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Comment by siavosh | Hacker News Reader

This is incredible, I put a cone in the middle of the road and realized the nightmare I had caused, and then quickly removed it. But the nightmare just continued to ripple through space and time. This explains all those mysterious traffic jams I've been in that seemed to defy reason.

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Here's a real world example of a traffic wave due to one person's sudden break

https://i.imgur.com/fLNs3k0.gifv

It's incredible to watch the emergent behavior out of real world systems. Completely counterintuitive, and yet there it is - a clear backward propagating compression wave, whose speed probably mostly depends on human physiology (e.g. reaction time, personal comfort level with distance to next vehicle, etc.) Fascinating.

>whose speed probably mostly depends on human physiology (e.g. reaction time, personal comfort level with distance to next vehicle, etc.)

I think it has more to do with the suddenness of the stop; each car behind it has to stop suddenly in response, as the change only spreads locally. I’m pretty sure all you need to modify is deceleration speed for braking and acceleration speed to catch back up, to see like 90% of the effect

I’d test it with the simulation but it was too difficult to setup on mobile :-)

> each car behind it has to stop suddenly in response

Yes. And simply maintaining good following distance gives you a buffer that can absorb this sudden change and prevent the tailback. This is one of the reasons that I long for the day that humans don't pilot cars on the highway.

The solution is simple. Lower the speed limit to 20kph, wait for the ripple to disappear and slowly raise the limit to 100kph. The reason this doesn't work in real life is lack of dynamic signage and because people don't adhere to the speed limit.

This gives me a newfound respect for variable speed signs on UK motorways (NB. not the actual speed limit signs, the black signs with the white matrix of bulbs giving advisory speeds).

To-date, if you see a sign saying "40", that basically means there's no traffic flow and you will have to come to a stop very shortly.

If you see a sign saying "60" that means that you probably won't need to stop but you're going to slow to a crawl.

If everyone just did as it said things would probably improve massively.

"40 - Queue ahead" by a junction about an hour after the traffic has cleared. A couple of weekends ago I drove past 3 lane closure warnings and low speed limit signs on a motorway with, you guessed it, no lanes closed.

To some extent this is my confirmation bias, but certainly as often as not the signage is spurious and that's why people don't pay any attention to them. I suspect most people agree with the principal of the things, but every time they obey a sign whilst everyone else flies past, only to discover it was inaccurate it does affect the amount of credence future signs are given.

Lowering your speed will technically resolve the congestion, but your average speed will still be the same as if you had simply "ridden the wave," that is, proceeded at normal speed up to the jam, then proceeded thru the jam at slow speed.

Essentially you are doing a "smoothing" operation if you proactively reduced your speed ahead of a jam.

Hmm, stop-and-go is less efficient though. I try to average things out myself to maximize fuel-economy in those traffic conditions.

Even with regenerative brakes, the regeneration isn't fully efficient. You lose energy to heat. So smoothly traveling one speed is superior.

And human psychology is an important factor here as well. Less stop-&-go means fewer temper flares & physical opportunities for collision.

This gives me a newfound respect for variable speed signs on UK motorways

I respect the idea, but the actual implementation feels a bit half hearted to me. They need to kick in a lot quicker and be more severe, all they seem to do is make a minor improvement and slow people down perhaps a half mile from the actual congestion.

We have variable speed signs in the US, but where I live, they aren't followed, to the point that it seems unsafe to obey them and go 30mph when the flow of traffic is 60mph

Those variable speed limit signs are meant to be enforced by average speed checks with cameras. I'm not sure what's going on with that, I know there are delays, but there's signage that says there are regular speed cameras.

So if the cameras work, we'll have to do exactly what they say or get fined for speeding.

On the new 'Smart Motorways' there is built in enforcement via cameras. On those, the speed limit signs are actually speed limits, with the red ring around them.

The ones I was referring to are advisory and look like this [0].

[0] https://www.drivingtesttips.biz/images/motorway-advisory-spe...

I got hit by this a while ago. Was driving down an empty freeway at night and there must have been road works off the side of the road during the day but I didn't see it until the second sign but the speed camera was after the first sign and I got hit with an $800 fine :/

Mostly the latter, in my experience. Germany has dynamic signage on several highways, but if the sign says 80 (even with a secondary sign notifying of traffic jams), most people will still drive 160 if it doesn't look congested yet.

That’s simply not true. Most people don’t even drive 160kph to begin with.

In some parts of the highway is very plausible. Even 200km/hour and more

People seem to have some terrible difficulty of understanding the difference b/w “Most people don’t do <x>”, and “It’s theoretically possible to do <x>”.

People seem to have some terrible difficulty ...

All / most / some people? Or just the person you responded to?

There are multiple reason why the way people communicate is important, one of which is the limited logical reasoning capability of most people.

Just look at politics / marketing / dark patterns / mortgage contracts.

I successfully changed my sister's mind from variable rate mortgage to fixed mortgage, as the language of the contract doesn't explicitly write how bad the worst case option can be 10 years from now for variable rate mortgage.

In unrestricted areas, this link says the average speed of cars is in the 140-150kph range, and about 20% of all vehicles above 160kph, so while you're right, it's not as extreme a difference as one may infer from your tone.

https://mil.brandenburg.de/cms/media.php/lbm1.a.2239.de/stud...

That depends on what country you live in. Some areas in Germany, 200 km/h is no problem for hours straight.

I live in Germany. And what you’re saying is (a) wrong, and (b) not even incompatible what I said.

"Some areas in Germany, 200 km/h is no problem for hours straight."

Please tell me of one road/route in Germany where you can drive '200 km/h for hours straight". I'll settle for tiny values of 'hours', as long as they're >= 1.

This ofcourse depends on time of day. An early morning somewhere around 2 am - 5 em we were driving from Sweden down to south-west germany, somewhere after Hamburg. We have a small car, top speed is about 200 km/h in downhill, wind in the back. I had the gas pedal to the bottom for several hours, most of the time it was four lanes and the only thing you saw was a truck every few kilometers. This was a few years ago so not certain if this has changed. Last time we drove down we ended up on the wrong highway and stuck behind a heavy transport for the same ammount of hours doing 70 km/h tops, when we weren't standing still. Friends told me the heavy transports blocking the whole roads have gotten a lot worse in the nights, the last years.

From experience this is rare. 20kph above the limit is standard (and probably expected by the system setting speed limits), 80 over would cost your license for several months.

Admittedly, 160 was exaggerated. But on stretches where there's no speed limit and you suddenly get this 80 km/h along with Stau signs, people tend to still drive 120 if there's no visible congestion. Since those dynamic limits are not really enforced (most speed cameras tend to be in places where there's a fixed limit at all times) there's rarely a downside to drivers. Same as with driving 80 in construction sites instead of the 60 that's signed.

This is starting to change now that the polizei have started leaving mobile panzerblitzers in place for several days at a time. I've noticed in the last few months significantly more respect for changes in speed limits from the folks with local license plates.

The ones still driving 160 are mostly foreign drivers in my experience. Most German drivers obey the speed limits. Note I'm not German but often drive through it.

As somebody who was using the Autobahn quite a lot I’d say most people go between 120 and 160 when there is no speed limit. But it depend on the conditions

Admittedly, the 160 was exaggerated. But as a German who does obey the speed limit, my experience seems to be that most drivers don't.

Because you never notice all the other cars that do. They travel as fast as you, so their distance remains constant. You only notice cars going slower or faster than you.

The thing that makes me most excited about self-driving cars is what with a little bit of coordination these kind of smoothing techniques could be applied with even a modest percent of cars with the capability.

Sure, as long as the rushed buggy insecure software doesn't accelerate all the cars into the traffic jam.

Wow, works like magic ! Actually Swedish drivers are somewhat more obedient and indeed traffic problems are due to road hazards are on average less severe.

For example when Swedes sees a lower speed sign or even just a warning before a road block they will slow and stay in one long lane, while in the mid eastern country I live in until recently everyone just speeds up trying to be first to pass the blocking. The results ? smoother flow and shorter queues, guess where ?

Swedish drivers are indeed obedient. Overly so, at times. I live in a heavily touristed part of Denmark - we get a lot of Swedish and German cars in the summerish half of the year. And I have long since learned to stay well clear of the Swedes whenever possible. They're too often going that extra bit too slow, adhering slightly too rigorously to every rule, not getting into the flow, too often being front Volvo going 70, the rest of us lined up behind, wanting to go home at 80 like the sign says.

In supercomputers, there are such mechanisms for reactive congestion management: messages are sent to emitters to temporarily reduce flow. It works at small network scale but doesn't scale up, because congestion can happen often and for short times, and the latency of reaction is similar to the latency of events means you are not reacting to congestion anymore, you're just reducing global bandwidth. To this day the best practical mitigation strategies are great deterministic load balancing, QoS through virtual channels and fine-tuned adaptive routing. The analogy with traffic doesn't really hold though, because we can theoretically propagate throttling instructions faster than the vehicles involved, and the problem lies more with getting instructions applied at all. Just thought giving some overview of a similar problem would give some food for thought.

In a sense this sort of adaptive throttling does exist in traffic flow via ramp metering. Upstream sensors detect the traffic density and the signal at the ramp is adjusted to allow only enough traffic to bring the freeway up to capacity.

This solution doesn’t work if the number of vehicles has met the road’s carrying capacity. There is no avoiding traffic once the upper limit is reach, due to the differences in every vehicle’s acceleration/deceleration, as well has inclines and declines in the road. At a certain number of lanes and a certain speed, each vehicle needs a certain amount of space at a specific point in time to pass through a specific point on the road. Once this has been exceeded, the vehicles must start slowing down, and this will cause a ripple effect backwards until this congestion has cleared.

We can try to smooth out this ripple, but if the congestion situation exists for long distances, all it will do is push back the ripple to the end of the congestion.

This only works if you are way under capacity on the road. Once there is enough traffic, no amount of tricks like this will fix the problem.

I have seen highway patrol officers doing what I assume is this in California. They turn on their lights and zigzag across all lanes, preventing anyone from passing them. Then they gradually slow down to a moderate speed, maintain that speed for a while, and at some point turn off their lights and drive off at high speed.

I've only seen it done to stop traffic to pick up a hazard on the road. I'll have to pay more attention now and see if I can see them do it for moderating traffic flow.

This is what's known as a "traffic break", and I see it on the 405 pretty frequently. Definitely threw me off the first time I encountered it.

https://en.wikipedia.org/wiki/Traffic_break

"and because people don't adhere to the speed limit."

If people can't follow the process maybe the process is wrong.

People can follow the process. Many people (most?) won't follow the process.

In the U.S., at least, it has been observed that roughly 10% of drivers adhere strictly to the limit. The rest drive the speed they find comfortable.

<England> Clearly the solution is to add Orwellian levels of automated ticketing</England>

Sarcasm or wishful thinking?

Both are valid points one could argue for, depending on underlying values that are not necessarily rational.

That's what the UK has done with their "Smart Motorways".

It works surprisingly well, both through modulating the speed of traffic and by allowing the hard shoulder to be used at peak times.

On the circular road, did you try clicking a single vehicle to slow it down just a tad? I did that (just one vehicle) and watched in horror as a phantom jam developed. Just 2 slow vehicles and there arose a bumper-to-bumper, wave of stationary cars.

This is why keeping distance and decelerating slowly is important (also saves your brakes and uses less fuel)

Alternatively, this is why not being that person going slower than the rest of the traffic is important.

This is a slippery slope. At least this puts pressure onto people to match the speed of people going faster. So this might lead to massively increased average speeds risking an increase in accidents.

Wouldn't it?

No, because average speed doesn't increase the number of collisions as much as speed differential does.

If everyone is going 70 mph, it's quite safe. It's the one moron going 50 mph or the one asshole going 90 mph that will cause the crash.

If you drive 100 and the limit is 100 you are perfectly fine. Anybody who goes over the limit has to live with the fact that many people stay within the limit.

I always do this but my mechanic is complaining that I have to use the breaks, they rust fast if not.

I am no mechanic so I don't make money from selling you new brake rotors, but I think coming to a complete stop a few times each drive should be more than enough to brush any rust away.

Depends on local weather conditions. Anywhere that road salt is used in winter, some hard braking regularly is extremely beneficial. Road salt is not only corrosive, it's also hygroscopic, so you really want to clean that shit off.

As for wearing out brake rotors prematurely - even with aggressive braking they last more than 50k miles. So on average you'd replace them less often than every 5 years, and they cost around around $250 for a set of four. Doesn't seem like a very good get-rich-scheme for the mechanic.

This. No mater how minimally you try and use them brake rust will not be a problem on a car that's driven at least weekly.

This depends largely on where you live. Salted roads are ready break-killers. If the car stands still a month and the breaks wasn't clean, there might be holes when you want to use the car again. I know, tried it a few times with my cars over the last 30 years. Repairing that can be expensive depending on car :-)

[CGP Grey did a video on this](https://www.youtube.com/watch?v=iHzzSao6ypE). Your goal, as a driver, should be to try to maintain equivalent distance between your car and cars in front of you and behind you.

reddit link syntax doesn't work on HN, btw.

Nevermind, raw markdown is good enough for this forum.

Next, increase following distance to improve the situation.

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