eli5 How does traffic work?


Interstate traffic. Highways are so full of traffic lights and sharp turns so I get that. But how does traffic work on an interstate? Every turn is designed so you don’t have to stop. Every merge is seamless.

Yet SOMEHOW every road-trip runs into the whole thing being in a complete standstill. How does that work? What does the front of that look like? What’s happening?

In: 0

Take a funnel.

Put it under your faucet.

Turn the water low.

The funnel never backs up.

Turn the water halfway.

Funnel starts to back up.

Crank the faucet to full.

Funnel overflows.

There’s only so much capacity on the road, same way there’s only so much diameter to the funnel.

When more water, more cars, attempt to access that same diameter of funnel, the same amount of road, things back up.

People get uncomfortable driving fast when they’re very close to other cars. That makes sense, because you have a lot less time to react to problems if other cars are close to you.

So they slow down. And when one car slows down, the one behind them does too, and the one behind that, and … This creates a cluster of slower-moving cars that are the start of a traffic jam. Once everyone is going slow, they can’t all just decide to go fast again. Only the people at the very front can do that.

[CGP Grey has an excellent video](https://www.youtube.com/watch?v=iHzzSao6ypE) about doing what you can to minimize traffic. The short version is: try to stay halfway between the car ahead of you and the one behind you. That keeps the cars as far apart as possible under the circumstances and means fewer people hit those brakes and cause that start of a traffic jam.

Vehicles take up room on the highway. At low speeds they take up a little more than their own side. But at higher speeds they require a lot more room around them so everyone can safely stop in an emergency.

Simply put traffic jams are times when there is just not enough space on the roadway for all the vehicles that are trying to use it.

Every time some inconsiderate asshole cuts someone off because they are impatient, it creates an exponential slowdown. Think about a formation marching, if 1 person gets a little behind and has to run to catch up it creates a gap slightly wider the next person has to catch up. Until you get to the end of the formation and the last guy has to run 15 feet to make up for the 1 foot a guy fell behind.

This same principle applies to traffic, anytime someone has to hit the brakes the next people have to deal with exponential slowing until you end up with bumper to bumper traffic.

Traffic involves different types of vehicles, such as cars, trucks, motorcycles, and bicycles, traveling on roads. Roads can have multiple lanes, traffic lights, intersections, and signs to guide and control the flow of vehicles. The number of vehicles on the road at a given time affects traffic flow. When there are many vehicles, especially during rush hours or in densely populated areas, congestion can occur, causing slower movement and potential delays.