On my evening commute on I-5 southbound from Everett there is always a
right-lane traffic jam at one of the Lynnwood off-ramps. Close-packed
cars must crawl along at 2mph for a very long time. Therefore I
intentionally changed to the exit lane as I approached that distant jam,
and I started
letting a REALLY huge empty space open ahead of me. By the time I hit
there was maybe 1000ft of empty road ahead. Sure enough, my big
empty space stopped traffic from feeding it from behind, while the front
of the jam kept dissolving as usual. By the time I arrived, the jam was
significantly smaller than it had been. Amazing. This wasn't any little
wave, yet one single driver was able to take a huge bite out of
the back of it.
Just moving jam around
Obviously my actions did more than just reduce the size of the jam.
In order to create the empty space, I was temporarily driving about 10 mph
below the speed of the heavy traffic. I did this for several minutes, and
therefore I caused a slight slowdown behind me. After I arrived at the
jam, the jam was smaller. When all was said and done, part of the dense
traffic jam had been removed. However, it was changed into a mild
slowdown, and it was spread backwards upstream over perhaps a mile of
traffic. Traffic behavior was changed. Rather than driving at 50mph only
to crawl along through a traffic jam for several minutes, everybody was
now driving at 40mph for a few minutes before the jam, but then having a
much smaller traffic jam to endure. The average traffic flow might have
improved, but also it might have remained unchanged. But some of the
nasty, frustrating part of the
2-mph jam was converted into a large "fuzzy" area of reduced speed. And
after I made these changes,
drivers as a whole would find it much easier switch lanes to avoid
becoming trapped in the jam, since the solid-packed region was much
smaller. If I had done it correctly, I
could have erased the whole jam,
transforming it into many minutes of slightly-slow driving for everyone
behind me. (If I could have started 30mi upstream of the jam, maybe I
would have only needed to drive 3mph slower than traffic... that is, if
other drivers didn't simply go around my slow car.)
Another thing that happened: by shrinking the region of solid-packed
cars, I made it easier for other cars to merge
into the exit lane, so I probably removed part of the backup in the
through-lane as well. By moving the jam backwards, I unplugged the merge
zone at that exit. The jam was mostly caused by drivers trying to merge
across lanes to reach the exit.
Drivers already in the exit lane weren't letting anyone in,
merging cars sat unmoving in one of the thru-lanes, waiting for a space to
and also forcing everyone behind them to halt.
So, by inserting a large empty space, I wasn't only
taking a bite out of the jam ahead of me. I was also easing the jam in
other lanes. At best, moving the jam backwards would entirely remove
the bottleneck and halt the
growing queue of stopped cars.
With the jam broken up, the clot of cars behind the merge zone becomes a
wave which freely moves
backwards. The traffic jam
was like downtown city "grid-lock," and
I was breaking up the gridlock and promoting free flow by putting spaces
between all the cars.
Here's a general principle I take from the above. (I guess it's obvious
hindsight!) ANTITRAFFIC DESTROYS TRAFFIC. Empty spaces can break up a
jam. While I was slightly slowing down to allow a space to gradually open
up before me, I was creating an empty pulse of "antitraffic" ahead of me.
my antitraffic finally collided with the dense "traffic" of the jam, the
two annihilated each other like a positron meeting an electron. It's
nonlinear soliton physics. The soliton waves destroy each other, leaving
only a slight fuzzy smudge behind. The fuzzy smudge may behave very
differently than the original bottleneck: it can travel backwards and
move off into the distance.
My next thought: if I took several friends along on my experiment, we
could have repeated the same jam-erasing process. No, not illegally
driving three abreast. Instead, each of us could have allowed a big blob
of anti-traffic to appear, and then those repeated impacts of the incoming
antitraffic could have completely erased the traffic jam at the Lynnwood
exit. Traffic at the exit would start flowing freely, and the long backup
would drain away. When traffic is sparse, we cannot keep a really large
space open ahead of us, since it's too easy for every to pass a
slightly-slow driver. But perhaps several separate drivers could bring
less-enormous spaces along with them, and any traffic jam would succumb to
the barrage of "antitraffic."
Another lesson I learned: plan ahead. Plan WAY ahead. When stuck in
traffic jams, I discovered that I cannot affect them by first making my
way through the jam and then "peeling out" at the end. I hoped to make
the far end of the jam dissolve faster. It never worked because I couldn't
get rid of the slow guy ahead of me. But as a commuter I'm encountering
the same traffic jams every day. I know their locations. I know what to
expect ...so if I planned way ahead and brought a big empty space along
with me into the jam, I could use that space to manipulate the jam. I can
control the traffic only by applying the brakes. But once I get myself
packed in with everyone else, I can do nothing. In order to have an
effect, I must behave differently BEFORE the jam, not while trapped inside
But won't other drivers simply go around me and fill my big empty space?
Hmmmm. After trying this many times in many places, I find that they
usually don't. I wonder why? (
see the FAQ page.)
And here's a final lesson: a tiny bit of math! (For some reason this
part took me years to figure out.) A traffic jam is a pattern in the
cars, but what *exactly* is a traffic jam? It's not congestion, but
congestion is usually required before jams will appar. A jam is two
things: it's a pattern of slowness which, with the same incoming flow,
might exist ..or it might not exist. Regardless of the number of cars
arriving, sometimes the jam just isn't there. Second: a jam is a traffic
pattern where the outflow from the pattern has become constant. In a jam,
the outflow DOESN'T equal the inflow. More and
less incoming cars doesn't affect the outflow rate, instead it will only
cause the length of the jam to continuously grow or
shrink. And individual drivers can't
affect the outflow rate of cars leaving the far end of the jam (they can't
dissolve the jam by "peeling out.") And amazingly enough, that's the key
thing that makes a jam *be* a jam.
In normal un-jammed highways, the outflow from a piece of road is
affected by the inflow. If inflow rate gets larger, then outflow rate
gets larger too. But once the non-linear jam-pattern appears, the outflow
rate becomes constant, and the jam will grow larger and larger if the
inflow rate is a tiny bit bigger than the outflow. (And of course the jam
may shrink to nothing if the inflow is a bit smaller.) And finally: once
the jam has been triggered, the outflow *must* be less than a similar
outflow from an un-jammed highway. (Think: if the outflow from a jam was
larger than the outflow from the unjammed highway, then whenever a
jam appears, the sudden improvement in outflow would drain out the cars,
and the jam would instantly evaporate before it had a chance to really get
While doing all of the jam-canceling above, I once caught myself behaving
creating a huge traffic wave. What a hypocrite! Bad habits die hard.
Traffic was heavy and I was in the left lane. I had to merge across
several lanes in order to get to my exit. I merged right once, but the
next lane was packed solid (but it was still moving, not jammed.) Nobody
would let me in. I drove like this for a long while, then started driving
fairly slowly in order to drift backwards along the solid lane. I found a
slot and got in, but now I had to merge right once more. Many minutes had
passed, and my exit was coming up. The right lane was packed solid,
NOBODY WAS LETTING ME IN. I drove slower and slower, and in a panic I
finally forced my way into a small gap, making the guy behind me jam on
brakes. After awhile I realized that I had just created a huge traffic
wave with my behavior. Just like any rubbernecker I had suddenly slowed
way down for no good reason. But I had an excuse, I had to get to my
exit! To make matters worse, I had nearly come to a complete stop, and
brought two entire lanes of traffic to a near halt too. I probably left a
long-term traffic wave at that spot on the highway. But it wasn't my
fault! Yeah, suuuure.
In stewing about this I realized that EVERYONE has this same problem at
that particular spot: an inability to merge in the dense traffic.
Others were probably doing the exact same thing that I did, and this would
make the "wave" near that exit worse and worse. Our inability to change
lanes would create a "dynamic bottleneck" which hovers near the exit.
Obviously the simple cure is to give up; not merge, and miss the exit. I
should never have forced the issue, I should have let my exit go past.
So should all the other merging drivers. But there is a bigger issue here.
People SHOULD be able to merge. Why was traffic packed so tightly? One
obvious reason: to punish the idiots who will jump into any little space.
I had always done the same myself. I never allow a space to appear ahead
of me, or some other driver will immediately swerve into it during their
quest to cheat by running down an empty lane to the front of the line.
But this sort of "closed-gap" driving would also prevent any
necessary merges at off ramps (and at on ramps too, of
course.) By eliminating the space ahead of me, I become part of the
impenetrable wall which creates the "dynamic bottleneck" and screws up the
traffic at highway exits. The gear teeth cannot mesh, so the whole
machine grinds to a halt. The "zipper" becomes jammed because the "teeth"
of the zipper are resentful about new teeth moving into the space ahead of
The jammed merging lanes are much the same as gridlock in a city.
Smart city drivers never block intersections, since blocked intersections
will freeze all traffic permanently. But we highway drivers are ignorant.
We close up the gaps when others need to merge. And our behavior creates
needless "highway gridlock" during every single rush hour.
So, if I keep a few carlengths of space open ahead of me, then not only
can I use it to help vaporize waves and jams, but I also eliminate one of
the major causes of the waves and "highway gridlock." I eliminate the
wall" of traffic at merge areas, and I let people merge without slowing
down and creating traffic waves behind them. Take a look at this animation on page
this article. Ideally a merge-area will act like high speed gear teeth.
A "Zipper Merge."
that everyone starts defending themselves against opportunistic drivers by
eliminating all gaps in traffic. In that case the valid
merges cannot take place either. A fight develops, and a traffic jam is
created. The jam appears at the merge zone, while a huge region of empty
roadway is created downstream. Sometimes this jam is the fault of people
like me who panic while missing their exit and who come to a complete
stop. Sometimes the jam is the fault of the huge blinking yellow
which blocks one entire lane of traffic during road work. But the
traffic jam is ALWAYS the fault of those who refuse to let anyone merge
ahead of them. "Just merge behind me." No, that doesn't work, since the
guy behind you doesn't want any merges either. Everyone in the whole lane
is saying "merge behind me!" It's a solid packed wall of hostility.
Poking any small hole in that wall can make a difference.
Bottlenecks: A Primer. US DOT Federal Highway Admin 2012
#2: KEEP SUFFICIENT GAPS.
"Keeping sufficient (or ideally, the largest possible) gaps leads to
uniform and free(er) traffic flow."
Delusions of Grandeur
Seattle suffers from many separate rush-hour traffic jams. Why should
I stop with
the jam at the Lynnwood I-5 exit? With enough people (maybe with
cellphone apps and GPS,) we could intentionally smooth out ALL the
traffic jams on all the
main Seattle highways!
This is all fantasy at this point. It's probably illegal for several
people to "conspire" to mess with traffic patterns (would we be arrested
under a drag-racing law?) This could be a "flash
mob" organized via email.
[NOTE: YEARS AGO IN TORONTO, TO
PROTEST THE LOW
SPEED LIMITS, SEVERAL PEOPLE FORMED A ROLLING BARRIER DURING RUSH HOUR.
THEY DROVE AT JUST UNDER THE SPEED LIMIT FOR HOURS, CREATING A HUGE
SLOWDOWN. THEY WERE ARRESTED BUT LATER RELEASED WITHOUT CHARGES.]
And while it is possible for a single driver to have huge effects on
traffic patterns, some things can't be done by a few people. For example,
suppose I want to eat the entire I-5 and I-90 traffic jam south of the
city. I would have to go all the way south to Tacoma before driving north.
I tried driving slightly slow, a space would never open up ahead of me
because that's sparse traffic down there and nothing stops other drivers
from passing me. In my experiments, I
could create my "antitraffic" spaces only because traffic was very heavy,
and because only a very few people had the opportunity and the ambition to
leave their lane and move into my empty space.
Rolling barriers made of State Troopers
OK, so here's how to dissolve a major interstate traffic jam. Start
many miles upstream from the jam. Put a row of State Trooper vehicles
across the road and have them drive towards the jam. They drive perhaps
at 55 rather than 70 as everyone else had been driving. Nobody can get by
them, and so all the traffic behind the State Troopers is moving at 55 or
so. In front of them a vast open space appears. After many minutes, the
traffic which had been feeding into the city traffic jams simply stops
arriving. There's little new traffic for many minutes. The huge jam
away. Just as the last of it is gone, the row of State troopers and the
55-mph traffic arrives, and the jam has been transformed into miles and
miles of slightly slow traffic upstream from the old location of the jam.
[Hey, since 1998
this has been tried, and found to actually work.]
The BIG question
But does this increase the throughput of the highway? YES!!! It allows
people to merge again! It actually changes the 'capacity' of the highway.
It wipes out a 'dynamic bottleneck.' By removing the close-packed region,
the two lanes of traffic at the exits and entrances are able to merge
at high speed...
so even though the
close-packed region is now a big fuzzy slowdown, the flow of traffic does
On the other hand, the situation is not so simple if lots of extra traffic
is entering from numerous
on-ramps. The "rolling barrier" can't affect these extra inputs, and if
nearly all of the traffic is from on-ramps, then
barrier" idea would be worthless. In that case it can only control the
main highway and not all the on-ramps.
Ah, but what about "rubbernecker slowdowns" at accident sites? A
rolling barrier could let the slowdown evaporate, and change it into a
wide region of slightly-slow traffic a few miles upstream from the
Would the slowdown just re-form? Would rubberneckers hit the brakes and
re-create the "traffic standing wave"? I dunno. Sometimes "rubbernecker
slowdowns" persist for hours after the accident has been cleared. This
suggests that the slowdown is self-perpetuating. Rubberneckers only
trigger it, but they don't keep it going afterwards. If so, then
"erasing" the slowdown might be worthwhile, because once it's erased, it
will only re-form very slowly (or not at all). If the slowdown normally
persists for several hours, yet it only takes half of an hour for the
police to erase it, why not erase it? True, the slowdown is not truely
since it has become a wide area of slightly slow traffic. However, over
many months of slowdown-erasure, this could prevent lots of fender-benders
and road-rage incidents, and eliminate thousands of man-years of anger and
Also, the average speed and traffic throughput on the highway MIGHT
actually improve if region of stopped traffic could be removed.
"Removing" the jam just spreads it out and does not immediately alter the
average speed. However, improvements in speed might be more than
you'd expect. After all, things are not "linear" in traffic flow, since
those who sit at 0 mph for many minutes in a jam cannot compensate by
driving at 120MPH afterwards. Also, some jams act like "virtual
bottlnecks" which create huge backups behind the jam while also creating
downstream. Removing the jam will remove the bottleneck and increase the
flow. (Note: if this is common, if jams commonly act as bottlenecks, it
means that highways don't have a known capacity. The highway's maximum
capacity is different whether a jam is present or not.)
Aha, one other point: if State Police, rather than driving slow,
instead came to a complete halt for a minute or to, this would force the
slowdown to turn into a "stop wave." Stop-waves always crawl backwards.
If this was done, the downstream end would start moving backwards. It
wouldn't be "pinned" to the location of the accident scene. And that
means that the growth of the jam would be entirely halted. The slowdown
would crawl off backwards into the distance: not dispersed, but with
having its constant growth entirely disrupted. Ooooo, does this actually
A ONE-MAN MULTI-LANE ROLLING BARRIER
In driving with huge empty spaces during rush hour, I find that the
space ahead of me doesn't just instantly fill up. Other drivers don't
change lanes to fill them. Weird! What's going on? First, if I'm
driving with a big empty space, sometimes (rarely) the car directly behind
me will pass me. Sometimes it happens twice. But this removes the lane
jumpers from behind me, and forms a row behind me of nonaggressive
drivers. Those drivers are like a plug, since any aggressive drivers many
cars back can't even SEE the big empty space ahead of me.
But what about the adjacent lane? Won't they all fill my empty space?
Nope. A few do change lanes, then they rush to the end of the empty
space. This filters out the aggressive drivers from the adjacent lane,
ones next to my empty space. They don't change lanes. They don't care
that there's a huge empty space growing and shrinking right beside them.
They form a big plug, and aggressive drivers behind them cannot get
big empty space.
This "plug effect" seems only to happen when traffic is highly
congested. When traffic is light, I can't maintain any really extreme
spaces, since aggressive drivers can easily swerve around to jump into the
space. But when traffic is light, traffic jams are minimal, so there's
not as much need to create an antitraffic bubble and perform some
Have you ever noticed when your highway lane ends, anyone merging
than you is a coward, and anyone merging later than you is a cheating
MAKING A REAL DIFFERENCE
During a year of practicing the "wave-smoothing" driving habits, I kept
looking for places where I could make a big difference in traffic flow.
Yes, I could always use an empty space to move a piece of the traffic jam
to another location. With a big empty space, I could even spread the cars
apart as I moved the slowdown, the same way I did it with the jammed
sections in the "traffic wave." But the genuine "bottlenecks" seemed all
too rare. Then finally I noticed that there was one
common situation where I could do some real good.
If you drive in heavy highway traffic, you've probably seen a traffic
wave develop at a construction site where one lane is blocked for a long
distance. You crawl
and crawl at 3 mph until you get to the start of the bottleneck, then you
turn merging as the two lanes sloooooooowly come together. Then you race
off down the single lane at 60 mph! No downstream congestion! The
merging lanes formed a terrible bottleneck, but they did not add together
overload the single lane downstream. The bottleneck was not the
downstream lane itself. Instead, a stalled "traffic wave" develops
at the merge-zone, and extends far back into the double lane.
After the wave, in the long single lane, it's clear sailing.
WHY must a bottleneck develop at a merge zone? Well, obviously because
there's too many cars on one road. And because everyone must take turns
slowly merging together. WRONG! Wrong wrong wrong. Even during
extremely low-traffic and high speed
conditions, still everyone must take turns. Yet everyone merges as
a high speed flow, like gear teeth or like a zipper. A
bottleneck never appears.
Traffic jams develop at a merge zone whenever the cars momentarily get
so close together that there are no gaps between them. Without gaps,
nobody can merge, and so the traffic suddenly comes to a near halt. The
"gear teeth" jams up, the "machine" halts, and a bottleneck is created.
The pile of pebbles all block each other and can no longer pour through
the funnel. Then traffic on the highway becomes something like a city
intersection, a "four way stop" where people pack together and take turns.
But whenever traffic comes to a near halt, people always
pack themselves together.
Huh. This is screwy. At the place where the lanes merge together,
close-packed cars cause the bottleneck, but... the bottleneck is the
CAUSE of the close-packed cars. And also, the close-packed cars keep the
bottleneck in existence. And the bottleneck causes the clot, it makes
drivers all pack together and start tailgating.
Do traffic jams CAUSE
THEMSELVES? After thinking about this even more, I realized that the
answer must be yes. It goes like this:
- Traffic is going slow.
- Everyone packs together and closes up the gaps.
- Fast merging becomes impossible.
- The incoming cars will grow a huge long back-up.
- Cars must slooooowly take turns merging.
- This makes incoming traffic slow down.
- Repeat this loop over and over.
This is absolutely fascinating because this self-caused situation has
- Traffic flows along rapidly.
- Nobody closes the gaps (they follow the 2-second rule?)
- Merging is easy.
- Streams of traffic flow together like a zipper.
- This allows traffic to go fast.
- Go back to the top of this loop and repeat.
At a merge zone, fast traffic causes traffic to remain fast, while
slow traffic causes a jam to persist. Weird! The difference in flow rate
between these two situations is enormous, yet EITHER ONE can arise on the
exact same highway under the exact same amount of incoming flow. In the
first one, the speed might be 2 mph, while in the second one it could be
40 mph. And here's the important part: because the situations create
themselves once they are established, sometimes they can switch from one
to the other. A smooth flow can hit a glitch and turn into a traffic jam.
Or somebody can switch them intentionally.
Suppose the traffic at
a merge zone was flowing fast as in Loop Number 2 above. Suppose I wanted
to wreck everything. I could get in the through-lane and slow way down.
Fight with drivers in the other lane, trying to block them.
Make all the cars pack together behind me. This would prevent any cars in
the other lane from merging into my closely-packed lane. Cars in the
merge-lane would then pile up too. Then I drive off laughing evilly,
have just CREATED MASSIVE LONG-TERM TRAFFIC JAM! The exit might stay
jammed for hours.
Or, I could do the opposite. Suppose everything is jammed up at the merge
zone. Suppose I accumulate a huge space ahead of me and bring it in
through the through-lane. When the huge space gets there, cars in the
ending-lane can suddenly change lanes into my space, spread out, and start
flowing fast. Next, I speed up and merge with them. The cars
behind me do the same. The "zipper-like" flow has begun. The switch has
have just ERASED a long-term bottleneck. As they say in those old Ranier
Beer ads, pretty cool, eh?