Chaos and Your Everyday Traffic Jam

An anonymous reader writes "What causes these mysterious traffic jams that continually appear throughout the day for no reason whatsoever? Is it simply the fact that most people just don't have a clue how to drive? That's very possible, and in reality there are so many variables involved in something like a traffic jam. But is it possible that the entire traffic jam could be both the continuing and end result of a chain reaction set in motion by a single driver who was in too much of a hurry?"

I know, I know!!

[quokkapox]

"What causes these mysterious traffic jams that continually appear throughout the day for no reason whatsoever?

Too many cars?

Roads and CSMA/CD

[rar42]

I'm inclined to compare roads to shared medium Ethernet. As the traffic builds up you get more 'collisions' and both systems have collision detection built-in.

With Ethernet, as the 'traffic' builds to about 40% of the theoretical capacity, collisions become the norm and the re-tries start to overwhealm the system and it locks. With roadways, as the traffic builds to a certain limit, then awareness of potential collisions magnifies in the drivers, so reactions to situations increases and the road stalls. This is why variable speed limits work, because the road and drivers can cope with more vehicles if there is a lower maximum speed.

WOW --- Just What we Need

[meBigGuy]

A pointless story with no data, no analysis, no facts, but lots of conclusions. Don't we usually leave those for Digg?

Chain reaction

[venicebeach]

One problem with tracing something complex like this back to a single event that was supposedly the cause of the "chain reaction" is that the event you choose was itself caused by something. For example, from the scenario used in the article, the event that triggers everything:

It is a clear, sunny day and the roads contain no obvious hazards that would cause problems with traffic. Traffic on this particular highway is pretty thick, but it is flowing smoothly and steadily. One of the drivers, let's say a man in a red car, decides that people in his lane are moving much too slowly for his taste. He quickly changes lanes in an attempt to get to a quicker moving lane. He fails to properly check his mirrors and cuts off another driver in the lane beside him. This forces that driver to apply his brakes to avoid getting clipped by the red car.

The story is that this man's decision caused an entire traffic jam. But his decision was a result of his interaction with the traffic conditions. One might find that given that traffic is moving at a certain slowness, there is some probability that any individual will find it too slow and make such a decision. Perhaps as traffic gets slower, that probability goes up. So what caused the traffic jam, this person's decision or the conditions that led to it?

Isn't it fairly obvious?

[eebra82]

Isn't it fairly obvious why we get traffic jam?

The only way to get consistent traffic throughput is to have cars that maintain the same speed at all times, do not switch lanes and do not turn left or right at all.

Since all drivers have different destinations, driving techniques, cars and intentions, it is impossible to achieve this. Someone's gonna change to the other lane, delaying the people behind him who have intentions to delay the traffic in some other way, which eventually triggers traffic jam. It's a gigantic chain reaction, really.

Re:On a very busy road...

[Alioth]

Oh, I do that. The thing that annoys me is when I see trouble ahead and start to slow, as soon as the tiniest gap opens, someone in an adjacent lane leaps in. It's so frustrating. Many of the problems are caused by poor 'me first' road discipline, for example, when approaching a constriction, so many drivers go right to the end and try and force their way in - and it's not helped by people not letting adjacent lanes merge in good time before the obstruction.

The art of proper merging should be something taught to drivers and tested on the driving test.

How to help unjam and jam

[weave]

This is why when in a jam the best thing for people to do is calm down and change their speed slowly to even out the speed. This means trying to predict the average speed ahead of you and do your best to maintain an even speed, even if slow. Yes, impatient drivers will move in in front of you, but they are also most likely to jump back out of in front of you in a bit too. Getting next to a truck doing the same thing helps. Pressing out the waves is what will get traffic moving again for those behind you.

Re:slow ass drivers

[dbIII]

it's the slow dumbasses that are the real cause of majority of wrecks. It's that asshole who is going 50 in the passing lane and won't move.

That's just what the idiot who drives too fast without paying attention and runs into them would have you believe. To make it easier to identify them they have often purchased personalised number plates identifying themeselves by name - at least in my country. If you ram into someone under ordinary conditions you are just not paying attention or do not know how to drive within the capability of your vehicle - if you can't even stop in time for a moving target what chance do you have against a stationary road hazard?

People who drive slowly in the wrong place may be annoying idiots - but they do not cause the accident.

Re:How to help unjam and jam

[Keeper]

You can even turn it into a bit of a game ... see how far you can travel without touching the brake.

A few other beneficial side effects:
* better gas mileage
* less stress
* less wear and tear on your brakes

Re:slow ass drivers

[beelsebob]

I'd agree with you partially, slow drivers getting in the way is a major cause here. But I'd suggest that it's usually actually the person in the expensive powerful car being impatient that causes the major tail backs.

Here in Britain, I drive a small Skoda, it doesn't go too fast, but it's certainly no snail (it can do well over 100 if I really wanted to). I tend to drive down motorways at 75 or 80 mph (very naughty, I know, the speed limit is 70). The thing that I observe most often is that if I pull out to overtake some slower moving traffic (a lorry, or someone doing 70), there's usually some ass hole in a beemer, a merc or an audi comes roaring up behind me at 100mph, slams on the breaks because he realises a bit too late that there's someone driving at a sane speed, and then proceeds to tail you 5m from your bumper until you deign to move over and let the selfish twat past.

Re:I know, I know!!

[SpinyManiac]

I'm guessing here, but I'd say it's a large amount of traffic in a small area. I've seen it happen on motorways (typically three lanes in each direction) when a truck overtakes another one very slowly and all the cars pile up in the other lane. Everything behind is forced to slow down and the congestion propagates back up the road behind them.

Re:On a very busy road...

[Anonymous Coward]

Another problem is that brake lights don't provide enough information, and what they do provide is provided at the speed of light.

BRAKE!!!!!!!

As opposed to

Take your foot of the gas and relax for a bit.

Re:slow ass drivers

[Lumpy]

REally? I always saw the asshole on the pocket rocket driving in between lanes at 120mph that eats a car or two that causes most of the problems. At very least he causes the panic-able drivers to hit their brakes when a moron flies past their door 8 inches away. The slower drivers rarely cause the traffic jams it's the guys that flip out and then cut off other people because they think they are more important and thus have the right to tailgate, drive 20-30mph over the speed limit and have a little kid hissy fit when they encounter a slower driver and then start their childish behaivoir all over again causing others to have to jam on brakes, etc....

The idiot that is 8 inches from the car in front of him is the cause. plain ans simple. Because if he/she is so stupid to tailgate, then they will cut others off at highway speeds and drive incredibly reckless causing more traffic behind them to slow down.

The one guy putting along NEVER causes the problem.

Re:On a very busy road...

[berj]

Umm. you may want to *read* the article at http://amasci.com/amateur/traffic/traffic1.html [amasci.com] (posted earlier). His evidence suggests that it *is* helped by people letting others in (in an effort to keep traffic flowing) and is actually *hindered* by behaviour such as yours.

Re:slow ass drivers

[dbIII]

Come on - you learned to stop in time when a car brakes in front of you to get your licence. Stopping in time for a car in front of you that you can see is the easy bit - it doesn't matter if they do something stupid, annoying or illegal if you are too close it is your body that can get injured so it is your own responsiblity to pay attention. Let one idiot sit in front of you being a pain instead of having two dead idiots. Cars come with horns and lights but not a battering ram even though some people drive as if they do.

Interesting justification for failing to pay attention - is it that everyone else should do it instead or am I seeing things the wrong way here? Why the death wish on a person in front? If someone is knowingly driving 6 inches away at 75mph on a public road with amataur drivers they should not have a drivers licence no matter how many times they've seen race drivers on TV.

Somehow replying to some guy that says the slowpokes cause all the accidents has been read as they cause no accidents - get off the absolutes guys but also remember that driving to put yourself into situations where you cannot control your vehicle can't be blamed on the tree, animal, child or slow driver in front.

Re:It's both!

[aderuwe]

Different issue.

The SUV driver should have at least touched the break to clue in those behind. That he didn't is deserving of having his driver's license taken away.
This has nothing to do with car separation. It just so happens that an idiotic separation of hundreds of feet would have helped in this case.

People who drive feeling as though they are _moving through traffic_ are what causes problems.
You should drive _with_ traffic, be a _part of traffic_. There are no individuals in traffic - we are all one whole. The Traffic.
Drive with this in mind, and you'll increase your driving skill by orders of magnitude.

(Of course none of this _really_ matters even if it is the truth, because of all the other individuals considering themselves the most important pricks in traffic / the universe...)

(And yes, this means that I break speed limits whenever moving with traffic requires it. This is a natural conclusion.)

Cheers.

Re:It's both!

[zero_offset]

If we planned for the worst case, driving would be illegal. :)

Seriously.

[aussersterne]

Everybody is busy pointing out the "first causes" of traffic jams without noticing that every one of these causes is caused by interaction with the roads in the first place.

The real "first cause" of traffic jams is differences in driving decisions and style. If everyone drove at 120mph, or everyone drove a 30mph, or everyone could anticipate exactly what the other driver would do before they did it, and adjust accordingly in advance, there would be no traffic jams.

Traffic jams happen because one guy is driving 45 mph thinking "look at all these damn idiots driving too fast for this weather and this level of congestion, well not me, I'm a good driver" and the guy behind him steps on the gas and passes at 80 mph thinking "look at all these damn idiots impeding the flow of traffic, well not me, I'm going to pass so that others behind me can pass afterward" and the guy in the next lane over is thinking "jesus, look at these idiotic fast and slow drivers passing each other and holding up traffic, I'm going to stay in my lane and not accommodate any of them or let them use my lane to pass, since they're all rotten-ass drivers."

It's the conflicting intentions and behaviors in similar situations that lead to brake-slamming, passing, swerving, wrecks, and the other causes of the density patterns that characterize traffic jams.

Re:The correct city driving speed

[zero_offset]

I also notice that the cyclists tend to take a rather liberal view of their obligation to obey traffic control devices. If you scaled the available road space down and traffic levels up, cyclists would have just as much trouble. The only reason they don't is because we build roads for cars, then we allow bicycles to use them.

Mod parent up

[Razed By TV]

Nice find.

So a 2 second gap is 205 feet.
2/5ths of a second then equates to 41 feet.
That leaves 205 - 41 = 164 feet left to stop, which is 6 feet less than the number cited for the average modern vehicle to stop. So the two second rule is deficient by about 3.5% for the average modern vehicle.
On a side note, modern isn't exactly defined, but I don't think my '87 station wagon (with no ABS) qualifies.

Re:It's both!

[Gr8Apes]

Actually, it's the fault of the moron that left his car in the road, most times. I've never figured out why a car is left in the lane, especially in an open road. I've seen this more than once, and almost hit an abandoned car in the left lane once with a perfectly available shoulder right next to it.

I've also been in an accident exactly like this - the van in front of me moved right - voila - about a 5 mph pickup in left lane, 18 wheeler right next to me - 55 mph smashup. It was a pretty hefty wreck, threw the pickup about 300 feet, totalled both vehicles. I was not found at fault, since it was a 55 mph road with no turns at that point, and pickup was moving far below stated and reasonable speeds. (FYI: No one was hurt in any meaningful way)

I've driven in the US, Canada, Mexico and a large segment of Europe, and by far, the best freeway drivers are in Europe. They drive right most of the time, trucks are limited to right lane(s) only and a max of 50 mph through large segments of freeway, and in general are far more considerate than the embecilic god-given left-lane road hogs in the US. (Then again, failure to drive right in Europe can result in incredibly horrible accidents - a mercedes plowing into a BMW launched over an Opel hatchback at a 100mph speed differential is not a sight you want to see. The Opel pulled into the left lane just in front of the BMW.)

Your largest problem on crowded US roads are trucks. Their braking/speedup times are significantly slower than cars, and will cause massive accordian effects the first time they brake. Move them to the right and limit their speed and following distances, and entire problem segments will dissappear. It may slow overall traffic but it will be a steady pace, which will be much faster than the accordian stops otherwise always experienced.

Re:slow ass drivers

[tuskentower]

there's someone driving at a sane speed

Don't take this the wrong way, but you just said what the guy/gal driving at 60mph (10 mph under the speed limit btw) thinks of you. They see the limit as the maximum, not the minimum. You should extend the same courtesy to the person driving 100mph that you would like to see from the person driving 60mph. Don't call him selfish just because drives faster than you. Maybe s/he shouldn't tail gate you, maybe s/he should. It's up to you not to give that person the opportunity to tail gate you.

The key here is to pass fast. For example, my cruising speed is 10-15mph + speed limit, but I pass somewhere between crusing speed and 90mph (depending how fast that person charging down the passing lane is going). When you pass fast you achieve passing speed before you enter the next lane to pass. If you are unwilling to achieve the speed to pass fast, don't attempt to pass.

tailgating turns slowdowns into stops

[Anonymous Coward]

Following distance is very important. People keep describing a situation where a driver has to hit the brakes even harder than the driver in front of him, where any small event leads to people standing on their brakes. That's all tailgating. If you ever hit your brakes harder than the guy in front of you, you were tailgating pure and simple.

Unfortunately, over-capacity roads lead to defensive tailgating, where if I don't tailgate, the tweaker/crackhead/cellphone/drunk behind me will pass me on the right, cut me off, and slam on his brakes 'cause he's too close in front. I don't mind that much, except that bad drivers keep being bad drivers. He'll nod off and slow down to 50 in no time, only waking up when I try to get around him. If I pass with a 20+ mph differential, they usually don't have time to notice and go back into hyperaggressive mode before I'm out of range (I don't care who's in front, but I just don't want to be near a bad driver) but that accelleration requires space.

The rule for tailgating is this: If you're regularly using your brakes on the freeway, you're probably tailgating.

Space cushion. Remember that from driver's ed? Oh yeah, we decided driver's ed was too expensive for public schools, so now we pay for that decision every day on the roads. There are plenty of non-obvious rules that make driving safer. Drivers really are worse than they used to be.

Re:Lane merges

[orgelspieler]

here's what I do: I drive in the closing lane, but only as fast as the person next to me (normally pretty slow). That way, the guy next to me doesn't think I'm "a cutter" and keep me from getting in when the time comes. Of course there's some idiot flashing his brights and honking, and doing sign language, because there's all this "open space" ahead of me. But it's only open for a quarter mile, and I'm not gonna let that idiot cut in front of all the other nice people who are trying to do the right thing. If he does, everybody will have to slam on their brakes and let him in.

Of course if everybody would just make space and bread-n-butter'd at the merge point like they're supposed to, I wouldn't have to go to all that trouble. But hey, it works. Neither me, nor neither of the people in the slow lane next to me have to come to a complete stop. And as for that honker behind me, he's only four or five cars behind where he wanted to be. What is that, like 20 seconds of travel time? Small price to pay for good flow, if you ask me.

BTW, I got the idea from watching how truckers do the same thing at a lane closure. I noticed that the fastest I ever got through the ship channel bridge was when two trucks drove side by side right up to the merge point, and then the one just hit the gas a bit and got ahead of the other. Nobody slowed down, and everybody pretty much got the point that there was no way to get around these guys. Sure we never went really fast, but we never stopped either.

Even this isn't foolproof though. Some idiot actually jumped a curb just to get around me. And you know what? When it came time for her to merge, nobody let her in until about one car ahead of me. Gee, she really got far! Idiot. And then there was another guy who was behind me, got in the merge lane, went over one more lane (causing much honking and screeching of brakes) and then got back in the merge lane where I was going to go just so he could continue giving me the finger. No bother. Just slow down a little more and take the next spot back.