# 4-way stop

#### Loren Booda

A car arrives from each direction of a four-way stop intersection simultaneously. How can the drivers determine who has the right of way?

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#### davee123

A car arrives from each direction of a four-way stop intersection simultaneously. How can the drivers determine who has the right of way?
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.

DaveE

#### Bohrok

A trick question? A car as in just one car coming from all four different directions?

#### skeptic2

In Chicago it was always the one with the oldest, beat up car.

#### Jimmy Snyder

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.

#### dilletante

A car arrives from each direction of a four-way stop intersection simultaneously. How can the drivers determine who has the right of way?
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.

#### regor60

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:

#### phlegmy

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!

#### Kronos5253

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.
I think this one's the right answer. lol

#### Phrak

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.
Yes, but the discussion can go on for days, and often stimulate similar discussion in related intersections that would have otherwise proceeded unimpeded.

#### BobG

Homework Helper
Yes, but the discussion can go on for days, and often stimulate similar discussion in related intersections that would have otherwise proceeded unimpeded.
And, naturally, the discussion wandered off topic, although occasionally passing back through the original topic - which is how PF invented the round about.

#### PhantomOort

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?

#### Alsdfj12

Are you serious? The stoplight/stop signs. How do you people not figure that out? I guess you were just thinking TOO hard...:grumpy:

#### Alsdfj12

or...get out, shoot the other drivers, and go.:rofl:

#### DaveC426913

Gold Member
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?
OK. All four of them attempt to slow down and speed up simultaneously, ultimately arrving at the intersection simultaneously, despite their best efforts.

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#### DaveC426913

Gold Member
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.
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?

#### DaveC426913

Gold Member
Argh!

All of them signal to make right turns. Now they have right-of-way, and may proceed.

#### dotman

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.

#### Sorry!

Why does it matter who goes first if they are all making right hand turns?

#### DaveC426913

Gold Member
Why does it matter who goes first if they are all making right hand turns?
Beat you to it.

#### Loren Booda

Find out which violates CPT symmetry.

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