
What Are Traffic Waves and Why Do They Happen So Much? - dhatch387
http://ww2.kqed.org/lowdown/2013/11/12/traffic-waves/
======
mixmastamyk
This reminds me of a piece on traffic I read quite a few years ago and might
have been on reddit or even perhaps slashdot. I still follow its advice to
this day when having to brave the 101.

It discussed traffic waves and our reactions to them. The repeatable waves
this article mentions come from (most of) the individual drivers driving
faster than the capacity of the road to carry them all. So they bunch up at
some point, catching those in front, everyone slows down for a bit and then
the bunch "evaporates" from the front... repeat. Those who get out of the
slowdown speed to the next one. It also discussed the effects of merging,
which have a high cost as lanes get fuller, also why you'll see lights at on-
ramps to mitigate.

The recommendation was to slow down, leave plenty of space ahead for others to
merge, and try to get going at as close to a single speed as possible to
conserve momentum, gas, wear & tear, nerves, etc. If packed enough you can let
the car idle push you along around 5mph/8kph, which is what I aim for in that
situation. Feels better than speed-up/break/slow-down over and over.

It also responded to questions such as, "what if everyone around me wants me
to go faster?" It happens, but those people tend to drive off in a huff. Once
one or two of them do, you'll find a more relaxed driver behind you for the
rest of the trip. The impatient driver won't get ahead much anyway so you can
wink at them when you catch up at the next slowdown.

~~~
ninkendo
How can you be sure the cars behind you aren't even more compressed than they
otherwise would be if you're leaving so much space in front of you like that?
One thing I hated about that article was that the author assumed he was doing
such a great service for everyone by "smoothing out" the traffic waves by
leaving a buffer, but he never really proved it; he just waved his hands and
said that it looked like the cars behind him were doing fine. (How can you see
more than one or two cars behind you? Are you driving in a literal ivory
tower?)

In reality it could be just the opposite, someone behind you could be blindly
accelerating because he sees the cars next to him doing so, then realizes you
in front of him aren't accelerating along with them and has to tap his brakes,
causing more traffic waves. And by definition the traffic behind you is more
compressed than it otherwise would be, because you're hogging tons of lead
space all for yourself. So you're dampening the waves in front of you by
making the waves that develop behind you even worse!

Given that the minimum amount of lead time drivers are comfortable with is
constant, if you pour tons of cars into a highway at rush hour, traffic
necessarily must slow down because there's too many cars per mile of road to
accommodate a safe lead time for each car at high speed.

So if everyone tried to smooth out traffic waves by leaving extra space in
front of them, they'd effectively be making the number of cars that will fit
in a mile of road even lower by artificially requiring more space for
themselves. This will always make traffic become slower overall. Say what you
want about the benefits of not having to stop-and-go traffic but you shouldn't
be under any illusion that it makes the commute faster overall.

~~~
morley
Whenever I see an article about traffic patterns, I inevitably see this
exchange:

Person 1: You can fix traffic waves by smoothing!

Person 2: Smoothing traffic waves makes no sense!

It's frustrating. I don't know what to believe. Is there any peer-reviewed,
simulation- and data-backed research that puts this issue to bed for good?

~~~
ninkendo
I think the sad truth is that it doesn't matter how you drive, traffic is
going to suck.

Given that everyone requires a safe lead time of (for example) 1-2 seconds,
the more cars there are per mile of highway, the slower everyone must drive.
(You can't drive 80 miles per hour bumper-to-bumper.) So if density is say,
50% (one carlength of open space per one car), you have to drive a speed such
that a carlength is 1-2s, in other words 10-20mph.

But traffic distribution is not uniform, there's exits and entrances, and cars
do occasionally need to change lanes. If traffic is going 10-20mph with 1
carlength of space between cars (steady state), and I change lanes, the guy I
merged in front of now has to slow down more to leave more room, and this
_will_ cause a traffic wave behind him. What happens at an exit when half the
cars change lanes? Standstill. No change in driving technique on anyone's part
will help this.

I think the only times where the way you drive matters is when the density is
kinda sorta high but still low enough for a safe following distance at
reasonable speeds, at which point "smoothing out waves" becomes a common sense
matter of "don't follow so close", which is effectively the same thing, and
something everyone should be doing anyway when the density is low.

~~~
btilly
You are thinking about traffic in terms of cars per foot. The right way to
think about it is in terms of cars passing a spot per second.

A lot of traffic problems stem from the fact that you get _more_ cars per
second from quickly moving smooth traffic than from slowly moving dense
traffic. Therefore if something bad happens to rapid smooth traffic, you
quickly get a phase transition into horrible traffic.

Now what does the transition from horrible traffic back to good traffic look
like? The first step is that you have to get back to smooth traffic, and then
that has to speed up.

If nobody smooths out traffic deliberately, then you don't get this until the
volume of people wanting to pass gets so light that all of the stop spots
"evaporate" on their own because there is no pressure on them. Then the road
speeds up. By contrast if traffic has been smoothed, the bottom speed steadily
increases, and you get back to full speed much earlier.

Plus smoothing traffic will save on your own car's wear and tear.

Oh, and on merging? Go read
[http://ops.fhwa.dot.gov/wz/workshops/accessible/McCoy.htm](http://ops.fhwa.dot.gov/wz/workshops/accessible/McCoy.htm).
You will find that in the real world when drivers are instructed to merge late
and politiely, merging happens significantly faster, with far fewer conflicts
between drivers. So how you merge actually does matter. A lot.

------
knodi123
Does anyone else remember the scene in Mission Impossible 3, where tom cruise
is hosting a dinner party full of guests who only know his fake cover identity
as a statistician working on traffic waves for the department of
transportation?

The movie portrays it as conversational kryptonite, where he briefly discusses
traffic issues in order to weaponize boredom and get some people to leave him
alone.

That part broke my suspension of disbelief, because I was like "Hey, why is
everyone wandering away?!? I want to hear what Tom has to say about how to
address the problem!"

~~~
irl_zebra
I remember this, and actually thought the same thing. It led me to do some
research and I came across a pretty interesting page on the topic of smoothing
out waves of traffic. It's an old site and I was just able to re-find it just
now, but perhaps you will find it interesting:

[http://trafficwaves.org/trafexp.html](http://trafficwaves.org/trafexp.html)

------
dismal2
If you're in the left lane and going slower than the guy behind you, pull to
the right. I wish people in America would be taught this, I'm amazed at how
often light traffic I encounter is from a few people meandering in the left
most lanes and then everyone behind just matching their speed. Obviously does
nothing about major traffic but still annoying. Besides that, people need to
stop thinking of driving as a competition and let others change lanes and
merge. I'd like to think those 2 things would help the situation a bit.

~~~
greggyb
In my driving experience in the US:

If a highway has two lanes, the average speed in the left lane is higher than
the average speed in the right, but the only major passing opportunities are
on the right when there's an empty stretch (because no one merges right).

If a highway has >2 lanes, the left is never consistently the fastest moving
lane, and almost every passing opportunity comes from moving right. It is not
uncommon to find the right lane near empty, and it is then possible to blow by
everyone in that lane.

The exception to the above is in areas with heavy merging, wherein the average
American driver seems to not understand how to merge (either blocking one
another, or coming to a stop as the merge lane begins). In this case, speed
seems to increase moving left.

~~~
pzxc
California is the worst about this and if that's where you live may be giving
you a false impression of the US as a whole. It's probably because in
California, 18-wheelers are required to stick to the right lane and have a
lower speed limit (55 instead of 65). That's not true in most other states.

I, too, have noticed that people here tend to just pick a lane (usually the
left one to avoid trucks) and stick to it no matter the speed they're going.
However, my experience in the other two states I've lived in (Florida,
Indiana) and the other 35+ states I've visited, is that most people in the US
know that the left lane is for passing and are somewhat more considerate about
this.

Drivers have trouble merging in all 50 states, though. Half the people merge
as soon as possible, thinking it's more polite to merge sooner rather than
later (or just not wanting to fight for an opening or get angry looks from
people who believe merging early is better), and half the people merge as late
as possible, attempting to do the zipper merge which is actually the most
efficient means of merging and what everyone should do.

~~~
dragonwriter
> It's probably because in California, 18-wheelers are required to stick to
> the right lane and have a lower speed limit (55 instead of 65).

Autos with trailers and trucks (vehicles subject to the 55mph limit where
other vehicles have a 65 or 70mph limit) are _not_ generally restricted to the
right lane in California.

They only are restricted to the rightmost lane on roads where: (1) where there
is no other designated (by sign) set of lanes for such vehicles, _and_ (2) The
road is not a divided highway with four or more lanes in the direction of
travel (where the default, without a designation, is that they are restricted
to the two rightmost lanes for normal traffic, plus the next lane out for
passing.)

~~~
pzxc
Okay, but the vast majority of roads do not have 4+ lanes in one direction,
and the vast majority of roads do not have a sign indicating a different lane
designation.

~~~
dragonwriter
> Okay, but the vast majority of roads do not have 4+ lanes in one direction

On a road with 1 lane per direction, trucks being restricted to the right lane
except for passing has no negative impact on your preferred practice regarding
use of the left lane in multilane roads.

On a road with 2 lanes per direction, trucks being restricted to the right
lane except for passing exactly matches your preferred practice, though the
lower truck speed limit (independent of any lane restrictions) might arguably
encourage other drivers to stick in the far left lane (though, except in areas
with high density of trucks, doesn't really provide any strong incentive not
to follow the use the right lane for traffic, except for passing, it just
increases the likelihood that non-truck traffic will want to pass trucks,
which is pretty high in any case.)

The lane discipline which doesn't apply on roads with 4+ lanes in one
direction thus is really even potentially even a factor in what you are
concerned with on roads with exactly 3 lanes per direction.

------
sinelaw_
Nice visualization! I made something similar a while back (refresh the page if
it doesn't start properly):
[http://sinelaw.github.io/jammed/src/](http://sinelaw.github.io/jammed/src/)

It includes drivers with different personalities (buses accelerate slower,
some drivers are daydreaming, etc.) and lane changing (people change lanes
when it's beneficial)

~~~
dEnigma
Very nice, but since there is no transition the lane changing looks a bit too
jerky so that at first I thought it was broken.

~~~
sinelaw_
Right - that needs to be fixed. Besides the graphical issue, the lane changing
is instantaneous, which is not realistic.

------
kokey
I find this happens a lot more in some countries than others, and it's not
down to levels of traffic on the road, it's down to driving habits. I think
the UK has done well with driver education and variable speed limits to
mitigate this, and zipper merging is the norm.

~~~
cones688
I think has a lot to do with manual transmission which allows for natural and
controlled engine breaking, thus not creating the brake light to come on (and
therefore the wave of copy-cat type braking).

~~~
kokey
The worst I've seen has been in Johannesburg in South Africa, where automatic
transmission is rare. I used to refer to it as concertina traffic before I
knew it was called a traffic wave, and the concertina is a popular instrument
in South Africa amongst the ancestors of the people who are usually
responsible for these waves.

~~~
Leszek
> and the concertina is a popular instrument in South Africa amongst the
> ancestors of the people who are usually responsible for these waves

I'm not sure what you're trying to imply there, but it seems a bit racist? Or
am I just being ignorant?

------
orik
Whenever the topic of self driving cars comes up in conversation I let people
know I think self driving cars are going to solve a lot of our traffic woes.

I'm not sure if a model exists yet for cars 'putting out' traffic waves, but
I'm sure once we have self driving cars and they start talking to each other
this is going to be of great interest.

I'm hoping we can effectively 'micro' away some degree of congestion.

~~~
stormbrew
I think people overstate the degree to which self-driving cars will actively
'talk' to each other. And I actually think it's somewhat detrimental to
advocating for self-driving cars, because the immediate thought is that
communication acting as a channel for malware. And that's a fairly justified
fear.

In the end, I think most of the communication will just be in terms of the
fact that self-driving cars interacting with other self-driving cars will be
much smoother and more normal than existing traffic.

Look at flocking birds. They're not talking to each other, really. They're
reacting in a complex but uniform way to their peers. I suspect self-driving
cars will look like that even without active communication.

~~~
jacquesm
And that's assuming error free communications, near latency free
communications and non-congestion as well as perfect hardware.

Is 'no signal' the same as 'no other cars' or 'broken radio(s)'?

Self driving cars have all the elements of a distributed computing system only
now it's moving at a pretty good clip and lives are at stake. 'Move fast and
break something' is very bad advice and 99.999% uptime is _not_ going to cut
it.

Hard enough to get autonomous (pun not intended) systems right, much harder to
make systems that communicate between vehicles right. The best way would be to
_not_ rely on inter-vehicle communication but to only work with the signals
that the car can pick up from the environment.

At least that data will be somewhat trustworthy because you have some idea
about the integrity of the system.

~~~
SomeStupidPoint
I'm not sure what either you or the person above you adds to the topic of
traffic congestion.

You certainly wouldn't depend on communication for collision avoidance, but
it's still likely that cars will have beacons which alert other cars to their
existence, and it has nothing to do with -- at the range of kilometers --
taking turns broadcasting how many cars you see around you (immediately, ie,
traffic packing), your velocity, and your location.

The other self-driving cars can listen for beacons about congestion, and
adjust strategic parameters (such as speed) in a safe manner. While I wouldn't
trust a random driver telling me to break suddenly, why not take a random tip
that there's a slowdown and adjust my driving to help mitigate the jam,
especially if 50 other people agree that there's a jam there?

------
tirant
Somehow I always knew this, and I was always driving in a specific manner to
combat this effect: I always leave a buffer of space between my car and front
car, and whenever I observe oscillations in the car in front, I use my buffer
to let my speed oscillate as less as possible.

------
jasmcole
Nice. I had a quick look at this effect in an old blog post, it's surprisingly
simple to derive the existence of waves in traffic flow (given some
simplifying assumptions) [http://jasmcole.com/2014/09/14/only-a-
fool/](http://jasmcole.com/2014/09/14/only-a-fool/)

~~~
eridal
TIL only a fool breaks the two second rule

[https://www.youtube.com/watch?v=mf5d2DP4Pp0](https://www.youtube.com/watch?v=mf5d2DP4Pp0)

------
nradov
Widespread use of adaptive cruise control basically solves this problem.
That's the technology where you can set the car to follow a certain distance
behind the car in front, and then it accelerates or brakes as necessary. The
systems use radar or stereoscopic cameras to measure distance. Once 20%+ of
cars on a road segment are using ACC then everything smooths out. Now that the
price of ACC has come way down I think the government should offer rebates for
purchasing the ACC option on new cars. It would be more cost effective than
building new roads or adding lanes.

[http://journals.aps.org/pre/abstract/10.1103/PhysRevE.69.066...](http://journals.aps.org/pre/abstract/10.1103/PhysRevE.69.066110)

~~~
wbeaty
ACC just imposes safe driving behavior: proper spacing for the reaction time.

So if just 20% of humans were safe drivers (spaced out properly,) then
everything smooths out.

That, or perhaps 0.5% of drivers could intentionally go at the average speed.

------
dmvaldman
An interesting point to mention is that the traffic "jam" moves backwards.
This is because at the front of the jam, cars are able to accelerate out of
it, while at the back o the jam cars are piling up into it. Seen from above
the jam is a shock wave moving backwards.

This often upsets people when they get out of a jam only to see there was
nothing causing it! In reality what caused it may have been much further up
the road.

~~~
AlfaWolph
It's usually high density traffic and someone pulling a move like a forced
merge which causes the person now behind them to slam on their brakes. I've
seen this hyperactive "weaving" happen too many times to have it be explained
only by "slow human reactions times".

~~~
cmdrfred
I heard a few years back (no source) that when they model traffic they can
design systems that work perfectly, its human error (not hitting the gas fast
enough after a green light, changing lanes to often, 18 wheelers getting in
the far left lane to turn right) that make the models break down.

------
blunte
One point that seems often forgotten (or undervalued) is the great different
in deceleration vs acceleration. Cars (especially driven by humans) decelerate
much more quickly than they accelerate. I didn't get the impression from the
simulation that this difference was realistically represented. I think reality
is even worse than this simulation illustrates.

------
marknow
I wish, for those of us that ride motorcycles in this type of traffic in the
US, that all states would adopt a sensible lane-spitting allowance for
motorcycles. In country after country, lane-splitting has proven to help
reduce congestion, and yet, if I try it here, I'll get a ticket. Ridiculous.

~~~
bequanna
Lane splitting is legal in California.

But honestly, how does this reduce congestion? My experience is that these
motorcycles often dart between cars causing drivers to swerve slightly and tap
their brakes.

I would argue this makes congestion worse as it causes more cautious drivers
to brake (sometimes abruptly) and almost always slow down.

Finally, it is important to consider that as a vehicle, most of us are not
looking for you and make small changes from left to right while in our lanes.
It is not a goal of mine to accommodate motorcycle riders while driving.
Splitter beware.

I'm all for sharing the road-- be we all need to subject to the same rules.

~~~
marknow
I'm aware of CA splitting -- it's not been adopted yet elsewhere in the US.
It's too bad that you had to experience someone who was "swerving in and out"
and not the majority of riders who would split sensibly and safely.

Lane splitting is only supposed to occur _below_ a certain speed; splitting
above that number would be a violation, of course.

Take a look at Europe to see how easily splitting can be integrated, for both
driver and rider. It absolutely does reduce congestion: bikes move smoothly to
the front and away, freeing space.

American drivers just aren't educated to realize this because negative
stereotypes about motorcycles are so prevalent in the US, and the many
positive benefits of riding aren't presented as options here.

------
giltleaf
That's why fuel efficient driving techniques are so great. Save you cash and
stop the stupidity
[http://eartheasy.com/move_fuel_efficient_driving.html](http://eartheasy.com/move_fuel_efficient_driving.html)

------
larrydag
This is something I love to think about when driving on the highway. I've read
so much on it. I've also had some thoughts on recommendations to solve it

1\. Divided lanes for 3+ lane highways. Basically a modified HOV lane but no
occupancy restriction. The far left lane would have lane dividers with
openings every 5 miles (or 3 exits). The middle lane opening every 2-3 miles
(or every other exit). The idea is that it would force a certain average speed
of traffic and hopefully distance between cars in the respective lanes.

2\. Smart roads to adequately distance cars from each other. Either a driver
assistance signal or actually controlling the internal cruise control system.

~~~
briffle
I believe there are highways designed like #1, I seem to remember visiting the
NY/NJ area, and I think it was the NJ turnpike that was this way. Several
lanes of "through traffic" with a wall, and every few miles, it merged with a
few lanes on the right of "getting off/on soon" lanes.

------
reuven
A model of this is included in the NetLogo models library, which comes with
the NetLogo modeling language:
[http://ccl.northwestern.edu/netlogo/models/TrafficBasic](http://ccl.northwestern.edu/netlogo/models/TrafficBasic)

It's fun to see children (and adults!) play with the controls on this model,
experimenting with different factors that they think might eventually reduce
the congestion.

This model is also a great way to introduce kids to complex systems, in which
the large-scale results aren't necessarily as obvious as the individual
actions that lead to those results.

------
athenot
Atlanta has rolled out variable speed limits on some highways (I285) which
attempt to slow down the flow BEFORE people reach the congested area.

I'm not sure how well most users understand how it's supposed to help them go
faster by slowing down; a bit more education could be helpful so everyone
plays the game.

It is a bit weird seeing the speed limit jump from 45 to 65 then to 35 within
the space of a mile…

~~~
leoedin
These are very common in the UK around large cities, and in my experience they
work really well. As long as there's sufficient threat of enforcement (speed
cameras), people stick to the speed and the whole mass of cars move along at a
reasonable speed. Compared to the horrible 80 mph, 5 mph, 80 mph, 5 mph thing
you get when everyone absolutely guns it to the next slowdown they're
brilliant.

------
shameerc
I would love to see a visualisation of lane change waves. How people changes
the lane on peak time. I always felt Elliott Wave Theory[1] is applicable to
this.

[1]
[http://www.investopedia.com/articles/technical/111401.asp](http://www.investopedia.com/articles/technical/111401.asp)

------
Inception
I would never get closer than a car's length away from the driver in front of
me if I was driving 60 mph; why should I act any differently when I'm driving
30 mph? Speed is trivial - the gap must be maintained so drivers have the
opportunity to react to cars around them without having to make any drastic
changes in speed.

~~~
sukilot
Because following distance is irrelevant. Following time matters, and that is
a function of speed.

~~~
Inception
Okay, but on-the-fly time calculations aren't exactly optimal. It's much
easier to establish a lower limit on distance.

------
eridal
can you see the animations?

my browser refuses to load them because of mixed content .. but html was
downloaded over http !?

    
    
        Blocked loading mixed active content "http://d3js.org/d3.v3.min.js"
        ReferenceError: d3 is not defined

------
lerouxb
I'm pretty convinced that self driving cars can fix this. Reaction times will
be much faster and they can network together to spread out the braking and
acceleration. Possibly even merging at high speed.

~~~
maxerickson
A more powerful potential mechanism is that they can be programmed to respect
do not enter directives from a traffic control system (keeping human drivers
off a freeway would take a lot of infrastructure). So the amount of traffic on
the road can be regulated to keep it below the capacity implied by the safe
response time of the vehicles.

------
egeozcan
The demo breaks after I click around a bit:
[http://i.imgur.com/OGvAO2Q.gifv](http://i.imgur.com/OGvAO2Q.gifv)

~~~
Pufe
this happens because the brake level when you click the brake button is
smaller than the brake level needed to not collide with the car in front of
it.

------
lewis500
this is one of the first things i ever programmed! cool to see it is seeing
the light of day again.

