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How traffic actually works

jliszka.github.io

81–90 of 186 posts

Re: How traffic actually works

#81

Earlier quoted context omitted.

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.

It also won't make you go slower. And rolling at a constant low speed is much less frustrating than constant stop and go traffic. Especially for people driving standard. It's also more fuel efficient.

I've always tried to smooth the wave on the fuel efficiency argument. I figure that maintaining a constant low speed has got to use less gas and be easier on my car than all the stopping and starting.

Re: How traffic actually works

#82

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!

Agree 100%.

From the article:

>Suppose you’re on a 2-lane (each way) highway and one lane is closed up ahead due to construction. Now the flow rate of your lane is cut in half (or there are twice as many cars in line in front of you, depending on how you want to look at it).

What I observe is that the speed is reduced to one twentieth of the speed, not just half. This is because people are merging very slowly while jostling resulting in needless braking. If everyone could agree upon a proper zipper merge, the speed for everyone would go up many times.

Or imagine a traffic signal at the merge point that only lets one lane go through for a minute each. The overall flow rate would be much higher than what it is without such a signal.

Discounting this based on theoretical flow rate (as if removing one lane reduces real traffic flow by only 50%) shows that the author totally ignores real traffic scenarios completely at odds with the title of the article.

Re: How traffic actually works

#83
post #12
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…

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.

Using the available lane-age/real-estate to attempt negotiating a merge at speed and minimize the problems caused by inertia is a good thing. This works in at least two ways: 1) The backup is less likely to extend past an off-ramp as pointed out. 2) Human-nature. If everyone in the backed up lane were paying attention and trying to get off the road as soon as possible, this benefit wouldn't exist. However, some people use the opportunity afforded by the congestion to check their email or a map or adjust their hair/makeup or turn around to yell at the kids or just day-dream. When the car in front of them moves, it takes them several seconds to snap out of whatever is distracting them. In these cases, it's possible to slip in front of them without adding to the number of inert vehicles.

This is true of unstopped vehicles as well. Not everyone is comfortable driving at the maximum safe speed. In fact, where I live, fines are enforced on all state roads from 6 to 13 MPH below engineering guidelines. Even so, many people choose to travel even slower than allowed by graceless statute. While these patient souls approach the bottleneck, several of the swift may de-queue themselves to the benefit of everyone who would otherwise wait behind them.

Oftentimes, the road hasn't actually reached saturation--it's just a compound case of live-lock and if people use the full measure of available lanes (instead of stopping at the first sign that says "Road Work Five Miles Ahead"), they have a greater opportunity of resolving the contention without promoting a cascaded slow down.

Of course, all of this is an edge-case. As the OP alludes, when the road is actually saturated, the real solution involves either decreasing demand for or increasing the supply of throughput at the chokepoint. Mere bickering and maneuvering will not avail anyone.

Re: How traffic actually works

#84
post #32

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.

But if you can spread the jam over a longer area with higher speeds then you should be able to prevent any part of the jam hitting that catastrophic threshold.

Re: How traffic actually works

#85

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!

The initial point about rate of vehicles through a specific point per lane per hour couldn't be more simple and valid.

Re: How traffic actually works

#86
This article is so horribly incorrect and fails right at the very beginning with the wrong assumptions and a idea that isn't thought through. What the author forgot is to take by far the most important variable into account: SPEED

So let's quote the author:

"The important fact: there is a limit to the number of cars that can pass by a given point on the highway in a given amount of time, and that limit is one car every 2 seconds, per lane. So imagine you are in heavy traffic during rush hour. There are a certain number of cars in line in front of you. Let’s pick a point on the road to call the front of the line — say, the point at which you plan to exit the highway. The line gets shorter by one car every 2 seconds. If there are 1,000 cars in front of you, it’s going to take a minimum of 2,000 seconds for you to get to the front of the line. It doesn’t matter whether people are kind and let cars merge in front of them, zipper-style. It doesn’t matter how much stop-and-go there is. The simple fact is that it takes 2 seconds per car for you to get to the front of the line, and there are some cars in front of you that have to get there before you do."

Ok this is correct. BUT: I don't care if it takes me 2,000s if my exit is 44 miles away (80mph) but I do if my exit is 10miles away.

So you drive along at 80mph and there is cars around you everywhere. Do you care? No, you're covering distance. It's the same as if you were all by yourself on the highway. The distance between cars is absolutely irrelevant and so is the time between cars if you still go 80mph. Nobody would call it a traffic jam.

Next quote:

"Leaving space in front of your car for people who are trying to merge won’t solve anything."

Yes it absolutely does. And it again solves one very very important piece of a traffic jam: Average speed. Leaving space is crucial. It allows people to do two things:

- Accelerate

- Not having to brake

These two are crucial to get speed up and thus resolve traffic jams. Incidentally he linked an article which visualizes exactly this:

http://www.smartmotorist.com/traffic-and-safety-guideline/tr...

(Section: Merging-lane Traffic Jams, A Simple Cure)

You can see on the right side how his "cars per second" is _exactly the same_ than on the left side. Yet, everybody will be much happier on the right side. Why? Because the speed's been increased and people in the cars cover ground.

Next quote:

"Suppose you’re on a 2-lane (each way) highway and one lane is closed up ahead due to construction. Now the flow rate of your lane is cut in half (or there are twice as many cars in line in front of you, depending on how you want to look at it). Road signs commonly ask you to use both lanes up to the point of the bottleneck. That’s reasonable advice, but it’s not going to get anyone home faster"

Yes it absolutely does. Why? We are all programmers here so let's talk in our lingo: This is our API. Our interface that we agree on. If people follow the interface (rule), then we have people agree on it and we are predictable and prevent braking. This is exactly what we want. Increase speed (with gaps), prevent breaking, be predicable. No surprising cars merging means no breaking and thus increase in average speed which in turn means no more traffic jam.

Please, disregard this article. The conclusions are wrong, the assumptions are wrong and the deductions and the terrible advice he gives at the end is wrong.

Re: How traffic actually works

#87

Given that ants don't have this problem, won't driverless cars be able to adopt ant-like traffic protocols to eliminate congestion? cf. http://www.phschool.com/science/science_news/articles/ant_tr...

Ants have no momentum, to a first approximation. It's not at all clear their algorithm would scale.

Re: How traffic actually works

#88

Earlier quoted context omitted.

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.

Okay, when do you think people should start merging? Thirty feet before the "choke point"? One hundred feet? One mile? Three miles? It's an arbitrary game, and everyone makes up their own "rules of decency" and gets mad when someone follows the rules of the road rather than the rules of their head. Yes, people "know" about choke points, but that doesn't mean it's wrong (or rude) to use open lanes. So long as those la…

During "normal" periods where there are no choke points because there is no traffic these "smart" people do not have a problem staying in the "right" lane. However, when the traffic starts growing up almost everybody, except them, go into what looks like a queue, one car at a time, waiting for their turn to move forward/leave the road they start passing everybody on the "wrong" lane and merging at the end.

This is not only rude, but also blocks the other(s) lane(s) bringing traffic to where it wasn't before.

Its almost like say it's ok to pass in front of other people in the theater line.

Re: How traffic actually works

#89

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!

I think lots of people, on both "sides" of this debate, have predetermined notions and are trying really hard to justify them.

Re: How traffic actually works

#90
post #86

This article is so horribly incorrect and fails right at the very beginning with the wrong assumptions and a idea that isn't thought through. What the author forgot is to take by far the most important variable into account: SPEED So let's quote the author: "The important fact: there is a limit to the number of cars that can pass by a given point on the highway in a given amount of time, and that limit is one car eve…

If you keep the same distance between you and the car in front of you regardless of your speed - which you seem to say -, I hope you are never behind me.
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