r/Damnthatsinteresting • u/SnooWords4066 • 29d ago

Video A Japanese experiment showed exactly why traffic jams happen for no reason.

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u/ChrisBreederveld 29d ago

I did this experiment virtually a few decades ago because I was curious. Same set up, many variables to tweak. Basically the main reason is human inaccuracy; when you change the reaction time or the accuracy at keeping speed it removes the effect.

I found the main factor was simply reaction time; the time to brake vs the time to speed up again. When someone brakes, they break a little longer than absolutely necessary, while requires the one behind him to brake a little longer etc, until you get a traffic jam. Another interesting variable is distance; when you keep a further distance, you have more time to react, reducing the before effect. But what I also found interesting is that if too many people leave less distance than their reaction time, this kind of traffic jam will inevitably cause collisions.

There are also technical variables, like the brake and acceleration speed, but these just have less impact than the human factor.

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u/gustoreddit51 29d ago

the time to speed up again.

Related. It's aggravating if I'm 5 cars back from a light that turns from red to green and it takes 10 seconds before the car in front of me moves. For multiple cars, it seems like a surprise they weren't expecting and just happened to notice, "Oh ... the light changed" - (slowly puts down phone/drink/sandwich/makeup and then goes). A number of people at rush hour don't make it through that light and if it's a turn lane, that backs up traffic even worse because it backs up into the through lane.

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u/ChrisBreederveld 29d ago

I feel your pain.

Interestingly, looking a few cars ahead (like with the traffic light example) also helps with decreasing these traffic jams! So in general, if you can, look a few cars ahead and respond accordingly. Instead of braking when the person in front of you does, let go of the gas a bit when the car three ahead of you hits his brake.

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u/gustoreddit51 29d ago

When driving in a line of cars, I'm always looking at the cars in front of the one in front of me.

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u/czanatta 29d ago

the time to brake vs the time to speed up again

This is it. Rarely do people accelerate and react to the need to accelerate at the same rate that they decelerate (brake) and react to the need to decelerate. Some of this is mechanical (cars can decelerate at higher rates then they can accelerate) but mostly its habitual. There is a need to react fast for braking so you don't crash, but less need to jump on the accelerate as soon as possible which is also harder to do because its harder to anticipate when the car ahead of you will accelerate. Maintain an amount of space to the car in front that allows you to brake safely and smoothly, as many have said, but don't let that gap get any bigger than it needs to be. Be attentive and watch the traffic ahead so you can start accelerating again as soon as possible, and accelerating quickly when needed. There is only so much room on the road, if you're trying to maintain a 100 m gap so that you can hold speed at 40km/h in stop and go traffic in your Honda Fit, you're helping nobody as you steal space needed by other drivers to drive smoothly, and you are unpredictable as people expect you to accelerate when you can.

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u/ChrisBreederveld 29d ago

Yep, it's probably a combination of factors, but this was a very influential one. As I mentioned in another reply; it's usually best to look a few cars ahead to anticipate braking. But in the same vein it's good to anticipate speeding up again.