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How do traffic signals work? (2019)

115 points · 86 comments · at1as

  1. Sohcahtoa82 · · focus · HN ↗
    > Actuated signal control [...] inductive loop sensors embedded into the road surface

    It blows my mind how many people don't realize that's what those circles near the stop bar of an intersection are for, and will then stop 3+ car lengths behind the stop bar, and then can't figure out why the light is never going green.

    > two signals closely spaced in a row on a major roadway. If one signal gives a green but the next one doesn’t, cars can back up

    This is a major problem at an intersection near me that I used to have to drive through frequently. Two intersections, only about 500 feet between them, and MOST traffic during rush out coming from just one of the directions, yet the lights were often exactly out of sync. The first light would be red while the next one was green. Then the first would go green while the other went red, and traffic would only move about 500 feet then stop for 2 minutes. The backup would last well over a mile.

    > The obvious next step in efficiency is coordination of most or all the signals within a traffic network. This is the job of adaptive signal control technologies, or ASCT. [..] These types of systems can dramatically reduce congestion

    My cynical ass would not be surprised at all if oil/gas companies were lobbying against ASCT because idling cars are buying more fuel. Wouldn't be too hard to convince people that an ASCT would cost too much to implement while not producing significant traffic benefits.

    1. at1as · · focus · HN ↗
      > This is a major problem at an intersection near me that I used to have to drive through frequently. Two intersections, only about 500 feet between them

      In San Francisco (as well as other cities I've lived in) they put police in the middle of intersections during rush hour to guide traffic (i.e., stop cars before they drive into an intersection that they'll get stuck in the middle of). Seems woefully inefficient

      1. WalterBright · · focus · HN ↗
        The traffic cops greatly improve throughput.

        Imagine how much gas would be saved if the city spent a little effort on the traffic lights.

        1. andrepd · · focus · HN ↗
          How much gas would be saved by turning half of each 4-lane stroad into a bike lane! :)
          1. WalterBright · · focus · HN ↗
            There are many bike lanes where I live, all installed in the last few years.

            The bike traffic remains 1% of the car traffic, and 0% when it's cold/rainy.

            1. andrepd · · focus · HN ↗
              Oh man, so much to be said about those two short, crass sentences... x)
              1. WalterBright · · focus · HN ↗
                The cyclists also tend to wander out of the bike lane into the car lane, without looking, and I've had many try drafting off of my bumper.
                1. andrepd · · focus · HN ↗
                  > The cyclists also tend to wander out of the bike lane into the car lane

                  This sentence alone tells me that it's not a bike lane. A painted line is not a bike lane.

                  Walter I'm seriously fighting the urge to write a wall of text in response. Why the knee-jerk reaction and the hyperboles? It's not interesting conversation...

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