resources, and planet-threatening
pollution.

99:59:59.999 --> 99:59:59.999
But new technology may help reduce 
these patterns,

99:59:59.999 --> 99:59:59.999
rendering our roads safer,

99:59:59.999 --> 99:59:59.999
our daily commutes more efficient, 
and our air cleaner.

99:59:59.999 --> 99:59:59.999
And the next time you’re stuck in traffic,

99:59:59.999 --> 99:59:59.999
it may help to remember that other drivers
aren’t necessarily driving spitefully,

99:59:59.999 --> 99:59:59.999
but are simply unaware of road 
conditions ahead—and drive accordingly.