Shockwave traffic jams recreated for first time on HackerTube: recommended by 1 Hacker News commenter across 1 month, 2009–2026.
demo · evergreen
A one-minute New Scientist clip showing a Japanese experiment in which drivers on a circular track produce a spontaneous traffic jam that travels backward. It illustrates phantom traffic jams without any obstacle.
It is the go-to demonstration HN links whenever traffic shockwaves, phantom jams or cascading failures come up.
“They told 22 drivers to drive in a circular path at 30 km/h. A traffic jam appeared, and moved backward.”
14 Hacker News comments link this video. Each one links back to its thread.
This article accounts for some of what causes traffic jams, but have a look at this video. It shows a study done where people tried to drive on a circular track at a constant speed. Things start out okay, but it quickly devolves into the back propagating wave. ▶ this video
in “How traffic actually works” · View on HN
▶ this video This is the natural response of tapping the brakes for any slowdown: people naturally over compensating and chain reactions happening behind them. This video shows how stop and go traffic forms and snowballs with no real impetus beyond mis estimated follow distance.
in “Hard-braking events as indicators of road segment crash risk” · View on HN
I'm going to double-down on this point because it does matter. In the real world some people are tailgating, but that does not cause traffic jams if and only if the road is running below peak capacity, where negative feedback self-corrects those traffic density deviations and gradually smooths everything back out. (Unless the tailgater causes a crash, of course). But if the average following distance is such that the road is exactly at the peak of throughput, or any smaller, then any momentary dip into tighter following distances pushes the road it into a positive feedback operating mode, which triggers a traffic jam. If you haven't seen it, here is a classic experiment demonstrating the effect: ▶ this video (But obviously the solution to traffic isn't 5 more lanes, it's viable alternatives to driving!)
in “A short post on short trains” · View on HN
as far as i can read, your argument is that traffic jams are impossible, because congestion acts as a dynamic cost function to keep the road at its highest utility, when the throughput is highest. unless you disagree with the that definition of the utility of the road? how do you explain phenomena like shockwave traffic jams, where otherwise high utility roads get sections of nearly stopped traffic. eg. ▶ this video in a closed system (30s of video) can you spend some time showing your work, and both propose and prove what the cost function of congestion is? then, it should be clear whether the government set cost is higher or lower, and under what conditions. id especially want to see the limiting behaviour - standstill traffic. my gur sense is that the cost of congestion should be going towards infinity, but im interested in how the constant value from the government is still higher.
in “Congestion pricing in Manhattan is a predictable success” · View on HN
▶ this video basically, people are dealing with a ghost. you have one person suddenly stop and now there's a permanent kink in the behavior of the traffic.
in “My Clients, the Liars” · View on HN
Even moving slower will not prevent stop and go traffic if the road has reached its capacity, due to the various different accelerations and decelerations of cars. My overarching point is something like this video: ▶ this video There simply is not a way to avoid the consequences of being near or above a road’s capacity. Of course, I recommend everyone to play it safe and stay far enough back to avoid any liability of being too close to the car in front. But the reality is we make lots of calculations and choices in driving and at various times, we choose farther distances and other times nearer distances (even though we know it is riskier) and these collective decisions will propagate down through the whole road.
in “Tesla driver dies after his Model 3 stops on highway, gets hit by two other” · View on HN
This was a topic of a few Asimov books. In describing the big cities of earth, the more complicated and JIT nature of getting food where it needs to be meant that where previously an hour delay was tolerable, that minute delays could severely impact everyone with massive ripple effects. It seems like what we are seeing WRT the supply chain is the equivalent of putting more and more traffic on a road and gradually increasing the speed limit to deal with delivery bottle necks (rather than making new roads/routes). Of course, the issue is that whenever someone cuts someone else off or whatever, you create massive shockwaves which can pull everything from 90mph to 10mph. [1] [1] ▶ this video
in “Looming food shortages is the next ‘slow-moving disaster’ to hit world” · View on HN
This model is overlooking some aspects of traffic flow. The most important one is that flow does not increase and decrease smoothly. As density increases there starts to be shock waves that bounce longitundinally[0]. It would also makes sense to also distribute some autonomous vehicles along the train to minimize these shock waves. A much simplier solution is to autonomously centre between the vehicle in front and rear[1]. Doing so smoothly is simple local logic which may improve flow rates dramatically but I don't know if it's been implemented anywhere. [0] ▶ this video [1] [link]
in “Altruistic Autonomy: Beating Congestion on Shared Roads” · View on HN
I think if it's annoying, you're not driving in traffic the right way. Sure, it's annoying for aggressive stop-and-go driving, but if you maintain a consistent speed with the traffic in front of you (see traffic shockwaves [0]), a manual transmission is better than an automatic. [0] ▶ this video
in “Fewer than 3% of cars sold in the U.S. have manual transmissions” · View on HN
Self-driving cars are the necessary but not sufficient condition to solve traffic problems. Start with that and remove 1-person-1-car model [0], add ride-sharing software, ban human drivers [1] and done, no more traffic problems. As for your example - this jam is a direct result of very high latency of human reaction. It causes stop-and-go in jams [2]. [0] Or it will rot and occupy parking space for 97% of time. [1] This will likely to average speed and reduce time spent waiting for lights. [2] ▶ this video
in “Why the focus should be mass transit instead of tolls to fix traffic congestion” · View on HN
Did you read the article? There is no single car that causes a jam. In fact according to previous research [link] (see "Conclusions" at the bottom), even if all cars follow the same rules a jam can still occur. They can also occur in very simple situations like in this video ▶ this video
in “Systemic traffic jams caused by small groups of drivers, study says” · View on HN
NASDAQ telling the market "you have until noon Monday to prove your losses" most definitely did cause people to dump stock in the first two days of trading. Confusion over whether trades were happening because NASDAQ wasn't confirming orders probably caused investors to be skittish, which could easily have had cascading negative effects. It's hard to say if FB's stock was overpriced. The GM thing couldn't have helped. But to say NASDAQ had nothing to do with it is silly. I look at FB's IPO and think of this[1]. Little (and not so little) permutations causing shockwaves through the system. 1. ▶ this video
in “Facebook Not Feeling Friendly With Nasdaq” · View on HN
TBH, if you are going to have smart intersections controlling autonomous cars, you might as well try to move to roundabouts as much as possible. Roundabouts provide continuous flow and probably represent a much more solvable problem since it involves attenuating the speed of approaching vehicles and vehicles currently circling. I imagine throwing stopping into the mix complicates things significantly I imagine that computers could completely prevent shockwave traffic jams. ▶ this video
in “Computers may control intersections for self-driving cars” · View on HN
Some Japanese scientists did a study to show how traffic jams can appear of of nowhere. They told 22 drivers to drive in a circular path at 30 km/h. A traffic jam appeared, and moved backward. The video is here: ▶ this video
in “Traffic Hacks: How One Driver Can Vastly Improve Traffic” · View on HN