Traffic Jams

Traffic Jams

The Physics Behind Traffic Jams

by William Beaty

I live in Seattle and my two daily commutes last about 45 minutes. (That's when I'm lucky; sometimes it's more like two hours each.) This has given me an immense amount of time for watching the interesting patterns in the cars. Boredom led me to fantasize about the traffic being like a flowing liquid, with cars acting as giant water molecules. Over many months I slowly realized that this was not just a fantasy. Why had I never noticed all the "traffic fluid dynamics" out there? Once my brain became sensitized to it, I started seeing quite a variety of interesting things occurring. Eventually I started using my car to poke at the flowing traffic. Observation eventually leads to experimentation, no? There are amazing things you can do as an "amateur traffic dynamicist." But first, some basic phenomena.

Have you ever been driving on an interstate highway when traffic suddenly slows to a crawl? You inch along for many minutes while waiting to see the accident which must have caused the jam. At the same time you also curse the "rubberneckers" who are causing the whole problem. But then all the cars ahead of you take off at high speed. The jam is over, but no accident, no police cars, nothing. WHAT THE HECK WAS THAT! A traffic jam with no cause? In the rear-view mirror you see all the poor saps behind you still stuck in the jam. But why? If all those people could just speed up at the same time, the whole traffic jam would evaporate. Why don't they ever do that? What caused the mysterious slowdown in the first place?

After experiencing many of these "invisible accidents", I came up with the following explanation. To best understand this, imagine that you look down on traffic from an aerial view point. Pretend you're in a Traffic Reporter's helicopter looking downwards.

Fig 1: Cars lining up behind an accident
Fig 1: Cars lining up behind an accident

Above in fig. 1 I've drawn a one-lane road, an accident, and a row of cars stuck behind the wreck. Other cars are approaching from the left and stopping too. Suppose that the "wrecked" car (the red one) has simply become temporarily stuck. Maybe it spun out on ice. What will happen when the red car moves and unplugs the flow?

Fig 2: A wave of 'condensed' traffic creeps backwards
Fig 2: A wave of 'condensed' traffic creeps backwards

Refer to fig. 2 above. In the top row (fig. 2A) the flow is suddenly unplugged. But not all the cars can move, since most cars are stuck behind drivers who are stopped. Figure 2B shows the traffic a few moments later, and figure 2C shows it a few moments after that. Notice the orange car in 2A, and see how it eventually becomes unjammed in 2D and begins moving. At the same time the red car in 2A approaches the jam and is swallowed up.

A MOVING WAVE OF "JAM"

After the wreck is removed, there seems to be no reason for the traffic jam to persist. Yet it does. The reason for this is sensible: if I am stuck behind a car that is stopped, then I have to stop too, and so does the car behind me. All the cars in the jam are in this situation. Even though the wreck is gone, they remain locked at standstill because if they want to move, they ALL have to move at once. They never do, because each driver is waiting for the car ahead to move. If I am in the traffic jam, I'm not going to move forward because I have no room to do so. I'd bump the car ahead of me. We all think like this, so none of us can move.

When the car in front of me leaves, I still cannot accelerate instantly, so I will remain stopped for a moment. I must delay leaving for a moment. If I started up instantly, I'd stay too close to the car ahead of me, and that would not be safe. Each departing car must delay in the same way, and this causes the jam to "evaporate" starting from the forward downstream end. It evaporates in a wave which begins at the forward end of the jam, (near the wreck). The wave eats into the jam from right to left.

Starting at figure 2A, the cars depart from the jam in sequence. In 2B the wave of "evaporation" has moved away from the wreck site, and in 2C and 2D it is far from the wreck. But notice an interesting thing: even though the CARS THEMSELVES are moving from left to right, the "wave of evaporation" moves in the opposite direction. It moves leftwards as it eats into the traffic jam.

There is a second important thing to notice. While some cars are still jammed, more cars are piling up behind them at the trailing end of the jam. Even after the wreck is removed, more cars are still "condensing" onto the back of the jam. The traffic jam is like a solid object whose front end is evaporating and whose back end is growing like a crystal. Cars move left to right, yet watch the group of stopped cars. The stoppage is creeping slowly upstream, in the opposite direction to the moving cars. The accident is gone, but a moving wave of stopped cars remains behind. It's not a traffic jam, it's a shock wave which propagates through the "automotive material". It's a traffic-clot in the blood vessel. It's a traveling wave of traffic-condensation.

NOT CAUSED BY ACCIDENTS

These sorts of traveling waves are common during heavy traffic conditions. An accident isn't needed to create them, sometimes they are caused by near-misses, by people cutting each other off, by merging lanes at a construction site, or simply by extra cars entering from an on-ramp. In traffic engineering lingo, they can be caused by "incidents" on the highway. A single "rubbernecker" could cause one by momentarily stopping to look at something interesting. Whenever you slow way down in order to merge across a lane to get to your upcoming exit, YOU could create one.

Sometimes they have no cause at all. They are like sand ripples and sand dunes, and they just appear for no clear reason. They are like ocean waves caused by the steady breeze, or like the waves which move along a flapping flag. They just "emerge" spontaneously from the writhing lines of traffic. In the science of Nonlinear Dynamics this is called an "EMERGENT PHENOMENON."

How long will the "traffic wave" last after the accident is cleared? Its lifetime depends upon the amount of traffic, and on the number of cars trapped in the jam, but sometimes these things can persist for many hours. When traffic is slight, the traffic jam might shrink rapidly to nothing. But if traffic remains heavy, then there's no reason for the traveling wave to ever dissipate at all. Also, if the conditions are just right (if the "condensation" happens faster than the "evaporation",) then even a tiny wave could grow large and larger. Sort of like dropping a tiny seed crystal into a supersaturated solution. When traffic is heavy and unstable, a single driver can cause the traffic to freeze into a gigantic crystal. Like Kurt Vonnegut's end of the world story CAT'S CRADLE it's the "Ice Nine" of the highways.

So, next time you are commuting and you approach a stoppage, don't think of it as a stupid f@#$% traffic jam. Think of it as a pressure wave which has approached your car and engulfed it. Think of it as a simple living thing which is made of cars rather than molecules. Stay hopeful that the crystalline amoeba poops your car out soon. Take an aerial viewpoint, and visualize the wave which is moving backwards as you move forwards.

 

Merging-lane Traffic Jams, A Simple Cure

DRIVERS WITH UNUSUAL BEHAVIOR: THEY ENCOURAGE OTHERS TO MERGE AHEAD OF THEM, AND THEY TEND TO MAINTAIN LARGE SPACES AHEAD, EVEN IF TRAFFIC SLOWS TO A CRAWL. MERGING IS EASY. SEE HOW MUCH FASTER THEY GO?NORMAL DRIVERS WHO PACK THEMSELVES TIGHTLY TOGETHER WHENEVER THE TRAFFIC COMES TO A STOP. NOBODY CAN MERGE EXCEPT AT THE END OF THE JAM. NOTE THEIR LOW SPEED.<<<<< ON THE LEFT: NORMAL DRIVERS WHO PACK THEMSELVES TIGHTLY TOGETHER WHENEVER THE TRAFFIC COMES TO A STOP. NOBODY CAN MERGE EXCEPT AT THE END OF THE JAM. NOTE THEIR LOW SPEED.


 ON THE RIGHT >>>>> DRIVERS WITH UNUSUAL BEHAVIOR: THEY ENCOURAGE OTHERS TO MERGE AHEAD OF THEM, AND THEY TEND TO MAINTAIN LARGE SPACES AHEAD, EVEN IF TRAFFIC SLOWS TO A CRAWL. MERGING IS EASY. SEE HOW MUCH FASTER THEY GO?

Traffic jams often occur on highways wherever two lanes must merge into one. Lanes of cars cannot merge if there are no large gaps between cars. Therefore, drivers who create large gaps between cars will ease this type of traffic jam.


To ease this type of jam:

  • Maintain a large space ahead of your car.
  • Encourage one, two even three cars to merge ahead of you.
  • If traffic slows to a complete stop, KEEP TWO CAR-LENGTHS OF SPACE OPEN AHEAD OF YOU.
  • Never "punish" merging drivers by closing your gap.
  • Other suggestions

Amazingly enough, it is not necessary that EVERYONE do this. If only a few drivers will maintain large gaps during heavy traffic, then merging traffic is not forbidden, and the situation in the left-hand diagram can be prevented.

Yes you're right, you cannot eliminate every problem by simply making a big gap in front of your car. When there are too many cars on the road, traffic slows down. But if we use these special driving habits, the smaller jams can be erased, and stop-and-go traffic can be smoothed out. Since many traffic jams are caused by merging lanes, many traffic jams can be improved by the actions of just one driver.


 

TRAFFIC "EXPERIMENTS" AND A CURE FOR WAVES & JAMS

My first 'experiment'

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 traffic, I decided to drive slow. Rather than repeatedly rushing ahead with everyone else, only to come to a halt, I decided to try to drive 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 brake lights 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, but sometimes 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 other 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, my car had been "eating" 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 "tube."

It's always a good idea to drive without changing speed and without competing with other drivers for bits of headway. But 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. Ironic that the angry people who drive as fast as possible might unwittingly participate in "amplifying" the very waves that they hate so much.

MORE EXPERIMENTS

I rarely commute on 520 where the good traffic waves appear. I started to miss having opportunities to cancel them. However, I soon realized that the same process could be used to affect small 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 the jam can be annihilated entirely by a single car, as I had done with traffic waves.

Now I remember something from years farther back. When leaving one of those "rubbernecker slowdowns", I always tried to accelerate like mad. I figured that if everyone did this, then the slowdown would evaporate. Yet this did little good, because the car ahead of me would not accelerate. I could not force the cars ahead of me to stomp on the gas, so I could do nothing to aid the "evaporation" of the traffic stoppage. Aha! I can control the people behind me by slowing down, but I cannot control the people in front of me by speeding up. Therefore, I can destroy a small traffic stoppage by slowing down long before I approach it, but I can do nothing to aid the "evaporation" at the other end of the jam. Accelerating out of the jam does nothing unless EVERYONE would do the same, and there is no way to change everyone's behavior. But just one single car, if it decelerates while approaching, can change the behavior of everyone behind it. It can bite a chunk out of the region of stopped traffic. If that driver gradually builds up some empty space before encountering the slowdown, the slowdown can be "eaten" just as the traffic waves were "eaten."

STANDING WAVE

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 approached that jam in the right lane, and 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 just a single driver was able to take a huge bite out of it.

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 was causing a slight slowdown behind me. After I arrived at the jam, the jam was smaller. When all was said and done, part of the traffic jam had been removed. However, it was changed into a mild slowdown, and it was spread backwards upstream over several miles of traffic. 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 nasty, frustrating part of the 2-mph jam was changed into a large "fuzzy" area of reduced speed. 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.)

"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 eat 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". When my antitraffic-pulse 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 smudge behind.

My next thought: if I took several friends along on my experiment, we could have spaced our cars out over many miles. Each of us could have allowed a big blob of anti-traffic to appear, and then the successive impacts of the antitraffic could have completely erased the traffic jam at the Lynnwood exit. When traffic is sparse, we cannot keep a large space ahead of us, since it's easy for cars to pass a slightly-slow driver. But a number of separate drivers could bring smaller 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 to affect them by "peeling out" after I'd made my way through the jam. 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 if I'd planned way ahead and brought an empty space with me into the jam, I could use that space to manipulate the jam. 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.

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 moving, not jammed.) Nobody would let me in. I drove like this for a long while, then started driving fairly slow 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. But I had an excuse, I had to get to my exit! To make matters worse, I had nearly come to a stop, and brought two 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 same thing that I did, and this would make the "wave" worse and worse. The simple cure is to give up, not merge, and miss the exit. I shouldn't have forced the issue, I should have let my exit go past. 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 fill it in their quest to get a couple of feet of headway. But this sort of 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 "waves" and screws up the traffic at highway ramps.

So, if I keep a few car-lengths of space ahead of me, not only can I use it to help vaporize waves and jams, but it also eliminates one of the major causes of waves and jams. It eliminates the "solid wall" of traffic at merge areas, and lets people merge without slowing down and creating traffic waves. Take a look at this animation. Ideally a merge area will act like gear teeth. But if everyone is defending themselves against opportunistic drivers by eliminating all gaps in traffic, then the valid merges cannot take place either. A traffic jam is created. Sometimes the jam is the fault of people like me who panic at missing their exit and come to a complete stop. Sometimes the jam is the fault of the huge blinking 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 everyone in the whole lane is saying the same thing!

Delusions of Grandeur

Seattle suffers from many separate rush-hour traffic jams. Why stop with the Lynnwood I-5 jam? 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?) 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 I-5 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 not open up ahead of me because nothing stops other drivers from passing me. In my experiments, I could make "antitraffic" spaces only because traffic was very heavy, and because only a very few people had the ambition to leave their lane and move into the 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 or 50 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 traffic jam simply stops arriving. The jam trickles away. Just as the last of it is gone, the row of State troopers arrives, and the jam has been transformed into miles and miles of slightly slow traffic upstream from the old location of the jam.

The situation is not so simple if extra traffic is entering from numerous on-ramps. The "rolling barrier" can't affect these extra inputs, and if the major portion of the traffic is from on-ramps, then the "rolling barrier" idea would be worthless. Ah, but what about "rubbernecker slowdowns"? 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 don’t know. Sometimes "rubbernecker slowdowns" persist for hours after the accident has been cleared. This suggests that the slowdown is self-perpetuating. 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 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. But the resulting 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 twice the speed limit afterwards. And once a jam is gone, the remaining region of slightly-slowed traffic might disperse fairly rapidly, whereas a traffic jam/stoppage is a different animal and can self-perpetuate once it has formed. And there's another thing that happens when we spread out a "jam"...

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 them, the way I did it with the jammed sections in the "traffic wave." Finally I saw 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 slooooowly come together. Then you race off at 60 mph! The merging lanes formed a terrible bottleneck. A "traffic wave" develops at (and behind) the merge zone. After the bottleneck, it's clear sailing.

Why?

WHY must a bottleneck develop at a merge zone? Well, because everyone must take turns. Wrong! Under low-traffic conditions, everyone still takes turns, yet everyone merges at high speed. 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 comes to a near halt. 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. But, but... do traffic jams CAUSE THEMSELVES? After thinking about this even more, I realized that the answer is yes. It goes like this:

  • 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 traffic go slow
  • Go back to the top of the loop.

This is absolutely fascinating, since 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 the loop.

At a merge zone, fast traffic causes traffic to be fast, while slow traffic causes a jam. 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, they can sometimes switch from one to the other. Or somebody can switch them intentionally.

Suppose the traffic at a merge zone was flowing fast as in 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 keep the other lane from merging into the close-packed lane. Cars in the merge-lane would pile up too. Then I drive off laughing evilly, because I have just caused a MASSIVE LONG-TERM TRAFFIC JAM!

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 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 Seattle, pretty cool, eh?

As you browsed 'Traffic Jams' you may find interest in following articles . . .

Comments

Nice job on a very interesting and thought-provoking article; I have a long commute myself over here on the East Coast, and I appreciate the usefulness of fluid dynamics when it comes to analyzing traffic patterns and the causes of traffic jams.
But I often find myself wondering if anyone's given any thought to the impact on traffic flow of the ever-increasing size of the 'molecules' themselves. Everybody knows tractor trailers, school buses and other big vehicles can't move nearly as fast as our zippy little Toyotas and Hondas, and many of us get extremely frustrated when we get stuck behind them. Simple physics is all you need to realize that the bigger and heavier a vehicle is, the higher its center of gravity and the longer it takes to get up to speed, slow down, turn and do all the other things you need to do in traffic. As a result, the more of them there are on the road, the slower traffic moves overall.
But what are we doing in America? We're buying bigger and bigger SUVs and minivans and more and more of them every day. I can't help but thiink this has to be a major contributor to traffic problems. To keep traffic moving, we're going to have to move slower and slower ...and slower...
Just something to think about.
Joe J.

I came to this same realization about a year ago and since then I have tried to leave a big space in front of me so that people can merge easily; however I regualrly encounter one signficant problem, which is this: The angry guybehind me who has no concept of why I'm leaving the gap in front of me and who is pounding on his horn and getting closer and closer to deliberatly rear ending me. This is something that happens about about 30% of the time and I must confess that it's stopped me driving in a manner that eases traffic. As much as I'd like to help everyone out I'm just not prepared to get shot for it :(

My opinion? Ignore them. I rarely get the guy right behind me because usually he can pass me anyway, and sometimes cut in front of me (thinking that I care, or that he/she showed me!).

Mostly tail gaters get tired of tail gating people that don't drive at a constant speed. Vary your driving speed almost imperceptibly by just by 1 or 2 miles per hour, and they eventually get tired of trying to figure out what speed you are going, and back off just a bit. Doing this slow enough that passengers can't notice yet force the person tail gating to match you is all that it takes.

Obnoxious I know, but what else is there to do? My other strategy is to leave space ahead of me for BOTH myself and my tail gating friend. That also makes one few friends among the rabbit drivers.

I just took a really fun high-speed driver training (N. Andover MA, "In Control"). Their solution to tailgaters:

1) Move over, let him pass, move back. Maintain 3-second rule (minimum 2-seconds).
2) If you can't move over, increase your following distance (between you and the car in front of you) proportionally to the closeness of the tailgater. In an emergency this enables you to begin braking gently at first so the tailgater has time to back off, *then* you can do an emergency stop. You need lots of extra space to do this.

The important thing *not* to do is try to "teach him a lesson." You're not gonna be able to teach him anything. Just stay safe, don't look directly at him, and relax, knowing that you will survive an emergency.

Thanks for posting such in informative article on the fluid dynamics of traffic. I'll be driving with a different outlook from now on.

When I moved to Texas from Oregon, I noticed that tailgaiting seems to be the norm on the freeways here. Even when the other lanes are open, many drivers choose to tailgate for miles rather than moving left and passing. Speeding up or slowing down doesn't seem to make a difference to them. They just stick to the car ahead.

There are also many drivers who move into any space big enough to enclose their car, even if it is only two car-lengths, which causes the car they cut off to slow, which causes the car behind that one to slow, . . .

I generally drive in the right lane of the three-lane freeway, at 5-10 miles below the speed limit. I seldom change lanes or have to slow down, yet I usually don't fall behind the traffic ahead of me, either. I don't see the logic in wasting $4/gallon gas to drive 85 mph for 10 miles, saving mere seconds on a clear road. In traffic, even the fast lanes develop slowdowns, so the time savings is even less, and the $ cost increases because of the frequent decelerating and accelerating.

My driving philosophy has changed since I was a teenager. Now I go with the flow, follow at least 3 seconds behind the car ahead of me and let those impatient folks fly past in the other lane. Quite possibly I will see them on the side of the road later on, either in front of the flashing lights of a police car, or backwards in the median.

Great writeup! I wish every motorist would read this (especially in Southern California where I live)!

I first learned of your theories from your hobbyist page. They made a lot of sense, so I figured I'd try them out on my Richmond to DC commute. Wow! I no longer feel powerless over traffic, and my blood pressure has never been better. I've lost count of the accidents I avoided by maintaining a cushion of space and actually studying the traffic patterns before me. I was so impressed I've taught most of my family and friends your traffic busting techniques.
It is worth all the dirty looks and honking to see the look of gratitude and happiness on the faces of the 'Desperate Mergers' to whom I bring the 'Gift of Space'. I actually pity those aggressive guys, I know I will see them again, jam packed like sardines whilst I drift on by, smiling in the knowledge that I will probably out-live them.
Bless you Sir, you have improved my life considerably.

Figured out the 'standing wave' aspect of traffic jams some time ago...about all I can still dredge up from fluid flow courses in college. Here's another one to work on: You're tooling along on a 2-lane expressway and the sign says, 'Right lane closed 2-miles ahead...merge left'. People take this command to heart and begin shoving themselves into a single lane on the left, way ahead of the restriction. So far ahead, in fact, that it leaves a conga line on the left and an enticingly free right lane. Inevitably, some motorists take advantage of this free lane and at the restriction are, of course, allowed to merge by some kind soul who isn't a mile back, fuming. Wouldn't it be better if the sign said 'Maintain lane to merge point', and at that point, allow motorists to alternately merge (I'm sure they would, since everyone has been treated equally and fairly). I recently heard that a Dutch city solved its horrendous traffic problems by eliminating all signs, traffic lights and parking restrictions...leaving it up to the motorists seems to work best in a libertarian way...

If you're interested in this as a science, you might look up the studies done from the 50s, such as by M J Lighthill and G B Whitham (1955) up to Nagatani (2002), and likely even more recent studies, where traffic is modeled as a wave, among other similar phenomenon.

My personal view--and I do not personally study such things scientifically, but only from personal experience such as yourself--centers on the idea of a feedback loop, where the medium is that of tail lights (stopping). Modeling from this micro-behavior should yield the emergent behavior of waves, etc.

I don't consider myself an aggressive driver, but in seeing the left lane empty because of a construction induced merge coming up 2 miles down the road, I wanted to use the empty left lane to drive to the merge point instead of standstill traffic in one lane. Some drivers take it upon themselves to pull out and block both lanes so that no one can get through. It seems that they are afraid of anyone using the left lane, even when the merge is not for a few miles. When they get into the merge spot, they don't want to allow anyone to merge, thus slowing traffic.

Why not encourage traffic to take up two lanes to keep traffic moving and then keep enough space to do a 'zipper' merge at the appropriate point?

Am I wrong to want to use the left lane until the merge point, while leaving adequate space to do a zipper-like merge? I thought that would actually help traffic along. I don't consider it 'cheating' behavior, but rather a more efficient flow of traffic for everyone.

My husband is convinced that staying in the right hand lane in one solid, slow moving line is equally as effective. I disagree, because it doesn't use all of the available lanes and creates longer backups.

Your thoughts, please :)

I don't think that merging at the "merge point" or end of the line will help to ease traffic at all. The problem is like the article suggests, that there isn't any room, anywhere, to merge. Waiting until the end of the line reduces your chances of successfully merging at a constant speed which forces you to decelerate drastically to match the slower speed of traffic in the jammed lane.

My favorite example of this is in Seattle on I-5 Northbound entering the downtown area. There is an exit only lane on the left side of a 3 lane road. When traffic is lighter, the speed is a steady 45 mph (speed limit is 60 mph) and there are gaps enough for a single car to merge/change lanes at almost point. Motorists in the left hand exit only lane (who receive over a mile notice of the impending exit) travel at a higher rate of speed until the last minute and merge "normally." This last minute merging would be at what you are proposing is the 'merge point'. The problem is that the motorist on the left inevitably must hit their brakes to decelerate to match the speed of the motorists on the right that they were passing. The motorist in the right lane who is trying to maintain a gap in front of them for safety and merging must then decelerate to continue to maintain that gap. The other motorist in the right lane behind the 'merge point' must then slow to match the slowing of the person who was forced to in front of them by the late merger, AND then they must be wary of the second late merging motorist who has passed them and expects to merger every other car. THIS is what causes the ensuing traffic slow down with slow reaction times multiplying the problem (same as when a traffic light turns green; in theory, everyone should be able to accelerate evenly and at the same time but you inevitably are waiting to accelerate depending on the number of cars in front of you).

I have personally tried leaving a large gap in front of me to allow the late and passing motorists plenty of space to merge. Unfortunately this requires traveling at the slower buffer speed for some time and all too frequently, left-lane-impending-exit-motorists STILL wait until the last minute to merger into a tighter gap.

I'm a firm believer of merging before you know you are forced to thereby elongating the merging/zipper point to allow for slow reaction times of slow drivers.

Actually when you do the zipper merge from the left at the last minute you cause everyone to hit the brakes one at a time behind you. You got in but you slowed everyone down behind you. The traffic would move smoothly otherwise. So you got an advantage at the expense of the 300 cars behind you all delayed 2-4 seconds. 4x300=1200 seconds or a long delay.

One thing I've noticed is, although as one person said, larger vehicles CAN make it tougher, most truckers tend to follow the same advice set forth by the author (although for different reasons). Ever followed a semi with a 13-speed or 18-speed into traffic? We go one speed, and try to smooth out the stops and go's. Why? Try getting 'er going ... shifting gears 4 times to go 0-10 gets annoying, and all the 4-wheelers just scoot ahead of you. So, what do we do? Block a couple lanes (takes a convoy to do this, but watch on 95, 80, 76, and other roads and you'll see it) and try cruising at one speed, all of a sudden, we don't have to burn oil in stop/go traffic. Same goes for local drivers on 2-lane roads; see it all the time.

Interesting to note that it also helps reduce traffic doing that ... and btw GREAT WRITE-UP!!!

Three Words: Variable Speed Limits. If instead of fixed speed limits, they were electronic and could be changed from 65 to 45 if need be, to smooth out traffic, people wouldn't need to take it upond themselves to be good citizens, they'd actually be required to drive more 'flow conciously'. I love all this in theory and in practice. Keep up the good ideas. One problem though, is when you do leave yourself that large space in front of you, you're either in a lane going slower than another (and therefore going slower than you could theoretically be going), or you're going in a lane faster than another lane, people in the slower lane will pull in front of you because they see a space in a faster lane. Sometimes you can't win for yourself, and truly have to make a tough sacrifice for the greater good. If only more people would read this

Thank you for doing these experiments. They have been very interesting to learn about.

However, despite your conclusions, I'm not convinced that leaving these gaps will help ease traffic congestion. The reason is that ahead of you still exists a traffic jam that you cannot change or ease. It would be impossible to do that from behind.

Also, in the Baltimore-D.C. area, a few motorists do leave gaps (not sure if they do it because of your experiments), but this is futile, as those gaps are quickly filled by other motorists. There is just too much traffic in this big metropolis to let any gaps in traffic exist for too long. To do this would actually be an inefficient use of the lanes.

This is why we need other solutions to ease traffic congestion. If you'd like, please visit trafficfrustrationblog.com, leave comments, and share with others.

AWESOME AWESOME ARTICLE!! I also live in western Washington State, just south of Tacoma in Lakewood. I worked for a few years as a Traffic Flagger and realized the same things about traffic flow as you stated in your article. But I never thought about it with scientific terminology like you phrased it. I used to come home from various parts of King County down 405, 18 or I-5. Very very VERY rarely did I use 167 because it's only two main traffic lanes with an HOV and is notoriously slow in the morning and evening. Most of the time I would drive across King county to get to I-5 instead of using 167.

Back to your article. After sitting in a lot of rush hour myself I too noticed the effect of maintaining a slower constant speed so I wouldn't get to the tail end of a back up until it was starting to move. A few times other people behind noticed why and how it worked and also started leaving a lot more space. Even though they could've passed me they didn't, just stayed behind and kept a large buffer. On the other hand there are many people that think because there's a large gap in a lane it must be faster. Well if that was the case than the traffic in that lane wouldn't be slowed too, even though there is a gap ahead!!

I've noticed sometimes it's not worth leaving a large buffer space of a 1/4 mile depending who is on the highway and how many of them are lane jumpers and just running ahead only to make the back up grow. If traffic is rolling at a constant speed like say 20 mph than I don't worry about leaving an extremely large gap for 2 reasons: 1. If it's moving slow but steady that's what's important, 2. If it's moving slow and steady and there's a large gap people keep jumping in and you have to slow down to regain the space lost. From what I've seen leaving the large gaps of 1/4 only seem to be most effective when traffic is extremely slow or stopped and usually people just stay put.

It’s funny how people tailgate me in a traffic jam cause there’s 4 or 5 car length’s of open road ahead. Than when the opportunity is right they jump lanes, pass me and than 5 seconds later we’re stopped again but now I’m behind them instead of them behind me.

That is very weird that you mentioned the idea of the State Patrol doing a rolling slow down to get the traffic back to a consistent speed. I had thought of the same idea while working as a Flagger except that’d it’d be easier to have DOT do it as long as they block all the lanes so the traffic can't race ahead. The problem is getting the vehicles to an area where they are needed. Of course they could have some units stationed in places that are notorious for back ups. But it would be expensive to have units stationed throughout the Puget Sound region and than having to use gas driving to a slow down and than more time spent maintaining traffic flow for say 45 minutes up to several hours.

I think the better idea would be variable speed limits. Have electronic signs so the speed can be adjusted as needed by traffic monitoring equipment like the equipment that controls the HOV toll rate on 167. Than to make sure drivers obey the temporarily lower speed limit have a State Patrol parked along the highway checking speed and issuing tickets for violators. Also doing an ad campaign on TV, radio, internet and articles in the newspapers would help raise awareness of how to drive in heavy traffic and what motorists can do make it smoother. With 3 large military installations in the Puget Sound and other large corporations we’re always getting new people. So the other challenge would be having to always educate new people by continuously publicizing how to drive in heavy traffic. Which would get annoying for those of us that know the techniques and what to do.

I realized a long time ago that mass transit is good. But here in Puget Sound the lifestyle has become when we want to go somewhere we have complete freedom to do that anytime 24/7 with our cars. To build a transit system to meet that same lifestyle would be very expensive cause there would have to be trains and buses available 24 hours a day. We definitely need mass transit, don't get me wrong. But there needs to be a lot more focus on better traffic management.

I have to come to think of the Puget Sound region (Pierce, Kitsap, King, Snohomish, Thurston and Lewis counties) as one HUGE GIANT suburb that spans tens of thousands of square miles and has a large variety of terrain from hills to valley's and lakes to inlets. And trying to develop a mass transit system that can efficiently transport someone from Buckley to work in Seattle or someone in Bothell that wants to see family in Yelm, just isn't feasible without a huge network of trains and buses and the willingness to do transfers and layovers at various stops. As compared to just driving a car directly and get from A to B in an hour or so.

I'm a firm believer in the zipper merge on the highway. But the DOT needs to post signs about using both lanes until the merge point AND just as importantly signs about leaving space so people can merge quickly and safely. When I'm on any of our various freeways and are approaching an on ramp and there's congestion on the freeway and on the ramp I make a point to leave A LOT of space to let 10-15 cars merge in and have noticed that once they're in the traffic lane if things aren’t completely stopped ahead traffic in the lane will get moving again fairly quick. I started doing this cause a lot of people get bumper to bumper when approaching an on ramp and those getting on can't merge cause there's no room. People on the freeway need to spread out and the people coming down the on ramp need to spread out so the two lines can zipper together quicker.

One thing that frustrates me to NO END is people that don't know how to get up to speed on a freeway on ramp. When I come down an on ramp I put my foot into the gas and have the mindset of watch out here I come. Of course I adjust my speed accordingly so I can merge with the flow. But my mindset is I'm coming down this ramp and I AM getting on the freeway. Why people come pokin down a ramp at 45-50 mph and traffic is moving at a steady 60+ mph and than expect traffic to slow for them is beyond me!! Another thing that irritates me about this is when I’m coming down a ramp at the speed of traffic and ready to merge and some idiot just refuses to let me merge. I read on another website that we learned this behavior from the lunch lines in school. That there are no cuts, you MUST get to the back of the line and wait your turn. Ironically enough though, when I get on the freeway I rarely stay in the far right lane. After I merge I usually get over at least one lane. So when it happens that someone won’t let me in and I finally find a merge point and get over and pass the person who wouldn’t let me in I wonder what they’re thinking now that I passed and are in front of them anyway.

I used to work nightshift in Federal Way and Maple Valley when I lived in Tacoma and now in Lakewood. (Which is why mass transit wouldn't work for me, cause there are very few buses and no trains that run at 9 or 10pm and could take me the 33 miles from Lakewood to Maple Valley.) Here's my point though. I would leave at 10pm to be to work by 11pm. I would get on I-5 and set my cruise at 60mph and just chill in the right lane and most of time had a relaxing commute to work in 35 minutes. (My commute Lakewood to Maple Valley during the morning or evening would be an hour if you're lucky, more likely an hour and a half, 2+ hours if traffic is bad.)

So anyway, I'm chillin in the right lane in my 4 cylinder Ford Tempo with my cruise on 60mph and approaching an on ramp all by myself with absolutely no other cars in my lane for miles behind me and none in front of me and here's comes an SUV down the ramp. I don't think much of it, they'll probably just speed up and go faster than me like most people do. But no, the other driver starts pacing me and it's getting to the point where they have to merge. They're not speeding up or slowing down. Several times it has happened at the very last moment they decide to step on the gas a little so they can cut me off and get right in front of me. After hanging there a few seconds they decide they wanna speed up and put their foot into the gas and take off for the horizon at a considerable speed. The opposite has happened an obviously much more powerful vehicle coming down the on ramp and starts pacing me and gets to the point where it's merge or wreck and they slow up to just below 60 so they can get behind me. After hanging there a few seconds and realize that I'm not going to speed up above 60 they put their foot into the gas and take off. It hasn't always been SUV's, big pickups etc. There are some cases where they were cars and some were V6 or V8 with obviously more power too.

In either case of being cut off or someone suddenly getting behind me I'm always dumb-founded why they found it soooo extremely difficult to merge with on single solitary car when all they had to do was accelerate more coming down the ramp and just merge in the lane before I approach the end of the on ramp too. Even with my little 4 cylinder Tempo I had no trouble merging into traffic I just come down the ramp with my foot in the gas and the mindset that I will be merging and start looking for a spot as soon as I start down the ramp, not at the last moment when it's time to get over.

A comment about Work zone merging from a Flagger's perspective. Waiting to do a zipper merge on the free way is a good thing if everyone plays nicely. The traffic barrels really don't care if you run up on them at 60 mph and merge over with only inches to spare. But when approaching a Flagger that is standing behind cones directing you to get over we really appreciate it when you do it sooner rather than at the last moment when you're next to the cones. The problem is when you're standing there in a lane of traffic and the only thing between you and the traffic are some rubber cones there's no way to tell who is paying attention and who isn't. Who is going to change lanes right at the cones and who is going to run through the cones cause they aren't paying attention? There's no way to tell and when a car is right on top of you it might be too late to get out of the way if they do run the cones. You would be shocked how many people I've seen driving distracted. Reading books or newspapers, doing make-up, talking on a cell phone, eating out of a food container with a fork, fooling with a laptop computer, fumbling with CD's. So when that car is running up on the cones there's no way to tell if they're going to get over, or run you over.

We also appreciate if you don't come running up to our stop signs at full speed and stop right next to us. Once again that's how people stop at regular stop signs. The stop sign at an intersection doesn't care if you stop at the last moment or come to a slow stop. But as for Flagger’s if someone is approaching to stop right next to us like a regular stop sign we can't tell if you're going to stop right next to us or run our stop sign. Flagger’s can't enforce traffic laws or issue tickets, we have no more authority to enforce traffic laws than any other joe on the street. Cops can only issue tickets for running a Flagger's stop sign if they see the violation, just like a regular stop sign. I almost had an unmarked Pierce County sheriff car run my stop sign on a side street in Spanaway. 35 mph road, all of our orange signs set up that some motorists ignore and here comes this cop and almost ran my sign at about 35 mph. I leaned my stop sign out and waved it a little to get his attention and sure enough he came to a very quick stop and than gave a quick abrupt wave at me cause he knew he was in the wrong.

Come to a slow safe stop anywhere from 20 to 30 feet back from the Flagger. If you get closer to the Flagger it doesn't mean you're going to get through quicker. Depending on what other traffic is doing or what is going on in the workzone you may want to stop even farther back. If for some reason you have the unquenchable desire to ask the Flagger a question allow your vehicle to roll slowly up to them and look at them and roll down your window. Some Flagger’s like lot's of personal space or maybe the workzone demands they keep traffic away from them. If they motion for you to stop than comply and wait patiently. Flagger's take a lot of sh*t from stupid and angry people and sometimes, like me, can have a terse very directive attitude like a drill sergeant. If they raise their voice or are very curt don't take it personally and flip them off, cuss them out, or swerve like you're going to hit them with your car (which is aggravated assault with a deadly weapon and you can be arrested if we get your plate & description to give police). Our job is to get cars through the workzone safely and efficiently and the job is stressful enough and dealing with motorists who don't understand what it's like as a Flagger makes it worse.

(I’m very happy this is an open forum. There are many websites I’ve come across where I’d like to leave a comment but they require registration. I don’t want to go through all that hassle cause I don’t do blogging and keep up on all these “threads” anyway.)

I first read these articles in 2006 and have religiously followed their principles. It changed my whole attitude towards driving.

My friends think I "drive too slow", but they never "beat" me anywhere. That means we drive at the same speed.

My new theory is that once these comments have a majority positive votes, traffic jams and road rage in the US will be a quaint memory.

Well sure some it was a nice read, and made some sense, but in real life things don't go so smoothly, because people don't cooperate when merging, so you have to fight ur way into traffic.

In my theory the ones at fault are usually the ones in front of the traffic jam, so if only people try to drive better(could mean a lot of things) during rush hours(I mean if u fight traffic daily.) and avoid braking as much as possible it would be better flow, cause some people brake and leaves a large block of space in between people which is unnecessary because it causes problems(people traveling may cut in front of u and thus the same guy leaves another larger block of space for the guy in front of him and so on...). and so more blocks of space are being built in the the front, but the people behind are getting slower and slower to a stop because everyone wants their large block of space... but if they think about it, they had already failed because now u got a hundred people right behind u.

Another issue is some people don't know how to pick their lanes, as I seen a freeway with 3 lanes and not much traffic, and these assholes are all driving about 45 in a 50mph and leaving no room for people to get over while there are almost no cars in front of them, so if u can't drive the speed limit stay in the slow lane only, because slow lane drivers tend to like to piss people off such as when they are going 45 in a 50 zone and when I try cutting in front of them they go 55 and then slow down to 45 again. Or that when they are going 50 some guy in front of them is going 45 and then they go to the faster lane but stays are 50 and stays in the fast lane even after passing the slower guy.

There are many reasons, but I still say it was bad drivers that causes most traffic jams other than natural traffic jams caused by police/accident/constructions

Of course traffic steams like a fluid, that what the field of "Civilian Traffic Engineering" is all about. A few notions:

1. TRAFFIC JAMS
The idea of easing a traffic jam by slowing down early and rolling through it is a well established on in the field of traffic engineering. Cars have powerfull brakes and generally stop five times as quickly as the can accelerate back up from a stop. Once traffic stops, the second row of drivers cannot accelerate before the first row has already moved on. The result is that traffic in jams moves in waves that go against the direction of traffic, and acroos gaps of about 50 meters or so.

The theory states that, once a column of car stops at a jam and stands still for a given amount of time, it will take five times more for it to return to flowing traffic. This builds up through the entire traffic jam as the cars stop and go. This style also results in increased fuel waste and a great risk of rear-end collisions.

2. MERGING
In many European countries, the driving culture is such that dictates that in areas where traffic merges from the right at an access ramp or from a blocked lane, it does so in a "zipper" fashion, where each car from the flowing lane, allows one car to merge in before it and than goes. This allows the traffic to distribute 50-50 without hesitations and problems.

Trying to apply this method when you merge or allow other cars to merge, is a good idea, even if one or two more cars jump on the oppurtunity and push their way in front of you.

Another good idea is to delay and smooth-out the merging action. The more flat is the angle in which traffic is merging, allows traffic to steam more freely. In merging left, the later you merge - the better. In merging right - the eariler you merge, the better.

So, as you get on the access ramp, don't just push your way into traffic. Instead, identify the gap you want to merge into and accelerate down the entire slip road while merging at a shallow angle.

If your lane is blocked, you should apply the same approach, but still merge early enough so that you don't need to slow down needlessly. If you merge too late, you will have to slow down or stop in front of the blocked part of the lane, which will make merging much more difficult.

3. LANE USAGE
Using the right lane as legally required does a lot to help traffic flow. In European countries where lane discipline is practiced commonly (due to higher speed differentials between lanes, as speed limits are much higher), you can see traffic flowing at speed in situations where in the US it would start to congense and slow down.

Keeping right and passing on the left is really important. Like with merging, lane changes should be gradual. When you spot a slow-moving vehicle, you should accelerate untill you are just two-three seconds short of it, while merging at an angle. Once past it, with a clearance of about two-seconds of a following distance, you should immediately merge back right, again at a shallow angle.

4. EXIT RAMPS
The exit ramp displays the same principle as the access ramp, which is to seclude speed changes, i.e. traffic which is accelerating or decelerating, aside from the steady-speed flowing traffic on the main carriageway. So, just like you are supposed to use the whole length of the access ramp, you are supposed to tuck right into the exit ramp as early as possible, and start slowing down only once inside it and not on the main carriageway.

5. SEPERATION SPACE
The seperation space between cars helps to reduce the so-called "anti-traffic", better known as the professional term of "ressistance." When tailgating the car in front, you are in fact blocking your view at the road ahead and cannot see any changes in the traffic flow or any congestions or slow downs. Also, once the driver in front so much as dips the gas or touches the brakes, you will have to brake ever more hard to maintain your small space.

Drivers tend to bunch up, as if driving in a group. In these conditions, you can create a "chain reaction" which will form a mysterious "shockwave" jam hundreds of meters to the back. So, the solution is to keep two FULL seconds at least, and more when unable to stop as quickly or when the driver in front is blocking your view, or in bad road or visibility conditions, or when the driver behind you tailgates you.

Even if other drivers push their way into your gap, it makes little difference, because they will usually be only few of those, and they usually leave you at least one full second of a following distance, as well as depart quickly and move on up the line. Even if two dozen drivers were to push their way in front of you and remain therein, the loss of time would be a mere minute!

The seperation distance is also important at stops. It's important to slow down and stop early to see that the car behind you, as well as the car or two behind it, slows down and stops without shounting your behind. Stopping early allows you to move forward to free up some more space, as well as manueverability to swerve aside, even to mount a curb, to avoid being shounted from behind. After they stop, you can move forward.

Even with a column of cars safely stopped behind you, it's important to keep a good few feet, almost a full car length, free in front. Do this by maintain such a space where you can see a few feet of tarmac seperating you from the car in front.

6. SPEED
Driving at the speed suitable to the conditions and the speed of the flow of traffic is important. Being slower than the flow of traffic or than the speed suitable to the conditions is not helpfull, only harmfull.

When traffic congestion begins to build up, speed has to be reduced so that the jam is allowed to "free up" before more cars can be fed into it from behind. This is why some highways are monitored and have electronically determind speed limits, meant to prevent the traffic from stopping needlessly, keeping it moving.

7. Awareness
Being aware to what is around you is really important. You should notice traffic congestions as early as possible by keeping a clearance in front of your car, and looking far ahead to the front of the queue. It's also about checking the mirrors, interior and side mirror - frequently.

For general use, you should check your interior mirror each five seconds, and your side mirror/s each seven seconds (i.e. two second after you check the interior mirror) which means a mirror check 10 times per minute. In thin traffic you can reduce this to five times per minute, and in heavy traffic, to fifteen times per minute!

You should also recheck mirrors at least twice at each lane change (before the lane change and once while merging), and when slowing down - slow down early and check mirrors at least twice. Also check mirrors before junctions and interchanges. Try to play with the gaps to keep a clear space to one of your sides at least.

A good gap behind is when, at speeds of up to 30mph, you can see the head-lights of the following vehicle. At higher speeds, you should see it's tires and at even higher speeds you should see some of the tarmac between you two.

8. ACCELERATION
It's important to accelerate freely at access ramps or lane changes, but it's important not to accelerate quickly in congestions, because it might make you brake just a minute later...

9. DECELERATION
Slowing down early, even if you do stop, is important because it gives you time to check your behind and allows to manage the traffic behind you and form a slow-moving column. If you slow down early enough, you will still be moving at speed when the car behind you catches up with you, and when the car behind it catches up with it. This protects your behind and allows you to lead them through congestions.

In Europe, it's customary to light up the hazard signals when you begin slowing down for a jam. It's a good habit and it eventually sticks to other drivers, too!

This is complex, isn't it? And "we" could understand it all we want, but that doesn't affect the vast majority of drivers who keep doing the same old thing but expecting a different result.

A couple of things may add to the discussion:

1) I think the gender of drivers has a lot to do with how one drives. I totally understand that women are "safer drivers" than men, for a variety of reasons. But that doesn't mean they don't cause wrecks, just that they don't have as many. Many women appear to not understand the concept of merging AHEAD of the car, at a safe speed just slightly faster than the car they're getting in front of. Instead, I see them decide to change lanes (to make their exit, for example) by coming to a complete and sudden stop, turning on their turn signal, cranking their steering wheel in the direction they want to go, waiting for someone to let them in, backing traffic up severely, in two lanes - the one they're in and the one that slows to let them in. Many times I've seen this result in wrecks several cars behind the lane-changer, of which she is likely totally oblivious. Presumably, many of these women do this every day on their commute, thinking they are safe drivers, when the truth is they are a big part of the problem. I'm quite aware that male drivers do this too, but in my experience women drivers tend to be overly polite (don't even get me started on right of way issues), causing at least as many problems as they think they're solving.

2) I've read all these posts and have yet to see anyone bring up the subject of highway design. Where I live, there are several interchanges where traffic is trying to get onto a major interstate highway at the exact place an equal number of drivers are trying to weave their way across and through the traffic to get off the interstate, and all this is happening in a very short distance. This results in dozens of wrecks each week at this location, not a few involving fatalities. Clearly, these interchanges are poorly designed, though I'm sure the designers would say their design was adequate for the times in which the interchange was built, to which I would reply that increasing traffic flow over time is a durable trend that needs to be planned for.

I'm a fluid dynamicist and throughout your text you've been missing one key point about continuum mechanics. Unlike an incompressible fluid, traffic density can change, and it increases under pressure. Bottlenecks cause pressure, and where you have high pressure you have low speed (similar to the Bernoulli Equation) by law of conservation. Physically what is happening is that as cars go faster, drivers keep larger following distances. Thus it would be impossible for all cars to merge into a single bottlenecked lane without slowing down, unless they were to pack together extremely/dangerously tight. This is a simple conservation of mass flow rate. It can only happen by increasing the traffic density if you want to maintain speed while reducing the number of lanes. However, there is one more option. Vehicles could speed up before/inside the bottleneck and possibly merge without causing waves behind them (this is why holding your thumb over a garden hose causes the water to shoot out faster, because the upstream pressure at the bottleneck CAN force the fluid downstream, whereas cars in traffic cannot.

I was thinking the exact same thing. If you have traffic flowing at 40 mph in 2 lanes with 2 car lengths gaps (3 total car lengths per vehicle), and you constrict it down to one lane, you have to either zipper together with only 0.5 car length gaps (1.5 total car lengths per vehicle) and remain that tightly spaced for the whole duration of the constriction, or speed up to 80 mph for the duration of the constriction and keep the 2 car length spaces, or some intermediate combination of speeding up and reducing spacing.

Anything else results in a higher rate of vehicles entering the constriction than leaving it, and will necessarily cause continually increasing traffic density.

The techniques in this article might help in moderate traffic jams if applied only during the approach to the congestion, not once inside it, but the real solution to a serious traffic jam is for the people to exit the congested area as quickly as possible.

I've witnessed traffic jams forming, and it seems to happen when traffic reaches a certain density. If there's a lane closed for construction, or simply an entrance ramp merging into the expressway, it's often enough to trigger it.

If people watched their rear view mirrors and tried to maintain an equal gap ahead and behind them, it might help because then upstream pressure CAN force the traffic downstream. As traffic gets heavy, you decrease the distance to the car ahead of you, and that car then decreases the distance to the car ahead, compressing the traffic fluid and increasing pressure. If everyone responded to "tailgaters" by speeding up as much as they could (within reasonable safety limits), then eventually this forward moving pressure wave would reach the front of the congested area, and the cars at the front with no constrictions would speed up to relieve pressure.

The difficulty with this is that everyone needs to be willing to drive fast in tight formation, and alert enough not to cause an accident while doing so. Also, people toward the front need to be willing to risk speeding tickets if necessary to relieve the pressure behind them.

I'm not a fluid dynamicist, but it seems like the normal "one-directional" pressure response of traffic would make it behave a bit differently from ordinary fluids, and encouraging drivers to engage in space-balancing the gaps ahead and behind them would allow a bi-directional pressure response and make it behave more like an ordinary fluid.

I myself have observed these same things over many years of driving. My basic conclusion has been that if people could see driving as a team effort, then traffic would be much bearable. If folks let others merge, kept a decent gap in front of them, let people smoothly merge, etc. then driving would be far more tolerable. Instead people see driving as a selfish act. All that matters is what they are doing and where they are trying to go. They won't let you merge, they cut you off, they jump into gaps you've created. Sadly we can't change human nature so traffic will be what it is.

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