You're cruising down the highway when all of a sudden, endless rows of brake lights appear ahead.

你正在高速公路上開車，突然間，前方亮起了無數個煞車燈。

There's no accident, no stoplight, no change in speed limit or narrowing of the road.

沒有發生交通事故、沒有紅燈、限速沒有變化，道路寬度也沒有變窄。

So why the fuck is there so much traffic?

但為什麼會塞車呢？

When traffic comes to a near standstill for no apparent reason, it's called a phantom traffic jam.

當交通沒有明顯的理由而停滯的時候，這個現象叫做「幽靈堵車」。

A phantom traffic jam is an emergent phenomenon whose behavior takes on a life of its own, greater than the sum of its parts.

幽靈堵車是一種新崛起的現象，它的變化難以控制，大於其各個部分的總和。

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.

即便如此，我們其實可以模擬這些交通堵塞，甚至了解造成這個現象的原因 — 而將來這種交通堵塞會比你想像中更容易避免。

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

要形成幽靈堵車的話，路上必須有很多輛車。

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.

這並不代表有太多輛車，導致無法順利通過一段道路，至少每個司機都保持一致的速度和間距的話，幽靈堵車就不會發生。

In this dense but flowing traffic, it only takes a minor disturbance to set off the chain of events that causes a traffic jam.

在這種密集但流動的交通中，只要有輕微的變動，就會造成連鎖反應，導致塞車。

Say one driver brakes slightly.

假設一名司機稍微踩了一下煞車。

Each successive driver then brakes a little more strongly, creating a wave of brake lights that propagates backward through the cars on the road.

後面的司機都會踩稍微大力一點，導致路上的車輛形成一波向後延伸的剎車燈波浪。

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

這種時行時止的波浪可以在高速公路上持續好幾英里。

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.

路上的車流密度較低時，交通會比較順暢，因為如果發生很小的變化，比如說某輛車換車道或在彎道減速，會被其他司機的調整吸收。

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.

一旦路上車輛的數量超越臨界密度，一般而言，當車距小於 35 公尺時，整個現象就會產生巨大的變化。

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

它會開始呈現動態不穩定，也就是小的變化會被放大。

Dynamic instability isn't unique to phantom traffic jams, it's also responsible for raindrops, sand dunes, cloud patterns, and more.

動態不穩定並非幽靈堵車所獨有 — 雨滴、沙丘、雲朵形狀等等也有。

The instability is a positive feedback loop.

這種不穩定性是正面反饋循環。

Above the critical density, any additional vehicle reduces the number of cars per second passing through a given point on the road.

超過臨界密度，任何多出來的車輛都會造成每秒通過路上某一點的汽車數量減少。

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.

這會導致道路區段上的塞車要花更多時間來清除，使得車流密度增加更多，累積成時行時止的路況。

Drivers tend not to realize they need to brake far in advance of a traffic jam, which means they end up having to brake harder to avoid a collision.

司機往往不會意識到他們必須遠在堵車之前就煞車，也就是說他們最後必須透過緊急煞車來避免撞車。

This strengthens the wave of braking from vehicle to vehicle.

這會強化車輛間剎車的浪潮。

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.

更驚人的是，司機在減速後往往都會加速過快，也就是說他們的車速會比下游的平均車流還快。

Then, they have to brake again, eventually producing another feedback loop that causes more stop-and-go traffic.

接著，他們必須再次煞車，最終會形成一個反饋循環，造成更多時行時止的路況。

In both cases, drivers make traffic worse simply because they don't have a good sense of the conditions ahead of them.

在這兩種情況中，司機讓交通變得更糟糕，僅僅因為他們無法預知前方的路況。

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.

自動駕駛汽車具備著從互聯汽車或路段感應器蒐集到的前方路況數據，這樣也許可以即時避免幽靈堵車。

These vehicles would maintain a uniform speed, safety permitting, that matches the average speed of the overall flow, preventing traffic waves from forming.

這些車輛會在安全情況下保持固定速度，配合總流量的平均速度來預防交通波浪再次形成。

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.

在交通波浪已經形成的情況下，自動駕駛汽車能夠提早發現，而它會比人們開車時更早逐漸地煞車，足以成功抵掉波浪的強度。

And it wouldn't take that many self-driving cars.

其實，防止交通波浪形成並不需要那麼多自動駕駛汽車。

In a recent experiment, one autonomous vehicle for every 20 human drivers was enough to dampen and prevent traffic waves.

在最近的實驗中，如果每 20 位司機中有一輛自動駕駛汽車，就足以抵銷和防止交通波浪。

Traffic jams are not only a daily annoyance, they're a major cause of fatalities, wasted resources, and planet-threatening pollution.

塞車不只是日常的煩惱，它是導致死亡、浪費資源和造成地球污染的主要原因。

But new technology may help reduce these patterns, rendering our roads safer, our daily commutes more efficient, and our air cleaner.

但是新技術或許可以協助減少這些狀況，讓我們的道路更安全、日常通勤更有效率、空氣更清淨。

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.

下次你遇到交通堵塞時，請記住其他司機並不是存有惡意地開車，他們只是不知道該如何因應前方的路況開車。

In a hurry but love physics?

時間有限但是喜歡物理怎麼辦？

We've got a book rec for you.

我們有一本書可以推薦給你。

Astrophysics for People in a Hurry is fun, accessible and may just blow your mind.

《宇宙必修課：給大忙人的天文物理學入門攻略》這本書非常有趣、容易取得，也有可能讓你大開眼界。

Download a free audiobook version from audible.com/ted-ed and get lost in the stars.

從 audible.com/ted-ed 這個網址下載免費電子書，沉浸在物理的奧妙中吧！