TRAFFIC WAVES
FREQUENTLY-ASKED QUESTIONS

A: Nothing, I am not a traffic professional. I'm a double-E and science amateur. Also I'm just a guy who is fascinated by the "traffic physics" I see during my commutes. Perhaps I'm that little kid from the story of the Emperor's New Clothes: I'm not an authority, but neither am I blinded by preconceptions. Also, I have no academic reputation to defend, so I have no inhibitions about posting my observations and reasoning, flaws and all.

Also, I grew up in the age of home-built 8080 kit-computers and spent gazillion hours futzing with cellular automata such as Conway's "Life." I'm especially fascinated by Game Theory, chaotic dynamics, and emergent phenomena, and so I'm "sensitized" to notice lots of it while commuting. I have the viewpoint of a physicist, not of a traffic engineer. Still, all the material on this website is nothing but speculation and "crazy experiments" performed by a total amateur. For info from actual experts, try some of these links, especially the article in Science News.

ANS: Wiping out traffic waves is a "theory?" I didn't start with any theory. My suggestions are based on a large number of actual testing, not on any pre-existing theory. I try various things, then I see what works and what does not. Rememer, I discovered the whole jam-busting effect by accident, when I was trying to drive at the average speed of traffic. Smoothing out the stop-go waves would tend to let a huge space open up ahead of me. I found on numerous occasions that the big space did far more than just eat traffic waves.

Those who haven't actually tried these techniques will disbelieve them ...on theoretical grounds. In theory, people will always go around you and fill your space. But if you actually spend some time in testing, you'll quickly find that this theory is wrong. In the real world something different happens.

In heavy highway traffic where all lanes are moving at about the same speed, whenever I bring in a large space ahead of me, a couple of aggressive drivers sometimes merge into it. If they don't just merge back out again, this takes them out of the adjacent row of cars. With aggressive drivers removed, that leaves behind the patient drivers who do not constantly weave and jump lanes. This is how the adjacent row gets filtered, leaving behind only the patient drivers. These patient drivers next to me will then act like a "plug". They keep the impatient speeders far behind me from getting to my big space. (The speeders directly behind me can sometimes pass me... which leaves only patient folks behind me as well.) Also, the ones FAR behind me in line won't even know about my empty space. The cluster of patient drivers behind me keeps anyone from even seeing the space.

The "plug" can only develop when traffic is heavy. In light traffic, there is no way to preserve a huge space ahead of you. Anyone can easily drive around your car. But in light traffic, jams don't usually arise, so there is no reason to create a big space. (Of course when you're driving at 60MPH, you naturally acquire a big space, same as everyone else. Don't forget this key fact.)

I think the real issue here is emotions, and "losing" versus "winning." If commuting is a car race, and if I let anyone jump into my big empty space, why, that makes me an inferior LOSER! That person is now ahead of me in the race for the finish line and they've stolen my glory. They'll arrive at the finish in first place, ten feet before I do! So, I must never open up a space ahead of me, that just makes me vulnerable. Right?

But if you think about it, you'll see that there is no race. There is no finish line, so any small gains are useless. I could spend my entire commute trying to get two carlengths farther than everyone else, but it would be stupid and pointless. Two carlengths is only 2 to 5 seconds in my 40-min commute. We can beat any driver on the road no matter what their skill level... by simply waking up a few minutes earlier in the morning! Just go and set your alarm clock 10sec earlier, then you won't have to spend your entire AM commute fighting for 10sec of headway. Drat, only able to set the clock by 1-min earlier. Does that mean I get to allow thirty cars to merge ahead? :) What if I should dare set my alarm clock an entire FIVE minutes earlier, what then? WHAT THEN?!

:)

See, this "race" is a bunch of stupid nonsense created by insecure immature drivers. Screw 'em. Instead do the opposite: look for ways which you can personally improve the traffic flow. Small increases in speed can mean minutes saved for everyone. Let people merge ahead of you whenever they need to, and you become the more professional driver.

ANS: I'm still just fine! :) Actually, I've had no problems with road-ragers, and for a simple reason. If the person behind me is a hostile idiot, eventually they'll change lanes and pass me. I can even help them do this. Then they quickly drive far forward, and they're happy. Remember, I still have that huge space ahead of me, and for aggressive drives it's too tempting. That often leaves a good driver behind me. If there are several road-ragers, they'll manage to get out of line too, and I'll be left with a long row of nice people behind me. Once this has occurred, I can maintain a large empty space for hours with no problems. But more frequently, the person directly behind me is not a road-rager in the first place.

When I first started driving with empty spaces, I was really afraid that road-rage would be a big problem. Like magic the problem cured itself. Those occasional road-ragers never even stop to give me the finger, they'd rather rush ahead to the end of my large space to "win the race." I think this even makes them happy, since they imagine that they're passing all those cars in the neighboring lane. Hey! Maybe these large empty spaces will let the speeders go 0.1MPH faster than everyone else, which would help to reduce road rage!!! Well, I can dream, can't I? :)

WARNING: NEVER DRIVE SLOW. If you suddenly slow way down in an effort to create a huge forward space, the people behind you are going to rightly get annoyed, and you might even present a danger to other drivers. On the other hand, if you only drive 1/2 mph slower than average, then other drivers might not even notice ...yet after 60 seconds you accumulate an extra car-length of space ahead of you. It's always better to bring space into a region of congested traffic, rather than trying to create space once you're packed in tight. When you hit the morning congestion, preserve the space you already had in the high-speed section of highway. But if you must slow down to avoid becoming a tailgater, do this so this doesn't trigger traffic jams. Do it over many minutes so nobody behind you even realizes it's happening.

ANS: Let's reprhase that.

"In a huge multi-lane traffic jam, if everybody would just move out of the passing lane, then I alone could drive really fast!"

Obviously that's not going to happen.

First, the people in the passing lane are not slowpokes. Actually they're just like you: they're all trying to drive fast, and they're angry at the "slow drivers" ahead of them. Ask yourself this: what does the person driving behind you think? He thinks you're the slow driver who should get out of the way! And you in turn think the person ahead of you is the slow one. All of you are in the same boat; driving slow, yet taking no responsibility for it, and blaming the drivers ahead of you. You believe you know the cause of the slowness. But you're wrong.

This brings up an interesting question: why is your lane slow? What causes the jam? Does each lane have one single very slow driver far ahead, a driver plodding along with miles of empty road ahead which grows ever larger? No, not usually, not in heavy congestion when all lanes are moving below the speed limit. Instead, the column of cars ahead of you is miles long, and they all drive slow because of a very natural rule. It's simple: the speed of each lane is controlled by the spacing of the cars. Let me say that twice. The spacing between the cars in a long congested lane determines their speed. The closer they pack together, the slower the lane. This is part of long-known traffic theory, and illustrated in the set of graphs called a " Fundamental Diagram"

If fast drivers in your lane are jammed close together, conditions become unsafe, and the entire group slows down by unspoken agreement. In other words, a clogged lane always gets slower, like magic. (Clogged lanes are slow? Who would have guessed!) Next, the impatient drivers will abandon the clogged lane and move to the faster adjacent lane. That adjacent lane then slows down instead. And by all leaving the slower lane, they made the slower lane become faster! After some time has passed, drivers weave back and forth, and eventually all the lanes end up going at the same speed. Finally there is little reason to switch lanes anymore, and the weaving mostly stops.

So the fast lane wasn't really blocked by slowpokes, instead all lanes had adjusted themselves to have the same traffic flow and the same speed. This effect is called "Synchronous Flow." It only appears during congested conditions. Here's a 1998 research paper by the mathematicians who discovered it: Helbing and Huberman.

ANSWER 1:

Right. That's the whole point. We want people to merge ahead of us before that other lane comes to an end. If I fear that someone will leap into the space ahead of me, or if this makes me feel angry or "inferior," then I close up ranks and prevent everyone from merging. I turn myself into a tailgater. Or, if I try to become the "vengance police" and punish the cheaters who zoom ahead, then I close up ranks and block all the necessary merges. The closed ranks create traffic jams. "Cheaters" don't trigger traffic jams, it's the tailgaters who try to punish the cheaters who do it.

Lane jumpers are not the real problem. In the merge-jam animations, the goal isn't to maintain the empty space under any circumstance. The goal is to allow people to merge ahead! The slower animation at left is slow because the only available space is at the end of the jam. When people line up and tailgate, there can be no high speed "zipper" or "gear teeth" pattern. But what about the times when we are far from the merging-lanes area... ?

I've found that other drivers frequently jump into my space... but then they jump across my lane and back out again! They weren't trying to beat me in the race. They didn't want my space after all. Instead they neededed some way to penetrate the solid "wall" of cars in my lane and get to the other side. Since most drivers close up ranks when driving slowly, my "traffic hole" is usually the only space in a close-packed lane of cars. During my commutes, sometimes my empty space helps quite a few drivers who otherwise would have to force their way between the cars in the adjacent lane and thereby halt two entire lanes of cars! To say nothing of collison danger; the screeching brakes and honking horns.

If you drive slowly, then everyone will want to pass you. So don't drive slowly! Often you don't have to create an empty space, instead you just have to maintain the large space that appears naturally when you'd earlier been driving fast. In other words, when you approach a slowdown, don't drive right up to the bumper of the car ahead of you and then hit the brakes. Instead preserve your large space whenever you're forced to slow down. Don't drive slow, instead just Stop Tailgating. Watch the professional truckers around you. If they can do it under those road conditions, then so can you. Bring large spaces into congested regions. Obviously don't try to quickly create a huge space when you're already in heavy traffic. That in itself could trigger a jam behind you.

Yes, there are many drivers who are always sure that the other lane is going faster. They welcome a chance to change lanes, and will jump into your space as soon as you pass them. However, these Lane Jumpers can't stand to stay still, and soon they will jump again, and your space will be restored. (And ten minutes later they'll still be nearby, since lane-jumping rarely gets you there faster.)

One solution: leave space for five or ten cars ahead of you. That way the lane-jumpers won't need to act so suddenly. If you maintain a tiny space, then you'll have to hit your brakes if anyone merges ahead of you. On the other hand, if your space is large, then cars will tend to jump into your lane slowly and much farther ahead of you, and you'll have plenty of time to respond.

I find that heavy highway traffic often makes it easy to maintain a big space. People will not fill it. In the heavy traffic, a handful of lane-jumping drivers might jump into my big space at the start, but this depletes the supply of lane-jumpers in that neighboring lane. Soon the lane-jumpers have moved to the far end of my space, and the entire lane next to me is now full of well-behaved drivers who don't mind watching my big space grow and shrink right next to them. Those thoughtful drivers in the adjacent lane act like a "plug", and lane-jumpers cannot get past them. In lighter traffic, the supply of lane-jumping drivers seems infinite. But in light traffic there is no reason to maintain a huge space!

Let me get this straight. You're saying that tailgaters relieve congestion? Should we punish the rare, non-tailgating driver?

:)

Wrong, but let's not listen to all the traffic research, and instead take a look at what actually occurs. There are two competing effects: eliminating all the tailgaters will speed up traffic [GOOD!], and eliminating all the tailgaters creates empty spaces which pushes everyone backwards away from their destination [BAD!]. Which one wins out? This is partly answered by an ancient graph of traffic behavior: the "fundamental diagram." This graph of car speed vs. flow-rate is shaped like a hump, and it both shows that congested conditions give a low flow rate, yet high speed driving on empty roads also gives a low flow rate. The highest flow is when light congestion brings everybody's speed down to 30-40MPH. And a lower flow will cause the backups to grow and grow ...and grow.

So, in order to quickly empty out a jammed highway, drivers should spread themselves out until, on average, they all have a "35MPH pattern" with two or three seconds between cars. Two or three sec spacing at 35MPH gives 100ft to 150ft between cars.

If nothing else is slowing down the traffic (no accidents or merge zones,) then this 35MPH-spaced traffic will automatically change its speed to 35MPH, and will have maximum flow. Whenever cars pack together closer than 100-150ft, they unconsciously slow down to maintain a minimum safe timing between cars. They're no longer at the peak of the "Fundamental Diagram" graph. And then, while this might mash lots more cars into each mile of highway, also they move very much slower. The two effects fight each other, with low speed causing low flow, but higher density increasing the flow. The graph shows that, below 35MPH, the lowered speed has a bigger effect than the increased number of cars, so the total flow gets reduced whenever drivers pack together closer than 100ft. If the flow is reduced, the long jam starts growing even longer as more cars pile in at the back of this low-flow highway section.

These backups push you back away from your destination. They grow or shrink slowly during rush hour. After a couple of hours, small differences in average flow will give us huge differences in the length of the backed-up traffic. (This also shows why the waiting line at the women's bathroom is almost always longer!)

So, what's the obvious way to reduce an enormous backup? Just get everyone to stop blocking the interlopers, and instead space themselves out at 150ft distances. As long as nothing else limits the flow, this will erase the bottleneck and drain out the jammed section as rapidly as possible (or at least slow its growth during rush hour.)

On the other hand, if everyone spaced their cars at far greater than 150ft, this also would reduce the average flow and would promote the growth of backed up lanes. On the Fundamental Diagram graph, this would put everyone down on the far side of the hump. So it's not a good idea to have every single driver performing the "jam erasing" trick with a 500ft space. They just need the usual safety-gap of two or three seconds. (Jut imagine if rush hour was all big trucks only, each trucker maintaining a quarter-mile gap! Hmmm. Maybe I could weave through them at high speed...)

Now don't lose sight of the reason for jam-erasure: it averages out the speed of traffic. When you maintain a space, you don't actually drive slow. The other lanes don't pass you, on average. First you start out with a large space, but then this space shrinks to nothing again as you arrive at the column of stopped cars. You haven't slowed any down behind you, you just made an empty space appear and then vanish again. But if you timed it right, the last of the jam would be gone just when you arrived at the jammed location. With the clot gone ...won't everyone take off at high speed? I sure would. After all, the highway downstream from the jam was fairly empty. Just make things change, so that "high speed" becomes the average speed of your lane.

Anyway, whenver you see a long lane full of vehicles which are driving close together in efforts to "pack more cars onto the highway," you can be certain that their efforts have backfired. They've programmed themselves to drive unusually slowly. Their flow rate is less than the ideal rate for that road. As long as their spacing is significantly shorter than 100-150ft, then their slow speed dominates the flow behavior, and it outweighs any benefits of packing more cars onto the road. So, all of our pushing ahead and closing up gaps only works at high speeds and sparse traffic. Once everyone slows down to the 35MPH and 150ft spacing pattern, it becomes counterproductive to try reducing spaces any further.

Here's another way to say the same thing:

Passing other cars does little, while small speed increases can greatly shorten your commute time. For example, during a half-hour commute, when congested traffic is going at 30MPH, if you could just drive only 1MPH faster, you'd shave off an entire minute from your commute time. Remember, 1MPH is tiny; it's five times slower than walking. On the other hand, leaping ahead by a few spaces doesn't do much, because the congested traffic usually has only 1 to 2 seconds between cars. So in order to shave a minute off your commute through aggressive pushing, you'd have to somehow manage to pass 30 to 60 other cars in the jam!

So you see, your place in line during heavy congestion is basically irrelevant. If you pass a couple of other cars, you'd better hope that your actions didn't slow the flow by a even a tiny bit. If drivers' efforts to gain a carlength of space would cause the lane to travel a couple MPH slower on average, then they've made their commute a couple minutes longer in their extensive efforts to make it five seconds shorter. The moral of the story: whenever really heavy congestion appears, change your behavior. Start doing everything you can to "lube the flow," and under no circumstances should you try to gain a few seconds (a few car lengths.) If you can make your lane a bit faster, it has a large effects on commute time. Of course don't stop looking for opportunities to gain entire minutes by passing vast numbers of other cars. (Good luck in finding any! Best wait until you get past the big daily jam, and hit the open road downstream.)

ANS: Traffic physicists know that this is a myth. Their simulations show the misconception. After all, the waves appear just as often when all the cars are inhuman driver-simulations obeying identical rules. So, perhaps we should call the brake-tapping idea by the name "Driver Blaming Theory" of traffic dynamics? Or the theory of, "My tailgating didn't cause it, somebody else tapped their brakes?" :)

If we simulate heavy traffic with perfect robotic drivers, traffic waves always appear. Merge zones still get jammed. Micro-fluctuations or "noise" gets amplified by the system until it creates huge jams from nothing. Driver stupidity and human psychology isn't the cause.

Let't take a close look at what happens. Suppose you're driving slightly faster than everyone in your lane. People behind you will notice this and speed up too. But then you approach the car ahead of you. Don't hit the brakes, just take your foot off the gas. You slow down slightly. After a short delay, the driver behind you notices, and they stop accelerating too. But because of their reaction time, they end up getting dangerously close to your rear end. The next car back does the same thing, but they get even closer to the car behind you. Perhaps they have to tap their brakes. If not, then the car behind them certainly does. And the one behind that one, they brake even harder. The lack of reaction time is stacking up, and it might cause someone to brake to a complete stop.

So, in the above case, the wave was caused by someone driving fast. This *forced* them to decelerate slightly in order to avoid a collision. They had no choice but to brake (but they did choose to drive slightly fast.) All the cars behind were forced to brake too, in increasing urgency. What if those were all robot cars? Doesn't matter. They'd still be forced to hit the brakes to avoid a collision. See what's going on here? The column of cars has become an amplifier! It's inevitable that any tiny change grows into a huge wave. The change might be caused by a driver who drifted a tiny bit too close to the car ahead. Or it might be caused by a slightly slow driver who gets too close to the car behind. But these are tiny changes in speed, changes that EVERYBODY creates all the time. In light traffic, with proper spacing between cars, they die away instantly. But when cars are packed too closely at too high a speed, the small "noise" grows larger and larger. The traffic waves are inevitable when tailgating is the norm. If you're looking for someone to blame, well, do you blame a particular sand grain for triggering the growth of that row of sand dunes? Or on flapping flags, which fabric-fiber caused the initial tiny bump that started it all? It's ridiculous! But I see this driver-blaming brake-tapping stuff in articles, some even written by professional traffic experts. They should well know it's a natural consequence caused by tailgating, not by brake-tapping.

ANS: Nothing's wrong with that! That is the easiest way to make a difference. Also, it won't get anyone angry. The road-ragers don't care about a tiny one-car empty space ahead of you. However, be aware that road-ragers are not a big problem.

On the other hand, if we only let only one person merge, it's like donating a single penny to charity: we could wipe out poverty if EVERYONE did this. Since everyone does NOT do it, then anyone who wants to make a real difference will have a big opportunity. But they DO have to bite the bullet to "donate" far more than just a single penny. Let five or ten people in if you can. (Or, just leave for work a few minutes early. Each sixty seconds is worth thirty cars merging ahead of you.)

When letting one person merge, here's one issue to think about:

Why do streets have stop lights? Wouldn't 4-way stop signs be just as good?

The answer is simple: stoplights are far better than stop signs. It's because cars taking turns can only drive very slowly. In heavy traffic, taking turns at a merge zone gives VERY low flow. Instead, if we let 20 cars move forwards during each "turn", then the traffic flows much faster. So, when you're driving in congested traffic, the merge-zones act like stop signs. People take turns verrrrry slooooooowly. Even if you don't erase the whole jam, you personally can convert this into a "stop light", at least temporarily. A single driver can improve traffic: simply let five cars merge ahead of you! Or ten or twenty. But DON'T just stop dead to let them through, since that will REALLY anger the drivers behind you. Instead, build and maintain a 20-car empty space long before you approach the daily jam. And whenever you approach a row of stopped cars, REFUSE to close your space and pack yourself in behind the last car. When traffic goes from 60MPH to 20, slow down prematurely so you preserve your big space ahead of you even when you're stopped dead. But only go from 60 to 20, and don't slow down any more than anyone else. Just slow down slightly early.

ANSWER 1:

Smoothing out the clots has no effect on flow? Possibly incorrect where traffic waves are concerned. Stoppages are not linear (they are similar to soliton waves.) The M. Treiber ring-road traffic simulation gives interesting results. With max congestion (80 cars per KM in a lane), the average speed depends on the presence of traffic waves. Without any traffic waves, the average speed rises to a little above 42KPH. With many small traffic waves (11 waves encountered for each KM of progress), average speed drops by 20%, down to 36KPH. With a great huge waves (3 waves encountered per KM of progress), average speed drops 30%, down to 30KPH. It seems that the "fast moving" sections of the waves don't make up for the time lost while waiting in the "stopped" sections. In other words, the "stopped" sections of the wave behave as serious bottlenecks; bottlenecks which can vanish if the waves are smoothed out.

But that's a simulation. Does real traffic behave like that? Do "traffic waves" create a bottleneck, or do the drivers simply drive twice as fast in the unjammed parts? Also, is it better to prevent any outbreaks of waves in the first place? Or, after waves have appeared, is it actually possible to speed up traffic by 30% via 'wave-smoothing' tactics? It might be possible. It seems sensible that the waves would seriously reduce the average speed, but we need some real-world measurements.

The average traffic flow cannot be improved? Incorrect where merging lanes are concerned. Look at the merge-jam animations again. Count how many cars are exiting off the top of the screen over time. The "jammed" animation at the left has a far slower traffic flow. A traffic jam can act like a plug. Drivers would love to get past that plug and rush into the nearly empty roadway which has opened up downstream of the jam. If the jam causes a tail-back to grow and grow, and also causes a huge empty region to appear on the other side of the jam, then that jam is a bottleneck. If the jam was removed, traffic flow will increase significantly. (But if I wipe out the jam, will it immediately reappear? Real-world testing is needed.)

It's obvious that a merging lane can sometimes act like a switch; under certain conditions can operate either in "jammmed mode" or "unjammed mode". More importantly, once it flips to one pattern, sometimes it persists in that state. If a traffic jam exists at the merge zone, then more cars will pack in behind, and the jam will get worse. On the other hand, if there happens to be no jam in the merge zone, and if the cars can easily merge at high speed, then no jam appears.

In my experience this is a very common situation in heavy traffic. If the merge zones could somehow resist being flipped into "jammed mode", city traffic would be far less congested.

If something should trigger the formation of a merge-zone traffic jam, the jam will persist long after the "trigger" has ended. Why is the antitraffic-bubble technique important? Because it tends to flip the jammed regions into their un-jammed state, and with luck, the jam will not reappear for awhile.

That's why a single driver can have an enormous effect on traffic. And a single person can light up a dark building if all they do is go around flipping all the light switches to "on!" When nobody knows that these "light switches" even exist, then a little knowledge can make a big difference, and a single person holds immense power to do some good.

Does this technique always work? Of course not. Sometimes traffic jams are caused by a backup such as a clogged exit or a true bottleneck. If two lanes merge into one, and if that one lane is also congested, then a traffic jam will certainly develop in the two merging lanes. Fancy driving techniques can't change this. However, in many cases the two merging lanes have a big traffic jam, yet the single lane beyond the jam is... totally empty! In Seattle traffic I see this all the time. The traffic jam itself is the bottleneck, and the single lane beyond the jam has hardly any traffic on it. In this common situation, "antitraffic" drivers can do some good.

"Jam busting" can't improve the average speed? Incorrect where "rubbernecker slowdowns" are concerned. Slowdowns at accident scenes often behave as genuine bottlenecks. Visualize the highway as seen from above. More and more cars are piling in from behind, and the jam grows longer and longer. At the same time, all the traffic downstream of the bottleneck drains away. The highway is empty after you get past the jam. The jam is creating a blockage. This is different than the situation with standing waves or with traffic waves. With the waves, the jam remains the same size as it moves backwards, and there is rapidly growing jam getting continuously larger while at the same time the empty space downstream is growing too. If the "rubbernecker jam" could be removed, the bottleneck would be gone, the jam would stop growing, and the overall traffic flow would greatly increase.

But is it possible to remove such a jam? Yes, I think so. But I've never experimented with this. Suppose we could move the entire lookee-loo jam backwards by a few hundred feet. In that case the cars would speed up after they left the jammed section. This means they would be moving fast when they went past the accident scene. Back in the jam itself, there would be no accident to attract drivers' attention... so they'd all start accelerating into the enormous empty space ahead. The jam would no longer be "pinned" in place. The jam would start dissolving at the same rate that it grows. It would become a traffic wave; a very large one. But the key point is that it would stop growing.

And this change would be accomplished by moving the jam backwards, so that the jam is no longer "pinned" in place. Remember, just one or two drivers can move a traffic jam backwards. (The big question is: would the slowdown just reappear again? Or would the fast drivers remain moving fast as they passed that police car sitting by the roadside?)

ANS: Yes, there are many situations where a single driver can create a traffic jam. "Traffic incidents" trigger jams every day. Might we accidentally make traffic worse by trying to erase "traffic waves" or "merge jams?" Yes.

C. Spencer points out that whenever a highway has a row of exits or entrances, if we try to erase the traffic jam at the most forward (most downstream) exit, we might briefly slow down to create the antitraffic-hole. If we slow down while driving past one of the earlier exits, we could trigger a permanent traffic jam there. It would be very unwise to intentionally drive slowly while driving within merge-zones of highway ramp! It's best to acquire your antitraffic-hole while driving far upstream, in the light congestion far from any highway ramps. Often you never have to drive slowly in the first place. Just bring your hole with you as you slow down from 60mph to 20mph at a traffic jam.

Another possible problem: if the traffic flows in waves, then people at the on-ramps are able to get in. Merging cars just have to wait for a wave of "emptyness" to arrive. However, if the waves are gone and the traffic is totally smooth, people at the on-ramps could wait forever and never see a big gap.

Another place where we could unintentionally cause problems: by increasing the throughput of a highway that feeds into a congested area. Suppose I erase a large region of traffic waves. Suppose this DOES cause the traffic to speed up on average (though many people here have argued that it does not.) If this traffic is feeding into a city, then by increasing the flow, I've probably triggered traffic jams in the city. (One way that the professionals cure congestion is to install electronic speed-limit signs on highways leading to a city, then lower the speed limit on that section of highway.) If a traffic wave is behaving like a speed-limit sign, then by raising the speed limit we can cause problems elsewhere.

The same is true of merge-zone traffic jams. If the traffic jam is on a highway leading to a highly-congested area, then by removing one jam we are increasing the capacity of that highway. If the traffic flow increases, it could trigger new jams downstream.

ANS: Partly true. A single driver has a disproportionally large effect on traffic. Small "traffic events" can trigger traffic jams, and sometimes a single driver can "un-trigger" them. For example, a single large space can temporarily break up a small merging-lane traffic jam. And a single driver can certainly wipe out many miles of traffic waves. I've done this myself. Try it! Yes, if everybody maintained a space, then the spaces would only need to be a couple of car lengths or so. But remember, if you let ten cars merge ahead of you, then you create the benefits of AT LEAST ten normal drivers. I suspect that the effects on merge-zone traffic jams are far greater than that. After all, traffic lights are much better than four-way stop signs! Let twenty cars merge ahead of you, and you've briefly converted an excruciatingly slow "4-way stop sign" into a traffic light.

Actually, you've hit on a central point in the controversy between traffic physicists and traffic engineers. The engineers dislike the invasion of NONLINEAR DYNAMICS into their field of expertise. It seems that they're certain that traffic is a "linear" phenomeon like water flowing through hoses. If traffic has "linear" behavior, then in order to improve traffic flow, we would have to change the behavior of every single driver on the road. And if we wanted to change traffic by 10%, then we would have to educate 10% of all drivers. That's linear-dynamics thinking.

On the other hand, if traffic is "nonlinear," then it can experience phase changes. It can display emergent patterns, spontaneous self-organization, and several other phenomena studied in Nonlinear Dynamics. If traffic is nonlinear, then a single driver can trigger a major reorganization of traffic patterns. If traffic is nonlinear, then a single driver has disproportionally large effects like the butterfly wings that create or cancel a hurricane. If traffic is nonlinear, then it might be full of "fluidic flip-flop" switches which can be "flipped" by the intentional actions of individual drivers.

Is traffic linear or nonlinear? I offer an "existence proof." Because I can personally erase traffic-waves, and sometimes can erase a small merge-zone traffic jam, therefore traffic sometimes obeys the rules of nonlinear dynamics. Sometimes a single driver can create vast improvements in the flow.

I have no theory, I only have explanations of what ACTUALLY HAPPENS during my commute.

Zipper merges work fine in medium-heavy traffic. I see them in Seattle, and I've seen them in Boston, Rochester, and other cities I've lived in. And in Seattle I've "busted" merge jams many times (they even break up ahead of me, BEFORE I arrive at the jamming point. The effects of a Big Empty Space are awsome to behold!)

On the other hand, in extremely heavy traffic conditions where a highway exit is overloaded and backed up, a zipper-merge cannot survive, and if I try the jam-busting trick on the merge jam, it doesn't work. But this type of overload is not common in Seattle. Instead these jams are the type where, once you get past the jam and enter the exit ramp, there are no cars ahead of you at all. You can take off at high speed. This type of jam can often be "busted" by a single driver.

In somewhat congested traffic, before any jams occur, the merging area at a highway exit becomes "sensitive," and highway incidents can set off a jam. A merge-jam can be triggered by a traffic wave on the highway. Or it can be triggered if traffic flow hits a momentary peak and the exit becomes temporarily overloaded. Or sometimes two egotistic idiots get into a race where the guy in the exit lane won't let the guy on the highway merge, and then one or the other has to hit the brakes, causing a jam to appear behind him. In any of these cases, once the merge-jam has been created, it continues to exist, sometimes for hours. There is no reason for the jam, instead it supports itself, and if something should interfere with the jam, it will evaporate.

So, if I arrive at a huge traffic jam at a highway exit, chances are that the traffic flow is far too low to cause a jam on its own. Instead the jam was triggered by something. And in that case a single driver can often un-trigger it again.

In Seattle there is one ABSOLUTELY HUGE jam which can be busted by a single driver. Sometimes the backup is over a mile long. When I try the Big Empty Space technique, I can evaporate that jam, and it breaks up ahead of me as I watch. It's truly stunning. And nobody but me knows why the jam broke up. It doesn't work every time (maybe one success out of five tries.)

But it's such a cool trick... that I'm not going to tell anyone where this jam is.

If everyone knew about it, then Seattle commuters would be triggering its evaporation all the time. The huge daily jam would cease to exist. Or maybe it would only appear when the exit ramp was actually overloaded (during conditions where jam-busting fails to work.) In any case, my fun would be ruined. Someday I'll have to do some videotaping and make some time-lapse footage of the jam and my effects on it. But until I get it on film, that jam is an endangered species, and I must take care to preserve it in its natural state! :)

But I'm not heartless; I'll reveal the location of a smaller one which also is sensitive to a single driver. Just north of Seattle, on the southbound "Express lanes" which go under the huge I-5 bridge over Lake Union, the center lane gets backed up during morning commutes. Sometimes the backup extends all the way to the bridge. This is the through-lane, and in the tunnel under the city center the Express Lanes neck down to a single lane which merges with I-5 just south of the city. If drivers maintain say a ten or twenty car space while trapped in that center-lane jam, sometimes the jam evaporates. This happens because the jam is kept alive by cars which get trapped on the wrong side of the clogged lane, and they're down there at the end, forcing their way through the backup so they can get to the city exits on the other side. Anyone who provides a huge empty space will let these cars flow through the jammed lane. They stop pushing slowly through the jammed lane. And this interferes with the process which keeps the jam in existence.

Yes, so why isn't it common knowledge?

And why do half the people reading my article object violently to everything I say? This stuff can't both be well-known trucker wisdom AND ridiculous foolishness at the same time!

Here's a thought. We look at truck behavior in highway traffic jams and perhaps we make a wrong assumption. We assume that trucks need large spaces because of braking distance. And so we imagine that cars should NOT preserve big spaces, since cars have short braking distance. Wrong. We totally misunderstood the reason that trucks leave big gaps ahead of them. Think for a moment: in slow traffic the professional truckers STILL maintain a huge forward space. This doesn't make sense unless the reason for the space has little to do with braking distance. And, if it's not about braking distance, maybe the people driving cars should adopt the behavior of the professionals?

SEE:
Gravel-pit etiquette

This brings up an interesting point. A macho "playa" who wants others to think they're a professional driver... how should they behave? Should they be a total competitive asshole who cuts into every tiny space and screws their neighbors every chance they get? After all, the real goal is to drive 1mph faster than everyone else, right? (...or make that 1/10 mph faster.)

Well, look at the most macho and professional drivers of all: interstate truckers. Why are they so unnaturally polite? Why do they let people merge ahead of them? Why do they group together and form rolling barriers? Why don't they act like properly competitive assholes who can drive 1mph faster than everyone in the traffic jam? The answer is simple. Truckers are genuine experts, and the guy in the hotrod who tears into your empty space while flipping you the bird is the total opposite.

Who would you rather be, a pro driver who knows the secret answers to extremely sophisticated questions of traffic dynamics? Or an ignorant little kid whose habitual selfish actions screw up things for everyone including themselves?

A: In the merge-jam's left-hand animation, the traffic jam occurs because people have trouble merging, right? Everyone in the through-lane is packed solid, so the other lane can't get in. But if people left big spaces between their cars, then the whole reason for the traffic jam would be gone. It would never form in the first place, so we would never have a chance to be trapped in a weird "wide spaces" traffic jam. But the only way those cars could maintain large spaces is if they refused to pack together as they slowed down.

On the other hand, if the traffic jam had another cause besides the "merging lanes" congestion, then the wide spaces between cars would not reduce the traffic jam.

Suppose that something caused the widely-spaced cars in the right-hand animation to all slow to a crawl. If they maintained their wide spaces, then they could all speed right back up again. They could keep merging as before, and no jam would form. However, if they packed themselves together as they slowed to a stop, then nobody could merge anymore. Two rows of solidly-packed cars would form, and a long-term traffic jam would be created. Once the cars have packed together at a merge area, they cannot unpack themselves. Yet the jam can be cleared if a bunch of widely-spaced cars could enter from the rear of the jam.

Even a single car, if it brings a large enough space into the jam, can sometimes "unplug" both lanes. I've done this several times on I-5 near Seattle.

It might feel stupid to sit in traffic with a big wide space in front of your car. But if merging lanes are ahead, then your space will help unplug the traffic jam. As far as I can tell, people with no space ahead of them are CAUSING the jam. Which type of driver should be feeling stupid?

A: A small space would ordinarily have just a small effect. But here's my main point... See all those people packed together in the left-hand animation? They are CAUSING the jam, although they might not realize it. If people IN THE JAM would spread out and stop trying to prevent merges, then the jam would go away.

As you yourself drive forward, soon you'll be in the jam. If you bring a space along with you, you will free up the people who need to merge. If your space is very large, or if several people bring in spaces, then that "plug" of cars will dissolve. (One single person CAN affect a traffic jam, see traf. experiments.)

However, this will not affect the people ahead of you in the jam. It will only affect the people behind you. If the people ahead of you allow merging, then YOU will not experience this type of traffic jam. If YOU allow merging, then you can dissolve the plug, and the people far behind you will never see the jam. On the other hand, if you care only about yourself, and if you refuse to aid the people behind you, then you should not complain when the people ahead of you do the same thing, and screw you up.

A simple Traffic Rule: if everyone tried to make traffic better for the people behind them, then traffic conditions would improve for all of us.

Note: sometimes you yourself will benefit from "antitraffic" driving.

When I manage to break up the daily jams at the express lanes south of Seattle on I-5, everyone ahead of me suddenly takes off at high speed. The plug at the express lanes exit evaporates because merging drivers are no longer driving down to the end and then butting in at the last minute. This happens because those merging drivers merged early, merged ahead of me into my empty space, and I was still about half a mile from the exit. The jam evaporated, and suddenly I could drive at 50mph.

Sometimes the "cheaters" are not cheaters at all. Sometimes they are innocent people who cannot penetrate the solid-packed row of exiting cars. If we provide space, they will merge early, and everyone will drive fast into the exit. But if we don't provide space, they'll take vengence by forcing their way in anyway, which brings our lane to a near halt.

ANSWER1:

Usually you don't have to drive slower. Just bring your big space with you when you approach congested traffic. At worst, we only have to drift backwards for a couple of minutes in order to open up several car-lengths of space. I suspect that some people believe that, since slow drivers always have an empty space ahead of them, therefore any drivers with an empty space ahead of them MUST be slow drivers. Wrong. If a driver is ACTUALLY slow, then the space ahead will grow larger and larger. If I drive with an unchanging 5-car space ahead of me, then I'm not driving slow. That space moves at exactly the same speed as the surrounding traffic, and so do I.

If you are in a burning building which is full of people, would you hope for an orderly exit, or would you want everyone to rush towards the doorway as fast as possible? They should go fast, right? ...since that lets them escape the building quickly. Wrong! Pushing ahead creates blockages. Traffic jams are created by people who attempted high speed and agressive manuvering. This has been called the "faster is slower effect." When the people in a burning building all rush for the exits, the exits get clogged. Faster really was slower. People die in fires while learning this fact. If they had exited the building "slowly" in orderly lines, then clogs wouldn't form, and they could escape more quickly. "Slower" was actually much faster. What most drivers don't realize is that this same burning-building crowd dynamics can apply to highway congestion.

When drivers pack together and refuse to allow anyone to slip in ahead of them, unfortunately they will take turns getting "stuck in the theatre doorway," and they slow down everyone behind them as well. When traffic is heavy, TRYING to drive faster can often cause stoppages, while intentionally driving non-aggressively can sometimes "flip" the stoppages into orderly single-file merge-patterns. (How often have you watched some idiot trying to drive fast in congested traffic, yet after many minutes they've only gained a few car lengths? So-called "fast" driving is not fast! If it causes traffic jams, then "fast" driving is actually FAR slower than patient, polite driving.) Also, "slow" driving is not slow! If we refuse to constantly switch lanes, refuse to push ahead and close up gaps, refuse to participate in stop-and-go wave cycles, then suddenly a traffic jam can spontaneously vanish.

Yes, to open up a TITANICALLY HUGE space, we must drive slowly for a much longer time. How can slow driving ever improve traffic? It's much like this question: "How can I give to charity, yet somehow end up with MORE money? Throwing away money is stupid, I would never give to a charity." Yet this represents short-term thinking. Giving to charity improves the whole world in the long run even though you lose money in the short run. Reciprocal altruists give to charity (they aren't true altruists, instead they are paying out money and expecting long term results!) With charity work, the return on investment is greater than the investment. It makes the world a better place over time. Commuter traffic is similar. Momentarily driving slowly while approaching any traffic jam helps to shrink the region where cars are actually stopped. A bit of driving slowly is an investment which expects a big return: hopes for a jam-busting event. A short-term thinker wants results NOW, and doesn't understand long-term investing. A short-term thinker will never drive even slightly slowly under any circumstances, and will refuse to experiment with these techniques, since the techniques seem to be used by suckers who throw away money for no reason.

In merging-lane traffic jams in particular, a big space in the "thru" lane can have amazing positive results. I've seen this actually occur on several occasions, it's not "just a theory." As your big space approaches the merge zone, everyone trapped in the jam merges into it, everyone is finally able to take off at high speed, and a wave of "unjamming" travels backwards through the jam. The traffic jam evaporates and the fast flow begins. In that case you did immediately help yourself by driving slowly to open up a big space. You broke the "clot" as you approached it. If there was no big space ahead of your car, then you'd still be inching along with everyone else, as is shown in the left-hand animation.

Traffic can be an ethics lesson in miniature: pursuit of short term gains causes greater losses in the long term. The Three Stooges can never get through the doorway because they cannot bear to surrender the lead and walk single file. Or instant Karma: the self-centered drivers often create their own traffic jams as punishment. Too bad they have to punish everyone behind them as well.

A: Yes, exactly. There's a famous UK version on the M-25 circular highway which surrounds London. But in some situations my "rolling barrier" idea is a bit less expensive, and can be used as needed when a traffic jam develops. For example, if an accident triggers a persistent traffic jam, then the police vehicles can move the jam upstream and away from the accident scene, and convert it into a large moving traffic wave which does not block the highway.

Perhaps someday ALL the speed limit signs will be programmable (just glue on some Digital Paper with a solar-powered internet satellite link?). In that situation, we could lower speed-limit signs far upstream, and create a VIRTUAL row of state trooper pace-cars. Or even create traffic-waves intentionally, which could give the merge zones a "virtual stop light" so they stop acting like 4-way stop signs with such low throughput.

PS For those who don't believe that a rolling barrier can help... do you also believe that the London M-25 programmable speed limit signs cannot alter the overall flow rate of traffic?

You may think that it is obvious that it has to reduce the time everyone spends in traffic but I don't buy it without some sort of mathematical proof. There must be some point where too many people leaving too large of gaps just slows everyone down. How do you know that your technique reduces the time people spend in traffic?

A good point! Sometimes removing the traffic waves or merging-lane jams will not affect the time spent in traffic. In this case the effects become a matter of personal opinion: would you rather drive for 20 minutes at high speed only to be stuck in a 0-mph traffic jam for 20 minutes? OR would you rather drive at half speed for 40 minutes? A slowdown without a "jam?" As far as travel-time is concerned, both are identical. Me, I absolutely HATE getting stuck in traffic and sitting still. I'd always choose the half-speed option even if it has no effect on my commute time.

As for traffic waves, the "stopped" part of the wave can trigger a huge jam if it moves past a merge zone on a highway. I've also heard that stop-and-go driving is a major cause of fender-bender collisions. And it eats gasoline, of course. If people enjoy traffic waves, if they see no need to smooth them out, then they should not complain if a hard-braking situation gets them rear-ended. Note: when I smooth out a traffic wave, I drive at the average speed. This doesn't increase or decrease the time that everyone stays on the road, at least at first.

Do large spaces between cars cause traffic to fill highways? Only if people increase their spaces on average, but that's not what I'm discussing here. Suppose traffic is moving at 40mph. Those cars will have several car-lengths of space between them. If those drivers encounter a slowdown... what happens if they REFUSE TO PACK TOGETHER? This doesn't increase the space between drivers, since they already were widely spaced when they were moving at 40mph. Yet if they maintain their wide spaces as they slow down, then they will have little trouble in speeding back up again. On the other hand, if they pack together bumper-to-bumper like a parking lot, then nobody can move until the car ahead of them moves. In that case we end up with a traffic wave which "dissolves" excruciatingly slowly. And if people need to merge as well, then the close-packing of the cars creates a permanent traffic jam: a "standing wave" trapped at the merge zone.

Removing the waves could make traffic worse if the traffic was approaching a congested area, and waves had actually slowed the traffic on average. SPEEDING UP THE TRAFFIC THAT FEEDS INTO A CONGESTED ZONE CAN CREATE TRAFFIC JAMS. If removing the waves doesn't affect the average speed, then there is no danger of creating jams downstream. If removing the waves DOES significantly raise the average speed, then sometimes the traffic waves might be beneficial, and act as a natural regulator where they accidentally improve the traffic far forwards (far downstream.) In that case the traffic waves improve regional traffic flow, and smoothing them out would be bad.

After messing with these techniques, I have the distinct impression that they speed up traffic often, have no effect sometimes, and slow things down rarely. If I saw evidence that they were causing more harm than good for much of the time, I would quickly stop doing them. My goal after all is to do my tiny bit to fix the screwed-up traffic, not to pump up my ego by showing how easily a single driver can manipulate traffic.

If EVERY driver was to constantly maintain a HUGE space regardless of speed, then it would probably cause problems. The merge-zones might stop jamming, but the capacity of major highways would be reduced. On-ramps would become choked as traffic backed up into them, and there would be slowdowns extending far out into the countryside. Yet if ALL drivers were to change their habits, then only small extra spaces would be required, yet waves and stoppages would be seriously squelched.

If we erase traffic waves and stoppages, would we spend less time on the road? Analogy: if the audience in a theater rushes for the doors, a plug develops and nobody can exit. If they exit in an orderly fashion, they exit more quickly. To exit fast, they must maintain order and not let any "jams" develop. "Jams" act as plugs, and orderly merging does not.

Smart drivers in heavy traffic are like "intelligent" molecules of fluid. They're like an additive which suppresses turbulence. If we adopt the right habits, then traffic waves and merging-lanes traffic jams will not be triggered as easily as they are today.

If some people insist that, since official proof is lacking, we should do nothing, then that's their opinion. My opinion is that the psychological benefits alone make these driving techniques incredibly worthwhile. I don't drive like a competitive idiot anymore. Instead I get to play the role of a playful altruist. Rushing forwards all the time would make me enraged and frustrated, and wouldn't increase my average speed by much at all. Even if removing traffic stoppages did turn out to have little effect on the time we all spend on the highway, I myself would still choose to drive in a way which leads to smoother flow.

A: "Traffic waves" and merging-lanes traffic jams are "emergent" collective phenomena. They are caused by the uniform behavior of the drivers. When a driver approaches a long row of stopped cars, that driver stops, and this certainly is not "widely varying behavior." Perhaps if the behavior of drivers had a much wider distribution, then traffic waves and merge-jams could not exist at all. Yet they do exist. Therefore, in many situations, drivers ARE like identical molecules after all.

It is my experience that heavy traffic forces people to act much more like identical molecules. In heavy traffic it becomes nearly impossible to pass other drivers, or to drive faster than the average speed. Uniform behavior is forcibly imposed, and the traffic behaves like a sort of compressible "crystal." When traffic is light, the limitations are removed, and our behavior becomes far less uniform. And when traffic is light, these "antitraffic" driving suggestions stop working, yet also they are no longer needed.

A:I drive that section too (and get to play with jam-cancelling techniques there).

I'm convinced that this clot is caused by all the people trying to merge left to get to the 520 exit. It's a runaway process: those who need to merge will start slowing down in order to find a hole in the adjacent lane of cars. Slow drivers pack themselves together. This closes the holes and prevents others behind them from merging, which makes merging drivers go even slower, which packs the cars even closer, etc. During rush hour this causes a jam. At other times it causes a mysterious slowdown.

If people knew the trick of maintaining large spaces, if they encouraged others to merge, then I bet the daily "ship canal" traffic jam would be greatly reduced. If I'm wrong, and if the slowdown is caused by something else, then my recommended large spaces between cars would obviously have little effect on that particular jam.

Here's a more exotic possible explanation. Traffic waves move backwards against the flow, correct? However, sometimes the waves move very slowly. If a slight inconsistency in the highway tends to momentarily slow down the individual drivers, then a slowly-drifting traffic wave might become "pinned" in place by the inconsistency. The backwards-crawling wave of slow traffic would halt its upstream motion, and would become a sort of standing wave. See the above animation. That's a "pinned wave."

If we could see this from above, we would see little blobs of "traffic waves" traffic slowly crawling backwards to a certain spot, then they would all stop moving and accumulate at that spot. It would be like dripping water filling up a bucket. This would only work with slowdowns. It would NOT work with waves of stopped traffic, since only a slowdown can remain "pinned" to one spot. A total stop-wave MUST crawl backwards away from the hill, it cannot sit in one place. A true stop-wave could not become pinned to a particular spot on the highway. However, a propagating region of slow/dense traffic might quit drifting backwards and instead accumulate at a hill. It could also accumulate at a merge zone, at a speed zone, at a jog in the highway, etc. The Seattle southbound I-5 shipcanal bridge has both a hill and a merge zone. Perhaps the slowdown is an accumulation of lots of little slowdowns being created downstream in Seattle by all the merge-zones of the many city exits. Those slowdowns creep slowly along until they become stuck at the hill leading to the bridge, where they build up to form one huge slowdown. It's a good example of (grin!) Nonlinear Dynamics in action.

OTHER SEATTLE SITES:

• Seattle Drivers, guide (humor)
• Seattle Drivers, ettiquette rules