Why Is This Traffic Stopped??

Update from the comments, Nov. 3: Harold gives us this brilliant link. Completely contradicts my surmise below. Definitely watch the video.

Being cursed with a curious mind, when I’m driving — especially on the highway — I’m always wondering about the hydrodynamics of traffic. Here’s an interesting insight:

With cars moving fluidly in a tight pack even a seemingly innocuous change of lanes may cause a tiny disruption which is propagated backwards for many miles.

[A new study fingers] timid and aggressive driver behaviour as the main culprit. … vehicle speeds drop to zero if just a few drivers accept shorter distances between their car and the one in front, and a handful of others in the same lane prefer a greater gap, relative to the “equilibrium spacing” which in theory ensures a steady ride.

It’s been my impression that one car leaving excessive distance —especially in the left lane — can clog up everything. I’m not advocating tailgating (I hate that), but…

Here’s my solution to highway traffic congestion: the government or some civic-minded nonprofit should give away these bumper stickers:






7 responses to “Why Is This Traffic Stopped??”

  1. Big Sis Avatar

    You’ll be happy to learn a tiny bit of information that I picked up in my physics undergraduate study, and which remained in my brain. If my memory serves, traffic behaves (mathematically speaking) like a non-collisional supersonic fluid. And apparently very small events, and sometimes even just random ones, will cause a shock-wave (aka a traffic jam) to propagate back through the traffic.

    There are a bunch of for-the-layman explanations around. Just for example see: http://www.failedsuccess.com/index.php?/weblog/comments/traffic_jam_causes/ and http://web.archive.org/web/20051220035011/http://www.nature.com/news/1998/981126/pf/981126-8_pf.html

    Unless we ever end up with automatically guided cars, where we become just passengers instead of drivers, I think we’ll have to live with the problem. (I.e. as long as flesh-and-blood humans are piloting the cars).

  2. Chris T Avatar
    Chris T

    I ran into this two weeks ago when I was driving up I-5 between Olympia and Fort Lewis. Traffic slowed to a crawl about Olympia and then sped up again for no apparent reason around Fort Lewis. No accidents or construction to explain it.

    Sometimes my preferred solution to traffic is rocket launchers.

  3. bj Avatar

    We always notice this when going over the I-5 bridge (north to south). The trafic clears mid way on the bridge. Since we haven’t observed a large number of cars exiting into South Lake Union, we always wonder where the clear up is coming from.

    Presumably it’s the result of some kind of clog that works its way out after it has been caused and before the bridge ends, but, we still don’t get it. Our current working hypothesis is that it has something to do with people moving over from the right entrances on to the I-5 before the bridge to the left exit for the 520 after the bridge slowing everyone down.

  4. Asymptosis Avatar

    > people moving over from the right entrances on to the I-5 before the bridge to the left exit for the 520

    Agreed. 45th and 50th. And it backs things up all the way to Northgate sometimes.

    Cap’n Dave’s theory is that there’s a limited sight line coming up the bridge, and people feel more comfortable after they get over the top. But I don’t think so.

    The shoulda put that 520 exit on the right side like a respectable exit.

  5. Chris T Avatar
    Chris T

    Although backed up traffic around exit 65 (S bound) did save me when my brakes failed last year. I’ve never been so glad to only be able to go five miles per hour.

  6. Harold Avatar

    http://trafficwaves.org/ has an interesting take on the topic. His solution, for certain densities of congestion: try to maintain a large gap in front of your car.

    1. Asymptosis Avatar

      Harold, this is brilliant. Hoisted to an update at the top of the post. I wonder how accurate his “ant’s eye” view of things is.