Friday, September 20, 2013

Passing fancy

I travel a lot.  When I'm driving, it's not unusual to encounter a construction zone where one lane is closed.  When a lane is closing, otherwise skillful drivers begin to exhibit some bizarre behavior.

I'm talking about attempts to get into a slow, single-file line before the "choke point" of the lane closing--sometimes miles before.

This behavior is wrong, inefficient, and frustrating.

It's our Puritan heritage at work.  I have, over the last few years, come to realize that much of the opposition to social progress in this country is rooted in the idea that NO ONE should get ANYTHING to which they are not entitled.  That's kept us from getting a single-payer health care system (unlike the rest of the civilized world, much of which enjoys greater efficiency and substantially healthier outcomes overall).  It's what causes some people consternation in the supermarket checkout line when someone pays with food stamps for something that's just a little bit too extravagant for someone who's getting help.

And it is extraordinarily frustrating to people who are "being good," sitting in a long single-file line of cars waiting for a lane to close, to see another car--usually a fancy low-slung sports car with a middle-aged driver--whizzing by to jump ahead in line.  I've even seen some extremely anti-social behavior, like when two 18-wheeler trucks collude to block both lanes so that line-jumpers can't pass, or drivers who refuse to let the line-jumper in at the merge point.

Some states have even made line-jumping a citable offense.

This is madness.  And here's why:

I offer you two propositions that I think you'll agree with in the abstract, plus one that you may have a little bit of trouble with.  First, the flow of traffic through any given point is affected by the number of lanes that are available.  An eight-lane (in one direction) superhighway in L.A. can carry more cars per unit of time than a single-lane (in one direction) country road.  It's just a matter of physics.

Second, without changing lanes, you can only move as fast as the slowest car that's ahead of you in your lane.

These two propositions combine to tell us that the more lanes there are to choose from, the faster the cars can go, and the fewer lanes there are, the slower they can go.

Now, the third proposition:  When the number of lanes is being reduced, the fastest way to get cars through the merge point is to have a car immediately ready to go through the merge point as soon as the preceding car is ready.

Think of it like marbles in a bottle with a neck that's a bit larger than the diameter of the marbles.  The marbles obviously fit through the bottle neck (they got in somehow).  When we turn the bottle over, the marbles will generally fall out one-by-one.  Each time a marble clears the neck, the marble that is closest to the opening will fall out next.

But notice that the marbles spread out across the widest part of the bottle.  They don't line up single-file.  That's because marbles pack in more efficiently when they are not lined up.

Cars are sort of the same.  Even when traffic is slow, or stopped, you're still keeping at least a few feet, maybe most of a car length, between you and the next car, just in case they stop suddenly.  You need time to react to stop.  Traffic studies have shown that people (thank goodness!) tend to leave more space than they actually need to leave.

The consequence of this is something interesting.  Other traffic studies have shown that the most efficient way to move from two lanes to one is something called "alternate merge," or the "zipper method."  Just like a zipper, which knits one prong from the left, one prong from the right, left, right, etc., the zipper method involves one car from the left moving forward, then one from the right, and so forth.  This is efficient because it allows us to use the "extra" space that people leave.  People intuitively feel more comfortable about this because traffic moves in a very predictable pattern.

It's also intrinsically and obviously and scrupulously fair:  One for you, one for me, one for's a method of divvying that everyone understands from a small age is fair.

The problem with the anti-line-jumping crowd is that when you force people too early into a single-file line, the efficiencies of the zipper method are impossible to realize because of the lack of predictability.  For the zipper method to work most efficiently, it has to take place at the point where everyone knows exactly what's going to happen--and that only happens at the point where the "extra" lane is ending.

I suppose that it feels like the line-jumpers are cheating.  And yes, I used to feel that way, too.  But the line-jumpers are the smart ones.  If we all waited until the merge point to merge, then no one would feel like someone else was getting an unfair advantage.  Line-jumpers are only taking the advantages that the cars ahead of them have voluntarily given up by getting over too early.

Now, I would never advocate that you do something that's against the law--although, to be perfectly fair, I don't think I've ever seen anyone pulled over for line-jumping.  But this is yet another example of one of those rules, whether it's an actual law or just some sense of collective morality, that just needs to change.  If we can all get on the same, efficient page, we can save ourselves a lot of time and a lot of heartache.

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