Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do pedestrian crossing signals count down then the traffic light stays green?

  1. Jan 15, 2012 #1

    DaveC426913

    User Avatar
    Gold Member

    Uh... what do I say now? Do I repeat it, like a dolt?

    Why do pedestrian crossing signals count down then the traffic light stays green?
     
  2. jcsd
  3. Jan 15, 2012 #2

    Moonbear

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It might be so the cars have a chance to actually turn at intersections with a lot of pedestrian traffic. I don't live in a place where the crosswalk signs do a count down, and the only place I've seen that is in D.C. and midtown NYC, so I can't be sure, but that seemed to be the situation. Of course, in reality, it doesn't work anyway, because the pedestrians just keep crossing until they see the light change. You'd think it was pointless to bother with a pedestrian crosswalk sign, until you'd been to a place where they didn't exist and realized how stupid the average pedestrian is. Someone forgot to teach a LOT of people, "Cross on the green, not in between."
     
  4. Jan 15, 2012 #3
    Maybe to give those still wanting to cross an idea of how long they have to cross before the cross traffic is let go.
     
  5. Jan 15, 2012 #4

    Pythagorean

    User Avatar
    Gold Member

    how long does it stay green in your town? It seems like it's up to the traffic engineers, ultimately.

    Our delay isn't that long, but I think the point is that people have different walking speeds and if you give them the feeling they have lots of time, the slower people might not make it all the way across. So you have to take the subjectivity of people's time predictions into account.

    Even when the lights go yellow then red for cars, the orthogonal lights don't instantly change to green. They delay a bit to give drivers with bad time perception time to get through.
     
  6. Jan 15, 2012 #5

    Moonbear

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Likewise for the pedestrians. If they are slow and start across when the countdown has begun, even if they're slow, there's a chance they'll make it across before the light changes. There will always be some idiot who starts crossing as the countdown is on 1...0... and it makes sense to have some lag for them to finish crossing too if the traffic engineers anticipate that there are so many stupid people. Ideally, the light will switch to Don't Walk with enough time for those in the middle of the intersection to get out of the way before the vehicles get the green light to start running them over.
     
  7. Jan 15, 2012 #6
    Does the countdown happen while the traffic light is green, or a blinking red?
     
  8. Jan 15, 2012 #7

    Moonbear

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Are you in Canada? In the US, there's no blinking red. We have green, to yellow (which is very brief, because most people ignore it or accelerate to beat the red), to red. Blinking red and yellow are for intersections where the lights aren't working right. Blinking red should be interpreted as a stop sign, and blinking yellow should be interpreted as a yield sign. Neither is interpreted right by most drivers, and the first collision brings a traffic officer to the scene to direct traffic more appropriately.
     
  9. Jan 15, 2012 #8

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    You walk 3 to 4.5 miles/hour. Cars, they go 30 to 45. The yellow light is timed for cars, not people.
     
  10. Jan 15, 2012 #9

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    The UK using sound warnings. When the road light goes red, a bleeper starts indicating it is safe to cross. After the bleep stops, the road lights flash red for long enough to give pedestrians already on the crossing time to get across, but nobody else is supposed to start crossing. If nobody is on the crossing, it's legal the cars to drive through the flashing red light.

    The bleep has the advantage over lights for blind pedestrians (in fact for everybody, since you don't have to look for the light) but it can confuse US tourists. The usual conversation goes something like

    "Hey, what does that bleep mean?"
    "When it stops, it warns blind people the lights are about to change soon."
    "Gee, you mean in the UK blind people are allowed to drive :surprised: ???"
     
  11. Jan 15, 2012 #10
    I was referring to the pedestrian traffic signal.
     
  12. Jan 15, 2012 #11

    Evo

    User Avatar

    Staff: Mentor

    The problem I find with the crosswalk timers here is that they start blinking and the hand to stop comes up before people make it halfway across. I assume this is to discourage people that have not yet started crossing to stay put. Those that started crossing as soon as the go sign appeared are lucky to make it across before the light turns.
     
  13. Jan 15, 2012 #12

    jtbell

    User Avatar

    Staff: Mentor

    Around here, the countdown tells you how much time you have left during which you can safely start to cross the street and reach the other side before the cross traffic receives the green signal (and parallel traffic receives the red signal). It's presumably timed for slow walkers; I can still start to cross for a few seconds after the countdown reaches zero, provided that I don't dawdle.
     
  14. Jan 16, 2012 #13

    DaveC426913

    User Avatar
    Gold Member

    I guess I thought this was common. Maybe only in Canada.

    Let me clarify what happens.

    On a plain old suburban road (in the outskirts of a major metropolis), at an intersection with smaller side roads, there will be traffic lights. These in are very common where I live. A main residential road will have a traffic light every half dozen blocks or so. Despite being a main road, the traffic is not at all heavy except perhaps during rush hour. Few pedestrians are likely to be around.

    Main road traffic light is green, pedestrian crosswalk says Walk.

    Then, with no one around, the pedestrian crosswalk signal will turn to Don't Walk and begin counting down from about 20. When it reaches 0, it changes back to Walk, and the traffic light stays green (instead of changing to yellow as one would expect in preparation for a stop).

    This happens routinely despite the complete absence of cross-traffic or pedestrians.
     
    Last edited: Jan 16, 2012
  15. Jan 16, 2012 #14
    My guess is they put them at intersections where people get hit often so that people feel more safe in crossing that said intersection.
     
  16. Jan 16, 2012 #15

    DaveC426913

    User Avatar
    Gold Member

    No. We have roads that are miles long and have these at every intersection. It is routine.

    I can think of one rationale that's marginally plausible. Traffic calming. Seeing signals counting down causes drivers to be more conscious of their speed and of potential cross traffic. With few major crossroads (i.e. no red lights), it is easy to get lulled into a sense that you're literally on a highway and can just whip along these roads at top speed (or more) all the way from start to finish.
     
    Last edited: Jan 16, 2012
  17. Jan 16, 2012 #16

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Here the sign changes from Walk, to Walk with a countdown, to Don't Walk.

    Your version sound like the person who programmed the light played one round too many of "beer hunter" the night before.
     
  18. Jan 16, 2012 #17
    If it is a pedestrian crosswalk, then a blinking red light means people who are already found on the path can finish their crossing. Other pedestrians are discouraged to start crossing. The countdown shows in what time exactly is left. People who can walk faster can still make it.
     
  19. Jan 16, 2012 #18

    DaveC426913

    User Avatar
    Gold Member

    Yes yes yes.

    Except 'make it' to what? The traffic light stays green. The pedestrian cross walk wasn't counting down to anything.
     
  20. Jan 16, 2012 #19
    Oh, so the pedestrian light shows green, counts down 10 secs, and then stays green again?
     
  21. Jan 16, 2012 #20

    DaveC426913

    User Avatar
    Gold Member

    Yes. Did I not make that clear?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why do pedestrian crossing signals count down then the traffic light stays green?
  1. Why does he stay? (Replies: 30)

  2. DO Look Down (Replies: 2)

  3. The final count down (Replies: 17)

Loading...