Traffic Experiments -- How to clear traffic jams

http://www.eskimo.com/~billb/amateur/traffic/trafexp.html

by cubix • 16 years ago

115 81 16 years ago

daten7• 16 years ago

The success of this technique can't be accurately measured by the driver who's trying "smooth" out the waves. It needs to be witnessed from outside of traffic. It usually just moves the problem to behind the "helpful" driver as other drivers can very easily continue to drive competitively and recreate the effect. Even if he's successful in removing the "wave", it's just by decreasing the average speed of traffic.

Please don't try this in the left lane.

keyle• 16 years ago

That's exactly why nobody would play along even if they knew. Big trucks eventually do that because it's painful for them to do the gears 1,2,3 constantly, they stay in second or third and just slow down the whole thing.

Here is the thing where it falls short though:

the guy at the front riding the wave really slow to "help lane locks" will also get merging traffic many many time right in front of him (people taking advantage of the free space), as a result, it won't help that lane but only make it worse if you're stuck behind such driver (or truck).

So if everyone stuck to their lanes, didn't change unless extremely necessary, and played the wave-canceling technique, it would work.

In other words, this will work when cars will be driven by computers hooked together via apis and maybe a traffic controller. I'm not signing for the beta.

maushu• 16 years ago

Yeah, I always thought that the only way to prevent traffic jams would be to remove the human factor entirely. Makes me wonder why people fight so much against this, I would love to be able to read while going to work (when I don't catch the bus as usual).

zepolen• 16 years ago

Actually, the fact that people in the middle lane are taking up his 'free' space helps the long term traffic.

The main cause of traffic wave is from someone stopping too abruptly, eg. if someone changes lane in tight traffic, he forces the other person to slow down too fast, which begins the wave.

If you allow more people to get in front of you in a smooth manner because you have given yourself a lot of distance, then there will be less 'cutting off' slow downs from the middle lane guys, which in turn causes less cutting offs in the right lane.

How much does it hurt over your entire commute to let even 100 cars in front of you? You'll be 500 meters later to your destination, what's that at 30kph...1 minute more? That's the worst case scenario. The normal case is you'll get there quicker because you've raised the average speed of the entire traffic wave.

lars512• 16 years ago

Actually, if the wave is successfully removed average traffic speed should increase. In fact, the whole post is an argument about ways to improve average traffic speeds.

He basically advocates estimating how fast traffic is really going, and going at that speed. Instead of reaching a bottleneck quickly, you should slow down appropriately in advance so that you never have to actually stop. Note that you (and the people behind you) still get through the bottleneck at the same speed, so there is no cost to you (or them), just less braking and accelerating.

For what it's worth, some freeway entrances near me (Melbourne, Australia) have traffic lights which rate-limit entry onto the freeway in peak hours. This is exactly the same as what this author is trying to do; by rate-limiting cars at entry, they're not actually slowing them down, since it increases average on-freeway speeds.

ergo98• 16 years ago

I'm not sure if you really got the point of the article: The wave happens because everyone wants to be {X} behind the person in front (usually because they desperately want to block anyone else getting between them and {X}). If you can enforce (for lack of a better word) a constant speed, people can maintain that {X} distance without constant accelerating and braking -- just by coasting at a constant speed.

>Even if he's successful in removing the "wave", it's just by decreasing the average speed of traffic.

How would it decrease the average speed in any practical way? Secondly, it would dramatically reduce the energy consumption of the waves (accelerating and braking constantly) and wear and tear on vehicles. The net benefit of optimizing highway traffic -- in real environmental and economic benefits -- is significant.

>Please don't try this in the left lane.

The left lane has no meaning during congestion. There is no "passing" when the highway is clogged. And yes, this is actually the law in many places. One of the biggest causes of highway inefficiency are the self-important (but usually going nowhere) people who pretend that by driving aggressively, they should be allowed through the congestion.

Sukotto2• 16 years ago

An oldie, but a goodie.

I base my commuting style on this guys articles. Drive slightly slower than everyone else and let large spaces grow in front of your car. It's a lot harder than it sounds (for me anyway) but when I get it right, it makes my drive much more enjoyable.

People that just HAVE to drive as fast as possible will go around you (you should never do this in the left lane) and everyone else will sort of get in line after you... it looks a bit like those nascar groups where all the cars go exactly the same speed.

loewenskind• 16 years ago

Except that people who want to go faster than you will pass you and get in front of you. This leads you to either keep slowing down to maintain the space or start shrinking the space.

Sukotto2• 16 years ago

Yes, many people will pass you and go in front. That's not such a big deal since many of them switch into yet another lane (Some will even exit the freeway only to immediately re-enter) as they try to gain even half a car-length on everyone else.

I don't feel like competing with them.

Personally, I'm not trying to fix traffic for everyone. I'm selfishly trying to make my own commute as pleasant as possible. By following the advice in this old article I've managed to greatly improve my own drive, and I find the amount of time I spend in the slow pockets of traffic greatly reduced.

philwelch2• 16 years ago

In the real world, where I have tried this, this doesn't turn out to be a real problem. It happens a little (not as much as you'd think, since traffic is jammed to the left and right of you) but by and large I'm consistently able to keep several car lengths open in front of me, even as people merge in from adjacent lanes.

10ren• 16 years ago

This space also enables you to time the lights, so you do less accelerating/decelerating, and increases your buffer for breaking, so driving is safer and less stressful.

naner• 16 years ago

Also somewhat related, if there's a slow vehicle in front of you when you're going down the on-ramp to the interstate -- slow down even more so you have many car lengths to merge.

Nobody else I've ridden with will do this (I've suggested it). Instead, they tailgate the slow car, curse like mad, and then almost cause a wreck when the try to pass the slow car in full-speed traffic as soon as they enter the interstate.

If you leave enough space you can easily get up to interstate speed and pass them quickly and effectively.

chopsueyar• 16 years ago

Old.

daten7• 16 years ago

He's right, this has been posted on HN before. I seam to remember the old article having more animated illustrations. I don't know if it's the same page, same author or just similar content.

Kliment• 16 years ago

The animations are linked at the bottom of the article.

DanielStraight3• 16 years ago

The solution to traffic jams is to not leave transportation in the hands of clueless, distracted, reckless drivers. The solution is fast, reliable, ubiquitous public transit.

stefs• 16 years ago

or jetpacks!

nphase• 16 years ago

or teleportation devices

nradov3• 16 years ago

Public transit is impractical and uneconomic in many areas. The real solution is smart roads and computer-driven electric cars.

rbranson• 16 years ago

Perhaps, but in the context of the article, traffic jams only exist because of commuters. The interstate & highway systems were not designed for commuters and should not be used for them. While I think computer-driven cars will be a boon to the environment and economy, the capital investment needed is pretty fantastic, which is going to be some decades off.

In fact, a real solution to this problem that could be applied in the design of our current urban landscape would be a multi-pronged effort to spread out the commuter traffic. Alternating work and school hours could widen commuting hours and distribute load across all commuter infrastructure. Increased levels of telecommuting would also contribute to this. This would be a sort of supply-side fix to commuter congestion. Local or regional governments could create incentives to encourage these changes.

DanielStraight3• 16 years ago

The intersection of the set of places that get traffic jams and the set of places in which public transit is impractical and uneconomic is very small.

fbnt• 16 years ago

Maybe the author never heard anything about green waves. http://en.wikipedia.org/wiki/Green_wave

superk4• 16 years ago

Green waves are a pet peeve of mine. They've tried to implement these where I lieve... a spectacular failure. Green waves only work when there is NO traffic. Speed is regulated and the flow moves fine. During rush hour they are a disaster. Every light is backed up to previous. My light turns green, then I have to wait 10 seconds for the next light to turn green and the entire line of traffic to move, before I can advance. This usually results in a stop at each light. A much better solution is what I remember from NYC: all the lights turn green at once, all the traffic moves.

ritonlajoie2• 16 years ago

In northern America, I think most of the cars have automatic transmission. In France, (where I live), we mostly (maybe 99% ?) have manual transmissions. Have you ever experimented a traffic jam with a manual trans. car ? You have to switch gears every 10 seconds and it's very very very annoying.

I'm telling you that, because as a frequent driver (well, today I drive a bike but still) in Paris, and kind of very observant, I clearly see that most people are not doing what these drivers do in this experiment. Most of the time, everybody will let a huge space between the car behind them in order to avoid changing the gears too frequently.

On the contrary, on automatic cars (I had one and drove another for 6 months in Canada), that's easier to go & stop & go & stop & go & stop, etc...

Not sure if it's related, but I think it is, in a small proportion.

edit: my english

maw2• 16 years ago

Pretty sure transmission type has nothing to do with it -- traffic is stop and go in plenty of places where most cars have manual transmissions.

I'm increasingly convinced that it's due to so many drivers optimizing for local minima, at the expense of not just everyone around them, but of themselves thirty seconds later.

maw2• 16 years ago

Duh, I meant local maxima, of course -- at least, that's what those drivers are trying for. What they get most of the time are more like minima, of course.

vaksel• 16 years ago

why exactly do you need to switch gears? Just leave it in first. And engage the clutch when you are stopped.

daten7• 16 years ago

If you're stuck in traffic for a long time, your leg engaging the clutch can get very tired. So you're constantly switching to neutral to rest your leg.

It's also not uncommon for traffic to go from stopped, to fast enough to warrant 2nd gear, back to stopped.

hippich2• 16 years ago

you will ruin your clutch bearing way faster. it's much better to keep constant speed (for engine/gearbox and for traffic in general). plus - you can't drive on highway on 1st gear. you still need to switch to upper gears.

ritonlajoie2• 16 years ago

Hm, just so you know, never drive in first. That's just the one to 'start'. Either, you'll kill your engine too early. Engaging the clutch every 10 meters is what is irritating. That's the point !

superk4• 16 years ago

In my experience that is not the case. Specifically because jams are waves not just slow moving traffic. A gap will open up and quickly I can be in the 3rd gear to catch up. Of course as soon as I do the flow stops again. I could just creep forward in 1st and let the rest of the flow fill in front of me...

Manual transmissions are definitely hell during rush hour... but doesn't seem to affect people contributing to the waves in my experience.

hippich2• 16 years ago

I am pretty sure this is one of the many reasons. I recently came to usa from eastern europe, where I had car with manual gearbox (and with ceramic clutch, turbo, etc rough stuff under hood keeping you busy in traffic =)). I quickly get used to do smooth driving and continue to do it here in usa too on my automatic trans car.

daychilde2• 16 years ago

I had the good fortune to do a three month internship in Seattle last spring. And I have to say - after growing up in Dallas, now living in Florida, and seeing traffic in other places... Seattle drivers are unique.

I've seen this [appear to] work in Seattle. While I was there, I tried it, and it appeared to work.

However, while I lived in Dallas, I was aware of this idea, and tried it there - and no way. People cut in, people tailgated - it completely doesn't work.

So I think it partially depends on the nature of the city's drivers. And there is something special about Seattle.

daten7• 16 years ago

I live and work in DC. I go to Dallas at least once a year and drive every day during the week and weekend. I agree this would be impossible in Dallas traffic. It doesn't feel as intentionally aggressive as DC traffic, instead it's like everyone in Dallas can't take their foot off the accelerator even if they wanted to. It's hard enough finding a space between two cars to merge, you usually have to just go for it and hope the rear car can find his brakes.

daychilde2• 16 years ago

It definitely took me a while after moving away from Dallas to change some of my driving habits.

I still definitely change lanes more than most drivers around here - although I do my best not to cut anyone off, and I actually let drivers in here... something definitely not done often in Dallas.

It also helps that I went from a 90 minute commute (each way) to usually a 10-15 minute commute here (depending on where I've worked). Smaller towns do have some benefits...

stuaxo• 16 years ago

Thats what those changable speed limit signs on the motorway are trying to achieve.

(I've also seen markings to keep cars 10 metres apart on some roads).

Luc• 16 years ago

Where I live there's something called 'block driving' (I haven't found a better translation for the Dutch word 'blokrijden'). On busy summer days, when the highway to the coast is jammed, a police car will block the highway, then drive at a constant speed (and slower than the maximum speed), with no one being allowed to pass the police car. Apparently this helps to resolve the traffic waves.

superk4• 16 years ago

That's actually one of the solutions suggested by the author. A rolling blockcade of police cars in each lane to regulate the flow.

0x5a177• 16 years ago

The California Highway Patrol appears to do this in Southern California as well. I've seen a patrol car start weaving between two lanes, then three, weaving more and more until it's blocking all lanes of traffic like a weavy pace car.

mds• 16 years ago

They do that to slow or stop traffic when there's debris or an accident ahead.

bmm6o• 16 years ago

It's called "running a break", and I've only seen it used to clear debris or disabled cars out of lanes, or so that a construction crew can close a lane. It'll freak you out the first time a cop car swerves in front of you across 5 lanes of traffic.

CaptainDecisive2• 16 years ago

Interesting observation is that what TFA calls the "cheaters", ie those who drive to the end of their lane before merging at the last minute, may actually be doing the right thing. See Tom Vanderbilt talk about it here http://www.vimeo.com/6779064.

If you're interested in this there's a nice longer interview with Tom at Streetfilms http://www.streetfilms.org/tom-vanderbilt-talks-traffic or an hour long Google talk.

ajb• 16 years ago

He doesn't say why in the short one (I haven't watched the long one). Late mergers are definitely bad in some situations - for example, when they are merging into a slow-moving turnoff lane, blocking a whole lane of ongoing traffic.

CaptainDecisive2• 16 years ago

He goes into the research a bit more in the Google talk and you see where he pulls the magic "15% improvement" from. Also he has a book (which I haven't read) called Traffic so he might explain it more or cite studies there.

I agree though that it doesn't really work in all situations. Tom uses the example of two lanes merging into one for roadworks. Maybe some situations where it doesn't work are an example of poor road design by traffic engineers?

mootothemax• 16 years ago

Heh, I didn't realise that it had such an effect, but this is the exact same game I play when caught up in a heavy traffic: the how-long-can-I-go-before-breaking game! ;-)

It could be related to driving a manual and being lazy, but it helps your concentration no end :)

Deestan• 16 years ago

No idea why you were at -1; your comment was informative and on topic.

On a side note, I also tend to play that "game" when stuck in slow home-from-holiday traffic over the mountains. Changing speeds all the time while driving manual is such a pain.

tomjen3• 16 years ago

The problem with slowing down miles in front of a traffic jam is that those who normally gets of before that space now has to suffer too.

In addition, most people would cut you of if you drove slower than the maximum speed.

kostko• 16 years ago

I remember reading this article while back: "Traffic jam mystery solved by mathematicians". http://www.physorg.com/news117283969.html

So now when I'm in a jam, I just slow down instead of engaging the speedup, break pattern. Haven't looked behind me though to see if I had any effect on the traffic behind me.

DanielN• 16 years ago

I've tried this in Boston. Mass-holes will not stand for you to screw with their traffic jams.

cakeface2• 16 years ago

I agree that this doesn't work well around Boston. My opinion is that the fluidity of traffic, ie. lane changing, doesn't break down at low speeds in Boston. If you let a cars length open in front of you it will fill up with a car. I tend to think that this makes efficient use of the pavement available and tends for a traffic flow that is as maximal as possible but I don't have any evidence about that.

asmithmd12• 16 years ago

If efficient use of pavement means park the most cars on it Then by all means fill every available space. Safe following distance increases with the square of the speed so higher average vehicle speeds means the road carries less traffic. The road moves the most cars around 30mph, so if traffic slows below 30 it will crash to 0mph

Natsu• 16 years ago

I've tried this in Phoenix with some success. You do get people passing you, but this helps traffic on average. We also have lights at the freeway on ramps which smooth oncoming traffic, but people ignore them until a line forms. I almost got rear-ended once because someone intended to ignore it and I chose to obey it.

If you do this when traffic is not quite jammed, people are far more upset. It's best used when traffic is being forced to actually stop regularly. You will still get people to honk at you and whatnot, especially once a person or two passes you, but I've long since given up expecting most people to understand the things I do.

bryanh• 16 years ago

So, what would happen on major interstates if highway patrols kept a steady pace? Is that a viable solution? Wouldn't they eventually run up on another wave of cars and ruin everything?

I would be really interesting to see if this actually improved interstate bandwidth during rush hours. But even more interesting, I bet a good portion of drivers would get upset at the fact that there are huge, unused gaps. I wouldn't be surprised to hear from people who think it is grossly more inefficient than regular old traffic jams...

parallax7d• 16 years ago

I used to do this every once in a while. One thing to be aware of, are large trucks behind you at the top of hills. They tend to want to increase speed going down hills to use their velocity to climb the next. It's probably a good idea to accommodate this.

RyanMcGreal• 16 years ago

Yes, absolutely. This is my experience as well. Related: http://news.ycombinator.com/item?id=785498

nwomack• 16 years ago

I've actually spent quite a bit of time pondering this. Here are my completely unscientific conclusions.

Imagine a [moving] wall, on an interstate, moving at 30MPH. No matter what is happening behind this wall, it is impossible to maintain an average speed faster than the wall. You can try any amount of tricks to smooth out traffic, but it will be impossible to ever move past this wall. As long as there is a steady stream of traffic, no car will ever do more than 30MPH until this moving wall gets out of the way.

So now let's, more realistically, replace the wall with a bunch of slow drives, and ask 2 important questions:

1. How to get out of this situation? -- We want these drivers to all SPEED UP! It's really as simple as this. Until these bottleneck drivers move out of the way, you can only smooth out traffic, but never increase the speed.

2. How to avoid this problem in the first place? -- In general, there will be some bottleneck. After too many cars are going through this bottleneck at once, there will be slowdown. Ideally, these bottlenecks should be identified and no more than the maximum amount of cars at a time should enter the bottleneck to decrease the speed of traffic.

Of course, that's easier said than done, but I think the important thing to take away from my ramblings is this:

The best way to get rid of a traffic jam is: if you are in the front of the pack and have a chance to accelerate, you should do so as quickly as possible. If everybody were to do this, average speed could then be increased.

wbeaty2• 16 years ago

But note that there aren't any slowpokes here, instead, every single car is blocked by the car ahead of them. The people behind you think that you are the slowpoke driver. But nothing is blocking the traffic, instead the average speed of the long column of cars is determined by the spacing between cars, and that spacing is determined by psychology.

In light traffic, some people go far faster than average, but nobody tries to stop them. When we arrive at heavy congestion, don't we suddenly decide to close up ranks to prevent aggressive drivers from passing, stop them from "cutting us off?" If everyone habitually blocks the speeders, then it means we've compressed the traffic pattern past the peak of maximum flow, and pushed it into the unstable realm of stop/go oscillations and standing waves or "ghost jams."

Want jams to vanish? Then do as the pro truckers do and just freaking back off. Happily encourage other cars to merge ahead. (They're not "cutting you off," there's no such thing as "cutting you off." They're just trying to merge into the adjacent lane!) If traffic momentarily slows way down, everyone just make sure to keep an open space that allows merging, and you'll find that it speeds right back up again.

tobtoh2• 16 years ago

In Melbourne, Australia, a similar concept was used to ease congestion entering freeways. We had a peak-hour problem where cars would flood the entry ramps, but struggle to merge with the existing freeway traffic - this caused people to brake/slow down which resulted in major jams.

To alleviate this issue "there are traffic lights at some freeway entrances to control the flow of vehicles onto the freeway when the traffic is heavy. When operating, the lights will change quickly, so that when the light is green only one vehicle in each lane will be able to enter the freeway." (taken from VicRoads website)

The lights basically cycle from red to green every 2 seconds. That's enough to regulate the flow onto the freeway and minmises jams on the entry ramp and at the merge point between the freeway and entry ramp.

tobtoh2• 16 years ago

In fact, here is a YouTube video of the lights in operation: http://www.youtube.com/watch?v=YFrEd8qSVCc

colonelxc• 16 years ago

This is used extensively in the SF bay area.

alexyim• 16 years ago

A side effect of doing this is that it is much more fuel efficient to not have to go into a cycle of breaking and accelerating again.

drv• 16 years ago

Somewhat different from the commuter-oriented topic of the article, I've seen attempts by truckers to enforce a similar pattern in construction zones (traffic reduced by one or more lanes) on long stretches of highway: In a situation where one lane has to merge because it is closed in the distance, the "nice" drivers all merge far ahead of time, while the aggressive drivers attempt to go all the way to the end of the lane and then merge ahead of everyone, causing the kind of grinding gears described in the article. To combat this, a couple of big rigs driven in parallel in the open lane and the lane that is closed ahead block all traffic from passing. This prevents the aggressive drivers from attempting to merge at the last second, smoothing out the wave. I've seen this happen several times on cross-country trips; I wonder if the truckers coordinate over radio or just choose to do it independently.

On a related note, I wonder if "lane closed ahead" signs placed too far ahead actually make this problem worse, since many drivers will immediately merge out of the closing lane far ahead of time, causing a wall of traffic if the front of the line has to slow down for aggressive last-second merges.

tectonic• 16 years ago

As a side note, I remember reading articles on this author's webpage almost 15 years ago. Some awesome stuff from a long-time netzen!

http://amasci.com

Traffic Experiments -- How to clear traffic jams

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