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
In: 8
/u/Cdurgin with the top post has it, tbh. Not much more to it.
I can’t think of enough specifics to find the correct video or even one that’s close to it, but I was a pretty entertaining time-lapse of a real highway that showed this.
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So it was from a highway traffic cam, and started super early in the AM while it was still dark out. A “normal” amount of cars, no actual traffic, everything moving smoothly at full speed, reasonable following distance between cars, etc. Not very crowded but not dead either.
A deer runs out of nowhere and the first car has to slam their brakes. The deer didn’t move immediately so they came to a complete stop by the time the deer ran back off the road. The car resumes, but that was plenty of time for the next car to catch up and so they *also* had to slow down since the lead car was still speeding back up.
Due to that slow down, now the next car has to. And the next. And the next.
The time-lapse then starts cutting hours at a time, and by the mid afternoon it’s bumper to bumper traffic for *dozens* of miles. Helicopter coverage and everything, huuuuuge backup.
And riiiiiight around that same area where the deer was, everyone is still having to slow down then speed back up. Because there was never enough of a break in the traffic for the problem to correct itself before even more cars came.
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And that’s pretty much what happens when your in dumb traffic and then get to the slowdown spot and there’s fricking nothing there.
YT how does traffic occur.
I will not lie there are some really amazing videos that cleanly explain this better than people can type.
It’s the wave behaviour. Next time you’re driving in slow moving traffic, try leaving 10 car lengths ahead of you. It’s sometimes difficult because the cars in the adjacent lanes will try to cut in front of you. If the car ahead of you stops, take your foot off the accelerator. By the time you get to the car’s position, traffic will start moving, and you won’t have to hit the brake.
That’ll show you what happens when one person gets too close to another, and then brakes. It causes a chain reaction.
Think of traffic like a slinkie. All together, it’s moving forward. No problem, then boom, the front stops for whatever reason, and the rest pile into it. Well, the back end doesn’t notice it because it’s so far away it hasn’t come up to the problem. The front has dealt with it and moved on before the back has had a chance to even see it, but the middle is dealing with it currently. So, while that obstruction is gone, the “momentum” of traffic gets slowed.
Bad drivers. This includes people not knowing how to merge or let people merge and people riding their breaks because other people are wanting to go 40 mph more than posted limit 8n their 6 foot lifted truck…
Sometimes there are reasons but they aren’t so obvious at first…
– There’s the “cascade of braking” thing that a bunch of people have already described pretty well.
– There’s sun glare delays… driving west on a east/west road at sunset and people are slowing down because they can’t see very well and don’t understand that sunglasses exist.
– There’s bad road design, which you might not recognize unless you’re very familiar with the area. For example… there’s one spot near where I live where the traffic goes from five or six lanes of highway down to two in the space of about a half mile. There’s almost always traffic in that spot, but depending on where you’re coming from the reason might not be obvious.
Lots of great explanations for WHY traffic waves start and propagate backwards! The reason they don’t easily disappear is two-fold.
1. Constant steady flow of traffic. Person A causes person B to slow down, causing C to slow down, causing D, etc, etc. If every single person has to slow down because person A slows down, then the traffic wave will never go away. There has to eventually be a significant gap in traffic for some person (let’s say Z) where Z doesn’t have to slow down, or has to slow down *less.*
2. Acceleration and reaction time. Cars can slam on breaks faster than drivers are willing to accelerate.
When you combine these two problems, you get the following scenario.
A nice steady flow of constant traffic moving at 65 mph. Let’s label the guy in front Person 1, followed by person 2, etc, etc, down to person 9999.
Person 10 encounters a raccoon on the freeway and slams on their brakes to avoid killing it. This causes the initial traffic wave of people braking. By the time person 10 gets moving again, a large gap has formed between #9 and #10. If there is a large gap in space somewhere in this constant steady traffic, there will *always be a crunch of space* (a traffic jam) somewhere else. Imagine a necklace of beads, with every bead evenly spaced around it. If you spread two beads apart somewhere, you are squishing two other beads somewhere else.
A possible solution to a traffic wave like this is for person #10 to speed up (go faster than 65mph) to *catch up to where they are supposed to be. They are supposed to be right behind person #9.* Doing this will let person 11 catch up to where *they* are supposed to be (right behind person #10), and so on and so forth.
So you either have to have some significant decrease in cars on the road so that someone downwind of the traffic wave does *not* need to slow down,
or
The people affected by the slowdown need to *rapidly speed up to get back to where they are supposed to be*.
In Formula 1 motor racing they call this the concertina effect; you see it most often on the approach to a corner where cars are slowing down, so the car behind can more or less catch up but then the other car accelerates after the corner while this one is now slowing down as it approaches that corner, so ultimately the gap between them is restored.
On the highway what probably happened was some lane-hopping, causing cars behind to brake, the cars behind them did as well and, if traffic is busy, this caused a chain reaction of cars slowing, pretty much to a standstill as the backlog extended.
Because there’s no actual obstruction (a collision) once the car in front starts moving, the rest of you can as well.
primarily caused by following too closely. the driver of the first car sees a cop ahead, or a shredded tire in the road, or, if in florida, for no reason at all, taps the brakes…..driver behind them following too closely has to do the same….and this just cascades back.
Tailgating. Seriously, that’s all it is. People tailgate and don’t have the necessary stopping distance to not stomp on the brakes.
Also means that people can’t enter or exit the highway because there’s no way for the traffic to ‘absorb’ the small adjustments in speed.
Tailgaters are the absolute worst.
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