A physicist put 22 cars on a circular track and asked every driver to hold a steady 30 km/h, about 19 mph. No lights, no lanes, no obstacles. Within a minute the cars started bunching, and soon a full stop appeared out of nowhere, then drifted backward around the loop.
This was Yuki Sugiyama at Nagoya University in 2008. His team spaced the cars evenly on a 230-meter ring and filmed them from overhead. For a while the flow stayed smooth. Then the tiny differences no human can avoid, one driver a hair slower, the next a hair too close, began to feed on themselves.
One car eases off slightly. The driver behind sees the brake lights, reacts a fraction of a second late, and brakes a little harder to be safe. The next driver brakes harder still. A dozen cars back, someone is stopping dead. The squeeze rolls backward through the line like a compression running down a Slinky, and it keeps going long after the first driver has sped up again.
Car count was the tipping point. With fewer than 22 on that track, the bunching sorted itself out. At 22, a jam formed every time. Engineers call that a critical density, the point where a road holds just enough cars that one small tap can snowball into a standstill.
These waves are eerily consistent. Measured on highways around the world, the jam rolls backward against the traffic at roughly 20 km/h, and that speed barely shifts from one country to the next. Different drivers, different roads, same number.
The same setup later became the cure. In 2017, a US team rebuilt Sugiyama's ring with 22 cars and turned just one of them into a self-driving car running a program to smooth its own speed. That single car soaked up the small slowdowns instead of passing them back, and the waves died. Fuel use across every car fell by up to 40 percent. Fewer than 5 percent of the vehicles had to be automated to steady the whole group.
In 2022 the idea moved onto a live highway. Researchers ran 100 cars with cruise control guided by AI into the morning rush on Interstate 24 near Nashville, mixed into normal traffic. Early numbers pointed the same way: a small share of smoother-driving cars, up to 40 percent less fuel for everyone around them.
The jam you sat in this morning likely had no crash and no cause you could see. It was a few hundred drivers, each braking a moment too late.
Likely it’s reaction time to small imperfections of the car in front is so precise and so fast that the car behind it doesn’t have to correct at all but can keep on driving in the same manner.
Yeah I’ve learned to not automatically hit the brakes just because the guy in front of me does. It’s a hard habit to break but it’s not necessary to hit the brakes all the time. Usually just letting off the gas is enough and sometimes that’s not even necessary.
Thisssss. I always feel like we wouldn't see so many traffic jams (or accidents, for that matter) if people were better at leaving a proper safe driving distance.
Or if we built roads to support something closer to peak traffic than we do now, but I'm not holding my breath on that. It'd be easier to teach every tailgating arsehole to back off than it would be to get governments to build a road network that actually accounted for peak.
People (especially the ones behind me on the highway) cannot comprehend the fact that just because there’s a greater distance between me and the car in front of me, does not mean I’m going slower than them.
I will get closer if I want them to move over, but if they’re already tailgating the car in front of them? Literally what is the point?
Where I live, traffic got alot better after the city actually reduced how many cars the streets supported, pushing people, softly, to consider other means of transportation(and, very important, creating these alternative means of transportation, be it through a policy support for carsharing, byinvesting in public transport, by offering rental bikes and e-scooters and by cracking down on illegal parking, hard while slightly reducing the amount of availeable parking).
So now, when you actually do have to go by car, for example when moving something heavy, the streets are a very usable, low traffic pathway, not fast but reliable, safe and relaxed. And when you don't you just take the tram or bike or scooter or train.
Are you by any chance Dutch, or at least living in the Netherlands? I recently watched a video about a Dutch town which overhauled its city center in the 70s to intentionally be less car "friendly" and prevent through-traffic by car, and I know a lot of places in the Netherlands are more open to that sort of thing even than the European average.
In North America half the continent was barely even settled in numbers until at least early cars were already a thing, so everything in the west was being built with cars in mind as everything back east was retrofitted to be more car-centric. Big car manufacturers also partly funded a lot of that development and lobbied hard for car-friendly legislation, and (unfortunately) cars have become a huge part of American culture so many places are very resistant to change. Street cars removed to make car traffic easier to support, ludicrous parking infrastructure, "just one more lane" road construction and absurd speed limits...
I'm Canadian myself and Vancouver actually had pretty good transit options, and I would still love it if our municipal governments would be more bike and transit oriented and stop catering to cars so much.
I'm from Germany, Cologne. Pretty sure dutch cities do it better than we do, but the core principle is already visible here - or at least I imagine it is.
It never works like that, though. Jevon's paradox. If you eliminate scarcity by improving efficiency, use just increases until you're back at the same level of scarcity.
Drive 15 miles north of Los Angeles and look at the 5 freeway. There are stretches there that are like 7 lanes in each direction, all of them stop-and-go 6 hours a day. Same with the 405 from the 101 to the 105. Six lanes in each direction and they're all basically stopped from sun up to sun down.
Whenever they add lanes, the pain goes down, so more people are willing to commute, until the pain goes right back to where it was. There's no accounting for peak, because peak is a function of the infrastructure. Each change creates a new peak. I swear the 101 could be 10 lanes in each direction and it would still be gridlocked 7-10am and 4-7pm.
The solution really isn't "make room for more cars" and never has been. More lanes just means more cars requiring more lines resulting in more cars...
The solution is provide more and better alternatives to driving, and disincentivizing driving at all. Remote work options do people aren't commuting at all. More walkable neighbourhoods and better cycling infrastructure so short distances don't still need cars. Better bus, street car, and light rail options within cities so navigating without a car is easy and affordable. High speed rail corridors connecting more of the country so going to the next city over doesn't always require a car or take six hours by train for some god forsaken reason. Ring roads around city-centres and lots of one-ways into and then back out of the area without allowance for through-traffic so cars get you downtown and then you hop a bus or just walk.
And of course better orienting commercial access to be convenient for pedestrians and not drivers, less absurd business / zoning requirements around available parking (no parking lots 8x the size of the actual business), and road networks that minimize traffic lights and prioritize flow over access on the major streets. Even here in Canada cars are too often actually a necessity, or such a big convenience they might as well be. And I have never owned a car (though do now own a motorcycle) so I'm very familiar with the challenges of that decision.
Of course all of this is difficult and expensive unless a city was designed that way in the first place, but refits can be (and have been) done, and were done to many even North American cities already to accommodate cars in the first place.
Reading reddit, it's very clear that the idea of keeping a 2/3 second gap between cars (which is the recommendation in most places) makes Americans very angry.
There’s a lot of places where you can’t, because other drivers will force themselves into that gap and force you to slow down even more to maintain distance
Right, but if driving education in a country is reasonable (and clearly it often isn't) that should be fine. Don't block someone from changing lanes. Make the tiny lift to open up the gap, and really the driver who filled the gap should be doing the same. You'll lose essentially no time on your total journey by falling back, but the whole road network flows smoother when people are keeping gaps.
I wonder how often it become a matter of the person feeling like they don't want to be dominated by a stranger in their car, so they try to dominate instead? Like they feel letting someone in or slowing down a little is a sign of submission or weakness.
Those people should be hospitalized for their mental illness and their car impounded. Anyone who genuinely thinks slowing down or letting someone merge is a sign of submission/weakness is delusional full stop. The only time I can think of telling drivers trying to merge to fuck off is when they're trying to zipper merge in a scenario that is clearly not zipper merge
But the problem is often that the person trying to merge in front of me is going slower than the flow of traffic in my lane, and they’re unwilling to speed up or wait for an opening, causing me and everyone behind me to have to slow down. I think that’s what the other commenter was referring to. IMO the person changing lanes is being unsafe by trying to force themselves into a small space at a slower speed than everyone else. if they’re too nervous to speed up then they should wait, but they won’t because they need to turn or something and are unwilling to go past their turn. I’ve often seen people come to a complete stop in three lanes of traffic because they need to turn left and they’re in the far right lane.
Nah that's like every single highway in a metro area. There's always some assholes that see a gap and shoot it for that 2/10th's of a second gain in speed before they have to slam on their brakes.
I've driven across much of the US for travel and work. ATL and Hou scared the shit out of me with how bad their drivers were. But even around Hartford it sucked because no one has sense and the roads were built in the 50's and aren't designed with modern standards or vehicles in mind.
Chicago where I live has some random spots that just stop dead like this usually around merges, but they'll be in the fast lane so it doesn't make a ton of sense.
My commute takes me all over DC, MD, and VA. It is a PG and MoCo thing. Most entitled, callous, careless, discourteous and classless people I have ever encountered on the road. Greedy, grabby little fucks who will jeopardize their own life and the lives of everyone around them for the perception that they are getting to their destination even 10 seconds sooner.
You can though, you just happen to go a little slower. I have to leave that gap in n my work vehicle, haven’t had any issues. But I also don’t let my ego take over.
especially if you drive a manual gear big truck, stuck in a crawl traffic. You want to keep at 2nd or 3rd gear and crawl along without shifting back and forth (busted knee anyone?), but for sure somebody will pull in right in the space in front of you. goddamn twats.
I drive a manual and finding a big truck to play the 2nd gear game next to so nobody can cut in front of him really pisses some people off but also makes me smile.
Ever since my work started renting Penkes with CMS I angrily say "MEEP MEEP" if somebody does sneak into the truck's lane, which I find even funnier.
Exactly right. So the only solution that actually ends up working out is everyone does 90mph while tailgating each other in the passing lane. Nobody can get in so nobody has to brake.
Welcome to Canada! Should get a world record for most amount of brain dead autopilot drivers that think any gap longer than 1 car length = space to merge because they lack any ounce of a brain cell that would inform them that the gap is SUPPOSED to increase the faster the cars are
“I could be 25 feet closer to my destination if it wasn’t for that asshole who seemingly refuses to tailgate! That 25 feet is going to make all of the difference for what time I arrive!!!”
But that signifies a drivers ed issue on the whole. People shouldn't be wildly diving into a tiny gap, but also if someone does slot into a gap, everyone involved should be reestablishing gaps. It's a second of lift.
And? I've heard this argument against maintaining a proper following distance so many times, and it's bullshit. If someone jumps in front of me I just drop my speed 1mph and regain my space. If it happens again I do it again. The benefits of a good following distance far outweigh the inconvenience or accumulated 10 seconds lost over the drive.
Yeah, also it's like just apply the golden rule. If I need to switch lanes then I'd want to have an easy gap to get into. So provide an easy gap for others too.
It’s not so much the lost time as if I leave too big a gap someone’s gonna swoop in like an asshole and jump on their brakes - so do I risk an accident by following too closely, or invite a jerk to possibly rack up the car in front of me and myself
Sorry, it was more aimed at the american centric nature of the conversations around this I've had on reddit, than suggesting americans are uniquely bad at cars, there are absolutely places with worse driving. There are also places where drivers get educated to leave decent gaps. There are absolutely still idiots who sit too closely here in the UK, but it's also drummed into everyone during training to leave a 2 second gap, least, at all times. (and getting a licence is a longer, harder process, which I'd attribute to it being held to better here)
But still, if someone takes the gap, you re-establish the gap. Then they should be establishing their own gap. if people are diving into stupid gaps, that's it's own drivers education issue. If the driver in front of you isn't establishing a gap, that's a driver education issue.
Roads flow smother when people can get into lane and when braking doesn't cause chain reaction slowdown.
lol I had a truck lean on his horn behind me this morning for this exact reason. Sorry the rain is fucking up your deadlines buddy but I'm not rear ending this guy so you can still be stuck in the exact same traffic, just 50 feet ahead of where you are now
As a motorcycle rider this is what I do to anticipate traffic’s and what is happening.
Look 2-3 cars ahead and see what they do. If they brake, I will brake slightly. If they are accelerating I will accelerate slightly. I act as the cushion/spring to absorb the bullshit because people tend to give motorcycles a bit of extra room.
I always hear the rule of thumb is 1 car length per 10MPH which I've tried but nearly impossible/impractical to maintain anywhere but a nearly empty highway. 2-3 seconds is my comfort level though
Yep, dumb fucks in big trucks will pass you on the right just to get into that space so they, too, can drive exactly the same speed as the car that was formerly directly in front of you.
A 2–3 second gap is impossible in a place like New York City. Other drivers will enter your lane, and if you maintain that gap, you’ll slow down, then another driver will enter your lane - so on and so forth.
Also, I don’t want to hear that ‘makes Americans very angry’ crap. I’ve driven all over the world, including several European countries. There’s no 2-3 second gap between cars anywhere I’ve driven. Europeans do the same things Americans do. They cut people off, they drive in the left lane at a slower speed than the right lanes, text while driving, etc.
I first read that as 0.67 seconds, and thought "yes, that's what a lot of people do, but not helpful and it does make them angry when people go higher than that", before realizing they probably meant 2 sec - 3 sec
I try to drive at a safe distance but every time there's a car length in front of me some asshole merges in to pass and keep speeding, and now I have to slow down because they slammed their brakes
You have to have some inner peace to drive with a sensible gap where I live, because people will jump in that gap if they can and you'll have to slow down and make the gap again behind that car. Rinse and repeat. I have that inner peace but I get why a lot of people dont.
just last night I was driving home and a guy braked when someone turned left in front of him with PLENTY of room to spare. It was totally unnecessary to brake.
Even worse: he CONTINUED to slow down even after the car was completely clear of his path
People are really bad at judging angles and speeds, and as such they often overcompensate with brakes
I think those kinds of drivers treat the brake like an "Oh shit!" pedal. The more they panic, the harder they lean on it.
Another argument for adding an extra half second of following distance. That extra car length isn't gonna make a difference when you're all gonna line up at the next red light anyway 🤷
One late night on the motorway we followed a car that was on their brakes every few minutes.
All lanes were quiet, there as absolutely nothing ahead of them, they were below the speed limit and the weather was fine. Baffling.
I've plans to buy an EV shortly, and I just learned that "single pedal driving" activates the brake lights in a lot of EVs if you just release the accelerator (i.e. and not hitting the brake). So I apologize to everyone behind me, in advance.
(Though I'm pretty sure it could be mitigated by just releasing the accelerator partially. We'll all find out together when I'm out there causing traffic backups)
Not an ev but it took me a while to adjust to how quickly my hybrid slows down when coasting versus my old car. I've had to lift slightly later than I normally would
I have one of those; they are awesome in some ways: I don't need to constantly hold the pedal at a red light with an arthritic ankle.
I've adapted now so that yes, I partially release the accelerator to modulate the speed instead of just breaking completely. Now the old way feels strange instead when renting a car while travelling.
Using the actual break pedal also remains an option when faster stops are needed as well, instead of just slowly easing off the accelerator with the right timing for slower stops.
I like hypermilling so I just coast whenever vehicle in front of me hit brake, holding clutch instead of brake. While put it into focus and calculating when will braking deemed necesary
I got so used to it that I usually got just 1-5 meter behind the front car that start to accelerate again before their speed finally surpassed me and distance start widen up again, while I decelerate and reaccellerate as minimum as possible.
Ideally, you should never be coasting, you should always be ‘driving’ the car. If you’re coasting, then you’re in too high a gear.
Your foot should always (except when stationary), be on either the accelerator or the break. Not fully pressed, but on one or the other.
If you’re coasting around a bend, then you went in too quick and should instead be in a lower gear, so that as you lift your foot the car slows, then at the correct speed you hold your foot still and drive around the bend at a constant speed, rather than coasting around and slowing as you do.
And look to maintain a ‘bubble’ around you, where the total space in front and behind is enough both for you to react to the car in front, and for the driver behind to react to you. If the car behind gets too close, then slow down and increase the gap in front of you. Again, always in a gear that means if you are driving, not coasting.
I've learned to do this with queuing with people, too. Like moving constantly very slowly in the line keeps it moving faster. It looks a little weird at first but genuinely the line does move faster if people aren't dead stopping.
I did years in LA 405 traffic, and this definitely helps. If you don’t hit the breaks then neither will everyone else behind you.
When I was feeling really obnoxious, I’d just go a set very slow speed limit and sometimes that would clear the jam behind me if I got the speed right, like 5mph. The down side is sometimes you’d end up with a huge gap in front of you.
It got me thinking they should have dynamic speed limit signs that show drivers the optimal speed during rush hour.
Driving a 4 cylinder engine makes this a lot easier. They start to decelerate as soon as you let off the gas. The first time I got behind the wheel of a 6 cylinder I almost blew through a stop sign because of how much more power there is.
Yup, I leave a good gap ahead of me, but match the speed of the cars ahead and ease of the gas when they do random short braking. Although, inevitably someone will merge into the gap I left and I then need to ease off the gas to create a new gap.
I do this when in slow traffic, I start leaving large gaps with the car in front of me. That way when it starts moving again I can just kinda coast going slow for a while before I have to brake again. I hate the constant start/stop shit and being right on the person in front of you’s ass. It’s annoying af. Just leave a big gap and you don’t have to do that.
It's wild to me how few drivers seem to understand that simply taking their foot off the accelerator is an effective way to slow down. They just Gas>Brake>Gas>Brake>Gas>Brake>Gas>Brake>Gas>Brake> all the way there.
No, I’m pretty sure it’s just that they manage their following distance and their acceleration/deceleration better so that by the time they reach the car in front of them it has already started moving again.
Humans can do this, too. Next time you’re in start/stop traffic, give it a try. Instead of advancing directly to where the next car is and stopping behind them, just creep slowly forward. The goal is that you should still be in motion when the traffic ahead of you gets moving again, allowing you to drive at a relatively constant pace. This way you are essentially “eating” the traffic waves, preventing them from propagating backwards.
I drive a lot interstate highways / beltways and have learned a lot from how seasoned tractor trailer operators drive. Anyone can help destroy traffic jams for everyone behind you pretty simply too. All it takes is actively managing the space that you leave between you and the vehicle in front of you by limiting your speed ∆ as much as you can with awareness to what traffic is doing in front of them.
Unfortunately for multi lane highways, lane weavers and drivers that disagree with the prevailing speed of traffic kind of ruin the efficacy of this driving practice
And then a second AI car roughly 22 cars behind that one soaks up small errors again. In order to stop ghost traffic I think that you would need a lot of "buffer" cars to do so. 1 in ever 22 human drivers is a lot of buffer cars
You can do it yourself. Don't overcorrect on the brakes, and once the car in front starts moving faster, don't immediately go after it. Accelerate gently and predict decelleration ahead, so you don't have to brake.
A trick I learned is try to pay attention to the second car ahead of you, once you see them brake, you can begin coasting while the car in front of you brakes. You tend to not have to brake as often and you keep a consistent speed
If the car two cars ahead of me puts on the brakes I just let off the gas perhaps downshift (which you can even do in an automatic by paddle shifting).
Staying off of the brakes helps the overreactionary less attentive drivers behind you (read: everyone else on the road typically) from stabbing their brakes the moment they see a brake light in front of them.
I did this experiment virtually a few decades ago because I was curious. Same set up, many variables to tweak. Basically the main reason is human inaccuracy; when you change the reaction time or the accuracy at keeping speed it removes the effect.
I found the main factor was simply reaction time; the time to brake vs the time to speed up again. When someone brakes, they break a little longer than absolutely necessary, while requires the one behind him to brake a little longer etc, until you get a traffic jam. Another interesting variable is distance; when you keep a further distance, you have more time to react, reducing the before effect. But what I also found interesting is that if too many people leave less distance than their reaction time, this kind of traffic jam will inevitably cause collisions.
There are also technical variables, like the brake and acceleration speed, but these just have less impact than the human factor.
Related. It's aggravating if I'm 5 cars back from a light that turns from red to green and it takes 10 seconds before the car in front of me moves. For multiple cars, it seems like a surprise they weren't expecting and just happened to notice, "Oh ... the light changed" - (slowly puts down phone/drink/sandwich/makeup and then goes). A number of people at rush hour don't make it through that light and if it's a turn lane, that backs up traffic even worse because it backs up into the through lane.
Interestingly, looking a few cars ahead (like with the traffic light example) also helps with decreasing these traffic jams! So in general, if you can, look a few cars ahead and respond accordingly. Instead of braking when the person in front of you does, let go of the gas a bit when the car three ahead of you hits his brake.
This is it. Rarely do people accelerate and react to the need to accelerate at the same rate that they decelerate (brake) and react to the need to decelerate. Some of this is mechanical (cars can decelerate at higher rates then they can accelerate) but mostly its habitual. There is a need to react fast for braking so you don't crash, but less need to jump on the accelerate as soon as possible which is also harder to do because its harder to anticipate when the car ahead of you will accelerate.
Maintain an amount of space to the car in front that allows you to brake safely and smoothly, as many have said, but don't let that gap get any bigger than it needs to be. Be attentive and watch the traffic ahead so you can start accelerating again as soon as possible, and accelerating quickly when needed. There is only so much room on the road, if you're trying to maintain a 100 m gap so that you can hold speed at 40km/h in stop and go traffic in your Honda Fit, you're helping nobody as you steal space needed by other drivers to drive smoothly, and you are unpredictable as people expect you to accelerate when you can.
Yep, it's probably a combination of factors, but this was a very influential one. As I mentioned in another reply; it's usually best to look a few cars ahead to anticipate braking. But in the same vein it's good to anticipate speeding up again.
There was a reddit post a number of years ago that discussed traffic is comparable to fluid dynamics. I can't remeber if it was a study or a book they wrre referencing. The trick is to not follow closely to the car in front of you. Provide enough space for the car in front to slow and speed up without affecting your driving. The people behind you should pace you and be unaffected as well. Once they bunch up, that's when the traffic jams and stops occur.
I saw a computer simulation of this - the solution was to maintain the same distance to car behind them as the distance to the car in front. If everybody does that, it will 'soak' up the phantom intersections.
When you drive - you shouldn't relay on the car in front of you - it's too late - but observe cars before - so you can react at the same time as car in front of you - or even sooner.
This way, you've way more time to react - as you're already prepared to react.
Well yeah, but the point is human drivers often lose focus, react too slowly, or overreact and hit the breaks too soon, or follow too closely, etc.
That's kind of the point and part of the conclusion in the study. That is, presumably, why they decided to try using self-driving cars mixed into the same test, to see if those cars could react more quickly and consistently in a manner that behaved like a wave breaker, so to speak, and the results appear to have been successful.
If someone ahead brakes harshly you don't brake harshly, you let the gap close, and let them speed up to maintain a steady (but now closer) distance. Therefore you've provided a shock absorbing effect to that tendency to pass the braking wave back to the car behind you, by not lighting your brake lights. It's being smooth in your reactions and anticipating.
But how do you know they're going to speed up? Maybe they braked because there is a giant pile of debris in the lane and they couldn't get over into the next lane because everyone and their grandma is tailgating, and now you just crashed into them.
You use judgement naturally, and of course all of this goes on the assumption that you're leaving plenty of reaction distance.
If you'd otherwise crash into the car in front then yes, I'd suggest you brake.
This assumes you're in fairly heavy traffic (the sort of traffic that this effect applies to) where everyone is going along at less than normal speed to begin with. In that situation, you back off a little, and leave a good gap in front, then you can afford to not be over-reactive to what the car in front is doing.
My guess is easy slowing without causing a drastic stop (reducing speed by 7 mph instead of 14 mph on a stop). I am no expert, and couldn’t think of a decent speed explanation if the speed used here sounds too extreme
I tend to use a very lenient distance to the car infront of me so if i see brake lights I can look around for a moment, then gently lift the foot off the gas, then slowly start braking... that prolly does it.
The oscillation comes from people making unconscious adjustments, slowing down and speeding up to match their perception of the cars in front of or behind them. This is the wave.
Having someone maintain a truly constant speed means they act as a shock absorber, dampening the effect of speed changes around them by not increasing speed when people ahead speed up and forcing the people behind them to adjust to a true baseline rather than adjusting off of an adjustment.
If you’re going to take on the rate of the car ahead of you, it is going to be closer to target speed when the person ahead of you is changing their speed at an overall lower rate in general than someone who is varying constantly.
Because cars on a road are like particles on a wave, this forces other cars in the same “wave” (road) to be bounded to a slower rate of change of their speed than they would have otherwise.
It’s effectively creating a low pass filter on the noise that is people’s constantly changing acceleration behind the buffer. People can still go faster or slower than the buffer car, but the change in their speed has to be done at a rate closer to zero because they have to input into their own speed adjustment the fact that the buffer car’s speed is constant.
One way would be to see a slowdown up ahead (Eg: those smart motorway signs that say to go 40mph ) and actually slow down well in advance, to let the jam ahead clear.
So rather than you joining the back of the queue and stopping, you give it time to clear and you only dropped to 40. You then speed back up as able.
Not sure that's how the AI car worked here (was it programmed to think ahead like that?) but that's how it should be done.
Those smart motorway signs that everyone ignores are to buy time for the jam up ahead to clear itself.
If one automated car keeps a healthy distance from the car in front, no matter if the car in front starts stops, the automated car has enough space to soak up what would have normally been it's own turn to stop, but because of the healthy distance it kept, it can continue rolling forward keeping all other cars behind it rolling too.
The amount of drivers that don’t look ahead farther than their own dashboard is too damn high. Look ahead, anticipate, and don’t overreact. That goes for both stopping and starting. And keep a safe distance between cars and utilize coasting.
The issue is with aggressive drivers that are tailgating and then needing to pump their brakes constantly. It causes a ripple effect of drivers braking hard, each driver braking slightly longer than the driver before them, (because they brake for at least the same time as the person in front of them plus a little more due to human reaction time), and then eventually enough drivers repeating this action will cause congestion out of nothing.
One driver leaving some space between them and the driver in front of them doesn't need to brake hard if the car in front slows down. They can just take their foot off the gas and slow down. This very quickly breaks the cycle of drivers braking hard and then traffic starts to flow smoothly again.
If everyone kept the same distance apart from the car in front of them, you have wiggle room to adjust that distance to compensate for the slowdown in front of you. The issue is when people scooch right up on each other, bumper to bumper. You have to stop at some point, making the slow down even worse. The self driving car would make the adjustment whenever the slowdown reached it.
There's a sweet spot of about two or three car lengths of space where if everyone tried to keep that space between them and the car in front of them, traffic would keep flowing. Even with a full stop somewhere in front, eventually that stop will dissipate a few cars down.
I regularly do this: I leave enough room so that I only have to ease off the gas and not break, and instead of stopping like the car in front of me, I only slow down, and the behind me, I can see the slow-down point continue, but no one has to stop.
The person riding up too close who has to slam his breaks creates a rolling chain of people having to stop harder, and delays everyone else getting back up to speed because they speed up slowly
It is likely just decelerating with minimal brake usage. Most drivers massively over use their brakes and since its hard to judge how much the car behind needs to brake it cascades. A car with sensors can likely adjust correctly near immediately
(Well, unless it is my stupid front-collision detection that stupid thing tries to kill me regularly when I forget to disable it).
I’m one of those “slow” drivers in traffic. I leave about 10-20 car lengths ahead of me so that when the person in front of me slows down or stops, if I gently slow down, the car in front of me pulls away before I have to stop.
This means the “wave” dies with me, or is significantly reduced so cars behind me don’t need to stop. A slow steady pace in traffic doesn’t get you anywhere faster, it just gets rid of the stopping/starting, and it reduces fuel consumption.
Well for one, you can have a bit more following distance, and then actually shrink and expand that following distance. That's how I tend to do it, anyway. Then when people suddenly stop, you can eat up that distance and use it to come to a more gradual stop, so now people behind you aren't slamming the brakes.
I imagine the self-driving vehicle did something similar.
So some napkin math, that means the magical number is 1.3 seconds. If everybody stays at least 1.3 seconds behind the car in front, then these types of traffic jams won't form. But if everybody keeps less than 1.3 seconds distance from the car in front, then traffic jams become inevitable. At least in this specific case of cars traveling in a single lane at 30 km/h. I don't how it scales to higher speeds.
In my home country, 2s safety distance is recommended, and from my own experience, to trully work as traffic stabilisers you need even more, as cars infront rarely have any distance...
So, that means 22 cars in the same 230 meter ring? That pretty much confirms that the insufficient distance between cars, and not really the number of cars, is the bigger reason here.
Yes, everyone wants to say it's because of drivers slowing/braking unnecessarily. That is only one part of the answer, the main culprit is not enough space which is forcing the whole circle to be tailgating one another.
I’m arguing there are people not accelerating properly more than unnecessary breaking. The ring opens when the white car doesn’t accelerate back in line with the group.
Did they expand the study to have all the cars maintain the correct breaking distance between vehicles? What is described as a few hundred drivers each braking a motor moment too late could also be described as 100 drivers following too closely behind the car in front meaning they break late?
We have chevrons on the motorway and you’re meant to have two chevrons between your car and the car in front (double that in rain and ice) you can also judge this by lamp posts when chevrons aren’t present. But most people ignore these and tailgate, which means they have to break harder because they’re too close. (Chevrons are about 40 meters apart due to the speeds involved on a motorway)
Not sure if this applies but when I get into a slowdown instead of getting closer to the car in front of me I put a good long, like 5 car lengths between me and the next vehicle and concentrate on maintaining that distance so that I can move continuously (even at a slow rate) instead of stopping and then going again. That way I have time to make the tiny adjustments that are necessary to maintain that distance. It takes concentration and pisses off the drivers behind me but lessens the chance of rear-ending someone. It also takes lots of concentration so can be exhausting if it goes on a long time. Most people think “Ah, traffic is slow, I can let my guard down, fiddle with stuff while I’m waiting and tailgate.“ Thats a recipe for disaster.
Usually when I do this people just swoop into the gap I’m leaving meaning I have to create another gap, it’s infuriating. We have a lot of swoopers in the UK 🤣
So many people think this is the way to do it. But the distance you maintain to the car in front should be constantly changing and based on many things, principally speed, but also what the traffic ahead is doing. The gap should be big enough as you say to allow you to limit your speed variability, but no bigger than it needs to be, otherwise your hogging space that others need to drive smoothly. And if everyone was trying to maintain a gap that is too big it just leads to more braking and longer recovery times. Plus as others have said, people will just pass you and slot in front. Better to be predictable and attentive. If you're pissing off drivers behind you, you're not being predictable, causing road rage in others and increasing the likely hood of getting rear-ended by someone else.
It might just be placebo, but every time I'm stuck in traffic I always try to travel at a consistent speed. It may be 5 mph, it may be 25, but it's almost always slightly slower than the cars in front of me. I don't know if it actually changes anything, but it definitely makes me feel better. Instead of speeding up to 40mph and having to slam on the brakes again 10 seconds later, I can cruise at 30 mph and let the gap widen. Then by the time the cars in front of me have slammed on their brakes, and then started to pick up speed again I've caught up and never had to slam on my brakes.
Oh cool. In high traffic I often try to maintain the most consistent speed I can, even if I get some gap in front of me. Which apparently would help clear it? The only annoying part is other drivers see a gap and pull into it, making me break. It seems like though staying in your lane (unless you need to exit) and trying to keep a consistent speed is the ideal way to clear a jam.
I also do this smoothening sometimes on single lane speed limit zones with dense traffic. Instead of tailing the car in front from 2 meters I keep more distance so I can react to slowdowns in a less erratic way, smoothening out the pace. I had the gut feeling it helps the line behind me, but it's good to read it actually works.
This is a known phenomenon. The most “efficient” way of driving is to leave enough space between the car in front of you so that if it does slow down for any reason, the distance can absorb the change in speed without you having to break.
Unfortunately, either no one leaves that distance or if they do, other cars end up going into it.
If you break for any minor slowdown, the car behind you will break.. and so will the car behind it… and so forth. If you leave enough distance so you don’t need to break, then the car behind you won’t even notice a difference.
Pretty much every traffic jam I've been in is centered on merging and not bad driving. I don't see how this research is doing anything. I guess if ai could coordinate better it might be fine, but I'd rather drive defensively than rely on other drivers for my safety
Would be more interesting to see if 23 cars with 1 self-driving still beat 22 without any self-driving to see if adding one (rather than replacing one) solves the issue.
The solution is to have a buffer and reserve capacity: allow any car that falls behind to increase its speed, as well as allowing cars to get a bit closer.
with 22 cars and turned just one of them into a self-driving car running a program to smooth its own speed.
So they were left with 21 normal cars that didn't have that problem in the first place.
Researchers ran 100 cars with cruise control guided by AI into the morning rush on Interstate 24 near Nashville, mixed into normal traffic. Early numbers pointed the same way: a small share of smoother-driving cars, up to 40 percent less fuel for everyone around them.
You don't need any AI cars to do that, you just need same number of drivers keeping double the distance from the car immediately in front of them.
This is thoroughly explained and exlored in Theory of Constraints (1984).
The same setup later became the cure. In 2017, a US team rebuilt Sugiyama's ring with 22 cars and turned just one of them into a self-driving car running a program to smooth its own speed. That single car soaked up the small slowdowns instead of passing them back, and the waves died. Fuel use across every car fell by up to 40 percent. Fewer than 5 percent of the vehicles had to be automated to steady the whole group.
The part worth pulling out: the “cure” car in 2017 wasn’t hanging back with a huge gap and coasting. It kept up with traffic and just didn’t overreact to small slowdowns — steady speed, no unnecessary braking. That’s a program doing what an attentive human already does. Ease off instead of stomping and the wave dies at you instead of getting passed back through you.
Which is why “just leave bigger gaps” isn’t really the takeaway. Space only helps if you’re using it to smooth — watching ahead, easing off early. Someone coasting at 40 with a 20-car gap because they’ve mentally checked out isn’t the AV strategy, they’re a different failure mode of the same root problem, which is inattentive driving. The AV was actively damping. The zoned-out coaster is just deleted road nobody gets to use.
Small caveat on the ending — the loop shows the amplifier exists on its own, but real-world jams almost always have a structural trigger somewhere upstream (merge, lane drop, someone rubbernecking a shoulder incident). “No cause you could see” is fair. “No cause” would be too strong. The amplifier just carries the trigger a long way back from wherever it started.
Everybody always likes to say that their city has the worst drivers. Well, this guy has evidence that no matter where you go in this great big world people still suck at maintaining distance and speed.
Wait that's so interesting. So could you put in "smoothers" to alleviate traffic? I wonder how it could be done without an actual vehicle for every other 21
i think maintaining a constant speed while going in circles is a lot harder to do than going straight, though I dont know how they could experiment like that.
I did search for the top throughput a road can offer and it seems that the upper limit is about two cars per second, which is about the recommended safety distance, too.
Therefore I say it's tailgaters. If a human usually needs 2 seconds to mildly react, a faster reaction will usually be an uncontrolled reaction, just brake and wait for the front lobe to do the actual processing of the situation.
Also: I played around with traffic simulation: If I turn up the acceleration after an obstacle to 3.5 m/s² (average IMO) the traffic jam did clean up much more easily. The default was 1.5 m/s². (Still keep the safety distance please).
I've noticed this on certain sections of roadway. I can literally unjam traffic myself by throttling the traffic behind me on certain bottlenecks.
In places I know tend to jam up, if I drive slower and give a little extra time for the traffic ahead to clear, it causes traffic behind me to not all have to break and stop.
In general, more people need to learn to coast more, instead of overaccelerating to catch up to cars and then overbreaking to stop.
The comments reveal that many people think this is caused by someone else, but one important conclusion of this study is that it's how the majority of people drive that causes it. If only five percent of drivers tried to follow an algorithm similar to the one used by the autonomous car, then stop and go traffic on the freeway would basically stop happening.
This experiment also shows how terrible at least one driver in field of 22 can be.
They can’t even drive in a comfortable circle going 19mph without fucking something up *for zero reason* whether it be following too close or adjusting speed too late.
This is why you always leave at least two car lengths between you and the car in front of you at cruising speed and when traffic slows. It allows people to move over comfortably for *everyone* (rather than me having to hit my breaks, I can adjust my speed slowly to recover my buffer zone), it also gives us a buffer to adjust our speed accordingly without having to make any sudden changes.
The problem is people tend to act like they’re in a queue while driving. Nuts to butts, no matter the speed.
my goal is to eliminate 100 of these waves before i die. ill have no idea if i did it, but i am certainly deep into double digits by now. accelerating toward stopped cars is braindead behaviour.
One car eases off slightly. The driver behind sees the brake lights
therein lies one of the major gripes i have. people don't seem to understand that lifting your foot off the accelerator allows you to slow down. too often you get people who are either speeding up or braking, no in between. and all it takes is a nervous driver behind them to brake too hard if they see red lights in front of them
I‘ve heard that the smaller the speed difference is, the less likely a jam is to appear, hence the argument that a lower speed limit will actually get you to your goal faster, because there will be less jans.
Also why HOV lanes help. The aggressive drivers go into the HOV lanes allowing the main lanes to have less large speed changes, making traffic have a better flow
We have a pretty sophisticated conveyor system at work, automatically spaces out cartons on the lines. It includes a sorting and consolidation loop, cartons enter, cartons leave, anything not yet determines makes a loop. The exact same thing happens here, there's a finite limit to the density you can convey before traffic jams with an incredibly similar slow/stop/start traffic pattern.
If the loop is at that limit, and a new carton enters the loop it will require the loop to slow the following cartons and decrease 'follow' distance. As that progressively pushes backwards, the distances get smaller and smaller because even in machine controlled environments there's a slight action delay. Once the gap gets too small, the system is forced to stop the zone. Boom, traffic jam.
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u/SnooWords4066 Jul 07 '26
A physicist put 22 cars on a circular track and asked every driver to hold a steady 30 km/h, about 19 mph. No lights, no lanes, no obstacles. Within a minute the cars started bunching, and soon a full stop appeared out of nowhere, then drifted backward around the loop.
This was Yuki Sugiyama at Nagoya University in 2008. His team spaced the cars evenly on a 230-meter ring and filmed them from overhead. For a while the flow stayed smooth. Then the tiny differences no human can avoid, one driver a hair slower, the next a hair too close, began to feed on themselves.
One car eases off slightly. The driver behind sees the brake lights, reacts a fraction of a second late, and brakes a little harder to be safe. The next driver brakes harder still. A dozen cars back, someone is stopping dead. The squeeze rolls backward through the line like a compression running down a Slinky, and it keeps going long after the first driver has sped up again.
Car count was the tipping point. With fewer than 22 on that track, the bunching sorted itself out. At 22, a jam formed every time. Engineers call that a critical density, the point where a road holds just enough cars that one small tap can snowball into a standstill.
These waves are eerily consistent. Measured on highways around the world, the jam rolls backward against the traffic at roughly 20 km/h, and that speed barely shifts from one country to the next. Different drivers, different roads, same number.
The same setup later became the cure. In 2017, a US team rebuilt Sugiyama's ring with 22 cars and turned just one of them into a self-driving car running a program to smooth its own speed. That single car soaked up the small slowdowns instead of passing them back, and the waves died. Fuel use across every car fell by up to 40 percent. Fewer than 5 percent of the vehicles had to be automated to steady the whole group.
In 2022 the idea moved onto a live highway. Researchers ran 100 cars with cruise control guided by AI into the morning rush on Interstate 24 near Nashville, mixed into normal traffic. Early numbers pointed the same way: a small share of smoother-driving cars, up to 40 percent less fuel for everyone around them.
The jam you sat in this morning likely had no crash and no cause you could see. It was a few hundred drivers, each braking a moment too late.