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So you’re sitting at an intersection in rush hour traffic, waiting for that light to turn green. But when it does, you can’t move.

There are stopped cars in your way still. Cars behind you start to honk, convinced that you are texting or sleeping or the incarnation of pure evil -- but you’re just as stuck as they are, and the cars in front of you... they can’t go anywhere, either. So you just sit there, exasperatedly watching the light turn from green to yellow, and then from yellow back to red.

And you didn’t move a centimeter. Traffic jams can be incredibly frustrating. And if you’ve ever been stuck in traffic with someone else in the car, you’ve probably talked through all kinds of ways that roads could've been designed better.

Maybe you’d change the timing of traffic lights so that nobody ever sat through another green without moving. Maybe you’d change where or how cars merge onto the highway. Or maybe you dream bigger, and you’d add new lanes or whole new roads to give everyone more room to drive.

But none of that is as simple as it sounds. Scientists studying everything from fluid dynamics to psychology have contributed to our understanding of traffic, which is a huge problem that our society has, and they have found that traffic jams don’t necessarily happen because of poorly designed roads -- or because everyone but you is a terrible driver. In fact, the seemingly obvious solutions to traffic sometimes make things even worse.

One of the simplest models of a traffic jam doesn’t involve humans at all. It involves a bag of rocks. If you cut a hole in the corner of the bag, you can pour rocks out of the hole.

Obviously. But if you try to force too many rocks through the hole, eventually they’ll go through what’s called a jamming transition, and no rocks will flow at all. There will be so much pressure on the rocks near the opening that they can’t move aside to let other rocks out and relieve the pressure, so none can fall out of the bag.

Jamming can happen whenever a bunch of objects are flowing through an opening or channel -- whether it’s rocks, or air bubbles in a liquid … or even cars moving on a road. They move pretty freely when there aren’t a lot of cars around, like on a remote highway at night. But when there are too many cars trying to squeeze onto the same roads, you get a jamming transition: There are so many cars that none of them can really go anywhere.

Which means that you don’t need bottlenecks or bad drivers to start a traffic jam; sometimes, they just happen because there are too many cars in one place. The similarities also make scientific models of those bags of rocks useful tools for engineers trying to mitigate traffic jams — although whether driving makes you personally feel like a rock in a bag probably depends on when and where you usually drive. But jamming transitions are far from the only way that traffic has been studied.

Mathematicians, computer scientists, physicists, psychologists -- they all have something to say about traffic jams. And a lot of it turns out to be pretty counterintuitive. Take Braess’ paradox, which involves a combination of statistics and human behavior.

When traffic is caused by too many cars on the road, you would think that adding more driving space would help — whether that means adding more roads, or more lanes to existing roads. Except in some cases, adding more driving space makes congestion worse, not better. That’s the paradox, and it’s why alleviating rush hour traffic isn’t as easy as just widening major roads or adding more big, connecting streets.

Braess’ paradox exists because drivers tend to be fairly selfish — and not just the ones who’d cut you off right when you’re trying to get on the exit ramp. We’re selfish simply because we take the route we think will be fastest. So if you build an additional road that starts out with lighter traffic or acts as a shortcut, people are going to take that route until it’s just as congested as the roads that it’s supposed to relieve.

If drivers worked together, with some people taking the shortcut and some not taking it, then yeah, we’d all get where we’re going faster. But we don’t work together, and that makes everyone’s commutes worse. New driving space also tends to lead to more people driving in the first place, congesting the roads even more.

So maybe it shouldn’t really be all that surprising when, for example, the state of. California builds new roads to relieve older roads with lots of traffic, and it doesn’t help very much. The new ones become just as gridlocked as the old ones within a few years.

On the flip side, removing a busy road can make traffic better in some cases. Seoul in South Korea and San Francisco both removed freeways that carried more than a hundred thousand people a day into and out of the cities. In both cases, commuting was either the same or even smoother after the massive freeways were removed and replaced with roads designed to hold way less traffic.

Some commuters gave up their cars and relied on public transit instead, while others found more efficient routes that they’d never explored when the highway was there. Of course, you don’t want to take Braess’ paradox too far. Like, we do need some roads for our cars to travel down, and some highway expansions have reduced traffic jams.

But planning and predicting traffic patterns definitely isn’t as easy as it might seem from the driver’s seat. And honestly, as a driver, you’re probably not helping, either. Different driving styles can have a big impact on the flow of traffic -- and drivers tend to do a lot of things that make traffic worse for everyone around them, including themselves.

First, and maybe most obvious, there are those really aggressive drivers who weave between lanes on the highway. Study after study has found that driving this way tends to leave behind what are known as traffic waves. When someone cuts in front of you, it’s natural to tap the brakes to keep from getting too close to hitting them.

That’s the safe thing to do, so I’m not saying you should stop doing that. But when the person behind you sees your brake lights, they also tend to hit the brakes. And they’ll probably slow down just a little bit more than you did, because if they didn’t, they might get too close to you or even hit you.

The person behind them does the same thing, slowing down even more, and … you probably see where I’m going with this. If there are enough cars traveling close together, eventually someone way behind you comes to a complete stop. All because someone cut someone else off, like, a few minutes ago.

Once one car stops, so does the next car, and the next. That’s the “wave” part of the traffic wave, crashing over each successive car. As traffic slows down in one lane, people are going to swerve out of it into the other lanes -- which makes the people in those lanes hit the brakes as well.

And before you know it, there’s a full-blown traffic jam. Events like these can cause those jams on the highway where you spend the whole time assuming that there’s some terrible accident or construction at the end, only to just… speed up after you hit, like, nowhere spot. What happened!?

You know, the times when you feel really bad that you feel frustrated that there was not an accident slowing everybody down. One of the best ways scientists have found to avoid getting hit by the traffic wave is to just leave more space between you and the car in front of you. With more space between cars, you don’t have to hit the brakes every time a car in front of you hits their brakes -- and it’ll make the drive easier both for you and all of the hundreds of people behind you.

Plus, when you follow further back, you’re less likely to get in a car accident. And that’s good for everybody. This might seem like a kind of weird strategy, especially in city traffic, because you’d think you’d miss things like traffic lights more often.

Like, if you’re sitting a few cars back at a red light, it seems like you’d have a better chance of making it through the intersection during the next light cycle if you’re as close to the light as possible. And the only way to get closer to the light is to get closer to the car in front of you. But when you crunch the numbers, it turns out that’s just flat-out wrong.

Any gains you might get from being closer to the light are balanced out by having to wait longer to accelerate. Because if you’re super close to the car in front of you, you have to wait for them to start moving before you can. But if you’re farther back, you can start moving as soon as you see the light change.

As a result, cars waiting further away from the car in front of them get through intersections just as quickly as the tailgaters. And like on the highway, leaving more space between you and the car in front of you makes things safer for everybody. On top of that, aggressive drivers, who accelerate sharply and brake a lot, use around 20% more gas than their calmer counterparts.

And they get in more minor and fatal car crashes -- without actually reducing their travel times all that much. But it isn’t just aggressive drivers making things worse for the rest of us. Driving too slowly or timidly can slow down traffic, too.

Some people leave plenty of room, but they still brake as soon as the person ahead brakes the tiniest bit to avoid getting any closer. Or, if the left lane is closed up ahead, most of us don’t want to be that car that just pulls in at the last possible moment before merging right. And we all know how tempting it is to punish the people who do that by not letting them merge with us when their lane ends.

But moving over way way before the lane actually ends leaves a whole bunch of unused road up there, and it forces the cars in your lane to slow down just to let you in. And because of traffic waves, you never just slow down the cars immediately behind you in traffic. There’s always that ripple effect.

When a lane ends, the safest and smoothest way to merge is what’s called zipper merging, where cars stay in their lanes until the merge happens, and then cars from both lanes take turns going forward like the teeth of a zipper. That maybe feels inconsiderate, but zipper merging can reduce traffic congestion by about 40%, which seems, like, worth a honk or two by people who don’t get it. With subjects as variable as humans, it’s essentially impossible for scientists to prescribe a one-size-fits-all driving strategy that will make traffic flow smoother in all situations.

But you’ve probably noticed some patterns throughout this video: the kinds of behaviors that make traffic worse also lead to more accidents, and that’s no coincidence. It comes back to those jamming transitions that happen with rocks: The more objects crammed into one space, the less each one is able to move, and the more likely they are to move into each other’s space. Which is fine when the objects in question are rocks.

It’s not as fine when they’re cars that have soft meat-bags inside. And if there’s one thing scientists have generally found reduces gridlock, it’s leaving more space between cars so they aren’t crammed so closely together, even in slow-moving traffic. For city traffic engineers, the trick for choosing speed limits, road patterns, lane restrictions, and new construction projects is balance:.

Balance between the physics of jamming and the psychology of driving; balance between rush hour and the middle of the night; balance between the ideal city for traffic and the ideal city for humans. But as drivers, we have our own balancing acts, too. We have to balance what we think will make our trips faster with the science of what actually will make them faster, and we have to balance our vindictive instincts with our empathy for the other humans around us.

So that hopefully when that next light turns red and you still haven’t moved, there’s just a little bit less yelling and honking. Thanks for watching this episode of SciShow! If you’re interested in more in-depth episodes on complex topics like this one, you might like our video about why scientists haven’t cured Alzheimer’s yet despite the massive amount of research being done.

And also if you want just more information in general don’t forget to go to youtube.com/scishow and subscribe! [♪OUTRO].