r/explainlikeimfive • u/[deleted] • Jun 11 '22
Other ELi5: How and why do traffic jams happen?
[deleted]
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u/LeDiamonddozen Jun 11 '22
Basically, one person will slow down a little bit. This could be because they are letting someone merge, there was something on the road, or they just don't want to drive any faster.
That car slowing down will cause the car behind them to slowdown and then the next car and the next and the next until there are no more cars. Then the car that had to slow down behind the first car takes a couple fractions of a second to get back to speed. This happens with every car and because each person has a different reaction time the amount of time it takes for this sequence to happen gets longer and longer.
Another reason could be based on how the roads are set up. For instance, if you are downtown you might hit a stretch with a light at every intersection. If someone is trying to turn left, but there is a lot of traffic they often have to wait until the light is red. This means the cars going the other way have to wait for them to go and the timings that engineers decided are wrong.
There are also a bunch of other reasons, but those two are probably the worst.
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u/MidnightAdventurer Jun 11 '22
To add to this, when there isn't too much traffic or when people aren't following too close, the car behind might change lane or slow only a little and be able to carry on pretty much as normal. When the change in speed is too extreme or the space between vehicles is too small then this will happen quite quickly.
It's possible to smooth it out again if a few vehicles in a row are able to keep good distance then each one is able to slow a bit less than the car in front.For intersections, most modern traffic lights run their own timings. There's a number of ways it can go wrong (like the phases getting shorter to try to give everyone a turn which wastes more time going to orange before the next green) but there also comes a point where no amount of timing can prevent a traffic jam - there are just too many people trying to get through so some of them have to wait.
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u/Cyberhwk Jun 11 '22
Reaction times. One person hits the breaks. Next one has to hit the breaks harder because they had to visually see brake lights and react. Since they brake harder, that means the 3rd guy REALLY has to put the breaks on. All the way down the line.
Then same with taking off. If a line of 20 cars are at a stop, each can't go until the car in front of them goes. That means even if someone goes in a single second after the car in front of them, the 20th car has to wait 19 seconds after the first car to finally move themselves.
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u/outerzenith Jun 11 '22
a visual explanation for OP
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u/8enevolent Jun 11 '22
That was a good help, thank you both. We really are "monkeys driving cars" aren't we?
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u/condor0067 Jun 11 '22
It's like an accordion. They've studied this, and like previous comment, people don't accelerate as fast as they stop. Hills and curves play a part in this too
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u/Toasty_93 Jun 11 '22
Check out this video: https://youtu.be/7wm-pZp_mi0
This is an experiment where a number of cars were asked to drive in a circle for a while. You can see how jams form on a small scale here, so imagine the same thing but on a motorway with thousands of cars.
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u/Truth-or-Peace Jun 11 '22
Pressure waves can travel through traffic in the same way that sound waves travel through the air: a car brakes, so the car behind it brakes, so the car behind it brakes, and so on.
Depending on traffic conditions, the clog can end up in a very different location from where the incident that caused it was, and/or persisting for a long time even after the incident that caused it is cleaned up.
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u/OwieMustDie Jun 11 '22
Driving too fast and too close.
If in a jam, keep your distance and keep rolling at an appropriate speed. You'll soon see that if everyone in that jam did the same thing...
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u/jimmyTheBlind Jun 11 '22
Traffic works like a rubber band. If everything is going well, the traffic stretches out and has just enough tension to keep its shape. If something happens to disrupt that flow, the rubber band snaps back and bunches up tight. It takes a long time to stretch back out into that right balance of taught, but not too taught.
A civil engineer friend did his thesis on this very topic.
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u/BurnOutBrighter6 Jun 11 '22
This gif shows an amazing example of the exact thing you're asking about.
In the interests of hitting the character limit, a brief explanation:
It happens when people are following to close (pretty much all the time on all highways everywhere). This creates a situation where if a car slows down, the car behind has to slow down a little more than them to not collide. And the 3rd car more still. This creates a compression wave like a slinky that moves opposite direction to the traffic.
Every time one car slows a little bit - to look at their phone, to rubberneck at an accident, to pick up a dropped French fry, to yell at their kid - it creates another one of those waves that rolls backwards, growing stronger as it goes.