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How traffic actually works

jliszka.github.io

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Re: How traffic actually works

#41

Post by CodeCube »

jameshart wrote: post #17
daniel_levine wrote: post #8

Earlier quoted context omitted.

This. I've studied traffic extensively and if you ever wonder how it works, it is actually quite similar to any other network. Imagine road traffic is the same as traffic through your servers. There are hard bottlenecks at various points, but in practice what matters is planning for variability. Real-life situations like someone cutting you off, people rubber-necking, the natural commute tendencies etc tend to be muc…

re spinning up a new lane: the A38-M Aston Expressway into Birmingham, UK dynamically multiplexes its lanes together into variable width channels depending on the required upstream/downstream bandwidth :) - http://i3.birminghammail.co.uk/incoming/article1294911.ece/A...

aww hell no. I'd be terrified of merging into that lane with no highway divider between me and oncoming traffic. Small two lane highways are bad enough! :P

Re: How traffic actually works

#42

Post by jbrooksuk »

jameshart wrote: post #17
daniel_levine wrote: post #8

Earlier quoted context omitted.

This. I've studied traffic extensively and if you ever wonder how it works, it is actually quite similar to any other network. Imagine road traffic is the same as traffic through your servers. There are hard bottlenecks at various points, but in practice what matters is planning for variability. Real-life situations like someone cutting you off, people rubber-necking, the natural commute tendencies etc tend to be muc…

re spinning up a new lane: the A38-M Aston Expressway into Birmingham, UK dynamically multiplexes its lanes together into variable width channels depending on the required upstream/downstream bandwidth :) - http://i3.birminghammail.co.uk/incoming/article1294911.ece/A...

I was watching Motorway Cops - or whatever it's called on BBC - just last night. There was a major crash, with a Mini and a Jaguar.

I didn't know that the direction of the Expressway changes, it's pretty cool, but has kind of put me off driving down it.

Re: How traffic actually works

#43

Post by DanBC »

unoti wrote: post #23

There are cases where leaving a greater-than-normal lane spacing makes sense, in the interest of safety. Safety is something utterly ignored in the fascinating analysis in this article. For example, if I'm in a left lane buzzing along at wide-open freeway speeds and the right lane next to me is congested and backed up and not moving, I leave more than normal spacing in front of me. Why? Because it's very likely that…

Shouldn't you leave less space, to dissuade people from jumping in the gap?

Re: How traffic actually works

#44

Post by dvanduzer »

libria wrote: post #29
hammock wrote: post #2

That was the clearest explanation of traffic I've come across. I'm inclined to buy into it (often it seems no one knows what they're talking about when it comes to traffic). It also runs counter to what is commonly said about traffic jams. Most people focus on restoring smooth flow by doing things such as driving really slow to create space in front of you. This guy makes the case that "flow" is not really the import…

Part of the problem is a lack of established convention. When a theme park opens its gates in the morning, we expect a large crowd to jostle in through a constricted entrance like a funnel. However, when we queue up at the bank, if the line extends past the cordon, nobody runs up to that point and stands shoulder to shoulder with other patrons; we maintain a single file. Which of these rules applies to cars merging?…

There is a protocol. Even in heavy traffic, if you observe the following distance that the DMV tells everyone to observe when taking a driving test there is always room to merge.

Re: How traffic actually works

#45

Post by analyst74 »

joosters wrote: post #33

What an arrogant article. How can you title something 'How traffic actually works' when you don't examine actual traffic. Instead, make up a simple model, run some code, claim it shows what you want. It's not like modelling traffic is so insanely hard that we need such simplistic models. Pick as complex a model as you like; we have the computing power to simulate it!

What author is trying to argue against is the idea of "smoothing-the-wave", which he does prove in his overly complicated article.

Yes, there is too much rambling for making a point, yes the tone of title is quite arrogant / link-baity.

But this terrible essay did prove the point: smoothing the wave is not going to make you go faster.

Re: How traffic actually works

#46

Post by chris_wot »

I'm wondering what if I'm missing something here:

"If anyone tries to tell you that if only drivers left space in front of them and took turns merging, traffic would flow smoothly, and it’s only because of jerks that there are any traffic jams at all, just ask them what’s going to happen at the next merge. Where is that extra space going to come from? You cannot keep 2 seconds back from the car that has just merged in front of you without, um, slowing down. If the car in front of you is also slowing down for the same reason, you have to slow down even more. This is basically the definition of a traffic jam."

However, if you have two lanes, with one lane blocked, then wouldn't not allowing for another car to safely merge into your lane prevent them from merging? Then you have one lane moving quite fast, but the other lane would be going more slowly...

Like I say, there may be something I'm missing here, I'm happy to be enlightened!

Re: How traffic actually works

#47

Post by tene »

bryanlarsen wrote: post #5

The central assumption for this argument is that the average time between cars is fairly consistent no matter the environment; small changes to this time do not matter. However, on a congested road very small changes can have huge effects. IIRC, one study showed that a 1% decrease in the number of vehicles resulted in a 28% decrease in commute time.

Did you read the whole article? That's explicitly discussed, even with graphs!

> At low occupancy (cars per mile), drivers can go as fast as they’d like. As occupancy increases, so does flow rate, even though speed decreases somewhat due to everyone trying to maintain following distance. At a certain point, when occupancy becomes high enough, speed dips low enough to where drivers are unable to maintain their minimum following distance, and — catastrophe! — the flow rate decreases dramatically.

Re: How traffic actually works

#48

Post by hamburglar »

jliszka wrote: post #32
jameshart wrote: post #6

The catastrophe theory model seems compatible with the anti-traffic solution to traffic 'standing waves' - by slowing down a little when approaching stationary traffic, allowing that stationary blockage time to 'evaporate', you avoid slowing down below the catastrophic threshold that puts your rear bumper more than 2 seconds behind your front bumper, keeping the road in the higher capacity regime. Same reasoning is w…

OP here, excellent point. Since so much is determined by occupancy (cars per km), I think the only way to switch domains is to lower the road's occupancy, which is exactly what you're doing when you leave extra space in front of you. However, I tried to point out that this probably comes at the expense of increased occupancy somewhere else, so you're just moving the jam around.

One thing your analysis leaves out is the human/psychological element. I've never bought into the idea of "erasing" the waves causing an overall throughput increase directly, but rather that it adds buffers so the traffic is a lot more predictable. The more people need to slam on their brakes, the more they deviate from the optimal-flow following distance, effectively wasting throughput. I guarantee you that when humans are involved, you'll get a lot better throughput from a line of nicely synched cars humming along at 25mph with a 2 second gap than the same cars constantly speeding up and slowing down, but maintaining an average speed of 25mph and attempting to maintain a 2 second gap despite wildly varying traffic conditions.

Re: How traffic actually works

#49

Post by dvanduzer »

HelloMcFly wrote: post #34
tucaz wrote: post #12

Earlier quoted context omitted.

This is the one thing that really pisses me off. It gets worse when the lane that you are merging into has a line of cars going back almost a kilometer. Doing this is like saying "Hey you idiots, look how smart I am" to everyone waiting for their way in the right lane.

I don't know why people get so mad about this. You use the term "right lane", but what does that mean? It only means what you want it to mean in a given situation. The lane is open until it is not, and as long as both lanes are open they are both "right" (in the non-directional sense, obviously). Why rely on clearly nebulous (sometimes variable, sometimes non-existent) social norms rather than concrete traffic norms?…

There are known choke points where traffic is forced to merge. There is the occasional tourist, but rush hour happens every weekday. People know about these merge points, see traffic backing up in all lanes, and will still jump into the lane about to end trying to rush around a few extra cars.

It's rude, and it demonstrably increases traffic.

Re: How traffic actually works

#50

Post by dvanduzer »

analyst74 wrote: post #45
joosters wrote: post #33

What an arrogant article. How can you title something 'How traffic actually works' when you don't examine actual traffic. Instead, make up a simple model, run some code, claim it shows what you want. It's not like modelling traffic is so insanely hard that we need such simplistic models. Pick as complex a model as you like; we have the computing power to simulate it!

What author is trying to argue against is the idea of "smoothing-the-wave", which he does prove in his overly complicated article. Yes, there is too much rambling for making a point, yes the tone of title is quite arrogant / link-baity. But this terrible essay did prove the point: smoothing the wave is not going to make you go faster.

He utterly fails to make that point. His simple model explains the idea of maximum throughput / congestion.

It utterly fails to explain the extreme variance in real traffic throughput during congestion. The original article does.

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