What Are Traffic Waves and Why Do They Happen So Much?

http://ww2.kqed.org/lowdown/2013/11/12/traffic-waves/

by dhatch387 • 11 years ago

118 91 11 years ago

ww2.kqed.org

mixmastamyk6• 11 years ago

This reminds me of a piece on traffic I read quite a few years ago and might have been on reddit or even perhaps slashdot. I still follow its advice to this day when having to brave the 101.

It discussed traffic waves and our reactions to them. The repeatable waves this article mentions come from (most of) the individual drivers driving faster than the capacity of the road to carry them all. So they bunch up at some point, catching those in front, everyone slows down for a bit and then the bunch "evaporates" from the front... repeat. Those who get out of the slowdown speed to the next one. It also discussed the effects of merging, which have a high cost as lanes get fuller, also why you'll see lights at on-ramps to mitigate.

The recommendation was to slow down, leave plenty of space ahead for others to merge, and try to get going at as close to a single speed as possible to conserve momentum, gas, wear & tear, nerves, etc. If packed enough you can let the car idle push you along around 5mph/8kph, which is what I aim for in that situation. Feels better than speed-up/break/slow-down over and over.

It also responded to questions such as, "what if everyone around me wants me to go faster?" It happens, but those people tend to drive off in a huff. Once one or two of them do, you'll find a more relaxed driver behind you for the rest of the trip. The impatient driver won't get ahead much anyway so you can wink at them when you catch up at the next slowdown.

vlasev• 11 years ago

One problem of the "car idle" in terms of humans is that it takes almost constant attention to make sure you don't bump into the cars in front. Compare that to the speed-up/break/slow-down which allows time to relax. In my own driving I tend to have a more relaxed speed-up/slow-down cycle which is somewhere in-between the two modes you describe.

leviathan• 11 years ago

Not to mention that, in the country I live in, it is guaranteed that while you're idly rolling forward, some asshole will cut you off and take your place so that he can win that precious 2 seconds.

Jach• 11 years ago

I think you're thinking of http://trafficwaves.org/trafexp.html

I read it too, years ago, and also follow its recommendations. (And I ended up in the Seattle area, so I've gotten to experience the same traffic the author did...) It makes traffic less annoying for sure, even if there were 0 other benefits I'd do it. The only freeway issue that annoys me these days is people who don't keep right except to pass during non peak times.

mixmastamyk6• 11 years ago

Yes, that must have been it. I don't remember anything about the look of the page, but his tone seems very familiar.

ninkendo2• 11 years ago

How can you be sure the cars behind you aren't even more compressed than they otherwise would be if you're leaving so much space in front of you like that? One thing I hated about that article was that the author assumed he was doing such a great service for everyone by "smoothing out" the traffic waves by leaving a buffer, but he never really proved it; he just waved his hands and said that it looked like the cars behind him were doing fine. (How can you see more than one or two cars behind you? Are you driving in a literal ivory tower?)

In reality it could be just the opposite, someone behind you could be blindly accelerating because he sees the cars next to him doing so, then realizes you in front of him aren't accelerating along with them and has to tap his brakes, causing more traffic waves. And by definition the traffic behind you is more compressed than it otherwise would be, because you're hogging tons of lead space all for yourself. So you're dampening the waves in front of you by making the waves that develop behind you even worse!

Given that the minimum amount of lead time drivers are comfortable with is constant, if you pour tons of cars into a highway at rush hour, traffic necessarily must slow down because there's too many cars per mile of road to accommodate a safe lead time for each car at high speed.

So if everyone tried to smooth out traffic waves by leaving extra space in front of them, they'd effectively be making the number of cars that will fit in a mile of road even lower by artificially requiring more space for themselves. This will always make traffic become slower overall. Say what you want about the benefits of not having to stop-and-go traffic but you shouldn't be under any illusion that it makes the commute faster overall.

morley• 11 years ago

Whenever I see an article about traffic patterns, I inevitably see this exchange:

Person 1: You can fix traffic waves by smoothing!

Person 2: Smoothing traffic waves makes no sense!

It's frustrating. I don't know what to believe. Is there any peer-reviewed, simulation- and data-backed research that puts this issue to bed for good?

andrewflnr• 11 years ago

The way I do it, I eventually catch up to the car in front of me. Not having the data to pick the perfect, average speed, I err faster so that the traffic behind me isn't, on average, going significantly slower than traffic in front of me. Note that waves behind you are the only waves affected. All you're doing is thinning out the medium so they don't propagate efficiently. The end goal is keeping actual speed close to average speed; spacing is a means to an end.

mixmastamyk6• 11 years ago

Someone posted the link, and noticed the part where he saw behind him. TL;DR: When driving downhill into a valley, you can see for miles behind you out the rear window.

Also, I didn't say that smoothing the waves increased road capacity, though improving merging may help a bit. Rather, the reduction of stop/go is to everyone's benefit.

In the city things are largely the same, people here tend to speed to the next red light, but it doesn't help.

wbeaty3• 11 years ago

How can you be sure the cars behind you aren't even more compressed

Because, uh, you don't drive slow.

You drive at the average speed of traffic. Not slow. To smooth out the traffic waves, you don't come to a halt, and neither do you rush forward. "Average speed:" that's when it's not fast, and also it's not slow.

---

Myth: if you have an empty gap, you must be a slow driver!!

Debunked: yes, genuinely slow drivers have big gaps ahead, but those gaps are continuously growing. If a clot of cars is going just 1/2MPH slower than average, then the gap increases rapidly: growing a half mile for each driving hour. If that clot of cars was 5MPH slow, then in just ten minutes the gap would increase by over four thousand ft.

But on the other hand, a constant-sized gap does not decrease your speed. Whether you're 6" from the car ahead, or 6ft, or 600ft, your gap has a constant size, and you're moving at the same speed as traffic ahead.

Slow drivers do make empty gaps, but empty gaps aren't any proof of slow driving.

Heh, so maniacal aggressive tailgating doesn't actually get you to your destination any faster? AMAZING! Who'd have thought!

zhte415• 11 years ago

I think it was on Slashdot years and years ago. I can't find a link. I also remember the recommendation for spacing out and making it all better. Taxi drivers tend to do this (personal anecdote - others' taxis may differ).

sinelaw_2• 11 years ago

Nice visualization! I made something similar a while back (refresh the page if it doesn't start properly): http://sinelaw.github.io/jammed/src/

It includes drivers with different personalities (buses accelerate slower, some drivers are daydreaming, etc.) and lane changing (people change lanes when it's beneficial)

dEnigma• 11 years ago

Very nice, but since there is no transition the lane changing looks a bit too jerky so that at first I thought it was broken.

sinelaw_2• 11 years ago

Right - that needs to be fixed. Besides the graphical issue, the lane changing is instantaneous, which is not realistic.

kokey3• 11 years ago

I find this happens a lot more in some countries than others, and it's not down to levels of traffic on the road, it's down to driving habits. I think the UK has done well with driver education and variable speed limits to mitigate this, and zipper merging is the norm.

smhg• 11 years ago

Is it based on your own experience? Or you read it somewhere?

In which countries did it happen more?

kokey3• 11 years ago

The last time I counted, I have driven in 12 different countries. I found this certainly happens a lot in South Africa and a bit in various parts of Spain and Ireland. It seems to happen a lot less in the UK and Portugal.

cones688• 11 years ago

I think has a lot to do with manual transmission which allows for natural and controlled engine breaking, thus not creating the brake light to come on (and therefore the wave of copy-cat type braking).

kokey3• 11 years ago

The worst I've seen has been in Johannesburg in South Africa, where automatic transmission is rare. I used to refer to it as concertina traffic before I knew it was called a traffic wave, and the concertina is a popular instrument in South Africa amongst the ancestors of the people who are usually responsible for these waves.

greeneggs• 11 years ago

I wish California would put traffic mitigation into its high-school driver's education classes. Knowing the rules of the road and driving safely are important, but so is reducing traffic, and some simple information could save a lot of time in traffic jams.

orik• 11 years ago

Whenever the topic of self driving cars comes up in conversation I let people know I think self driving cars are going to solve a lot of our traffic woes.

I'm not sure if a model exists yet for cars 'putting out' traffic waves, but I'm sure once we have self driving cars and they start talking to each other this is going to be of great interest.

I'm hoping we can effectively 'micro' away some degree of congestion.

stormbrew• 11 years ago

I think people overstate the degree to which self-driving cars will actively 'talk' to each other. And I actually think it's somewhat detrimental to advocating for self-driving cars, because the immediate thought is that communication acting as a channel for malware. And that's a fairly justified fear.

In the end, I think most of the communication will just be in terms of the fact that self-driving cars interacting with other self-driving cars will be much smoother and more normal than existing traffic.

Look at flocking birds. They're not talking to each other, really. They're reacting in a complex but uniform way to their peers. I suspect self-driving cars will look like that even without active communication.

jacquesm• 11 years ago

And that's assuming error free communications, near latency free communications and non-congestion as well as perfect hardware.

Is 'no signal' the same as 'no other cars' or 'broken radio(s)'?

Self driving cars have all the elements of a distributed computing system only now it's moving at a pretty good clip and lives are at stake. 'Move fast and break something' is very bad advice and 99.999% uptime is not going to cut it.

Hard enough to get autonomous (pun not intended) systems right, much harder to make systems that communicate between vehicles right. The best way would be to not rely on inter-vehicle communication but to only work with the signals that the car can pick up from the environment.

At least that data will be somewhat trustworthy because you have some idea about the integrity of the system.

rajivm• 11 years ago

While I largely agree, I think that some layer of _untrusted_ communication could be beneficial. Think along the lines of how turn signals & honking can benefit humans. Similar types of information could be conveyed more precisely "over the air", even if it was not presumed to be perfect (i.e. just likes human's signals aren't and a smart car isn't going to trust someone's turn signal, but it might react to it). Examples of this could be 1) relayed traffic ahead (more time to break smoothly) 2) lane switching plans (cars can coordinate such that it reduces slow downs & conflicts) etc. This is very similar to how network layer algorithms behave (i.e. TCP can hint to slow down, but you don't have to listen).

melloclello2• 11 years ago

I expect in the short run we'll see different tribes of self-driving cars - say Google vs. Uber - who will actively communicate cooperatively with their in-group, but actually compete against the out-group, depending upon whether or not their host companies decide to cooperate or not.

In the long run, I expect cars to become smart enough to be self-owning, in which case it's every car for himself.

bryondowd• 11 years ago

I would think that active coordination could be beneficial, when not guaranteed. I'm sure most people have been in a situation where they are waving or gesturing with another driver to coordinate something in a friendly way. Maybe you need to move into their lane, so you point and make a questioning face. They nod as if they understand and make room for you. You move over.

They didn't need to back off, and in that case, you would have just waited for an opportunity to open up, or a friendlier driver to be next to you. Even if they nodded, but didn't back off, you'd just assume a mis-communication and not try to merge in.

So, self driving cars could do this on a much more regular and reliable basis, while still not relying on it or trusting one another. They broadcast that they want to get over, and if another car happens to get that signal and happens to be willing/able to comply, it opens a space. If the first car sees a space, it moves over, which is what it would have done anyway, regardless of whether the space is coincidental or intentional.

Could also be handy for determining right of way in edge cases where humans often get confused and play the you-go-no-I-guess-I'll-go-no-wait-now-he's-going game. Two computers might be playing the same game, but they can resolve it in a tiny fraction of a second. Of course, the flocking birds example works well here as well. If the rules are clear enough, the cars will just know who should go.

But yes, no matter what, a car couldn't trust what another will do, and should be prepared with an evasion strategy for every possibility, thus not doing anything that would leave it no way out if another car did something stupid or there was a mis-communication.

toast0• 11 years ago

I hope that self-driving cars will broadcast their intentions, so other cars can use that to get a better prediction of where the car will be. But the broadcast processing definitely needs to be done carefully: the data may be corrupted or malicious, so the receiver needs to carefully process it to avoid crashing (software or vehicle), and also check it against the picture of reality it gets from its sensors.

If the vehicle in front of you tells you it will accelerate, you shouldn't accelerate too, unless you can verify the acceleration and that it's safe for you to do so.

blunte• 11 years ago

One point that seems often forgotten (or undervalued) is the great different in deceleration vs acceleration. Cars (especially driven by humans) decelerate much more quickly than they accelerate. I didn't get the impression from the simulation that this difference was realistically represented. I think reality is even worse than this simulation illustrates.

tirant• 11 years ago

Somehow I always knew this, and I was always driving in a specific manner to combat this effect: I always leave a buffer of space between my car and front car, and whenever I observe oscillations in the car in front, I use my buffer to let my speed oscillate as less as possible.

egeozcan• 11 years ago

The demo breaks after I click around a bit: http://i.imgur.com/OGvAO2Q.gifv

Pufe• 11 years ago

this happens because the brake level when you click the brake button is smaller than the brake level needed to not collide with the car in front of it.

reuven• 11 years ago

A model of this is included in the NetLogo models library, which comes with the NetLogo modeling language: http://ccl.northwestern.edu/netlogo/models/TrafficBasic

It's fun to see children (and adults!) play with the controls on this model, experimenting with different factors that they think might eventually reduce the congestion.

This model is also a great way to introduce kids to complex systems, in which the large-scale results aren't necessarily as obvious as the individual actions that lead to those results.

jasmcole• 11 years ago

Nice. I had a quick look at this effect in an old blog post, it's surprisingly simple to derive the existence of waves in traffic flow (given some simplifying assumptions) http://jasmcole.com/2014/09/14/only-a-fool/

_lce02• 11 years ago

TIL only a fool breaks the two second rule

https://www.youtube.com/watch?v=mf5d2DP4Pp0

lerouxb• 11 years ago

I'm pretty convinced that self driving cars can fix this. Reaction times will be much faster and they can network together to spread out the braking and acceleration. Possibly even merging at high speed.

maxerickson• 11 years ago

A more powerful potential mechanism is that they can be programmed to respect do not enter directives from a traffic control system (keeping human drivers off a freeway would take a lot of infrastructure). So the amount of traffic on the road can be regulated to keep it below the capacity implied by the safe response time of the vehicles.

dmvaldman• 11 years ago

An interesting point to mention is that the traffic "jam" moves backwards. This is because at the front of the jam, cars are able to accelerate out of it, while at the back o the jam cars are piling up into it. Seen from above the jam is a shock wave moving backwards.

This often upsets people when they get out of a jam only to see there was nothing causing it! In reality what caused it may have been much further up the road.

AlfaWolph• 11 years ago

It's usually high density traffic and someone pulling a move like a forced merge which causes the person now behind them to slam on their brakes. I've seen this hyperactive "weaving" happen too many times to have it be explained only by "slow human reactions times".

cmdrfred• 11 years ago

I heard a few years back (no source) that when they model traffic they can design systems that work perfectly, its human error (not hitting the gas fast enough after a green light, changing lanes to often, 18 wheelers getting in the far left lane to turn right) that make the models break down.

larrydag• 11 years ago

This is something I love to think about when driving on the highway. I've read so much on it. I've also had some thoughts on recommendations to solve it

1. Divided lanes for 3+ lane highways. Basically a modified HOV lane but no occupancy restriction. The far left lane would have lane dividers with openings every 5 miles (or 3 exits). The middle lane opening every 2-3 miles (or every other exit). The idea is that it would force a certain average speed of traffic and hopefully distance between cars in the respective lanes.

2. Smart roads to adequately distance cars from each other. Either a driver assistance signal or actually controlling the internal cruise control system.

briffle• 11 years ago

I believe there are highways designed like #1, I seem to remember visiting the NY/NJ area, and I think it was the NJ turnpike that was this way. Several lanes of "through traffic" with a wall, and every few miles, it merged with a few lanes on the right of "getting off/on soon" lanes.

_lce02• 11 years ago

can you see the animations?

my browser refuses to load them because of mixed content .. but html was downloaded over http !?

    Blocked loading mixed active content "http://d3js.org/d3.v3.min.js"
    ReferenceError: d3 is not defined

knodi123• 11 years ago

Does anyone else remember the scene in Mission Impossible 3, where tom cruise is hosting a dinner party full of guests who only know his fake cover identity as a statistician working on traffic waves for the department of transportation?

The movie portrays it as conversational kryptonite, where he briefly discusses traffic issues in order to weaponize boredom and get some people to leave him alone.

That part broke my suspension of disbelief, because I was like "Hey, why is everyone wandering away?!? I want to hear what Tom has to say about how to address the problem!"

irl_zebra• 11 years ago

I remember this, and actually thought the same thing. It led me to do some research and I came across a pretty interesting page on the topic of smoothing out waves of traffic. It's an old site and I was just able to re-find it just now, but perhaps you will find it interesting:

http://trafficwaves.org/trafexp.html

joshdance• 11 years ago

Any time my friends and I talk about traffic, someone mentions that scene. :)

thunderbong• 11 years ago

I don't know if you've seen this video[0]. It helped me understand traffic waves quite a bit

[0] https://www.youtube.com/watch?v=iGFqfTCL2fs

wppick• 11 years ago

https://youtu.be/5SMYognL2t4

giltleaf• 11 years ago

That's why fuel efficient driving techniques are so great. Save you cash and stop the stupidity http://eartheasy.com/move_fuel_efficient_driving.html

shameerc• 11 years ago

I would love to see a visualisation of lane change waves. How people changes the lane on peak time. I always felt Elliott Wave Theory[1] is applicable to this.

[1] http://www.investopedia.com/articles/technical/111401.asp

dismal2• 11 years ago

If you're in the left lane and going slower than the guy behind you, pull to the right. I wish people in America would be taught this, I'm amazed at how often light traffic I encounter is from a few people meandering in the left most lanes and then everyone behind just matching their speed. Obviously does nothing about major traffic but still annoying. Besides that, people need to stop thinking of driving as a competition and let others change lanes and merge. I'd like to think those 2 things would help the situation a bit.

greggyb2• 11 years ago

In my driving experience in the US:

If a highway has two lanes, the average speed in the left lane is higher than the average speed in the right, but the only major passing opportunities are on the right when there's an empty stretch (because no one merges right).

If a highway has >2 lanes, the left is never consistently the fastest moving lane, and almost every passing opportunity comes from moving right. It is not uncommon to find the right lane near empty, and it is then possible to blow by everyone in that lane.

The exception to the above is in areas with heavy merging, wherein the average American driver seems to not understand how to merge (either blocking one another, or coming to a stop as the merge lane begins). In this case, speed seems to increase moving left.

pzxc2• 11 years ago

California is the worst about this and if that's where you live may be giving you a false impression of the US as a whole. It's probably because in California, 18-wheelers are required to stick to the right lane and have a lower speed limit (55 instead of 65). That's not true in most other states.

I, too, have noticed that people here tend to just pick a lane (usually the left one to avoid trucks) and stick to it no matter the speed they're going. However, my experience in the other two states I've lived in (Florida, Indiana) and the other 35+ states I've visited, is that most people in the US know that the left lane is for passing and are somewhat more considerate about this.

Drivers have trouble merging in all 50 states, though. Half the people merge as soon as possible, thinking it's more polite to merge sooner rather than later (or just not wanting to fight for an opening or get angry looks from people who believe merging early is better), and half the people merge as late as possible, attempting to do the zipper merge which is actually the most efficient means of merging and what everyone should do.

michaelchisari• 11 years ago

I wish people in America would be taught this

I remember being taught this when I started driving in the 90's. And I remember people following it until the mid/late 2000's. It's as if a switch was flipped, and nobody cared about this practice anymore.

Zikes• 11 years ago

I was taught this and practice it regularly.

I think it's an attitude thing. As often as I see drivers in my area blowing through red lights and pulling other ridiculous or dangerous stunts every single day, I can only imagine it's due to a lack of care or respect on the drivers' part.

drzaiusapelord• 11 years ago

Maybe on some tiny two lane rural highway can you get away with the left being only a passing lane, but in an urban congested 10-12 lane expressway with exits on both the left and right, well, those rules just don't apply.

The reality is that cars are becoming very ineffcient methods of transport. Most carry only one driver, take up a lot of space, etc. The days of driving on a rural highway with two lanes and having the left lane designated as just the passing lane are long behind us. We don't have the capacity for that outside of rural areas and as more areas become congested with car traffic, the less polite people will be.

andegre• 11 years ago

This is my biggest "pet-peeve". It seriously pisses me off! What makes it worse, in Iowa, half the time there ARE NO CARS in the lane to the right. They're just retarded and drive in the left lane.

ljf• 11 years ago

Just a note - not sure if it is the case but the downvotes are likely due to your use of 'retarded'. Outside of the US, using it in a negative context as you have here is considered offensive.

athenot• 11 years ago

Atlanta has rolled out variable speed limits on some highways (I285) which attempt to slow down the flow BEFORE people reach the congested area.

I'm not sure how well most users understand how it's supposed to help them go faster by slowing down; a bit more education could be helpful so everyone plays the game.

It is a bit weird seeing the speed limit jump from 45 to 65 then to 35 within the space of a mile…

leoedin• 11 years ago

These are very common in the UK around large cities, and in my experience they work really well. As long as there's sufficient threat of enforcement (speed cameras), people stick to the speed and the whole mass of cars move along at a reasonable speed. Compared to the horrible 80 mph, 5 mph, 80 mph, 5 mph thing you get when everyone absolutely guns it to the next slowdown they're brilliant.

marknow2• 11 years ago

I wish, for those of us that ride motorcycles in this type of traffic in the US, that all states would adopt a sensible lane-spitting allowance for motorcycles. In country after country, lane-splitting has proven to help reduce congestion, and yet, if I try it here, I'll get a ticket. Ridiculous.

bequanna• 11 years ago

Lane splitting is legal in California.

But honestly, how does this reduce congestion? My experience is that these motorcycles often dart between cars causing drivers to swerve slightly and tap their brakes.

I would argue this makes congestion worse as it causes more cautious drivers to brake (sometimes abruptly) and almost always slow down.

Finally, it is important to consider that as a vehicle, most of us are not looking for you and make small changes from left to right while in our lanes. It is not a goal of mine to accommodate motorcycle riders while driving. Splitter beware.

I'm all for sharing the road-- be we all need to subject to the same rules.

marknow2• 11 years ago

I'm aware of CA splitting -- it's not been adopted yet elsewhere in the US. It's too bad that you had to experience someone who was "swerving in and out" and not the majority of riders who would split sensibly and safely.

Lane splitting is only supposed to occur below a certain speed; splitting above that number would be a violation, of course.

Take a look at Europe to see how easily splitting can be integrated, for both driver and rider. It absolutely does reduce congestion: bikes move smoothly to the front and away, freeing space.

American drivers just aren't educated to realize this because negative stereotypes about motorcycles are so prevalent in the US, and the many positive benefits of riding aren't presented as options here.

mateo411• 11 years ago

You can lane split in California.

Inception2• 11 years ago

I would never get closer than a car's length away from the driver in front of me if I was driving 60 mph; why should I act any differently when I'm driving 30 mph? Speed is trivial - the gap must be maintained so drivers have the opportunity to react to cars around them without having to make any drastic changes in speed.

sukilot• 11 years ago

Because following distance is irrelevant. Following time matters, and that is a function of speed.

Inception2• 11 years ago

Okay, but on-the-fly time calculations aren't exactly optimal. It's much easier to establish a lower limit on distance.

nradov• 11 years ago

Widespread use of adaptive cruise control basically solves this problem. That's the technology where you can set the car to follow a certain distance behind the car in front, and then it accelerates or brakes as necessary. The systems use radar or stereoscopic cameras to measure distance. Once 20%+ of cars on a road segment are using ACC then everything smooths out. Now that the price of ACC has come way down I think the government should offer rebates for purchasing the ACC option on new cars. It would be more cost effective than building new roads or adding lanes.

http://journals.aps.org/pre/abstract/10.1103/PhysRevE.69.066...

wbeaty3• 11 years ago

ACC just imposes safe driving behavior: proper spacing for the reaction time.

So if just 20% of humans were safe drivers (spaced out properly,) then everything smooths out.

That, or perhaps 0.5% of drivers could intentionally go at the average speed.

lewis500• 11 years ago

this is one of the first things i ever programmed! cool to see it is seeing the light of day again.

What Are Traffic Waves and Why Do They Happen So Much?

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