# The Tetris theory of traffic flow

I live in a medium-sized town just on the edge of the DFW congestion zone. I don’t go to the big city much, so I don’t have a lot of experience on the urban freeways. I was in Fort Worth today, crossing the south side both ways along I-20, which has 4 through lanes. The speed limit here is 60. I almost never speed, but I do there at times to prevent a constant stream of passing traffic, which is dangerous. Today traffic wasn’t too bad and moved fairly freely, so I went 60 in the right lane.

I noticed everyone seemed to be short on time and had somewhere to get really quickly. But as I watched other cars, I started to wonder if the drivers weren’t just looking to go as fast as they can, but if they were trying to fill in the space ahead of them.

In heavy traffic I try to keep a steady speed and accomplish this by leaving a buffer ahead of me. People constantly fill in that buffer, so I’m constantly falling back further. This doesn’t bother me at all. Unless traffic is really thick, I can avoid most of the stopping and starting that way. Many people seem to be bothered by gaps and want to fill them in, even if it leads to less efficient operation. I often see people change lanes when a light ahead turns red, in order to get out from behind other cars and be the first car in that lane. Often this is because they want to accelerate and get to the next red light as quickly as possible, but with some frequency these drivers don’t even accelerate rapidly when the light turns green. It’s as if they just want to fill in that empty space.

On urban freeways, I see drivers going fast until they get close to the car ahead. Then, instead of passing in order to maintain speed, sometimes they slow to the speed of the car ahead and follow. This makes me wonder if some people are less interested in maintaining speed, or going as fast as they can, and more interested in filling in empty spaces. There are certainly people who simply want to go fast, and these are the ones constantly passing, but many people accelerate into a gap and then maintain position behind another car.

Seeing this today reminded me of the game of Tetris. The cars represent falling blocks. These blocks don’t stop and float, but fall until getting to the bottom or falling onto another block. They are also moved in order to fill in gaps that are further down. It’s similar to what some drivers seem to do.

I know people who simply want all the speed they can get. I had the misfortune of riding along at 95 in a 30 because it was night and no one was around. But I wonder if there’s another class of driver that goes fast in some situations, motivated not by speed itself, but by a need to fill in the space ahead and catch up with someone. I’ve long noticed that some drivers approach rapidly from behind on a quiet freeway, slow down to my speed and follow for a while, and then apparently realize they don’t like my speed and pass in the left lane. They do this even though the left lane was empty the whole time. Maybe for a while they were content just knowing that the gap was filled.

These observations are based on infrequent experiences. I prefer rural areas over big cities, so I don’t have a lot of city driving experience in spite of my geographical proximity. And I really only thought about this today, so the asserted possible phenomenon described has only minimal support. Perhaps more frequent users of urban freeways have more insight.

Well it seems that driving consists of of many contests of will.You deal with the aggressive types who are competeing against you when you are not competeing,my favorite are the ones that shoot by at warp speed and slam on the brakes and turn off 100 yds ahead of you forcing you to almost stop.I also have a certain fondness for those who carefully merge on interstate at a blistering 30-35 mph,not to mention those who choose to ignore right on red.The “Clovers” who make everyone drive legal ,the “Regulators” who hang in the left lane for miles and wouldn’t consider pulling over and letting the Half mile train of traffic by on a winding road.You are right,someone will fill that safety zone you try leave in front of your vehicle and got to love the new traffic lights that appear in the middle of nowhere,that add several minutes to the commute(I despise the smart lights as a truck driver I find myself stopping at practicaly every light) not to mention the Folks that like to stop in the in the “offtracking arc” of trucks rear axles(they sometimes become acquainted first hand with the crumple potential of several 11R -22 tires-those lines are there for a reason).All I can say is drive careful,have a hard shell, keep the middle finger down,stay off the"toot,toot" if you are angry stay home.My Wife has to be the biggest "Killjoy"in the world,so as you have probaly figured out by now she does our driving when we go some where,a lot easier on the ears.But by and by you are most likely right,there has to be Algorithms and formulae that deal with traffic flow ,taking into consideration “Mob psychology” and plain old human nature.My mantra is “flow smoothly” not .“fits and starts”.I’ll stop for now,some of these sage people on here can really give you a one up on this matter-Kevin

In a previous life I worked a night shift and drove on highways in the wee hours.
Many’s the time I would encounter a multi-lane highway empty except for two or three cars and they were constantly weaving around each other and tailgating in a cluster, almost as if they were magnetic.

I have also noticed the phenomenon that circuitsmith stated. When you have 4 lanes to choose from and want to tailgate someone in a small cluster there is definitely some brain cells missing in the driver. I usually just slow down and watch for headlights in the rear view mirror. I go back to cruising speed and let the cluster go on without me.

I have watched the phenomena and surmised that the normal mix of personalities and driving styles seem to cause the situation. When cruising just above the speed limit and overtaking a driver cruising just under the limit it often seems the timid driver will increase their speed and follow if the increase is not too great. As these two cars continue on they often overtake more timid drivers who sheepishly fall in line at the speed they would never drive on their own but together, following the pace car, they keep up the “break neck” speed.

Very interesting. Thank you for the insight. I intend to look more intently for what I can learn from traffic patterns in the future.

Rod Knox wrote:
When cruising just above the speed limit and overtaking a driver cruising just under the limit it often seems the timid driver will increase their speed and follow if the increase is not too great.

The driver being passed may feel that it’s safer to speed now that there’s a lead car to get hit by radar first.

Have youall ever encountered the California rolling road block ?
At least in L.A., the C.H.P recognize that tetris effect and attempt to prevent some of the worst by having one patrol car, lights flashing and weaving accross all six lanes, slow the next pack of wolves down to just under the speed limit creating a mile or two of space between the packs.

Durring the rush hours they repeat this procedure about every fifteen minutes or so.

I’ve even seen it done once or twice in Albuquerque.

And then there’s “traffic waves”:

http://www.smartmotorist.com/traffic-and-safety-guideline/traffic-jams.html

I’ve been in my share of traffic snarls and believe it or not, total capacity increase if you leave about 5 car lengths between cars. Everything moves along nicely then. Humans though and other animals seem to at times do things that are illogical just to fool themselves into thinking they are making progress.

Certainly the traffic moves much better in that scenario, circuitsmith. But it doesn’t take into consideration the range of egos and sensibilities of drivers. I often find that as I approach a closed lane ahead and move out of that lane long in advance there is always a number of aggressive drivers who rush ahead in the emptying lane that will soon end and force themselves into the front of the line of traffic entering the narrowed roadway. It is those selfish, aggressive drivers who cause the problem. But what can be done to eliminate the problem? Likely nothing. We must just live with it… Don’t sweat the small stuff."

Hasn’t it already been said that it’s actually better for traffic flow to occupy both lanes until the end and each lane takes a turn going. This dosn’t always happen, though.

I have noticed the fill in any space available. I think it is the I’m going nowhere in this lane, OOh OOH, There’s a space maybe I can gain a car length. Changing lanes to gain a car length with little change in going from point a to point be seems to be some peoples preferences. And yes I hear the pains of previously posted, and in the 3 lane tollway I commuted for many years, my best analogy is a snake race of 3 lanes. A snake has to recoil before moving forward. The snakes are not in sync, any lane can go or stall at any minute. Leave a car length space and somebody will take it, but imho it seems to work out pretty even in the end.

I’ve been through Fort Worth and Dallas quite a few times and I agree with the Tetris analogy; especially on that stretch from near downtown Fort Worth south to about Alvarado.

On occasion it also reminds me of those contests where thousands of small plastic ducks are dumped into a stream. A few get caught on a rock and the logjam begins…

Yeah I agree with all these statements…Interstate 26 and 526 leading into Charleston, South Carolina were like that as well! And to add all the construction in as well. What a mess! I now live in Rural Nevada and this isn’t much of a problem until you drive in Reno, where again…construction and road widening has been backing traffic up and people do the same thing!

Rod Knox wrote:
I often find that as I approach a closed lane ahead and move out of that lane long in advance there is always a number of aggressive drivers who rush ahead in the emptying lane that will soon end and force themselves into the front of the line of traffic entering the narrowed roadway. It is those selfish, aggressive drivers who cause the problem.

Studies have shown that traffic moves best if the merge is done at the point where the one lane ends. Around here, they put up signs saying “use both lanes until merge point” so that everyone knows what to do. I guess they don’t do that where you live.

“Studies have shown that traffic moves best if the merge is done at the point where the one lane ends.”

I’ve read that before. I think the idea is that when there’s heavy traffic it’s better to have a single merge point, since merging inevitably results in disruption. It’s better to have one point of disruption rather than a mile-long zone.

Of course, it would be even better if people wouldn’t crowd together and follow so closely. But we haven’t figured out a way to stop that. People just feel better being 20 feet further ahead, even if it makes them have to stop.