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.

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.

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.

You are thr traffic.

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.

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.

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.

The people at the front are morons and probably in the wrong lane.

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

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.

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.