
Traffic Experiments -- How to clear traffic jams - cubix
http://www.eskimo.com/~billb/amateur/traffic/trafexp.html
======
ritonlajoie
In northern America, I think most of the cars have automatic transmission. In
France, (where I live), we mostly (maybe 99% ?) have manual transmissions.
Have you ever experimented a traffic jam with a manual trans. car ? You have
to switch gears every 10 seconds and it's very very very annoying.

I'm telling you that, because as a frequent driver (well, today I drive a bike
but still) in Paris, and kind of very observant, I clearly see that most
people are not doing what these drivers do in this experiment. Most of the
time, everybody will let a huge space between the car behind them in order to
avoid changing the gears too frequently.

On the contrary, on automatic cars (I had one and drove another for 6 months
in Canada), that's easier to go & stop & go & stop & go & stop, etc...

Not sure if it's related, but I think it is, in a small proportion.

edit: my english

~~~
vaksel
why exactly do you need to switch gears? Just leave it in first. And engage
the clutch when you are stopped.

~~~
daten
If you're stuck in traffic for a long time, your leg engaging the clutch can
get very tired. So you're constantly switching to neutral to rest your leg.

It's also not uncommon for traffic to go from stopped, to fast enough to
warrant 2nd gear, back to stopped.

------
Luc
Where I live there's something called 'block driving' (I haven't found a
better translation for the Dutch word 'blokrijden'). On busy summer days, when
the highway to the coast is jammed, a police car will block the highway, then
drive at a constant speed (and slower than the maximum speed), with no one
being allowed to pass the police car. Apparently this helps to resolve the
traffic waves.

~~~
0x5a177
The California Highway Patrol appears to do this in Southern California as
well. I've seen a patrol car start weaving between two lanes, then three,
weaving more and more until it's blocking all lanes of traffic like a weavy
pace car.

~~~
bmm6o
It's called "running a break", and I've only seen it used to clear debris or
disabled cars out of lanes, or so that a construction crew can close a lane.
It'll freak you out the first time a cop car swerves in front of you across 5
lanes of traffic.

~~~
davidu
I've definitely seen it used in both Nor Cal and So Cal to slow traffic down.
They do it on I-5 fairly often during rush hour, of all times.

------
Sukotto
An oldie, but a goodie.

I base my commuting style on this guys articles. Drive _slightly_ slower than
everyone else and let large spaces grow in front of your car. It's a _lot_
harder than it sounds (for me anyway) but when I get it right, it makes my
drive much more enjoyable.

People that just HAVE to drive as fast as possible will go around you (you
should never do this in the left lane) and everyone else will sort of get in
line after you... it looks a bit like those nascar groups where all the cars
go exactly the same speed.

~~~
loewenskind
Except that people who want to go faster than you will pass you and get in
front of you. This leads you to either keep slowing down to maintain the space
or start shrinking the space.

~~~
Sukotto
Yes, many people will pass you and go in front. That's not such a big deal
since many of them switch into yet another lane (Some will even exit the
freeway only to immediately re-enter) as they try to gain even half a car-
length on everyone else.

I don't feel like competing with them.

Personally, I'm not trying to fix traffic for everyone. I'm selfishly trying
to make my own commute as pleasant as possible. By following the advice in
this old article I've managed to greatly improve my own drive, and I find the
amount of time I spend in the slow pockets of traffic greatly reduced.

~~~
daten
I'm concerned that when cars doing this are the minority, with other traffic
constantly going around them, they just become an obstacle in traffic. If you
inspire other cars to change lanes to pass you, because you're intentionally
driving slower, you may be making traffic less safe.

~~~
Xurinos
This is a fair concern. I have not seen this in practice, though, and it might
be a matter of the part of the world where I drive vs where other people
drive. But let's cut to the chase with how the system works:

You are not supposed to be driving slower. You are supposed to be driving in
average speed with the traffic. You may have had to slow down in the first
place to do this. If you are just starting out, a good rule of thumb is to
just leave space between you and the vehicle in front of you; keep pace.

The key is to maintain enough space between you and that vehicle that you can

(1) allow people to freely pass through (they are going to do something crazy
to do it anyway if they do not have the space)

(2) absorb traffic waves

It is likely you will not be able to do #2 consistently. But you can let off
your gas and not hit the break. Hitting the break is an instantaneous warning
signal to people behind you, causing something of a reflex for them to also
hit the brake. You are trying to avoid the brake-light wave. Letting off easy
generally does not cause as much reaction.

What if you end up in a traffic jam anyway? Leave space anyway. It does help.

Aren't more accidents are caused by people getting panicky as they try to
cross lanes of traffic that are closed to them? They force their way in or
speed up to get around a bunch of people?

It is all about leaving that space so they are comfortable. Everything begins
to move faster. And you are more relaxed in your driving because you are not
freaking out at what is going on in front of you, allowing you the freedom to
react as necessary to all the other kinds of problems traffic can bring.

Intentionally driving slower over a long distance is a misapplication of the
principles. As the article points out, it is all about the average speeds.
Frankly, very rarely does anyone get "pissed", and all they do is pass around
you. And then they end up hitting their brakes and shifting out quickly.
Passing != bad thing; we are not trying to win a race here. I have never been
honked at.

~~~
shadowflit
"But you can let off your gas and not hit the break. Hitting the break is an
instantaneous warning signal to people behind you, causing something of a
reflex for them to also hit the brake."

...As long as you're careful about doing this, of course. If the people behind
you aren't paying enough attention to notice a slow decrease in speed (and
can't see the reason you're slowing down), they'll be hitting the brakes hard
to slow down when they're suddenly too close.

In hilly city driving (hi, SF), I will actually intentionally use my brakes to
signal to the people behind me I'm trying to slow down, because there's often
not enough space to notice when your line of site effectively stops at the
next intersection's brief period of flat road. And yeah, this behavior was
inspired by seeing someone rear-ended right behind me.

~~~
Xurinos
Yeah, I am sure I am not alone in despising tailgating. It is the exact
opposite of what we are talking about in this thread, a product of impatience
and bad assumptions. You simply cannot stop some people from hitting the
brakes repeatedly. You can just work to reduce it by not causing the reflex
response in most situations.

I also refuse to be pushed by people who ride my tail; I maintain my speed and
ignore them (which is oodles better than slowing down to teach them a lesson,
which actually does the other thing I was warning against here, causing a
ripple in the traffic). It is my rebellious nature against their several ton
weapon.

------
parallax7d
I used to do this every once in a while. One thing to be aware of, are large
trucks behind you at the top of hills. They tend to want to increase speed
going down hills to use their velocity to climb the next. It's probably a good
idea to accommodate this.

------
CaptainDecisive
Interesting observation is that what TFA calls the "cheaters", ie those who
drive to the end of their lane before merging at the last minute, may actually
be doing the right thing. See Tom Vanderbilt talk about it here
<http://www.vimeo.com/6779064>.

If you're interested in this there's a nice longer interview with Tom at
Streetfilms <http://www.streetfilms.org/tom-vanderbilt-talks-traffic> or an
hour long Google talk.

~~~
ajb
He doesn't say _why_ in the short one (I haven't watched the long one). Late
mergers are definitely bad in some situations - for example, when they are
merging into a slow-moving turnoff lane, blocking a whole lane of ongoing
traffic.

~~~
CaptainDecisive
He goes into the research a bit more in the Google talk and you see where he
pulls the magic "15% improvement" from. Also he has a book (which I haven't
read) called Traffic so he might explain it more or cite studies there.

I agree though that it doesn't really work in all situations. Tom uses the
example of two lanes merging into one for roadworks. Maybe some situations
where it doesn't work are an example of poor road design by traffic engineers?

------
daychilde
I had the good fortune to do a three month internship in Seattle last spring.
And I have to say - after growing up in Dallas, now living in Florida, and
seeing traffic in other places... Seattle drivers are unique.

I've seen this [appear to] work in Seattle. While I was there, I tried it, and
it appeared to work.

However, while I lived in Dallas, I was aware of this idea, and tried it there
- and no way. People cut in, people tailgated - it completely doesn't work.

So I think it partially depends on the nature of the city's drivers. And there
is something special about Seattle.

~~~
daten
I live and work in DC. I go to Dallas at least once a year and drive every day
during the week and weekend. I agree this would be impossible in Dallas
traffic. It doesn't feel as intentionally aggressive as DC traffic, instead
it's like everyone in Dallas can't take their foot off the accelerator even if
they wanted to. It's hard enough finding a space between two cars to merge,
you usually have to just go for it and hope the rear car can find his brakes.

~~~
daychilde
It definitely took me a while after moving away from Dallas to change some of
my driving habits.

I still definitely change lanes more than most drivers around here - although
I do my best not to cut anyone off, and I actually let drivers in here...
something definitely not done often in Dallas.

It also helps that I went from a 90 minute commute (each way) to usually a
10-15 minute commute here (depending on where I've worked). Smaller towns do
have _some_ benefits...

------
DanielStraight
The solution to traffic jams is to not leave transportation in the hands of
clueless, distracted, reckless drivers. The solution is fast, reliable,
ubiquitous public transit.

~~~
nradov
Public transit is impractical and uneconomic in many areas. The real solution
is smart roads and computer-driven electric cars.

~~~
rbranson
Perhaps, but in the context of the article, traffic jams only exist because of
commuters. The interstate & highway systems were not designed for commuters
and should not be used for them. While I think computer-driven cars will be a
boon to the environment and economy, the capital investment needed is pretty
fantastic, which is going to be some decades off.

In fact, a real solution to this problem that could be applied in the design
of our current urban landscape would be a multi-pronged effort to spread out
the commuter traffic. Alternating work and school hours could widen commuting
hours and distribute load across all commuter infrastructure. Increased levels
of telecommuting would also contribute to this. This would be a sort of
supply-side fix to commuter congestion. Local or regional governments could
create incentives to encourage these changes.

~~~
nradov
The first-generation interstate highway system wasn't designed for commuters,
but everything added or upgraded in the past 40 years has been. Many new
luxury cars already come with limited computer driving, in the form of
adaptive cruise control. The capital investment is small and rapidly dropping,
and just that one innovation could deliver a major reduction in traffic
congestion. <http://www.springerlink.com/content/p56714553g82r2h7/>

------
DanielN
I've tried this in Boston. Mass-holes will not stand for you to screw with
their traffic jams.

~~~
cakeface
I agree that this doesn't work well around Boston. My opinion is that the
fluidity of traffic, ie. lane changing, doesn't break down at low speeds in
Boston. If you let a cars length open in front of you it will fill up with a
car. I tend to think that this makes efficient use of the pavement available
and tends for a traffic flow that is as maximal as possible but I don't have
any evidence about that.

~~~
asmithmd1
If efficient use of pavement means park the most cars on it Then by all means
fill every available space. Safe following distance increases with the square
of the speed so higher average vehicle speeds means the road carries less
traffic. The road moves the most cars around 30mph, so if traffic slows below
30 it will crash to 0mph

~~~
cakeface
Cool! What is the science backing this? Is it coming out of studies or some
theoretical modeling? I've been wondering what the most efficient speed for
moving traffic was on a typical highway for a while.

~~~
wbeaty
Google on "Fundamental diagram" of traffic, or
[http://en.wikipedia.org/wiki/Fundamental_diagram_of_traffic_...](http://en.wikipedia.org/wiki/Fundamental_diagram_of_traffic_flow)
It's a set of graphs. Density plotted against speed, etc. It's old stuff,
empirically derived. As flowing cars slowly get closer together on average,
drivers slow down a bit for safety, yet with more cars passing per second, so
the flow increases. But then as they approach even closer, drivers must
greatly slow down because of reaction time issues, even though more cars are
packed onto the roads ...and so the slowing dominates the extra cars per mile,
and average flow decreases. The peak flow ends up being 30MPH - 40MPH. That
flow will most rapidly drain out the backups. I hear that it's a different
value in different countries.

But worse is the problem that once everyone slows past the 35MPH hump, traffic
becomes unstable, and you get oscillations or standing waves which slow things
far more than you might expect.

Rule of thumb (but I've never seen it stated anywhere) 35MPH=51FPS, 3 seconds
between cars, gives 150 FEET BETWEEN CARS. Closing up gaps will wreck the max.
flow pattern. During congestion, if you start trying to stop others from
"cutting you off" or merging ahead, then you're the one causing the jam. (Or
to be fair, 30MPH and 2sec gives 90ft spacing, perhaps the bare min.)

Another issue: in 1998 Helbing and Huberman discovered the existence of
distinct 'phases' in simulated multi-lane traffic. When all lanes seem to lock
together like a moving crystal, that's now called "Synchronous Flow," and
gives maximum throughput. But it's on the edge of collapse, and
"condensations" are triggered by anyone who tries outrunning neighbors by
switching lanes, or who approaches too close and has to tap the brakes.
[http://www.nature.com/nature/journal/v396/n6713/abs/396738a0...](http://www.nature.com/nature/journal/v396/n6713/abs/396738a0.html)

------
mootothemax
Heh, I didn't realise that it had such an effect, but this is the exact same
game I play when caught up in a heavy traffic: the how-long-can-I-go-before-
breaking game! ;-)

It could be related to driving a manual and being lazy, but it helps your
concentration no end :)

~~~
Deestan
No idea why you were at -1; your comment was informative and on topic.

On a side note, I also tend to play that "game" when stuck in slow home-from-
holiday traffic over the mountains. Changing speeds all the time while driving
manual is such a pain.

------
nwomack
I've actually spent quite a bit of time pondering this. Here are my completely
unscientific conclusions.

Imagine a [moving] wall, on an interstate, moving at 30MPH. No matter what is
happening behind this wall, it is impossible to maintain an average speed
faster than the wall. You can try any amount of tricks to smooth out traffic,
but it will be impossible to ever move past this wall. As long as there is a
steady stream of traffic, no car will ever do more than 30MPH until this
moving wall gets out of the way.

So now let's, more realistically, replace the wall with a bunch of slow
drives, and ask 2 important questions:

1\. How to get out of this situation? -- We want these drivers to all SPEED
UP! It's really as simple as this. Until these bottleneck drivers move out of
the way, you can only smooth out traffic, but never increase the speed.

2\. How to avoid this problem in the first place? -- In general, there will be
some bottleneck. After too many cars are going through this bottleneck at
once, there will be slowdown. Ideally, these bottlenecks should be identified
and no more than the maximum amount of cars at a time should enter the
bottleneck to decrease the speed of traffic.

Of course, that's easier said than done, but I think the important thing to
take away from my ramblings is this:

The best way to get rid of a traffic jam is: if you are in the front of the
pack and have a chance to accelerate, you should do so as quickly as possible.
If everybody were to do this, average speed could then be increased.

~~~
wbeaty
But note that there aren't any slowpokes here, instead, every single car is
blocked by the car ahead of them. The people behind you think that _you_ are
the slowpoke driver. But nothing is blocking the traffic, instead the average
speed of the long column of cars is determined by the spacing between cars,
and that spacing is determined by psychology.

In light traffic, some people go far faster than average, but nobody tries to
stop them. When we arrive at heavy congestion, don't we suddenly decide to
close up ranks to prevent aggressive drivers from passing, stop them from
"cutting us off?" If everyone habitually blocks the speeders, then it means
we've compressed the traffic pattern past the peak of maximum flow, and pushed
it into the unstable realm of stop/go oscillations and standing waves or
"ghost jams."

Want jams to vanish? Then do as the pro truckers do and just freaking back
off. Happily encourage other cars to merge ahead. (They're not "cutting you
off," there's no such thing as "cutting you off." They're just trying to merge
into the adjacent lane!) If traffic momentarily slows way down, everyone just
make sure to keep an open space that allows merging, and you'll find that it
speeds right back up again.

------
RyanMcGreal
Yes, absolutely. This is my experience as well. Related:
<http://news.ycombinator.com/item?id=785498>

------
drv
Somewhat different from the commuter-oriented topic of the article, I've seen
attempts by truckers to enforce a similar pattern in construction zones
(traffic reduced by one or more lanes) on long stretches of highway: In a
situation where one lane has to merge because it is closed in the distance,
the "nice" drivers all merge far ahead of time, while the aggressive drivers
attempt to go all the way to the end of the lane and then merge ahead of
everyone, causing the kind of grinding gears described in the article. To
combat this, a couple of big rigs driven in parallel in the open lane and the
lane that is closed ahead block all traffic from passing. This prevents the
aggressive drivers from attempting to merge at the last second, smoothing out
the wave. I've seen this happen several times on cross-country trips; I wonder
if the truckers coordinate over radio or just choose to do it independently.

On a related note, I wonder if "lane closed ahead" signs placed too far ahead
actually make this problem worse, since many drivers will immediately merge
out of the closing lane far ahead of time, causing a wall of traffic if the
front of the line has to slow down for aggressive last-second merges.

------
kostko
I remember reading this article while back: "Traffic jam mystery solved by
mathematicians". <http://www.physorg.com/news117283969.html>

So now when I'm in a jam, I just slow down instead of engaging the speedup,
break pattern. Haven't looked behind me though to see if I had any effect on
the traffic behind me.

------
tobtoh
In Melbourne, Australia, a similar concept was used to ease congestion
entering freeways. We had a peak-hour problem where cars would flood the entry
ramps, but struggle to merge with the existing freeway traffic - this caused
people to brake/slow down which resulted in major jams.

To alleviate this issue "there are traffic lights at some freeway entrances to
control the flow of vehicles onto the freeway when the traffic is heavy. When
operating, the lights will change quickly, so that when the light is green
only one vehicle in each lane will be able to enter the freeway." (taken from
VicRoads website)

The lights basically cycle from red to green every 2 seconds. That's enough to
regulate the flow onto the freeway and minmises jams on the entry ramp and at
the merge point between the freeway and entry ramp.

~~~
tobtoh
In fact, here is a YouTube video of the lights in operation:
<http://www.youtube.com/watch?v=YFrEd8qSVCc>

------
tomjen3
The problem with slowing down miles in front of a traffic jam is that those
who normally gets of before that space now has to suffer too.

In addition, most people would cut you of if you drove slower than the maximum
speed.

------
bryanh
So, what would happen on major interstates if highway patrols kept a steady
pace? Is that a viable solution? Wouldn't they eventually run up on another
wave of cars and ruin everything?

I would be really interesting to see if this actually improved interstate
bandwidth during rush hours. But even more interesting, I bet a good portion
of drivers would get upset at the fact that there are huge, unused gaps. I
wouldn't be surprised to hear from people who think it is grossly more
inefficient than regular old traffic jams...

------
daten
The success of this technique can't be accurately measured by the driver who's
trying "smooth" out the waves. It needs to be witnessed from outside of
traffic. It usually just moves the problem to behind the "helpful" driver as
other drivers can very easily continue to drive competitively and recreate the
effect. Even if he's successful in removing the "wave", it's just by
decreasing the average speed of traffic.

Please don't try this in the left lane.

~~~
keyle
That's exactly why nobody would play along even if they knew. Big trucks
eventually do that because it's painful for them to do the gears 1,2,3
constantly, they stay in second or third and just slow down the whole thing.

Here is the thing where it falls short though:

the guy at the front riding the wave really slow to "help lane locks" will
also get merging traffic many many time right in front of him (people taking
advantage of the free space), as a result, it won't help that lane but only
make it worse if you're stuck behind such driver (or truck).

So if everyone stuck to their lanes, didn't change unless extremely necessary,
and played the wave-canceling technique, it would work.

In other words, this will work when cars will be driven by computers hooked
together via apis and maybe a traffic controller. I'm not signing for the
beta.

~~~
zepolen
Actually, the fact that people in the middle lane are taking up his 'free'
space _helps_ the long term traffic.

The main cause of traffic wave is from someone stopping too abruptly, eg. if
someone changes lane in tight traffic, he forces the other person to slow down
too fast, which begins the wave.

If you allow more people to get in front of you in a smooth manner because you
have given yourself a lot of distance, then there will be less 'cutting off'
slow downs from the middle lane guys, which in turn causes less cutting offs
in the right lane.

How much does it hurt over your entire commute to let even 100 cars in front
of you? You'll be 500 meters later to your destination, what's that at
30kph...1 minute more? That's the worst case scenario. The normal case is
you'll get there quicker because you've raised the average speed of the entire
traffic wave.

~~~
superk
Absolutely! I've thought about this so many times... what is 5 seconds for you
(allowing someone to merge in front of you) would translate to much much
faster commute to _everyone_ else. It is all about the balance between my
personal greed and what's better for the common good (if only we could solve
this on _and_ off the road).

Where I live there is terrible traffic... and I've contemplated causes and
solutions many a mind-numbing commute. I realized we behave differently when
we are behind the wheel... Very few people would behave or maneuver the same
if it where lines of people instead of cars. That is why your best chance to
merge is to make eye contact with the other driver - if they recognize you as
another human not just another vehicle than they are much more likely to move
over or make space.

When I drive (during rush hour) I suffer a lot to justify my actions. What I
have settled on... is a simple rule: In any situation the person who is in the
wrong is the one who has contributed to the impediment of the overall flow of
traffic. This comes back to what zepolen says (and I agree with). If we could
ever make the connection that what's good for my fellow (wo)man is good for me
too... we would be so much better off in general.

~~~
daten
I agree in the situation where all other drives on the road are equally
intelligent and paying attention when they drive.

Sadly, in the area I drive in, many drivers are constantly paying more
attention to their phone than the road. Those drivers cause all of the traffic
problems I see. They likely contribute to many of the accidents as well. They
will end up leaving huge gaps in front of them at the expense of traffic
behind them even when traffic ahead of them is moving smoothly and quickly.

The cost of letting cell phone drivers in front of you can be many minutes to
your commute per each driver. Add a few minutes for each traffic light or
merge lane that you will spend behind the inattentive drivers.

In a 5 mile commute, following the flow of inattentive drivers can add 45
minutes to my drive. Attentive driving and following the flow of other
"aggressive" drivers can make it a 10 minute drive total.

So what I have actually found helps, in a real world scenario, not a fictional
one where all drivers are equal, is trying to identify and cooperate with
other "aggressive" drivers helps the group of aggressive drivers get through
traffic efficiently and gives the cell-phone drivers plenty of time to
concentrate on their conversations.

------
tectonic
As a side note, I remember reading articles on this author's webpage almost 15
years ago. Some awesome stuff from a long-time netzen!

<http://amasci.com>

------
alexyim
A side effect of doing this is that it is much more fuel efficient to not have
to go into a cycle of breaking and accelerating again.

------
fbnt
Maybe the author never heard anything about green waves.
<http://en.wikipedia.org/wiki/Green_wave>

~~~
superk
Green waves are a pet peeve of mine. They've tried to implement these where I
lieve... a spectacular failure. Green waves only work when there is NO
traffic. Speed is regulated and the flow moves fine. During rush hour they are
a disaster. Every light is backed up to previous. My light turns green, then I
have to wait 10 seconds for the next light to turn green and the entire line
of traffic to move, before I can advance. This usually results in a stop at
each light. A much better solution is what I remember from NYC: all the lights
turn green at once, all the traffic moves.

------
stuaxo
Thats what those changable speed limit signs on the motorway are trying to
achieve.

(I've also seen markings to keep cars 10 metres apart on some roads).

------
chopsueyar
Old.

~~~
daten
He's right, this has been posted on HN before. I seam to remember the old
article having more animated illustrations. I don't know if it's the same
page, same author or just similar content.

~~~
Kliment
The animations are linked at the bottom of the article.

