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  • The CGP Grey video on Traffic is a decent explainer on how traffic happens to begin with and how it gets relieved, kind of like a traffic snake that grows and shrinks, travelling in the direction opposite to traffic.

    This City Nerd Video explains how traffic gets exponentially worse as it increases.

    It usually starts with someone making a dumb move at a merge, changing lanes or another person forgetting to brake until the very last moment. That's part of the reason I don't see much benefit in adding more lanes to a highway save for very few exceptions, since you'll just have more changing lanes leading to slowdowns and extending a section that was a bottleneck often just shifts the bottleneck somewhere else.

    So anyways, I'll keep preaching to the choir: trains, trains, we need more buses and trains.

    • While things like merging movements and so on is part of the story, it's not the whole story.

      You see, by saying "traffic jams are caused by merging mistakes and so on" it kinda implies that if everyone drove perfectly a highway lane could carry infinitely many cars. In actually a highway lane has a finite capacity determined by the length of the vehicles traveling on it, the length of the gap between them (indirectly determined by how fast they can start and stop), and the speed they're moving.

      There are finite limits for gap widths and speed determined by physics and geometry. As the system approaches these limits it becomes less and less able to deal with small disruptions. In other words, as more cars move on a freeway a traffic jam becomes more and more likely. The small disruption which is perceived as the cause was really just the nucleation point for a phase change that the system was already poised to transition through. If it wasn't that event then something else would trigger it.

      It is interesting to note that once a highway has transitioned from smooth flow to traffic jam its capacity is massively reduced, which you can see in the graphs in the above link. Another interesting thing to note is that the speed vs volume graph, if you flip it upside down, resembles a cost / demand curve from economics, where volume is the demand and time spent commuting (the inverse of speed) is cost. If you do this you see something quite odd, which is that the curve curls up around itself and goes backwards.

      This is less like a normal economic situation (the more people use a resource the more they have to pay, the less people use it the less they have to pay) and more like a massively multiplayer version of the prisoner's dilemma. For awhile the cost increases only slightly with growing demand, until a certain threshold where each additional actor making a transaction has a chance to massively increase the cost for everyone, even if consumption is reduced. Actors can choose to voluntarily pay a higher time cost (wait before getting on the freeway) to avoid this, but again, it's the prisoners dilemma. People can just go, trigger a traffic jam anyway, and you'll still have to sit through it + all the time you waited trying to prevent it.

      Self driving cars are often described as a way to eliminate traffic jams, but they don't change this fundamental property of how roadways work. It's true that capacity could potentially be increased somewhat by decreasing the gap between cars, since machines have faster reflexes than humans (though I'm skeptical of how much the gap can really be decreased; is every car going to weigh the same at all times? Is every car going to have tires and brakes in identical conditions? Is the condition of the asphalt going to be identical at all times and across every part of the roadway? All of these things imply a great deal of variability in stopping distance, which implies a wide safety gap.), but the prisoner's dilemma problem remains. The biggest thing that self driving cars could actually do to alleviate traffic jams would be to not enter a highway until traffic volumes were at a safe level. This can also be accomplished with a traffic volume sensor and a stop light on highway on-ramps.

      Of course trains, on top of having a way higher capacity than a highway lane, don't suffer from any of this prisoner's dilemma stuff. If a train car is full and you have to wait for the next one that's equivalent to being stopped at a highway on ramp. People can't force their way into a train and make it run slower for everyone (well, unless they do something really crazy like stand in the door and stop the train from leaving).

      • You could carry a near infinite amount of cars on a highway if you could instantly accelerate to near the speed of light!

        In all seriousness, yes you're right that there is a max throughput of people per hour even with ideal drivers and cars on a given highway. You simply do not have enough space.

        The article was very interesting and informative, but that too assumes many ideal conditions. Re: zipper merging, the author really discounts the affect of confusion causing on cumulative delay. Of course that never letting anyone get in front of you, and decreasing your headway will theoretically let you get to your destination earlier, but you run the risk of needing to detour to an auto collision center. In a 2 to 1 merge, one of the lanes must delay themselves 2 more seconds, everybody playing chicken instead of sharing the delay across the two will cumulatively slow things down on the whole.

        This can also be accomplished with a traffic volume sensor and a stop light on highway on-ramps.

        This kind of traffic metering does already exist, as you're probably aware!

        But the fact that even just a single rail car holds 360 commuters, equivalent to 180 cars or more on the highway changes the math completely.

    • https://youtu.be/oafm733nI6U?si=dUBMco9Ql-QtLF2a

      Broke: thinking cgp greys video is informative and good

      Woke: realizing cgo greys video is fucking stupid and car brained

  • They have to get up to speed and there are so many on ramps that don't give enough space to actually get up to highway speeds. And some places (like my fucking dumbass city) have 30 of them spaced only 10 feet apart, so you have a lot of people going really slow getting on the freeway, so everyone else has to slow or stop to avoid hitting them.

    • Quite a few cities in the Northeast are designed this way, because they were done so in the 1950' swith the intent of highway traffic doing 40, maybe 45 MPH through town at maximum. To conserve expensive urban land, they have short ramps and merge areas that were appropriate for those speeds, not to mention the lower overall volume of traffic in those times. And now we're stuck with it, because it'll be a 200 year long court battle to eminent domain the 427 landowners who are all clinging to five square feet each in the patch you'd require for a longer ramp, all hoping for a fat payout to let go of it.

      And nowadays, of course, everyone takes it as their god given right to do 90+ MPH on the freeway at all times, and get frothingly pissed off if they can't for any reason whatsoever. So the ramps aren't long enough anymore because no one is using the highway as designed.

      • I'd never heard that point and it's really interesting. I drive around the part of NJ that is close to NY, Bergen, Essex, Hudson Counties, and sometimes it's a straight up stop sign at the edge of a highway. And the problem is, there's no other way to go, I'm not cutting through a residential area or nothing, this is me coming from the turnpike onto Rt. 9 or something, massive thoroughfares with insane volumes. And you just do commit and that's it. Terrible design, but with the light you've shown on it, I can understand it a bit better.

    • You live in a shitty car centric designed city with poor public transportation.

      Ftfy, fucking suburban shit ass sprawl is even spreading to my country like a plague, fuck that shit

  • I feel like HOV lanes are more trouble than they’re worth because they disrupt traffic flow since people need to slow down when the HOV drivers enter/exit

93 comments