Ok let me try and make this make sense; when driving on the motorway/highway, why does traffic come to a slow or stop at some points and then speed up (with no obstructions/traffic lights etc) and flow freely with no change in the road or conditions?
If it’s clear up ahead, why do cars slow down or stop when nothing is blocking them in the distance? It’s like it slows down for no reason then just regains speed and traffic flows fine. Drives me insane
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Traffic can be modeled as compression waves (like sound in air) in a fluid.
What does that mean? Lets imagine a line of traffic 2km long, with an average distance between each vehicle of around 1-2 meters. Lets say the car at the front slams on its brakes for some reason, any reason. Moments later they accelerate again and continue. This is like making a noise in the metaphor above, what happens when you make a noise? The noise travels through a medium, like a wave in water.
The car behind the lead car now has to slam on its brakes, or hit the car ahead, and so on and so on down the line. In reality it’s not usually just one incident, maybe there’s an accident and a series of drivers slow down or stop near it. Maybe there’s a traffic signal ahead that causes these “waves” of slowdowns or stops.
But the bottom line is that once you initiate that wave, it travels back through the traffic and you get period slowdowns/stops, and resumptions of travel.
It doesn’t slow down for no reason. Somewhere up ahead someone slowed down,hit the brakes a little, or something similar. Thay has the effect of having the cars behind it do the same thing. It doesn’t are long for the entirety of traffic to follow suit and everyone ends up slowing down. I believe it’s covered in traffic flow theory.
I’m driving, a guy cuts in front of me, so I hit the breaks. The guy following too close behind me has to hit the brakes harder to avoid hitting me. The guy behind him sees the slow down and cuts left to avoid it. The guy behind him that he just cut off hits the brakes. Rinse and repeat.
It’s exactly why roads can’t have more than three lanes without actually decreasing the amount of traffic that can go down it per hour
There’s a delicate balance between how fast cars can go on the highway and how densely packed they can be. People naturally want some buffer room ahead of them, and the amount of buffer room increases with speed. As you take away buffer room, cars slow down.
But it isn’t exactly a linear relationship. At a certain density, the potential speed will absolutely plummet.
If a highway is near this density, then something as simple as a light changing color and 15 cars coming on the highway all at once can be enough to shock the system and make everyone slam their brakes. Everyone slamming their brakes then increases the density behind them, causing more people to brake, and so on and so on.
When you see traffic squeeze down like that, that’s typically from a shock to the amount of people on the highway at once.
Here’s a video with some more information: https://www.youtube.com/watch?v=30uzZRSVxXQ
CGP Grey did really nice ELI5 ish video regarding this.
There’s a phenomenon in traffic flow, present even when the road is clear called concertina, where drivers continually over and under adjust to the distance they perceive themselves to be from the vehicle in front. Add to this the distraction provided by GPS, passenger interaction, driver fatigue and so on and you end up with basically a body of traffic that is in a continual act of over or under reaction. Then add to this the frequent merge lanes and exits and you are now in the situation where you are stopped for no good reason and somebody better be bloody dead up there, but they aren’t. The only thing that died today was your hope of a quick dart down the 401 to that boat place that’s been on TV.
So it’s basically people behind others having anti slow down a little bit more than the person in front of them. Magnify that by dozens or hundreds of cars and the speed hits zero. [check out this video for a visual](https://youtu.be/Suugn-p5C1M?si=s8LexZK5TmPW3KC7)
everyone slows down fast, but afterward everyone speeds back up slowly to put distance between them and the car ahead.
I’ve noticed that these things happen on hills, curves, merging traffic, police car parked on side, etc.
Myth Busters did an episode about this phenomenon. All it took was one person to hit the break pedal for one second to cause a snowball effect and eventually caused the cars behind them to come to a stop
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