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Comment by gbadman | Hacker News Reader

I've always felt that driving in traffic is a sort of prisoner's dilemma.

Consider a two-lane highway that splits into two directions, a very popular direction and an unpopular direction. Inevitably, we will see a group of people who will wait to the last second to cut into the popular lane. These late-comers cause two problems: 1) people to hit the brakes who get cut off in the popular lane, and 2) people to hit the brakes in the unpopular lane who are forced to slow down to allow the selfish drivers to merge.

When I take the generous approach and wait in line, I hate people who cut in. On the other hand, I find myself clever for cutting in at the last second and saving time when I take that approach. I'm still not sure which approach is 'right'. All I know is that the latter will increase overall delays while doing the former will have little to no impact on overall delays but a major impact on my own.

Traffic is a pretty interesting subject.

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> " I'm still not sure which approach is 'right'"

If you have a circumstance where 2 lanes are merging into 1 lane, merge as late as possible. It sounds counterproductive, but it's actually best for traffic as a whole.

See http://trafficwaves.org/seatraf.html and the HN discussion at http://news.ycombinator.com/item?id=785259 . http://drmomentum.com/aces/archives/003800.html also has a number of nice links.

Yes. It's paradoxical because merging early seems like the polite thing to do -- but actually, if you do that, you're allowing people who were behind you to get in front of you, which makes the merge unfair.

My interpretation is that gbadman did not mean the situation where two lanes merge into one, but rather where you have two lanes traveling in a single direction approaching an interchange where the lanes split such that the left lane goes one direction and the right lane goes another.

In gbadman's example one lane (let's say the right hand lane) goes to a road that does not back up, whereas the left hand lane goes to a road that will back up through the interchange. In this situation, you have a left lane congested with people waiting to go on the busy road, and a right lane that should be almost free flowing. By most understandings (including the execellent book, Traffic) people wanting to go to the busy road should wait in line in the left lane, allowing people going to the less busy road to pass freely. However, many drivers will stay in the right lane until the very last minute, then attempt to move into the left lane before the right lane splits off, backing up both lanes.

You are right in your interpretation of what I meant.

I think the lane split is the better example of the prisoner's dilemma. As others have mentioned that in the two-lane merge situation the global optimum is the selfish approach.

>If you have a circumstance where 2 lanes are merging into 1 lane, merge as late as possible. It sounds counterproductive, but it's actually best for traffic as a whole.

I think it's important to stress the caveats of that recommendation.

It applies to heavy, congested (as opposed to flowing or merely partially congested) traffic with a merge that is orderly, zipper-like.

So, don't just merge as late as possible. Rather, when faced with heavy, congested traffic perform an orderly merge, as late as possible.

except when presented with a chance, someone would attempt to disrupt this order in order to get ahead themselves. The pure selfish (or greedy?) method doesn't lead to the optimal result.

As i mentioned in some other comment, self-driving cars taht you "hail" are the best compromise between public transport and private transport.

I've debated that sort of thing several times with my dad. He argues that if everyone waited until the last possible point to merge, things would be better, because there'd only be one point of congestion. I.e., nobody could "cut" the line because everyone would be merging at the end, instead of the people in the slow lane being slowed down not just by people merging in ahead of time but also by people merging at the last possible second.

I can sort of see his point, but the downside to me seems to be that if you wait until the very end, and don't get to merge in cleanly, you have to slam on the brakes, and nothing kills traffic like someone having to merge in from a complete stop. So my tentative position is that it would be best if everyone merged exactly as soon as they could do so without significantly slowing down (speeding up to merge seems like it would be a good thing, in comparison?). But good luck with that, heh.

According to the book traffic, your dad is right. But the effeciency only works if everybody waits.

After reading Traffic, I always merge at the last minute (and I have never had to "slam the brakes" from this behavior)

When you say major impact, do you mean something like 45 seconds, or something else?

I'm asking in all seriousness, I have no idea what sort of traffic you are driving in.

Not sure where gbadman is, but a similiar situation I often see at the circle interchange in Chicago waiting in line (as you should do in that case) can easily cost you several minutes.

I'm in Montreal. We have a serious traffic problem during morning and evening rush hours. In fact, TomTom has put our city at #4 in North America for worst traffic and it seems #1 for both morning and evening traffic peaks [1].

Two places that I can think of this being brutal are taking Decarie Blvd exit from Decarie North and taking the 20W exit off of the 13S. I'm sure that are some other pretty good examples out there.

1: http://www.tomtom.com/lib/doc/congestionindex/2012-1003-TomT...

Consider a two-lane highway that splits into two directions, a very popular direction and an unpopular direction.

Heh, years ago I wrote a comment here about a situation like that, but with three lanes (and one more for the exit), that I suffered.

Then, as if they were reading, in a few weeks the output was changed as I wanted: two lanes in each direction. The results were excellent, no more jams in that exit.

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