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How traffic actually works

jliszka.github.io

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Re: How traffic actually works

#91

Post by colanderman »

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...

There are highways in the US that do this with dividers. A specialized vehicle drives over them and shifts them one lane to the left/right twice each day.

Re: How traffic actually works

#92

Post by rdmcfee »

cwoods wrote: post #82
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!

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 j…

I think you're may be thinking of this as one mechanism instead of two separate mechanisms.

You are dealing with a queue as well as a through-rate at the merge-point. The through-rate with one lane can still be 1/2 of the through-rate with two lanes, but because you have a queue waiting to reach the merge-point you can end up waiting much longer. More spacious merging will not change this because of the principle stated in the first paragraph of the article.

Increasing the speed at the merge-point will not decrease the queue. It will only decrease the density of the queue but move it back further in traffic. Your time to cross the merge-point will be basically the same.

Re: How traffic actually works

#93

Post by susi22 »

joelangeway wrote: post #90
susi22 wrote: 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.

You're right. But in case of a traffic jam we operate at speeds that allow us to do this. Look, the Article's author actually mentions it too:

Quote:

"The difference between these is negligible at high speeds, but at a low enough speed, it becomes difficult to maintain a 2 second following distance from the front bumper of the car in front of you without impinging on the rear bumper of the car in front of you, especially if said car is more than 0 feet long. So under these circumstances the flow rate of the highway decreases below 1 car every 2 seconds — maybe to 1 car every 5 seconds. So now you have to wait 5 seconds for every car in front of you in line."

There you have it. We have plenty of space/time to play with in case we're in a traffic jam since we approach very very slow speeds.

Obviously I'm not saying you should stay 10ft behind somebody when going 80mph.

Re: How traffic actually works

#94

Post by peterwwillis »

Kluny wrote: post #70
DanBC wrote: post #43

Earlier quoted context omitted.

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

No - because you don't know what's going on in front of you, and you may need to stop suddenly.

If you want to prevent people from jumping the gap, speed up.

If you want to be safe from random edge cases (like someone in front of you slamming on the brakes), slow down.

Re: How traffic actually works

#95

Post by pertinhower »

As a matter of general policy, if you're going to declaim another article as wrong, you really should bring clearly stronger evidence—generally in the form of citations—to the table. Otherwise it's just "he said she said" as in this instance.

Re: How traffic actually works

#96

Post by DanBC »

peterwwillis wrote: post #94
Kluny wrote: post #70

Earlier quoted context omitted.

No - because you don't know what's going on in front of you, and you may need to stop suddenly.

If you want to prevent people from jumping the gap, speed up. If you want to be safe from random edge cases (like someone in front of you slamming on the brakes), slow down.

I'd leave at least a minimum safe distance between my vehicle and the one in front.

But Parent suggested that it's a good idea to leave more space in case someone jumps in. I'm not sure I understand that - if they jump in it's always dangerous because you don't know what the person behind you is going to do and you don't know when the jumper is going to jump in. Thus, you want to try to prevent them jumping. This is achieved not by giving them more room (which encourages jumping) but by leaving less gap (reducing jumping) but still leaving the safe minimum.

Re: How traffic actually works

#97

Post by sk5t »

GigabyteCoin wrote: post #80

The article claims that the maximum theoretical throughput for traffic per lane is 2,400 vehicles per hour. What if every vehicle is separated by 2 seconds of stopping time, but all vehicles are moving at the exact same speed of 200 Miles per hour? It would take one vehicle 0.005 hours to cover a mile (5,280 feet). Assuming each car had a length of 10 feet, it would take 0.005/528 = 0.0000095 hours for a vehicle to c…

Hmm? This shouldn't be surprising at all given the rather large assumption that one car passes a fixed point each two seconds. Go as fast as you like... an observer on the roadside must still wait two seconds for each car to rocket (or crawl) by. Imagine having an ethernet card that could only send one frame a second to a party on the other side of the globe: would you care if the additional network latency due to switches and the speed of light were 1msec or 200msec?

Where we may see great improvements in road throughput is in driverless cars that greatly reduce that gap.

Re: How traffic actually works

#98

Post by lisper »

samatman wrote: post #87
polynomial wrote: post #55

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.

> Ants have no momentum

And collisions between ants are not catastrophic.

Re: How traffic actually works

#99

Post by jere »

I'm having a very hard time accepting the claims of original article, "The Physics Behind Traffic Jams."

Recently, I've tried to perform a few basic hypermiling techniques, especially "timing a light": if I'm the first car to encounter a red light, I leave extra space and can sometimes be moving through the light at 20mph or more when it turns green instead of stopped. It seems analogous to leaving space in front of you during a traffic jam, so that you can exit the jam at a faster speed.

The problem with this is that other drivers will often take advantage of your behavior. They think the most appropriate thing to do is race up to the red light. So instead of profiting from the faster speeds out of the light, they instead cut you off and start the light cycle at a predictable 0mph. You haven't done anything except swap places with these jerks.

Re: How traffic actually works

#100

Post by gknoy »

DanBC wrote: post #96
peterwwillis wrote: post #94

Earlier quoted context omitted.

If you want to prevent people from jumping the gap, speed up. If you want to be safe from random edge cases (like someone in front of you slamming on the brakes), slow down.

I'd leave at least a minimum safe distance between my vehicle and the one in front. But Parent suggested that it's a good idea to leave more space in case someone jumps in. I'm not sure I understand that - if they jump in it's always dangerous because you don't know what the person behind you is going to do and you don't know when the jumper is going to jump in. Thus, you want to try to prevent them jumping. This is…

I prefer to be pessimistic about other drivers' ability to judge whether they have enough room to cut in front of me. I'd rather not try to go fast enough to "discourage", as there will inevitably be some person who either doesn't notice me, or misjudges my speed (via a side view mirror) and mistakenly thinks he can cut in front of me safely.

A lower speed increases the chances of this, but reduces the risk of a catastrophic result. ;)

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