Circulation

Circulation, as I think of it in this context, is what you do when you take advantage of the fact that it's easier to turn right than left. It's sort of like the circulation of blood, in that you're going with the flow of traffic, and it's sort of like going in circles … clockwise circles, to be precise.

Why is it easier to turn right than left? The underlying reason is, of course, that we drive on the right side of the road; but I find it more instructive to look at specific situations.

• When turning onto a road from a side street or parking lot, you need both sides of the road to be clear to make a left turn, but only one side to make a right.
• When turning at a stoplight, you need to wait for a green light, or, worse, a green arrow, to make a left turn, but in most states you can make a right even on red.
• Even when “turning” from one interstate to another via a cloverleaf, you have to go around a whole loop to turn left, but only around a short arc to turn right.

Here's an example where circulation would work well. Suppose you're driving south on some street, and need to get to a destination one street over. The natural thing to do, I think, would be to follow the red path and turn just before the destination.

An alternative, that might well be simpler and faster, would be to circulate and follow the blue path.

Unfortunately, circulation doesn't always work so well. Suppose you now need to get back on the original street, northbound. In this case, the alternative suggested by circulation is exactly the same as the natural path, only with an extra loop.

Maybe, just maybe, if the streets had two or more lanes in each direction, and the traffic were very dense, the alternative would be better, but that seems like a stretch even to me.

Anyway, the point isn't that circulation is always the best method, but rather that it's sometimes a better method. The best method, it seems to me, will be a combination of the two … perhaps, in the example above, circulating to get to the destination, but returning directly.

In downtown areas that have alternating one-way streets, some different idea of circulation must be necessary, but I haven't had enough experience to figure out what it is.

Finally, speaking of natural paths, here's a little tidbit that has almost nothing to do with circulation. When I walk from one place to another, there's usually a natural path that I follow, one that seems like the obvious choice given the geometry of the streets and buildings and so forth. However, when I walk between the same two points in the other direction, the natural path doesn't always come out the same. If, for example, I have to walk around a rectangular building, I almost always prefer to take the long side first, like so.

I don't think this means a whole lot, just that I'm using local rules rather than global analysis to choose a path, but I still like to think about it.

A while back, I ran across an article that explained that UPS uses circulation! It didn't put it that way, of course, but still, you can imagine how pleased I was. Apparently circulation really is more efficient, saving both time and gasoline. (The article also says that it improves mileage, but that seems like a bad metric to use, since circulation will probably also increase the number of miles driven.)

Note that the article is from June 2008, when efficiency was on everyone's mind because gas prices had just gone through the roof. They've settled back down for now, but … well, let me just link you over to The Age of Transportation.