589: Designated Drivers

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Designated Drivers
Calling a cab means cutting into beer money.
Title text: Calling a cab means cutting into beer money.


Ambox notice.png This explanation may be incomplete or incorrect: Diagram is not analysed at all - does it match the stated problem?
If you can address this issue, please edit the page! Thanks.

There is an intricate kind of strategy and logical thinking required for going out with friends. As the third panel illustrates, there are three people who have yet to get to the bar that these four are at already. That whole group, after having imbibed will be splitting up, some going to a dinner party and others going to a non-descript kind of party. After dinner, everyone is moving on to the other party, and from there people are going to head home, or off to the bar. The enormous complexities of planning who car pools with whom, from where to where, and when make an excellent logic puzzle.

Speaking of logic puzzles, Randall alludes to the classic logic puzzle, in the last panel with the man with the goat saying he can't be in the car with the wolf. In terms of xkcd, Randall brings this up again in 1134: Logic Boat.


[Four people are outside a bar.]
Man #1: Wait, who's driving?
Man #2: Why?
Man #2: Tom, right?
Man #1: Yes, but we have to leave in two groups. One of which will need at least two drivers.
[There is a complicated flowchart with arrows between a group of people and 3 locations, labeled 'bar', 'dinner', and 'party'. Lines point from the group of people to the bar, then to the party or dinner, then from dinner to the party and vice versa, as well as leaving the panel or entering the panel in several other directions.]
Someone has to get Paul, and Julia and Emily have to leave by 10:00.
The logistics of who can get drunk are nontrivial.
[The third man has an animal on a string behind him, which was previously not visible.]
Man #3: Yeah, and I can't ride in a car with the wolf because he'll eat my goat.
Man #1: Dammit, guys.

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I think I read a reference to the goat/wolf puzzle in an older comic. 18:08, 1 January 2014 (UTC)

Why no explanation of the third panel? Is it because it doesn't make sense? That seems unlike Randall, so I'll have a go.

Proposed Order of Events:

  • On the diagram, we see three figures entering the bar, and three lines entering dinner (probably Paul, and Emily and Julie). That makes six people altogether.
This makes no sense as there are four at the bar to begin with. Also one of the people already there would have to go and pick-up Paul and thus arrive at the party with him (two lines entering). Since Megan could be one of the two girls, the last girl could be the one arriving alone. But the above takes the first Cueball out of the equation! Thus if what I wrote here is true, then all the rest of this explanation below falls --Kynde (talk) 19:17, 31 October 2015 (UTC)
  • Of the three at the bar, two go to dinner and one goes straight to the party
  • All five people leave the dinner and go to the party, joining the sixth.
  • Two of the six leave the party together (Julie and Emily at 10pm)
  • One of the six leaves the party and goes back to the dinner venue, and from there goes home.
  • One of the six leaves the party and goes back to the bar
  • The last two leave the party and on their way home, appear to join up with the one who went back to the bar

Because there are no times or identities assigned to the paths, other interpretations are possible.


  • The diagram as a whole has six entries and six exits, as does each venue. There are definitely six people who all enter from the outside, and eventually leave.
  • Why are there four people visible in the first panel, but only three people start at the bar? That part makes no sense. To match the diagram to the comic, we have to pretend that there are only three people in the first panel.
  • Who is Tom? He must be one of the people visible in the first panel, even though he is spoken about as though he wasn't there.

Ignoring the goat for now, how many drivers do we need?

  • Most cars can take five people at a pinch, so sheer numbers don't seem to be the issue.
  • You need two drivers (and two cars) if people were leaving at different times, or heading in entirely different directions.
  • Presumably, Julie and Emily arrive in their own car and also leave in it. Ignore them for now..
  • Someone drives back from the party to the dinner, and then home. This must be one of the people from the first panel, and they must have their own car.


  • There are only three people at the bar, Tom, Megan and (I'll say) David. They each have their own car.
  • David drives to the party. Tom and Megan drive separately to the dinner, one of them collecting Paul on the way.
  • Tom and Megan drive from the dinner to the party; Paul rides with one of them.
  • Megan later drives back to the dinner venue, then home.
  • Paul later drives Tom's car back to the bar, and proceeds to get drunk.
  • David drives Tom a ride to the bar, to collect his car and drive Paul home.

Note that this scenario implies that everybody who is initially at the bar has to be a designated driver.

And I have still not considered the goat.

Any other interpretations would be welcome! I am not really satisfied with this but got tired of thinking about it. 18:52, 7 February 2014 (UTC)

tl;dr or Wikipedia:Too long; didn't read. Please calm down just to the essentials. And please try to keep an explain just straight forward. --Dgbrt (talk) 21:08, 7 February 2014 (UTC)
I have made a suggestion that could explain the information and the diagram, but it assumes that the party is held at Cueballs place (the one who talks first). And that he starts and ends there. The three drivers is though not easy to explain. --Kynde (talk) 19:17, 31 October 2015 (UTC)

<sigh> everyone forgets the cabbage... Brettpeirce (talk) 15:12, 27 February 2014 (UTC)

- For those unfamiliar, the goat/wolf reference is an old logic puzzle. You have a goat, wolf and head of cabbage. Using a rowboat, how can you get them all safely across a lake? Sometimes the assumptions are given: Goat eats cabbage, Wolf eats goat. Sometimes only 1 item at a time in rowboat, sometimes two. Solve. (Goat, empty, ...)

- BTW, I think the diagram description above is ok, shows how lots of interpretations possible. If it is inaccurate, well, has the artist already been to the bar? Arranging outings with friends, sometimes it's just a hassle, eh? And people will try to solve problems with the tools they know, hence a flow-diagram from the head XKCD geek. Just needs a state table with optimizations to make it complete! (-: 18:25, 9 June 2014 (UTC)

I like the double meaning of "non-trivial" in the comic. There's the math sense of the term and also the human sense. To fully appreciate the human sense it helps to be old enough to know that, say, 35 years ago, drunk driving was commonly regarded as a trivial problem. MADD (Mothers Against Drunk Driving), founded in 1980, helped change this, as did increasingly drastic penalties and public shaming of people who drove drunk. When I was growing up (1960s) and a young adult (1970s) there was no concept of Designated Driver. Npsych (talk) 09:52, 9 October 2015 (UTC)

I believe the reason one group needs two drivers is that said group may be due for a longer, cross-continental trip (and driving hung over with minimal sleep in a sedan isn't much easier than driving drunk)... Maybe we finally solved anothe logic problem? Papayaman1000 (talk) 03:22, 6 March 2016 (UTC)

Just overhauled the last two sections. If anyone has anything to add or I made a mistake in my solution, I'd love to hear feedback on it. Toadfart (talk) 21:46, 14 September 2016 (UTC)