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Comment by velosol | Hacker News Reader

So should I leave space on the highway so my lowest speed through the occasional slowdown is higher than average or am I hurting the traffic flow?

I recall reading about (highway) traffic being a queuing problem where the arrival rate exceeded the clearance rate due to a transient action (someone tapping their brakes because they were distracted or a bright glint from a glass truck going the other way). If you decrease the arrival rate slightly such that the clearance rate can remove the stopped/slowed cars then the entire traffic flow will return to the norm for the volume of traffic on the road.

Sound? Valid?

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Stop/go is bad. Gliding is better for the collective and for your fuel efficiency.

CGP Grey has a great video on traffic flow: https://www.youtube.com/watch?v=iHzzSao6ypE

They do this in the UK with variable speed limits, the M25 (ring road around London) is the bottleneck so you will regularly find that the roads joining it during rush hour will have a limit when there is no apparent problem.

If the drivers are rational then going at the average speed works best.

But in real life people would cut you off making you slam the breaks and making the problem worse.

Brakelights lead to brakelights which leads to parking lots on the highway. Most people really only need to let off the throttle and coast, but its either throttle or braking for some people while driving.

There is an excellent youtube video on this (won't be able to look it up until later) but researchers had people drive at constant speed in a circle in a parking lot. Little by little, people bunched closer and closer, and once people started hitting brake lights it became stop and go. With 12 people driving in a small circle.

Capacity should be proportional to speed over car-distance. I wonder if mitigating ripples can really overcome that much increased distance.

Shouldn't it be proportional to sqrt of that, or something else that's sublinear? In places where people have respect for law and safety of themselves and other human beings, the distance between cars should increase with speed.

Braking or a quick lane change causing traffic jams is a well known and refuted myth. Similar to the idea that driving slower on the highway is more dangerous. We have massive amounts of data on both of those. I wouldn't put too much value in that source.

And that would be my fault - my parenthetical was just imagined reasons why there might be a saturation-related slow down.

In trying to look for the source I did come across [1] from a few years ago and may have been remembered incorrectly.

Edit: More details at [2], [3], and [4] some noting transient braking may be a factor.

[1]: https://www.npr.org/2013/11/29/247825768/phantom-traffic-jam...

[2]: https://www.vox.com/2014/11/24/7276027/traffic-jam [3]: http://math.mit.edu/projects/traffic/ [4]: https://www.sciencedaily.com/releases/2007/12/071219103102.h...

Right, those are brief disruptions that cause no significant variation in total transit times.

Much like the idea that slower drivers cause more accidents, this is based on other drivers experiencing discomfort and having to be more careful to avoid the slower car. It feels like it is more dangerous, but the data we have shows that it does not result in more accidents.

Traffic jams that cause meaningful changes to total transit time are almost always based upon significant disruption events(accidents) or uneven capacity(merging traffic at a higher rate than the freeway can absorb).

We know from psychology that a person will perceive something that requires more attention and effort as taking longer and perceive something that requires less attention and effort as taking less time, even if the times are equal.

> Much like the idea that slower drivers cause more accidents

Slower vehicles are more likely to end up in an accident. Speed - accident rate curve is IIRC called Solomon curve. I haven't heard this theory disproved.

> this is based on other drivers experiencing discomfort and having to be more careful to avoid the slower car

I have heard experts saying that observations indicate that slower drivers cause accidents indirectly. They are usually slow enough to irritate other drivers, but fast enough to make it hard for an average car to overtake them safely, which pushes some faster drivers into dangerous situations, sometimes resulting in accidents from which the slower drivers escape unscratched.

> Much like the idea that slower drivers cause more accidents

I think this might be statistically valid, due to correlation with the root cause. Drivers who are inexperienced or uncomfortable due to poor eyesight, poor reflexes, unfamiliarity with the vehicle or area naturally slow down.

These are the same sorts of “driver disadvantages” that would cause more accidents.

No references, just intuition based on decades driving in Chicago traffic.

You are continuing to make assertions without references.

My experience of traffic jams is that they are normally caused by density of traffic. There are a lot of simulations out there that show how small variance in high density traffic acts like a wave that propagates backwards. You even see it on escalators on the tube here in London, where people are supposed to walk on the left but often get stuck in a standing state at busy rush hour times.

http://www.traffic-simulation.de/ring.html is a good one - push up the density and see how the wave propagates backwards. It's not merging that causes the problem, it's density.

Of course standstill traffic that stops everything for minutes at a time is usually something else. But that isn't an everyday jam. And everyday jams do make a meaningful difference in transit time. They even affect me on my motorcycle especially as roads get narrower to accommodate bicycle lanes. I do however still get through and it's a rare day that there is any kind of event or interesting artifact at the front of the jam.

Can you cite some sources supporting your statement or providing analysis of the massive amounts of data you mention?

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