[wordup] Hacking Traffic for the Common Good

[Adam Shand]

This is great, I used to play with similar things when I had a crappy  
12 mile, 45 minute commute in Portland, but I never figured it out to  
this extent.  Neat!

Source: http://trafficwaves.org/trafexp.html

TRAFFIC "EXPERIMENTS" AND A CURE FOR WAVES & JAMS
1998 William Beaty Electrical Engineer

My first 'experiment': accidentally erasing waves!
Once upon a time, years ago, I was driving through a number of stop/go  
traffic waves on I-520 at rush hour in Seattle. I decided to try  
something. On a day when I immediately started hitting the usual  
"waves" of stopped cars, I decided to drive smoothly. Rather than  
repeatedly rushing ahead with everyone else, only to come to a halt, I  
decided to try to move at the average speed of the traffic. I let a  
huge gap open up ahead of me, and timed things so I was arriving at  
the next "stop-wave" just as the last red brakelights were turning off  
ahead of me. It certainly felt weird to have that huge empty space  
ahead of me, but I knew I was driving no slower than anyone else.  
Sometimes I hit it just right and never had to touch the brakes at  
all. Other times I was too fast or slow. There were many "waves" that  
evening, and this gave me many opportunities to improve my skill as I  
drove along.

I kept this up for maybe half an hour while approaching the city.  
Finally I happened to glance at my rearview mirror. There was an  
interesting sight.

It was dusk, the headlights were on, and I was going down a long hill  
to the bridges. I had a view of miles of highway behind me. In the  
neighboring lane I could see maybe five of the traffic stop-waves. But  
in the lane behind me, for miles, TOTALLY UNIFORM DISTRIBUTION. I  
hadn't realized it, but by driving at the average speed of the traffic  
around me, my car had been "eating" the traffic waves. Everyone ahead  
of me was caught in the stop/go cycle, while everyone behind me was  
forced to go at a nice smooth 35MPH or so. My single tiny car had  
erased miles and miles of stop-and-go traffic. Just one single  
"lubricant atom" had a profound effect on the turbulent particle flow  
within the entire miles of "tube."

It's always a good idea to drive without changing speed and without  
competing with other driversfor bits of headway. I'd always assumed  
that the reasons were philosophical rather than practical (i.e. try to  
be a calm, nice person.) But my above experience shows differently. A  
single solitary driver, if they stop "competing" and instead adopt  
some unusual driving habits, can actually wipe away some of the  
frustrating traffic patterns on a highway. That "nice" noncompetitive  
driver can erase traffic waves. I suspect that the opposite is also  
true: normal competitive behavior CREATES the traffic waves.

Suppose we push constantly ahead, change lanes to grab a bit of  
headway, and always eliminate our forward space in order to prevent  
other drivers from "cutting us off". If tiny traffic waves appear, we  
will rush ahead and then brake hard, leaving larger waves behind us.  
Repeated action causes the waves to grow huge. Ironic that the angry  
people who push ahead as fast as possible might unwittingly  
participate in "amplifying" the very conditions that they hate so  
much. The solution seems obvious: drivers with a smooth "calm" style  
will tend to damp out the waves and produce a uniform flow... and the  
few drivers who intentionally drive at a single constant speed will  
wipe out the waves entirely.

MORE EXPERIMENTS
I rarely commute on 520 where the good traffic waves appear. I started  
to miss the opportunities to cancel them. However, I soon realized  
that the same process could be used to affect the smaller traffic jams  
too. "Traffic waves" are simply a series of small traffic jams with  
even spacing. Each little jam is destroyed when a large empty space  
approaches it from behind. If no new cars are feeding into the jam  
from behind, yet cars are leaving from the front, then the jam is  
eroding away. If the jam is small enough, or if the empty space is  
large enough, then a single car can entirely annihilate the jam, as I  
had done with traffic waves.

Now I remember something from years back. When trapped in one of those  
"rubbernecker slowdowns", I always tried to accelerate like mad when I  
escaped at the end. I figured that if everyone did this, then the  
slowdown would evaporate. Yet this did no good, because the car ahead  
of me blocked my move. It would not accelerate. I could never force  
the cars ahead of me to stomp on the gas too, so I could do little to  
aid the "evaporation" of the traffic stoppage. Aha! I could control  
the people behind me by slowing down, but I couldn't control the  
people in front of me by speeding up. Therefore, I can smooth out a  
small traffic stoppage. I just have to acquire a huge empty space long  
before I approach it. But if I'm already inside the jam, I can do  
nothing to aid the "evaporation" at the far end. If I cannot predict  
where jams will arise, then I'd better drive all the time with a huge  
empty space. (Which is just what many truckers do. Did they figure  
something out that I didn't know?)

Just one single car, if it decelerates while approaching a jam, can  
change the behavior of everyone behind it. And soon these people  
behind that single car will take the place of everyone in the jam.  
Your single car can bite a huge chunk out of the region of stopped  
traffic. If one car refuses to pack together with everyone else to  
form a "parking lot," the jam can be made smaller. Or if one driver  
gradually builds up lots of empty space before encountering the  
slowdown, perhaps that driver can "eat" the whole slowdown just as the  
many traffic waves were "eaten" by my own car.

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STANDING WAVE: a "rubbernecker slowdown" ...without rubberneckers!  
Traffic-waves need not drift backwards. Sometimes they become pinned  
in one place. Once this type of traffic jam is established, it can  
last nearly forever. It can also grow to enormous size as more cars  
arrive from behind. New cars MUST slow down as they enter, and that's  
what makes the jam persist. You cannot dissolve the jam by  
accelerating out at the end, since you're still blocked by the driver  
ahead of you. The solution: bring a huge space in as you approach it.  
This temporarily cuts off the flow of incoming cars which feeds it and  
keeps it alive. A single driver can easily erase a small jam, removing  
the bottleneck and turning it into a wave which moves backwards.
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 up ahead of  
me. By the time I hit the jam, there was maybe 1000ft of empty road  
ahead of me. 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 about half the size it had  
been. Amazing. This wasn't any little traffic wave, yet one single  
driver was able to take a huge bite out 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 several miles 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 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,  
so the merging cars sat unmoving in the thru-lane, waiting for a space  
to open, and stopping everyone behind them. 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.


"Anti-traffic"
Here's a general principle I take from the above. (I guess it's  
obvious in hindsight!) ANTITRAFFIC DESTROYS TRAFFIC. Empty spaces can  
break up a traffic jam. While I was slightly slowing down to allow a  
space to gradually open up before me, I was creating a pulse of  
"antitraffic" ahead of me. When 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 behaves very differently than the original  
bottleneck: it travels backwards and moves 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. Each of us could  
have allowed a big blob of anti-traffic to appear, and then the  
repeated impacts of the 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 large space ahead of us, since it's too easy  
for cars to pass a slightly-slow driver. But 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 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 it.

But won't other drivers simply go around me and fill my big empty  
space? Hmmmm. After trying this many times, I find that they usually  
don't. I wonder why? (see the FAQ.)

And here's a final lesson: a bit of math! (This part took me years to  
figure out.) A traffic jam is a pattern in the cars, but what  
*exactly* is a traffic jam? Simple: it's a traffic pattern where the  
outflow from the pattern has become constant. Drivers can't affect the  
outflow rate from a traffic 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 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 the outflow from an un-jammed highway. (Think: if  
the outflow from a jam was better than the outflow from a normal  
highway, then the sudden improvement in outflow would drain out the  
cars, and the jam would instantly evaporate before it had a chance to  
really get started.)

Ooops! Damn!
While doing all of the above, I once caught myself behaving normally  
and 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  
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:  
an inability to merge in dense traffic. Others were probably doing the  
exact same thing that I did, and this would make the "wave" near the  
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. And 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 to the front of the line.  
But this sort of "closed-gap" driving would also prevent any  
necessarymerges 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 ramps. 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 them.

The jammed merging lanes are almost a city's gridlock. 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 waves and "highway gridlock." I  
eliminate the "solid 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 three of this article. Ideally a  
merge-area will act like gear teeth. But suppose 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 arrow which  
blocks one entire lane of traffic during construction. 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 the same thing! It's a solid packed wall of hostility.  
Poking a hole in that wall can make a difference.

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 cellphones and GPS units), 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: 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 to Tacoma, then  
drive north. But if I tried driving slightly slow, a space would never  
open up ahead of me because 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 space opens up. After many  
minutes, the traffic which had been feeding into the city traffic jams  
simply stops arriving. There is no new traffic for many minutes. The  
huge jam trickles 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.

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... so even though the close-packed region is now a big  
fuzzy slowdown, the flow of traffic does greatly increase.

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 the "rolling 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 area of slightly-slow traffic a few miles upstream from the  
accident. Would the slowdown 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 "gone," 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 frustration.

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 empty highway 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.)

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 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  
several 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,  
letting them move to my lane at the end of my space, and leaving sane  
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 to my big empty space.

This "plug effect" only happens when traffic is highly congested. When  
traffic is light, I can't maintain a big empty space, since aggressive  
drivers can easily swerve around to jump into the space. But when  
traffic is light, there's no traffic jams, so there's no need to  
create an antitraffic bubble and perform jam-busting.

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. You  
crawl and crawl at 3 mph until you get to the bottleneck, then you  
take your turn merging as the two lanes sloooooooowly come together.  
Then you race off at 60 mph! No downstream congestion! The merging  
lanes formed a terrible bottleneck, yet the open lanes ahead were not  
a bottleneck. A "traffic wave" develops at (and behind) the merge- 
zone. After the bottleneck, it's clear sailing.

But why?

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 conditions, everyone still takes turns, yet  
everyone merges as a high speed flow,like a zipper. A bottleneck never  
appears.

Traffic jams develop at a merge zone whenever the cars 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" jam up, the "machine" halts, and a bottleneck is created. The  
pile of pebbles can no longer pour through the funnel. Traffic on the  
highway turns into a city street intersection, where people merge at a  
"four way stop sign."

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 the close-packed cars keep the  
bottleneck in existence. And the bottleneck makes drivers all pack  
together.

But, but...

Do traffic jams CAUSE THEMSELVES? After thinking about this even more,  
I realized that the answer is yes. It goes like this:

No. 1

	? Traffic is going slow.
	? Everyone packs together and closes up the gaps.
	? Fast merging becomes impossible.
	? Incoming cars create a huge back-up.
	? Cars must slooooowly take turns merging.
	? This makes incoming traffic slow down.
	? Go back to the top of this loop and repeat.
This is absolutely fascinating because this self-caused situation has  
a counterpart:

No. 2

	? 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 between  
these two situations is enormous, yet EITHER ONE can arise on the  
exact same highway under the exact same amount of traffic. 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 slow way down  
and make all the cars pack together behind me. This would prevent any  
cars in the other lane from merging into the closely-packed lane. Cars  
in the merge-lane would pile up too. Then I drive off laughing evilly,  
because I 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  
into the jam. When the huge space gets there, the other lane of cars  
can suddenly change lanes, spread out, and start flowing fast. Next, I  
speed up and merge with it, and so do the cars behind me. The "zipper- 
like" flow has begun. The switch has flipped. I have just ERASED a  
long-term bottleneck. As they say in those old Ranier Beer ads, pretty  
cool, eh?