Exactly, it's entirely because the people in front of you are going slow.
this post was submitted on 28 Aug 2025
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Hey I studied this in grad school for a bit, and it really is just "someone does some dumb shit which leads to a cascading wave of additional people doing dumb shit which propagates backwards for miles." Basically when the offered load is getting close to the maximum load, all it takes is one person aggressively changing lanes to throw that section of highway into gridlock, and it will remain that way until the total integrated traffic flux across that incident boundary again falls below the critical offered load inflection point.
Basically, pick a lane and just stay in it. Maintain proper following distance. Counterintuitively, the following distance should be for the speed you want to drive, so even in traffic it should be like 5+ car lengths even though you are going slow. This is because it reduces the offered load, and once that number falls below the critical point, speeds will increase again. Bumper to bumper traffic basically prevents that from happening because it dampens the ability for a "speedup" wave to propagate.
Of course this is all impossible for humans. All it takes is a few idiots to throw off the balance.
"Pick a lane and stay in it" leads to slow drivers blocking the left lane, no?
Yep! All it takes is one person braking, and then the person behind braking, then the person behind them, and eith each braking the overall speed slows down more and more. It creates a wave of traffic. The wave passes through. The starting point I think moves back further and further.
I think about it a lot while I sit in traffic.
I think the issue is more or less slow drivers. One asshole is going 60 in a 70 in the left lane which caused people to pass them which in turn cause the cascade from the maneuvering around the slow person.
Slow drivers are far more dangerous than people don't 10 15 over the speed limit.
You are incorrect. You are the cause of the traffic jam lol, literally.
so even in traffic it should be like 5+ car lengths even though you are going slow.
Other drivers: "It's free real estate"
Secret is to play the game next to a semi. Some semis kinda do it too by engine braking as they see the wave approaching instead of waiting until theyre close to even slow
It all starts with someone in the passing lane, not passing, and one or more pissed off people behind them :)
The pissed off people trying to get around causes the wave of people behind them to brake and it snowballs from there.
Yeah I drive around 3 hours on the highway every several weeks. Sometimes on my drive, there's obviously traffic. A lot of times it will be something like rush hour traffic, a crash, construction, etc.
But then like...a good portion of the time when I come to the very front of the "clog", I find that it is just a blockade of multiple people going incredibly slowly and taking up all lanes of traffic, refusing to move over despite the fact that they are going under the speed limit.
People drive too close to each other, someone has to slow down and then the car behind slows down a bit more. Repeat until you get to the point someone completely stops. Then the next car stops for slightly longer.
If you leave a safe distance then it wouldn't happen.
Only ever have to ask, my friend
Heres an overview shot of a traffic pulse.
One person brakes for no reason which leads to everyone else braking. The pulse travels despite there being nothing there. The longer it can take for someone to start up again also can delay the whole thing.
It happens when people tailgate. They over react and it causes an accordion effect.
Or when people drive too aggressively and cut someone off, causing the person being cut off to slam on their brake.
Iirc, the answer is to have someone drive slowly and let other cars pass. It creates a buffer zone that regulates the flow back to normal pace. Or at least that's what I remember from New Scientist's video from like a decade ago.
I used to just idle when traffic moved. Slowed down way before i was even close to the car ahead. Played a game where i was trying to move at a constant speed or max fuel econ. Much less stressful to always be moving than gas/brake every 10s, even if you're moving 5mph.
Really helps to look 3-4 cars ahead for brake lights.
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You are thr traffic.
Most traffic jams actually act as a kind of compression wave moving backwards through traffic. Something as small as a squirrel running across the road can cascade into an hour-long jam.
One person brakes, then the person behind them, then the person behind them, but each time they are getting closer to each other (nobody stays equidistant from the car in front of them when braking). This causes a greater and greater slowdown as more cars are compacted into a tighter space, which travels backwards in traffic like a wave. Often the person who caused it doesn't even realize anything happened.
A lot of mapping software actually estimates a given traffic slowdown by treating traffic as a fluid with a wave moving backwards through it.
That's also why the best way to relieve traffic is to go at a slow even pace without braking. Every time the someone in heavy traffic runs up the ass of another car and brakes hard, or swerves into the "faster" lane and make someone else brake to not hit them, they cause another brake wave. If you have a few cars intentionally just hanging back and cruising with a big enough gap between them and the cars jocking for position in traffic in front of them, then their brake waves do not propogate behind you and eventually traffic just picks up pace again.
Edit: side bonus, you still get there just as fast, but with a lot less stress fighting assholes for position (minus the ones who fly past you thinking you're the asshole for not riding someone else's bumper)
Yeah, in theory it's great but every time I try it people just cut in front of me then slam on brakes causing me to have to brake then adjust then repeat ad nauseam. People suck.
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3 fucking seconds
The answer is a simple 3 second gap.
That's it, just 3-mississippi (or 3-onethousand) seconds behind the car in front of you and most of the avoidable jams go away.
If you do that, someone will move into the gap. If someone moves into the gap you can slow down to make another gap to them, but then someone else will drive into that gap. I don't know of any major city where you can maintain a 3 second gap during rush hour.
Even worse, if you ever brake to try to create a gap, you're likely to cause a traffic jam behind you.
Sure, if everybody did follow the suggestion and allowed a 3 second gap you wouldn't have traffic jams, but that's just not human nature, apparently.
Teach people to drive on the right lane unless they want to overtake somebody. Whoever overtakes you on the left won't drive into the gap because they also want to overtake whoever is driving in front of you.
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A few years ago, I was bitching and moaning about a jam, and my pal just said "you're not in traffic, you are traffic".
I know it's nothing more than a cheeky soundbite but just reframing it like that and knowing I'm part of the problem rather than the exception has made me a lot calmer on slow moving roads.
Plus it has encouraged me to either use public transport more, or just drive to a park-and-ride a mile or three out, and run the rest - facilities permitting of course.
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Reaction times of humans.
It's the people not zipper merging correctly. You have idiots entering that are not up to speed and you have idiots breaking for the idiots not up to speed.
On ramps should be required to have their lane not end abruptly which causes the panic. The on ram should continue for at least a 1/4 mile.
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It's usually a complex crowd effect created by many participants trying to maneuver among each other in slightly disperate ways.
In Portland OR, it really is because some dingbat slowed down to 20 MPH on the interstate for literally no fucking reason at all.
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https://www.mdpi.com/2076-3417/11/9/4278
TLDR: modeling traffic as a gas leads to fairly accurate predictions. If that doesnt mean anything to you, here's a decent visualization of how gasses move around in a system. In this analogy, each of the gas particles models a car on the road. https://youtu.be/Hr5Baj3lXFA
CGP grey did a video on it: https://www.youtube.com/watch?v=iHzzSao6ypE
Basically one car breaking too much will make the following brake even more and so on until one stops and there's a jam. There is no clear reason like a road blockage or an accident, just compounding slow down.
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