
How traffic actually works - jliszka
http://jliszka.github.io/2013/10/01/how-traffic-actually-works.html
======
joosters
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!

~~~
analyst74
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.

~~~
jerf
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.

~~~
SilasX
>I'm not sure who actually thought that "smoothing the wave" did make you
actually go faster though,

Probably all the people that passed around the critiqued link and suggested
that it gives a way to improve traffic flow?

~~~
j-kidd
The gist I get from the link is that the act of "smoothing the wave" will not
benefit the one who performs the act, but it benefits the drivers behind the
lane. You can't fix the traffic jam in front of you, but you can do your part
to prevent one from forming behind you.

And that's why people pass around the link.

~~~
SilasX
Right: like I said -- and like the GGP (jerf) just denied them saying -- the
claim is that it will hasten the traffic flow. Individual vs group benefits is
beside the point.

------
ynniv
_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 effective than preventing people from merging in front of you.

 _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._

No, but if your line is twice as long it is twice as likely to cause
contention on the roads behind you. ie, if an exit road has a traffic line
twice as long, it is probably causing traffic on the non-exiting highway. So,
do everyone a favor and (a) don't tailgate (b) ignore people merging in front
of you (c) don't drive in rush hour, and (d) ignore traffic advice from people
who don't drive.

([http://tomvanderbilt.com/traffic/the-
book/](http://tomvanderbilt.com/traffic/the-book/) has actual good advice)

~~~
hnrandom
>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.

~~~
ynniv
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.

------
unoti
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 someone stuck in the right
lane is going to dart out in front of me or the driver in front of me, and
very likely that I'm going to need to make a very sudden stop. It's just
unsafe to cruise along at 75 mph next to hundreds of cars that are looking for
an opportunity to jump in front of me.

The prisoner's dilemma also applies to this. Behavior that is "optimal" for a
single individual in terms of getting to destination is not what's optimal for
the collective in terms of safety. And optimal behavior for getting to
destination is not the optimal behavior for safety or lower stress driving,
either.

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

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

~~~
peterwwillis
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.

~~~
DanBC
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.

~~~
peterwwillis
The stopping distance of my car (a Lexus IS300) at 70mph is 171 feet. So I
know I need over 14 car lengths in front of me for me not to hit anyone with
brakes at full power.

Do I actually leave that much space in front of me every time I drive 70mph?
Hell no. But the slower the cars beside me are going, the quicker i'm going to
hit one if it pulls in front of me. If they were going 70mph too, I would only
have to brake down to, say, 65mph, which would should only take a second. But
if they're going 10mph, I now have to come to an almost complete stop to
prevent from hitting them, which as you can see above is a lot of car lengths.

------
bryanlarsen
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.

~~~
daniel_levine
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 much more impactful. Unlike with servers,
you can't just spin up a new lane, and "packet loss" is pretty much
unacceptable :)

EDIT: For all you infra folks out there, imagine not being able to dynamically
provision resources, no packet loss and worse, the packets can basically do
what they want. That includes things like crashing into each other and
stopping any other packets from making it through.

~~~
jameshart
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...](http://i3.birminghammail.co.uk/incoming/article1294911.ece/ALTERNATES/s615/astonexpressway-1294911.jpg)

~~~
CodeCube
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

~~~
maxerickson
There are less terrifying, less dynamic alternatives:

[http://en.wikipedia.org/wiki/Barrier_transfer_machine](http://en.wikipedia.org/wiki/Barrier_transfer_machine)

~~~
CodeCube
Nice! yeah, this would be preferable. If there's a catastrophe in the opposite
lane, I'd really love it if it could stay over there while I'm going 70 miles
an hour in the opposite direction :P

------
jameshart
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 why UK motorways signpost 'advisory speed
limits' \- and some even have variable enforced speed limits - to manage road
capacity.

~~~
jliszka
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.

~~~
hamburglar
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.

------
durandal1
For a blog post that that claims to explain "How traffic actually works" I
find it disappointing that the author doesn't even briefly touch on the
dynamic properties of this problem.

No, this is not how traffic works, the model is much more complex and you're
missing very large dynamic effects.

------
petenixey
This is such interesting stuff. If you go just one step further and plot the
carrying capacity of a road against the speed of traffic on it you get a hump
which tends towards zero as speed reaches infinity.

The significance of that hump is that it represents a maximum carrying
capacity for a road - an optimum speed. If you do the numbers I believe it's
about 50mph. What I think is really cool is that when the road is completely
at-capacity, traffic is stable above 50mph (as cars can just slow down and
capacity increases) but collapses to a jam below below 50mph (because as the
traffic slows down to absorb new cars, capacity falls still further).

This is way why, when the M4 motorway funnels down to only two lanes as you
approach London, there is a 50mph speed limit - it literally allows the road
to carry more vehicles per hour. I also suspect that it's another benefit
(other than safety) of setting 50mph speed limits in contra-flows and during
roadworks - it allows the road to carry more vehicles despite having fewer
lanes.

------
johlindenbaum
I'll have to try and find the article, this was years ago, but there was a
study about people just tapping their brakes lightly that would cause the
person behind to do the same, a little longer, etc. etc. that would cause a
big traffic jam over time. The study calculated some figure that showed if
people just kept their distance up to the person in front, and instead of
lightly tapping the brake as a reaction to a merge, and just let the car idle
a bit to slowly decelerate, you could reduce traffic slows / jams by a lot.
This seems more reasonable than the solutions presented by this article, don't
let people merge.. really?

~~~
dvanduzer
I believe the article you seek is the one being critiqued by this author.

Currently I drive for about 15 miles on a major highway (during rush hour at
least once) on most days of the week. I rarely _need_ to use my brakes.

------
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.

~~~
sk5t
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.

~~~
dclowd9901
It'll be really interesting to see how driverless cars negotiate things like
natural merges (a freeway losing a lane), and how that affects traffic flow.
Ostensibly they'll be as efficient as possible, but if two lanes become one,
there's going to be some slowing regardless. Furthermore, in a vehicle that
also ostensibly wants to get you to your point of interest in as timely a
manner as possible, how does it decide whether to subject itself to staying in
a right lane where it might be subject to moreane merges, or does it ever
decide to go around a particularly bad or congested spot for the sake of
getting you to the destination?

------
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.

~~~
losvedir
Flying through a light that _just_ turned green for you is a good way to hit a
pedestrian or someone coming in from the left or right trying to catch the
tail-end of their yellow.

Please don't do this.

~~~
corin_
Surely most (all) lights leave a significant gap between one set of people
going red and the next going green. Of course, people noticing this gap may
think "well it only went red two seconds ago, so I have time" \- which would
mean not shooting off at the green makes you safer - but if people get used to
everyone else waiting 2 seconds after it turns green, then suddenly they have
even more time after their turn goes red to make it through...

------
susi22
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...](http://www.smartmotorist.com/traffic-and-safety-
guideline/traffic-jams.html)

(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.

~~~
teddyh
> 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.

~~~
susi22
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.

~~~
teddyh
The lamp is irrelevant. Look at the _cars_.

------
Lagged2Death
_Don’t let people merge in front of you, ever._

When you merge, you're merging in front of _somebody_ , so a rule like this,
universally followed, is effectively a ban on merging. How's that going to
work, exactly?

Travel time isn't the only consideration. Things like safety, fuel
consumption, and stress levels are important too.

------
methodin
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 there to make any empirical evidence
useful.

~~~
privong
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.

~~~
methodin
I get that we want it to be consistent but having driven the same roads for
years, in the same times of day, in varying but similar weather patterns the
driving is completely sporadic so no amount of normalization well give you
anything more than a crapshoot on any given day other than "it will probably
take me anywhere from 20 to 50 minutes." Statistical analysis is great for
many things but understanding the "now" of a traffic pattern is not one of
them.

~~~
privong
I think it depends on what one wants to model. For a general understanding of
how traffic flows, a statistical model is okay. On the other hand, if you want
to know why a specific traffic jam is happening and how it's affecting the
flow, a statistical model won't be sufficient.

------
jwallaceparker
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.

~~~
CWuestefeld
I blame The Eagles. By popularizing the meme of "fast lane" in preference to
that of "passing lane", they've badly damaged the laminar flow of highways,
and likely contributed significantly to road rage.

~~~
kens
It _is_ the fast lane in California. According to the California Driver
Handbook, the left lane is known as the "fast" lane (p32). They suggest if
there are three lanes to use the left lane to drive faster, pass, or turn
left. They also say "Do not drive slowly in the left (fast) lane." (p64)

[http://www.dmv.ca.gov/pubs/dl600.pdf](http://www.dmv.ca.gov/pubs/dl600.pdf)

~~~
dmckeon
The California fast-vs-passing lane pattern also emerges due to two things
that California requires that other US states tend not to require: semi-trucks
must go slower and keep right.

From the handbook cited above, or see also signage on many CA highways: _The
maximum speed limit on most California highways is 65 mph. You may drive 70
mph where posted. Unless otherwise posted, the maximum speed limit is 55 mph
on two-lane undivided highways_ and for vehicles towing trailers.

... _When you tow a vehicle or trailer, or drive a bus or three or more axle
truck, you must drive_ in the right hand lane _or in a lane specially marked
for slower vehicles._ [emphases added]

On divided highways of 2 or 3 lanes in each direction, such as I-5 in the
Central Valley, these requirements strongly separate traffic into 2 sets -
semi-trucks doing 55-ish in the right lane, and non-trucks doing 65-70-ish in
the "fast" lane. (okay, yes, 85-ish, except in Kern County).

The results are that 1) over-the-road truckers dislike driving in CA, and 2)
CA non-truck drivers quickly learn to avoid the right lane.

The other 49 US states tend to have more of a "keep right except to pass"
pattern, depending of course on other local variations, especially population
density, traffic volume, and similar.

Source: someone who drives (and tows) in 18+ states per year.

------
hammock
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 important factor here.

For the record, I am "that guy" who runs all the way down to the end of the
closed lane before merging over. Except I don't consider it an asshole move
because as OP points out, I'm making the best use of the road ahead (while
allowing fewer people to merge ahead of me)

~~~
tucaz
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.

~~~
cecilpl
What pisses me off is having to wait in a kilometer-long line of people
merging when what I really want to do is turn right down a side street.

Please keep that line as short as possible by using all the lanes available.

~~~
jessaustin
I thought we were talking about freeway/tollway/highway traffic? If there are
side streets, that's a different situation.

------
Evolved
I've tested this theory and others on my commute. Whether I speed up and never
let anyone in (nearly impossible since someone won't care and will merge
knowing you'll most likely not hit them) or keep a good 7-10 car distance, it
doesn't matter. My commute on the freeway with either of these two ways or on
side streets is always 45 minutes +/\- 1 minute.

The difference is I'm using less gas and putting less wear on my clutch,
brakes, etc. by leaving a big gap between myself and the cars in front of me.

Furthermore I find that my demeanor is much more calm and relaxed when I leave
a big gap because I'm not tensing up to avoid rear ending someone every time I
stop.

------
themodelplumber
Great. I really hope that like 100% of jerk drivers do not find out about
this. Never let people merge in front of you...geez man, really?

~~~
jliszka
That was a joke ;) Onramp regulators are the real answer, letting only 1 car
merge into traffic at a time at a rate well under 1 every 2 seconds.

~~~
isaacaggrey
> That was a joke ;)

I found your article interesting in light of the popular traffic piece you
mentioned, but ending it with jokey suggestions after a fairly serious
analysis doesn't really make it clear if you're joking or not.

But hey, maybe @themodelplumber and I are the only two who thought you were
serious.

~~~
jliszka
Yeah, I failed there. The whole article started out jokey but I ended up
cramming a bunch more "serious" analysis in.

------
acomms
"At the risk of being helpful, here are some things YOU can do that are
actually guaranteed to improve commute times for everyone: ... 2\. Don’t let
people merge in front of you, ever."

How will the commute times of those who sit there waiting to merge be
improved? If traffic flow was heavy enough they would never merge.

If they didn't pack a sandwich they'd starve!

------
confluence
Smooth flow doesn't get you there any faster than highly variant turbulent
flow. This is obvious. Now given that there will be little difference in
overall throughput would you rather a) have a relaxing smooth drive or b) a
stressful highly variant aggressive drive? The answer should be a, because the
increased of risk of a fatal crash vs. the miniscule return of a few car
lengths is not worth the effort you put in.

You can verify this yourself. Drive as smooth as possible. Smoothly accelerate
and decelerate. Leave large gaps. Try to use both your accelerator and brakes
as little as possible. Now observe aggressive drivers and memorize their car's
make, model and license plate. Observe the amount of risk and effort they
undertake to get ahead. Then observe them only getting about 5-10 car lengths
for their trouble. Finally observe them stopped behind you two traffic lights
down.

Laminar flow = turbulent flow, but laminar is so much better.

This is besides the fact that laminar flow reduces traffic waves, reducing the
variance for all those behind you, leading to a reduced risk of death and
injury for everyone in your lane. My driving style has probably saved a life,
or two.

------
szirka
I honestly believe it's the quick decrease in speed by drivers that causes
non-accident traffic jams during rush hour.

Don't hit the brakes before merging and when someone merges in front of you,
you should have kept enough space so you do not need to hit the brakes.
Really, I think traffic would move much smoother if drivers leave at minimum 2
car lengths of open space between the vehicles in front of them and only hit
the brakes on the highway when it is absolutely necessary.

If all the cars are put to a halt because of quick and hard braking cars ahead
of them, its going to cause a chain reaction that is way more time consuming
to drivers behind you than the author's "2 seconds slower to destination"
factor due to a merge in front of you.

------
exue
The purpose of making traffic flow smoother is that it'll push out the point
where flow breaks down and you can extend the zone of (relatively) uncongested
operation with higher vehicle throughput. Smoother flows leads to fewer abrupt
stops and rear-end accidents that make the wait even worse. (Cars in a
'beautiful line at 35mph' behave differently from stop-and-go and for instance
a fleet of self-driving cars would make the highest throughout, and extreme
example)

As for late merging, evidence shows that it's more efficient in congested
areas and by reducing the line, you help prevent spillover to other areas and
importantly accidents due to conflicts. I can see one super suboptimal
situation where a car completely stops to change lanes (into a stopped/slow
one) due to bad flow, resulting in more than just a 2 second impact on one
lane. MN signs say "use both lanes / take turns" and dynamically adjust based
on traffic conditions. In LA, the "zipper merge" seems to be standard for most
construction zones in traffic jams, with the only distinction being where you
merge in the last 200 feet

[http://www.dot.state.mn.us/trafficeng/workzone/doc/2004DLMS-...](http://www.dot.state.mn.us/trafficeng/workzone/doc/2004DLMS-
Evaluation.pdf) [http://www.dot.state.mn.us/trafficeng/workzone/doc/When-
late...](http://www.dot.state.mn.us/trafficeng/workzone/doc/When-latemerge-
zipper.pdf)

------
meatwad
Thanks for explaining this for us. What is missing from this compared to what
I see on the roads is trucks. The truckers always leave large gaps and it's a
sure bet that jerks will cut in front of these rigs. I don't think the trucks
are leaving that much space to "smooth out" the traffic but it's more a matter
of safety and efficiency (saves fuel) when all the nimble passenger vehicles
are stop and go and zipping all over the place. Truck drivers are so patient,
they have to be, especially in rush hour I've seen cars jump in front
constantly and the trucks keep making more space for them. The other half of
people seem to just follow at the same pace instead of cutting in front, and
every time some one does cut in front it slows them down another 2 seconds. So
my theory is aggressive drivers aren't much affected by trucks but easy
drivers are slowed down because they won't get in front... But if nobody gets
in front of the trucks then everyone makes it at the same time. Basically the
trucks are blockers for the aggressive drivers, so if you use them they'll get
you home ahead of others. I don't imagine any truckers are on HN?

~~~
timthorn
If ever I hit a jam on the motorway, I always pull to the left lane (UK), join
the trucks, and imitate their driving style. Almost always, over the length of
the jam, I beat my neighbours in the middle lane without doing any lane
hopping.

------
gqvijay
Hmm. Interesting. I have always contemplated this problem. First, I want to
simplify the problem a bit and hope it'll apply to traffic jams. I think the
problem is human reaction time. I must be missing something but hardly people
talk about this. (although my solution might not be feasible). Let me explain.

So, lets say you are stopped behind 30 cars and the first car is waiting for
the red to turn green. Now, as soon as it turns green, it takes a second (or
two) for the first car to start moving. After that second, second car adds
another second before starting to move. Based on this, you won't move your car
until 30 seconds have passed. Isn't this one of the big issue? If all of 50
cars instantly start driving once it hits green, wouldn't it reduce traffic
jam greatly. Now, I do realize this is not feasible but if computers were
controlling the car, 50th car can move instantly after green.

On a side note, I realize the bottleneck situation can't be solved easily BUT
again, I think human reaction (response) time is a huge part of it in most of
these scenarios. If computers were driving, I wonder if we can have 10 times
the car we have today without traffic jams.

------
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!

------
polynomial
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...](http://www.phschool.com/science/science_news/articles/ant_traffic.html)

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

~~~
lisper
> Ants have no momentum

And collisions between ants are not catastrophic.

------
mmoche
Isn't the real problem with traffic more perception than reality? I'd be much
more interested in how much additional stress/unpleasantness is caused by
stop-and-go traffic that would be alleviated with the concepts mentioned in
the "wrong" article.

------
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.

------
Osiris
Has anyone tried to use genetic algorithms with traffic simulations to try to
determine if there's a better way to manage traffic, especially in regards to
the design of merge lanes, on/off ramps, etc?

I've thought for a long time that it would be fascinating to build a complex
traffic simulator that could be used by people designing freeways, neighbors,
shopping malls, etc to plan for traffic. Right now those designs are just
based on best guesses (or terrible guesses) by architects and not simulations.

------
galaxyLogic
"... there’s not much benefit to trying to 'cancel out' a traffic wave by
leaving a ton of space in front of you."

What do you mean by a ton of space?. What do you mean by 'not much'?"

The real question we should try to answer is "What is the optimum distance all
cars should try to keep between them? And that can't be answered
qualitatively, only quantitatively.

------
DigitalJack
This article accounts for some of what causes traffic jams, but have a look at
this video. It shows a study done where people tried to drive on a circular
track at a constant speed.

Things start out okay, but it quickly devolves into the back propagating wave.

[http://www.youtube.com/watch?v=Suugn-p5C1M](http://www.youtube.com/watch?v=Suugn-p5C1M)

------
StavrosK
I don't understand the claim that front bumper to front bumper is a bigger
problem the faster you go. When you're going fast, the 4 meters of a car are
much shorter (in terms of time), because 4 meters at 4 m/s is a much smaller
amount of time than 4 meters at 0.2 m/s.

~~~
maxerickson
It says it is a bigger problem at low speeds.

~~~
StavrosK
...oh. Wow, major reading comprehension fail there. Thanks.

------
tlrobinson
I suspect that a critical mass of self-driving cars will improve traffic flow
for everyone.

I don't have a clue what the percentage to achieve critical mass is, but my
wild-ass guess would be 20-50%.

------
maplebed
My two favorite parts of this article: the author lives in New York and "Don’t
let people merge in front of you, ever." Now I understand New York drivers!

------
Evolved
Shockwave effect is a big contributor to traffic. Even moreso than
accidents/road construction.

------
titaniu
How to avoid traffic: work from home.

------
orthecreedence
"Don't let people merge in front of you, ever."

Californians have this one covered.

------
amalcon
Everyone has their own pet theory of how people should behave in traffic. The
problem isn't even that most of them are wrong -- most of them are "correct"
in the sense that, if everyone behaved in this way, traffic jams would be less
common and have shorter durations.

The problem is that you don't know which theories other drivers have, so you
don't know how they will react to different situations. If you don't know how
other drivers will react, you need to drive conservatively -- which usually
means slowing down.

Take a construction merge down to a single lane. It might be the case that
it's best to use both lanes up to the merge, so that the traffic jam takes up
less space. It might be the case that it's best to merge as soon as you figure
out what's going on, to provide more time for the merge negotiation and reduce
the risk of accidents at the merge point.

In reality, it doesn't matter either way. If I try to use both lanes, I don't
know that I'll be allowed to merge at the lane closure. If I merge early, I
don't know if someone is going to use the now-unoccupied lane to zoom up and
cut someone off, making both lanes slower and more accident-prone. I can't do
either one and confidently say that I've made the correct choice, because I
don't know how other drivers will react. What I do is just pick one, and slow
down when something unexpected happens.

This article also generalizes some things incorrectly (it seems based mainly
on German data, presumably because they have better infrastructure for
measuring such). For example, I live in the northeastern U.S. We are
notoriously bad drivers up here, even among other east coasters. Here, a two-
second following distance at 65MPH is almost unheard of -- actually, a two-
second front-to-rear distance probably qualifies as a "large space", as some
people here seem to think such a space can fit multiple vehicles. That's about
100ft or 30m, so I suppose it technically can, but it is not safe.

I can react to this either by leaving a very large space (5-6 seconds seems
sufficiently large that people get impatient and leave before it fills up) or
by leaving a small space (.75-1.25 seconds is the largest I can leave and have
a good chance of nobody trying to use it). The former is much safer, so that's
what I do.

The key is that, over a period of about two minutes, I need to maintain this
following distance relative to the same vehicle. Usually this is fine: at most
three cars tend to use that space at a time, and if anything they're more
likely to tailgate the vehicle I'm following than to stay back near me.
Occasionally I need to slow down in a situation where many vehicles fill that
space, or someone moves in and immediately slows down. This is the exception,
rather than the rule.

Yes, I know this behavior can cause traffic jams under certain conditions and
with certain other drivers -- but so could the alternative, under other
conditions. Given the choice between two bad options, I optimize for my
personal safety.

