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How traffic actually works

jliszka.github.io

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Re: How traffic actually works

#71

Post by hnrandom »

ynniv wrote: post #36

Since occupancy determines flow rate, there’s not much benefit to trying to “cancel out” a traffic wave This is so fundamentally mistaken as to make the rest of the analysis useless. Traffic waves account for the majority of highway traffic (excluding on and off ramps), and cause the flow rate for a road to be decimated. It should be obvious that avoiding rate decimation is (literally) an order of magnitude more effe…

>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.

If you simply kept reading:

>This is good because it is less likely to affect other traffic by spilling out onto onramps and surface roads. Also maybe there’s someone on the highway who’s planning to exit 3 miles before the bottleneck. If the backup is 2 miles instead of 4 miles, that person doesn’t have to wait in traffic.

Re: How traffic actually works

#72

Post by jff »

jwallaceparker wrote: post #53

I wish more drivers would follow the driving etiquette that advises one to "stay right except to pass." On every highway trip I see countless cars camped out in the left lane but moving at the same rate as the rest of traffic. One should only use the left lane for passing. Otherwise, stay right. Traffic flows smoothly when people follow this system.

It's evolved sufficiently here in California that I believe Californian drivers now prefer to pass on the right rather than the left, given a choice.

Re: How traffic actually works

#73

Post by privong »

methodin wrote: post #19

I thought the whole point of these "fixed traffic jam" articles was to make your undoubtedly terrible commute go faster by giving you something to do, or at the very least providing a false sense of control over a situation (like a superstition). The science behind traffic is completely arbitrary and nonsensical no matter how many graphs or numbers you put behind it. There are simply too many different drivers out th…

I think the idea is to give it a treatment analgous to how statistical mechanics works for properties of gas, for example. True, we can't easily say what individual atoms/molecules might do, but we can say something about the gas as an ensemble.

It's true that free will may make drivers tougher to predict than individual molecules. But molecules obey quantum mechanics and one could conceivibly construct probability distributions for how that particle might behave. In principle, probability distributions could also be constructed for the behavior of individual cars/drivers, so a statistical approximation for the behavior of the ensemble of drivers in a system is probably a reasonable aim.

Re: How traffic actually works

#74

Post by thrownaway2424 »

jwallaceparker wrote: post #53

I wish more drivers would follow the driving etiquette that advises one to "stay right except to pass." On every highway trip I see countless cars camped out in the left lane but moving at the same rate as the rest of traffic. One should only use the left lane for passing. Otherwise, stay right. Traffic flows smoothly when people follow this system.

In practice this cannot work. "Except to pass" allows for any nonzero differential speed. Someone will be doing 55MPH in the rightmost lane and someone will pass them at 55.1MPH, and someone will pass them in the left lane at 55.2MPH.

Re: How traffic actually works

#75

Post by msane »

leothekim wrote: post #40
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!

If you RTFA, he compares the results of the model to actual data collected by the Federal Highway Administration. Why use a complex model when a simple one suffices?

Not a fan of the use of "RTFA".

Re: How traffic actually works

#76

Post by admax88q »

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.

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.

Re: How traffic actually works

#77

Post by jliszka »

dvanduzer wrote: post #50
analyst74 wrote: post #45

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.

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.

OP here. That's a really good point. I just think the original article overreached in saying that zipper merging is a "simple cure" for traffic jams, without taking into account that the road might at or near capacity (and you hit capacity in a hurry when you lose a lane).

My article doesn't account for variance in flow rates in congested traffic, but variance in car length (% of trucks on the road) might explain it. I'm not convinced that merging behavior is the culprit; the original article only speculates that that's the case. I'm putting forward a reasonable explanation for why that isn't the explanation, and a basis for evaluating whether it might be — in particular, whether a zipper merge results in higher flow rates after the bottleneck.

Re: How traffic actually works

#78

Post by ynniv »

hnrandom wrote: post #71
ynniv wrote: post #36

Since occupancy determines flow rate, there’s not much benefit to trying to “cancel out” a traffic wave This is so fundamentally mistaken as to make the rest of the analysis useless. Traffic waves account for the majority of highway traffic (excluding on and off ramps), and cause the flow rate for a road to be decimated. It should be obvious that avoiding rate decimation is (literally) an order of magnitude more effe…

>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. If you simply kept reading: >This is good because it is less likely to affect other traffic by spilling out onto onramps and surface roads. Also maybe there’s someone on the highway who’s planning to exit 3 miles before the bottleneck. If the backup is 2 miles inste…

His point is a little different: that someone exiting the highway might exit before reaching the traffic if the line were shorter. My point is that a line on an exit ramp which reaches back onto the highway will cause additional traffic for everyone on the highway. Since his model doesn't take traffic-causing-traffic into account, he never reaches my conclusion.

Exit lines are particularly costly because adjacent lanes of traffic generally only sustain a 10MPH difference in speed. Once the right lane is stopped, a 4 lane highway has a maximum safe speed of 30MPH in the left hand lane. This reduces the flow capacity of the highway and may cause the left lane to lock up because left lane drivers get frustrated at slow speeds and are the more likely to tailgate and cause traffic waves.

His model also is only concerned with the self: "can I reach my goal any faster", and he reaches the mostly correct conclusion that he basically cannot (even though avoiding traffic waves by driving in the rightmost non-exit lane would help him significantly). He does not appear to value his contribution to the delay of the people behind him, which is where all the really interesting conclusions are found, including the reason why traffic wave busting is beneficial.

Re: How traffic actually works

#79

Post by jerf »

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.

I'm not sure who actually thought that "smoothing the wave" did make you actually go faster though, since a moment's thought shows that "going faster" would necessarily involve going through the bumper of the car in front of you. I think it's about a "laminar" traffic flow being a lot less annoying and dangerous to drive in, if you are going to go slow.

I think he fell down on his own argument when he claims that doing that causes a "big traffic jam" downstream of you; how can it, if you're only 10 or 20 seconds max further behind than you otherwise would be? You can only cause that much additional "traffic jam" in the back, and it may well be worth it to create a more laminar flow.

Re: How traffic actually works

#80

Post by GigabyteCoin »

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 cover it's own length at 200 mph.

2 seconds is the equivalent of 0.0006 hours. So each vehicle would take approximately 0.0006095 hours to cross any given point.

Providing a number of 1/0.0006095 = 1,640 cars per lane per hour according to my calculations.

I expected the number of cars per lane per hour to increase with speed, which is why I did the calculations, but apparently it does not!

If you could actually drive 200 mph on your way to or from work however, you would get there sooner than you typically do in traffic. It's an odd problem to think about.

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