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by susi22 4645 days ago
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.

6 comments

Having smoother traffic is good on many grounds: happier drivers, better fuel efficiency, and what not.

But it does not make people go home faster. Let's put a simple deductive thinking practice:

If you are the first car in the jam, you are stopped and slowed by traffic lights, police, accidents or whatever, but the traffic jam does not affect you, you are going as fast as you can.

For the second car, you will not get to your destination faster than the car you're following. So however you drive, you will not be faster than following with minimal distance.

The same applies to the third car, and on ward.

Basically, the fastest way to get through a traffic jam is to follow closely, and that means stop and go.

Thank you for that comment. You're 100% correct. However, it again shows that it's important to distinguish between theory and practice:

IF your model is correct, the traffic jam will clear since everybody will at some point reach their desired speed of approx ~80mph again. However, the reality is differnt. The model doesn't work since people break. That's the thing that makes your "deductive thinking practice" break down: People break. To prevent this....

Wait, let me just link this:

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

I think you're not considering the full problem.

You are describing a particular greedy algorithm wherein each individual car gets home as fast as they can. 1) Car #1 gets home as quick as they can by following closely. 2) Car #2 gets home as quick as they can by following closely, given #1. 3) Car #3 gets home as quick as they can by following closely, given #1+#2. ... Yes, given 1..N, the course of action which minimizes your own time is to follow closely. I agree. And people will probably act that way.

But that doesn't make it an optimal solution. Consider that car #1 introduces several seconds of delay due to braking and acceleration. Then car #2 does. Then car #3. By the end, you've got hours of delay built-in.

Optimal is obviously hard to define. But most people would agree that it's not every driver maximizing their personal interests to a globally shitty end. Everyone following closely is the prisoner's dilemma where both betray each other.

Consider the following alternate thought experiment. Flow of traffic is very much non-linear. That's what stop and go does -- it introduces cascading delay where there was none. Suppose roads are just 10-15% over capacity, and it results in a global average commute time of 1h instead of 45 minutes. This is not unrealistic. What if every day, 20% of drivers leave 1h later, thus preventing stop and go and keeping commutes to 45 minutes. Even if you counted their extra hour as commute time, the average is still less 1h.

I'm not saying this is a good idea (though strategies like this have been used in some cities). I'm just saying that are ways to get everyone home faster than everyone maximizing self-interest [co-operative self-driving cars please!].

Yes it does get people home faster if only because there is no latency on speeding up after breaking.
This. Smooth driving will make for a higher average / continuous speed, not to mention it'll cause less wear (brakes, clutch) and lower fuel consumption (accellerating costs fuel, braking is a waste of motion energy)
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.

Leaving space does not increase the average speed, because the average speed is dictated by the MAXIMUM speed in addition to the number of cars on the road.

--

First we must assume we live in the 3rd dimension of a physical realm, wherein objects like cars cannot pass through other objects. Cars can not drive through other cars, thus, if one car stops or slows down, all cars behind it stop or slow down.

Second, 5000 cars cannot travel 65mph over one mile of road, because it is physically impossible to fit 5000 cars inside a one lane 1-mile stretch of road. This is the bottleneck that causes the initial traffic jam.

In order to get all 5000 cars to travel the distance of a mile in the same TIME as 1 car, your only option is to increase their speed. But our speed is limited (by the speed limit of the road), so the only thing we can do is squish them together and hope for as fast a speed as possible given the dimensions.

If all the cars were linked and communicated in unison, they could react immediately and maintain a high degree of coordination in moving around each other and traveling as fast as the number of cars and speed limit will allow over a given area. Unfortunately, humans suck ass at driving, and their communication and latency make the movements of cars much much slower than is theoretically possible.

--

So what does "leaving space" actually do within our constraints of number of cars and max mph? It smooths out the errors humans make in driving. Specifically, it adds a time buffer for communication and decision-making; more time is given to driving decisions in order to prevent unnecessary stops, which would slow or stop the cars behind it.

What "leaving space" does NOT do is change the laws of our physical world. The given number of cars still have a maximum possible speed in a given distance, due to the traffic laws and physics.

Is it possible that the buffering from leaving space in the road will cause traffic to actually move faster than the starting-and-stopping of human error? Yes.

But it also uses up additional space, which is equivalent to more cars on the road, which as we know from above means a lower speed to travel the same distance.

--

You can not get away from it - until you reduce the number of cars or increase the max speed, your traffic jams will remain.

As for my personal opinion, I believe the one thing that could reduce traffic slowness is to speed up merges. If you want to help a traffic jam, merge at a speed faster than the cars around you and accelerate away from the merge area. It won't permit more cars on the road, but if a car is going 45mph and you move in front of it going 35mph, physics only allows for two results: the car behind you will hit you, or the car and all the cars behind it will have to slow down to 35mph.

The other thing that could be done is sensors in the road could detect the level of traffic (number of cars) and provide on a digital display the maximum speed humans could manage for that many cars on the road. If all cars then traveled at that speed, we would slowly but at a continuous pace travel down the highway without any stops. Until the first asshole that merges slower than that speed, which would make everyone slow down to his speed, defeating the whole thing....

Agreed 100%

I'm sorry if my post was confusing. I didn't really mean to go to infinity with my theory. I really only wanted to point out the flaws of the article. I didn't mean to say that traffic jams can be avoided. I simply disagree heavily with the author and agree with what's been out there in traffic theory for decades. Part of what you described well in your post!

Again, I'm not suggesting anything radical or want to emerge a new theory. The existing ones are good. I'm just trying to point out flaws from the OP.

There is a huge benefit related to your point about smoothing out human error:

People who pick the wrong lanes can change back immediately and travel the right way rather than either (a) slowing to a crawl with their indicator on hoping to be allowed into the correct lane by some kind soul (who also has to stop/slow down) or (b) looping back round on themselves and trying again.

Obviously, both of the latter solutions add to congestion, and the most common solution I see is an awkward combination of both (a) and (b).

EDIT: In addition, smoother-flowing traffic is desirable in-and-of itself, as fuel economy is greatly improved.

> You can see on the right side how his "cars per second" is _exactly the same_ than on the left side.

Is it my browser, or is this completely wrong? For me, the right side moves about double the amount of cars into the upper edge of the image. And I believe that this is the point of the section — it wouldn’t make any sense otherwise.

You are correct. And it's not just the images either, it's to illustrate the point that difficulty merging lowers the average flow-rate below that of "maximum lane occupancy".
I have to correct myself: Yes you're correct. I thought that the top left blinking arrow indicated that a car passed but that's very wrong. It's just displaying the road construction arrow and the rate or cars is indeed about twice as high.
There are considerably more cars on the Left GIF though. I guess if the number of cars in both lanes and in both GIFs was equal, the average speed of cars in the right GIF would be lower such that flow (throughput) would be the same.
Cars per second are the same. Open the two gifs next to each other. Each has 8 frames and the top right lamp is on for 4 and off for 4.
The lamp is irrelevant. Look at the cars.
> You can see on the right side how his "cars per second" is _exactly the same_ than on the left side.

Yes, but there are way more cars on the left side. That is the difference. Occupancy (density) determines speed when flow rate is fixed, and that graphic is actually a perfect illustration of this. You can't make the cars on the left travel at 80mph because it would not allow for a safe following distance.

One thing that is the same on both sides is that if you're the nth car back from the merge, it's going to take 2n seconds for you to get to the merge. Yes, the important thing is how many miles you can cover in that time, but this way of thinking places an upper limit on how many miles that can be, based on the occupancy of the road.

Also, pretty sure the advice at the end was tongue-in-cheek ;)

I'm sorry for the harsh critique. I think new radical ideas are great and people should question everything that's out there. But this way was not thought out well enough IMO.

I hope you keep it up and maybe have a follow up post where you can fix some of the flaws.

Some of your ideas are correct and noble. But always remember to take into account the real world driver and his/her behavioral flaws.

Few thoughts: Interesting would be a stochastic behavior model of drivers + driving strategies and in connection with red lights, accidents, rush hour to see what happens. Thought, that model would be quite some work in python.

Cheers

Really appreciate the feedback, comes with the territory :)

I added an update to the article looking at what happens when you vary the average car length (corresponding to the % of trucks on the road), fwiw.

> I added an update to the article looking at what happens when you vary the average car length (corresponding to the % of trucks on the road), fwiw.

That's silly, at most reasonable traffic flow-rates the difference in a couple of feet between a short car and a long car (esp with how uncommon the outliers are) is insignificant compared to the constant overhead of the per-car padding. And below reasonable traffic flow you're already at a high enough density to cause catastrophic traffic speeds.

I just read some of your older blog posts. They're close to my research and well written and very informative. Keep up the good work!

Cheers

Thanks! What's your research on?
Please allow me to unify the two theories, because both seem correct.

The free flow merge theory applies only in open areas with maybe temporary blocks that flow is unrestricted in any other way.

The no-merge theory applies on situations with traffic lights (and/or other special flow control such as a policeman or toll booths etc) where you suppose there are cut-off points that allow a specific number of vehicles per second.

I'm sorry. I didn't mean to lay out any "theory". I really just meant to criticize. I'd suggest to just stick with the theories that researchers have come up with and improve upon them.