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You’re cruising down the highway [br]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 [br]or narrowing of the road.

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

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

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it’s called a phantom traffic jam.

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A phantom traffic jam [br]is an emergent phenomenon

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

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

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even understand the principles [br]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[br]in the future.

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

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That doesn’t mean [br]there are necessarily too many cars

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to pass through a stretch [br]of roadway smoothly,

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at least not if every driver maintains [br]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 [br]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 [br]a little more strongly,

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creating a wave of brake lights [br]that propagates backward

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through the cars on the road.

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These stop-and-go waves [br]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 [br]because small disturbances,

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

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are absorbed by other [br]drivers’ adjustments.

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

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generally when cars are spaced [br]less than 35 meters apart,

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the system’s behavior [br]changes dramatically.

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

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Dynamic instability isn’t unique [br]to phantom traffic jams—

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

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The instability is [br]a positive feedback loop.

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Above the critical density,

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any additional vehicle reduces [br]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 [br]for a local pileup

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

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which eventually adds up [br]to stop-and-go traffic.

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

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

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This strengthens the wave of braking [br]from vehicle to vehicle.

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What’s more, drivers tend to accelerate[br]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 [br]downstream of them.

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Then, they have to brake again, [br]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[br]sense of the conditions ahead of them.

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Self driving cars equipped with data [br]on traffic conditions ahead

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from connected vehicles [br]or roadway sensors

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

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These vehicles would maintain [br]a uniform speed, safety permitting,

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that matches the average speed [br]of the overall flow,

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preventing traffic waves from forming.

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In situations where there’s [br]already a traffic wave,

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the automated vehicle [br]would be able to anticipate it,

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braking sooner and more gradually [br]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 [br]self-driving cars—

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

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was enough to dampen [br]and prevent traffic waves.

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Traffic jams are not only [br]a daily annoyance–

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they’re a major cause of fatalities,

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wasted resources, [br]and planet-threatening pollution.

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But new technology may help reduce [br]these patterns,

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rendering our roads safer,

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our daily commutes more efficient, [br]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[br]aren’t necessarily driving spitefully,

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