What Are Traffic Waves and Why Do They Happen So Much? • Hacker News Board

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

#31

Post by ghaff » Thu, May 21, 2015, 12:05 PM UTC

rajivm wrote: post #14

stormbrew wrote: post #8

Earlier quoted context omitted.

I think people overstate the degree to which self-driving cars will actively 'talk' to each other. And I actually think it's somewhat detrimental to advocating for self-driving cars, because the immediate thought is that communication acting as a channel for malware. And that's a fairly justified fear. In the end, I think most of the communication will just be in terms of the fact that self-driving cars interacting w…

While I largely agree, I think that some layer of _untrusted_ communication could be beneficial. Think along the lines of how turn signals & honking can benefit humans. Similar types of information could be conveyed more precisely "over the air", even if it was not presumed to be perfect (i.e. just likes human's signals aren't and a smart car isn't going to trust someone's turn signal, but it might react to it). Exam…

One of the comments I've heard about how to start addressing some of the harder aspects of autonomous vehicles is changing the environment to help them in various ways. I suspect radio beacons communicating various information to complement vision systems or to hand off control (within some defined interval) are one of those things. Of course, they'll have to operate in mixed environments for many decades so they can't be dependent on, for example, coordinating with other autonomous vehicles.

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

#32

Post by dmvaldman » Thu, May 21, 2015, 12:23 PM UTC

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

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

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

#33

Post by happyscrappy » Thu, May 21, 2015, 12:52 PM UTC

leviathan wrote: post #26

vlasev wrote: post #9

Earlier quoted context omitted.

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

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

This happens to an almost comic degree in Boston. One car, invariably with Maine plates, insists on leaving five car lengths between them and the driver in front of them. By Boston standards this is a monster opening which is quickly filled and the Maine driver continues to back off. This happens the entire way through the city.

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

#34

Post by larrydag » Thu, May 21, 2015, 12:57 PM UTC

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

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

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

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

#35

Post by AlfaWolph » Thu, May 21, 2015, 1:02 PM UTC

dmvaldman wrote: post #32

An interesting point to mention is that the traffic "jam" moves backwards. This is because at the front of the jam, cars are able to accelerate out of it, while at the back o the jam cars are piling up into it. Seen from above the jam is a shock wave moving backwards. This often upsets people when they get out of a jam only to see there was nothing causing it! In reality what caused it may have been much further up t…

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

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

#36

Post by _lce0 » Thu, May 21, 2015, 1:15 PM UTC

jasmcole wrote: post #21

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

TIL only a fool breaks the two second rule

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

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

#37

Post by _lce0 » Thu, May 21, 2015, 1:20 PM UTC

can you see the animations?

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

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

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

#38

Post by gpvos » Thu, May 21, 2015, 1:21 PM UTC

jacquesm wrote: post #11

stormbrew wrote: post #8

Earlier quoted context omitted.

I think people overstate the degree to which self-driving cars will actively 'talk' to each other. And I actually think it's somewhat detrimental to advocating for self-driving cars, because the immediate thought is that communication acting as a channel for malware. And that's a fairly justified fear. In the end, I think most of the communication will just be in terms of the fact that self-driving cars interacting w…

And that's assuming error free communications, near latency free communications and non-congestion as well as perfect hardware. Is 'no signal' the same as 'no other cars' or 'broken radio(s)'? Self driving cars have all the elements of a distributed computing system only now it's moving at a pretty good clip and lives are at stake. 'Move fast and break something' is very bad advice and 99.999% uptime is not going to…

Some research has been done with the result that even if only a certain percentage of cars coordinate with each other, this already causes a remarkable reduction of congestion. (Sorry, don't have a link.)

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

#39

Post by briffle » Thu, May 21, 2015, 1:26 PM UTC

larrydag wrote: post #34

This is something I love to think about when driving on the highway. I've read so much on it. I've also had some thoughts on recommendations to solve it 1. Divided lanes for 3+ lane highways. Basically a modified HOV lane but no occupancy restriction. The far left lane would have lane dividers with openings every 5 miles (or 3 exits). The middle lane opening every 2-3 miles (or every other exit). The idea is that it…

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

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

#40

Post by ninkendo » Thu, May 21, 2015, 1:41 PM UTC

mixmastamyk wrote: post #2

This reminds me of a piece on traffic I read quite a few years ago and might have been on reddit or even perhaps slashdot. I still follow its advice to this day when having to brave the 101. It discussed traffic waves and our reactions to them. The repeatable waves this article mentions come from (most of) the individual drivers driving faster than the capacity of the road to carry them all. So they bunch up at some…

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

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

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

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