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4-way stop

  1. Sep 24, 2009 #1
    A car arrives from each direction of a four-way stop intersection simultaneously. How can the drivers determine who has the right of way?
  2. jcsd
  3. Sep 24, 2009 #2
    This depends on what country they're driving in, and possibly in what state/province/etc, since driving laws can change regionally within a country.

    In Massachusetts, there is no legal recourse for determining who goes first of which I'm aware. Someone just takes the initiative.

    If you're asking as a hypothetical, one could decide based on any one of a number of statistics, such as the alphabetic precedence of their license plates. However, this is not realistic, as not all cars have license plates that are visible to other drivers on their front bumpers. One could also make the determination based on the closest compass direction, with (say) North/East/South/West taking precedence. But that assumes that you can tell the compass direction. There are a bunch of other ways that might work, but nothing that I can think of offhand that's fool-proof from a realistic standpoint.

  4. Sep 24, 2009 #3
    A trick question? A car as in just one car coming from all four different directions?
  5. Sep 24, 2009 #4
    In Chicago it was always the one with the oldest, beat up car.
  6. Sep 24, 2009 #5
    The normal rule is that the one on the left has the right of way. However, in this case that would cause an infinite loop and result in a black hole sucking in cars forever. So I'll say that the one on the right goes first.
  7. Sep 26, 2009 #6
    Since all 4 drivers belong to the Physics Forum, they are aware that there is no such thing as absolute simultaneity. So they exit their vehicles to determine exactly in whose frame of reference this simultaneity occurred, and during the course of this discussion they agree on an order in which to proceed.
  8. Sep 28, 2009 #7
    Since there are no established accepted rules and presumably they haven't communicated beforehand, some form of universally understood communication needs to take place, so this teaser is :yuck:
  9. Sep 28, 2009 #8
    they sit there forever, or untill they come up with a superior alternative to a 4 way stop, and so the round about was born!
  10. Sep 28, 2009 #9
    I think this one's the right answer. lol
  11. Sep 28, 2009 #10
    Yes, but the discussion can go on for days, and often stimulate similar discussion in related intersections that would have otherwise proceeded unimpeded.
  12. Sep 29, 2009 #11


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    And, naturally, the discussion wandered off topic, although occasionally passing back through the original topic - which is how PF invented the round about.
  13. Oct 9, 2009 #12
    It's a moot point. If you see that three other drivers are going to reach the stop at the same time you do, just slow down or speed up to make sure the problem doesn't arise. That's what I do. Like I can't wait a few more seconds to get where I'm going?
  14. Nov 1, 2009 #13
    Are you serious? The stoplight/stop signs. How do you people not figure that out? I guess you were just thinking TOO hard...:grumpy:
  15. Nov 1, 2009 #14
    or...get out, shoot the other drivers, and go.:rofl:
  16. Nov 1, 2009 #15


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    OK. All four of them attempt to slow down and speed up simultaneously, ultimately arrving at the intersection simultaneously, despite their best efforts.
    Last edited: Nov 1, 2009
  17. Nov 1, 2009 #16


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    I think there's an implicit proviso: the drivers have not agreed to any rules beforehand.

    Loren, is it safe to say that getting out of their car to discuss it is effectively ruled out as an impracticiblity?
  18. Nov 1, 2009 #17


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    All of them signal to make right turns. Now they have right-of-way, and may proceed.
  19. Nov 1, 2009 #18
    They look at each other, and one waves to another to go, who edges out, and sees he's being allowed to go, and goes. The rest repeat.

    We've all been in this situation dozens of times, usually with just another car or two, however, in mall parking lots or the like.

    It's situations like this where we should be happy that people are driving the cars, and not computers.
  20. Nov 1, 2009 #19
    Why does it matter who goes first if they are all making right hand turns?
  21. Nov 1, 2009 #20


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    Beat you to it.
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